Ver. 3.1.2
Maersk Training - DP Training Manual
Ver. DPMAN.0400.04
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TABLE OF CONTENT
COPYRIGHT ............................................................................................................................... 7
Complaint procedure.................................................................................................................... 7
Health and Safety ........................................................................................................................ 7
Contact Maersk Training .............................................................................................................. 7
Chapter 1 - Course Descriptions .......................................................................9
Disclaimer .................................................................................................................................... 9
DP Induction/Basic Course ........................................................................................................ 11
DP Simulator / Advanced Course .............................................................................................. 14
DP Sea Time Reduction / NI Shuttle Tanker Course C.............................................................. 16
Chapter 2 - Glossary and Abbreviations ..........................................................19
Chapter 3 - Introduction to DP ..........................................................................23
Background................................................................................................................................ 23
The Mohole Project .................................................................................................................... 26
Overview .................................................................................................................................... 27
Chapter 4 – Principles of DP .............................................................................29
The DP/Vessel System .............................................................................................................. 29
Vessel Movement ...................................................................................................................... 30
Positioning Principles ................................................................................................................. 32
Chapter 5 - The KONGSBERG DP System .......................................................33
Block Diagram of a Kongsberg DP System ............................................................................... 33
The Vessel Model ...................................................................................................................... 34
K-Pos 11/21/31 system layout ................................................................................................... 39
Integrated Kongsberg System ................................................................................................... 43
Kongsberg DP system main modes ........................................................................................... 44
Chapter 6 - The Marine Technologies DP System ...........................................45
The Power Supply System......................................................................................................... 46
The Fail-safe Communication Network ...................................................................................... 47
The Fail-safe Control Computer (CC) ........................................................................................ 49
The Fail-safe Operator Station (OS) .......................................................................................... 50
The Thruster Controller (TC)...................................................................................................... 50
Operational Flow Chart .............................................................................................................. 52
Chapter 7 – Other Manufacturers of DP Systems ...........................................53
AutoNav IVCS 2000 ................................................................................................................... 53
EasyDP 100 ............................................................................................................................... 54
Rolls-Royce - Icon ..................................................................................................................... 55
Wärtsilä – DPI, Dynamic Positioning Inc. - Platinum .................................................................. 56
Chapter 8 – Planning, Setup and worksite approach ......................................57
Operation planning .................................................................................................................... 57
Activity Operational Planning - CAMO / TAM / ASOG ............................................................... 58
Contingency planning ................................................................................................................ 61
Risk Assessment ....................................................................................................................... 63
Worksite Approach .................................................................................................................... 65
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Chapter 9 - Thrusters........................................................................................ 69
Thruster Types ........................................................................................................................... 69
Thruster failure modes ............................................................................................................... 73
Chapter 10 – Environmental Sensors .............................................................. 75
Wind Sensors ............................................................................................................................. 75
Tide or Current ........................................................................................................................... 80
Roll & Pitch................................................................................................................................. 80
Chapter 11 – Position Reference Systems...................................................... 83
Notes for use of Position Reference Systems ............................................................................ 83
The Hydro-Acoustic Position Reference System........................................................................ 84
The Artemis Microwave Position Reference System .................................................................. 91
The Taut Wire position reference system ................................................................................... 97
Fanbeam System ..................................................................................................................... 107
Fanbeam Maintenance............................................................................................................. 111
CyScan system ........................................................................................................................ 112
RadaScan system – FMCW principle ....................................................................................... 113
RADius system – FMCW principle ........................................................................................... 114
GNSS(Global Navigation Satellite System) - Overview ............................................................ 117
GPS History ............................................................................................................................. 117
GPS System Description .......................................................................................................... 118
Glonass History ........................................................................................................................ 128
Glonass System Description .................................................................................................... 129
Galileo History .......................................................................................................................... 134
Gallileo System Description ..................................................................................................... 134
DARPS (Differential Absolute and Relative Positioning System) ............................................. 138
Sources of Errors in GNSS systems ........................................................................................ 139
Sum of errors ........................................................................................................................... 144
Differential GNSS ..................................................................................................................... 145
SBAS - Satellite Based Augmentation System......................................................................... 148
Chapter 12 – Positioning Weighting .............................................................. 153
Position-reference systems Handling ....................................................................................... 153
Establishing Reference Origin .................................................................................................. 155
Messages, warnings and Alarms ............................................................................................. 156
Tests on position measurements ............................................................................................. 156
Weighting ................................................................................................................................. 163
Operator Considerations .......................................................................................................... 163
Position Information in the Kongsberg DP systems ................................................................. 165
Marine Technologies Reference Systems ................................................................................ 167
Chapter 13 – The UTM Coordinate system ................................................... 173
Universal Transverse Mercator ................................................................................................ 173
Chapter 14 – DP Operations........................................................................... 179
Pipelay Operations ................................................................................................................... 179
S-lay Operations ...................................................................................................................... 181
J-lay Operations ....................................................................................................................... 182
Reel-lay Operations.................................................................................................................. 182
Seabed Tractors and Trenchers ............................................................................................... 183
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Rockdumping operations ......................................................................................................... 184
Dredging operations ................................................................................................................ 185
Cable lay and repair operations ............................................................................................... 186
Dive support operations ........................................................................................................... 187
Survey and rov support vessels ............................................................................................... 191
Shuttle tanker........................................................................................................................... 193
Submerged turret loading operations ....................................................................................... 198
FPSO unit operation ................................................................................................................ 199
Loading Operations From FPSO Units .................................................................................... 199
Offshore support vessel operation ........................................................................................... 201
Riser angle monitoring ............................................................................................................. 202
Chapter 15 – Power Systems .......................................................................... 205
Power generation, Power management and distribution systems ........................................... 205
Diesel engines ......................................................................................................................... 205
Switchboards ........................................................................................................................... 205
Power management ................................................................................................................. 206
Electric system single line diagram from Maersk Achiever ...................................................... 208
Simplified electric system single line diagram from DSS 21 semi sub ..................................... 209
Chapter 16 – The UPS system ........................................................................ 213
UPS (Uninterruptable Power Supply)....................................................................................... 213
DNV requirement for numbers of UPS ..................................................................................... 213
Chapter 17 – Shallow / Deep Water and Tides ............................................... 215
Shallow water .......................................................................................................................... 215
Deep water .............................................................................................................................. 218
Solitons .................................................................................................................................... 219
Chapter 18 – Rule and Regulations ................................................................ 221
Disclaimer ................................................................................................................................ 221
Redundancy rules and regulations offshore safety .................................................................... 223
Consequence classes .............................................................................................................. 224
IMO, Equipment Classes ......................................................................................................... 224
Overview of authorities ............................................................................................................ 227
Client Requirements ................................................................................................................ 229
Chapter 19 – Failure Mode Effect Analysis (FMEA)....................................... 231
FAILURE MODES AND EFFECTS FOR DPS ......................................................................... 231
Chapter 20 - Human Factors in the DP operation .......................................... 235
Project 1................................................................................................................................... 235
Project 2................................................................................................................................... 240
Chapter 21- DP incidents ................................................................................ 251
IMCA DP Event bulletin2016: .................................................................................................. 252
Classic DP incident .................................................................................................................. 255
Chapter 22 – Read more, Various links .......................................................... 256
Chapter 23 – Drawings .................................................................................... 257
Dan Swift – cable layout .......................................................................................................... 258
NB 190 – cable layout .............................................................................................................. 259
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NB 190 – Cable Layout C-Joy system...................................................................................... 260
NB 190 – Cable Layout Position Reference Systems .............................................................. 261
NB 190 – Cable layout DP Alert system ................................................................................... 262
NB190 – Single Line Diagram Power Set Up ........................................................................... 263
Kongsberg – Power and wiring for panel.................................................................................. 264
Kongsberg – Network Structure 2xOS and DPC2 .................................................................... 265
Kongsberg – Principle block diagram DPC 2 ........................................................................... 266
Kongsberg – Principle Block Diagram I/O signals DPC2.......................................................... 267
Kongsberg – Power and wiring diagram DPC2 ........................................................................ 268
Kongsberg – Power and wiring diagram RBUS A and B section.............................................. 269
Kongsberg –cC-1 principle block diagram and I/O ................................................................... 271
Page 6
COPYRIGHT
This manual is the property of Maersk Training and is only for the use of course participants at
Maersk Training conducted or approved courses.
This manual shall not affect the legal relationship or liability of Maersk Training with or to any
third party and neither shall such third party be entitled to reply upon it.
Maersk Training shall have no liability for technical or editorial errors or omissions in this
manual; nor for any damage, including but not limited to direct, punitive, incidental or
consequential damages resulting from or arising out of its use.
No part of this manual may be reproduced in any shape or form or by any means electronically,
mechanically, by photocopying, recording or otherwise, without the prior permission of Maersk
Training.
Copyright  MaerskTraining
Prepared by: TMM
Modified & printed: October 1st 2016 Version no.: DPMAN.0400.04
Revision History:
Revision History.xlsx
Internal reference: \\MTDKSVB-FILE\Data\Coursemat\01 Maritime\1 General MARITIME\01 DP\02 Course materiale\01 Manuals\DP manual\DPMAN.0400.04 DP Training
Manual.doc
Complaint procedure
Maersk Training are operating a complaint procedure system. Please contact Maersk Training
as mentioned below, in case of any complaint you might feel have not been handled during the
course
Health and Safety
Health and safety information will be provided by your instructor during the start of the course. If
you are in doubt of any health and safety issues or have comments, please contact your
instructor or Maersk Training below.
Contact Maersk Training
Maersk Training
Dyrekredsen 4
Rantzausminde
5700 Svendborg
Denmark
Phone:
+45 70 263 283
Fax:
+45 70 263 284
Email:
svendborg@maersktraining.com
Homepage: www.maersktraining.com
Page 7
Chapter 1 - Course Descriptions
Course descriptions for course accredited by The Nautical Institute are reproduced below, due
to requirements by The Nautical Institute. Please notice Disclaimer below.
Course description for special courses, can be handed out upon request.
Course descriptions for other open course in which this manual is used can be found on
www.maersktraining.com .
Disclaimer
The below course descriptions are to be considered as uncontrolled copies.
Please look up the latest version of the course descriptions at www.maersktraining.com
Page 9
DP Induction/Basic Course
Purpose
At the end of the course the student should:
• Have acquired knowledge of the principles of DP.
• Have acquired a basic understanding of how to set up a DP system.
• Have an understanding of the practical operation of associated equipment, including
position reference systems.
• Be able to recognise the various alarm, warning and information messages.
• Be able to relate the DP installation to the ship system, including (but not limited to)
power supply, manoeuvring facility, available position reference systems and nature
of work.
• Be able to relate DP operations to the existing environmental conditions of wind, sea
state, current/tidal stream and vessel movement.
Content
The training is based on the program outlined by the Nautical Institute Scheme of Training.
Training is a mix of theoretical sessions and practical exercises on Kongsberg DP standalone simulators. All training is conducted as classroom training.
• DP CONTROL STATION.
• POWER GENERATION AND MANAGEMENT.
• PROPULSION UNITS.
• POSITION REFERENCE SYSTEMS (PRS).
• SENSORS.
• DP OPERATIONS.
• PRACTICAL OPERATION OF A DP SYSTEM.
Objective
Clear objectives are set in Nautical Institute’s Training Scheme. These objectives
can be summarised as follows;
By the completion of the training session, the trainee must be able to:
•
DP CONTROL STATION
o Define Dynamic Positioning and use of same.
o Define main components in the DP system and role of same, including
advantages and disadvantages in the use of the DP.
o Discuss concept of mathematical model, and consequence analysis.
•
POWER GENERATION AND MANAGEMENT
o Describe the power generation and distribution arrangements in a typical hybrid
diesel/diesel-electric DP vessel. (Main CPP or Az drive which are direct drive),
with particular reference to system redundancy as described in IMO MSC Circ.
645 and vessel FMEA.
o Describe the functions of a power management system as installed on Class 2
and Class 3 DP vessels and explain the concept of available power and spinning
reserve in worst case failure.
o Describe the provision of uninterruptible power supply to the DP system.
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•
PROPULSION UNITS
o Describe the types of propulsion systems commonly installed in DP equipped
vessels: Main propellers and rudders, azimuth thrusters, Azipod thrusters and
tunnel thrusters, common failure modes and where to detect/monitor the
values of same.
•
POSITION REFERENCE SYSTEMS (PRS)
o Describe the operation, principles and limitations of position reference systems
in general, including differential corrections and inertial navigation.
•
HEADING AND MOTION REFERENCE SYSTEMS
o See sensors.
•
ENVIRONMENTAL REFERENCE SYSTEMS
o See sensors.
•
SENSORS (HEADING, MOTION,ENVIROMENTAL AND EXTERNAL)
o Describe all sensor inputs, how they work, what they do and limitations.
o Describe the use of external force reference systems such as hawser tension,
plough cable tension and pipe tension monitoring.
•
DP OPERATIONS
o Describe/explain the need for procedures, checklists and logbook entries,
emergency and contingency planning.
o Describe the alarm messages provided on the DP system displays and on the
DP printer, and recognize the alarms/warnings associated with loss of
redundancy after worst case failure.
o Explain the use of worksite diagrams using Universal Transverse Mercator
(UTM) coordinates, projections and datum.
o List various providers of documentation and classification societies.
o Describe the hazards associated with DP operations conducted in areas with
very deep water, shallow water and/or strong tidal conditions.
•
PRACTICAL OPERATION OF A DP SYSTEM
o Demonstrate the correct procedure for setting up on DP, and maneuvering in
the different modes.
Assessment
In order to be awarded a certificate of completion for the Induction course the trainee must
pass an online assessment at the training center. The exam is composed of 40 multiple
choice questions and shall be completed in 1 hour and 15 minutes. 70% of the questions
must be answered correctly. Students who fail the first attempt are allowed to have
another two attempts within six months of the first attempt; however, the second attempt
must be undertaken within 24 hours of the first attempt. Failing these three initial
attempts, the student is required to repeat the Induction course and undertake the
assessment again.
Page 12
Admission Requirements
Minimum requirements for entering the DPO scheme are:
The participants must hold either a STCW Regulation II/1 ‐ II/2 ‐ II/3 Deck, Regulation
III/1 – III /2 – III/3 – III/6 Engine or Regulation III/6 for ETOs. Certificate must be
presented on the first day of the course as proof.
Participants with alternative appropriate marine vocational qualifications (MVQs)* are
allowed to participate on the course if holding a NI DPO request letter to enter in the DP
scheme approved by the NI.
*Marine Vocational Qualification: is a non-STCW Certificate of Competency issued by a
white list Maritime Administration for use in the administration’s local waters only.
Prospective Offshore DPOs on the new scheme who are in the process of training for an
STCW certificate may complete, the Induction course (Phase A), the 60 days DP sea time
(Phase B) and the Simulator course (Phase C). The remaining 60 days of DP sea time
training (Phase D) and the subsequent suitability sign-off (Phase E) may only be completed
after they hold an appropriate STCW Certificate of Competency. A letter from an employing
company or approved college or a cadet seaman’s book must be presented on the first day
of the course as proof.
Duration
4 days.
Participants
Maximum 8 pr. instructor
Language
Course material is in English
Course is conducted in Danish or English
Venue
Maersk Training Svendborg
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DP Simulator / Advanced Course
Purpose
To give the student knowledge of the practical aspects of DP operations. He/she should be
capable of planning and conducting any DP operation, including risk assessment,
contingency planning and assessment of vessel capability. He/she should also be able to
recognise conditions which will cause degraded operational status or emergency status,
and alarms and warnings associated with worst case failure.
Content
The training is as outlined by the Nautical Institute Scheme of Training.
The Simulator course principally involves simulated DP operations including errors, faults
and failures, giving the participants the opportunity to apply the lessons learned in both the
Induction course and the 60 DP sea time days required for completion of qualifying DPO
training tasks. It covers the following topics:
• Practical operation of the DP system
• DP operations, including planning
• DP alarms, warnings and emergency procedures
• Refresher of theory:
o Elements of the DP system
o DP Block diagram
o Basic rules and guidelines
o Position reference Systems, their limitations and handling in the DP
system
Objective
• By use of simulator equipment, the student must demonstrate ability to set up and
operate DP system, including special functions as Auto track, follow target, minimum
power heading mode, sensors and associated Position reference systems.
• The student must demonstrate ability to carry out a DP operation. All elements of the
operation must be adequately covered. This includes planning using all relevant
information, contingency planning, worksite approach, use of checklists,
communication, DP watch schedule, DP hand over and DP log book requirements.
• The student must describe the limitations of the DP systems during operations in
shallow and deep waters.
• The student must be able to recognise conditions that will cause degraded or
emergency status, and recognise warning and alarms associated with Worst Case
Failure.
• The student must demonstrate correct behaviour in connection with failures and take
appropriate action to stabilize the vessels position after a failure.
Assessment
The DP Simulator Course has a three part assessment process.
• Practical assessment, according to DP set up practical assessment skills table
• Online multiple choice exam. (From 1/1/2017)
• The third part of the assessment process is the feedback given to students throughout
the course, based on their performance during simulator exercises. The assessment is
according to NI Standards.
Page 14
Admission Requirements
Participants must have attended an approved Dynamic Positioning induction course (DP
basic course) and passed the NI online examination.
For participants entered on the scheme AFTER 1st January 2015:
Minimum 60 DP sea time days after the induction course and completion of all tasks in the
“Task Section” – and signing of relevant section by vessel master - onboard a certified DP
class/ unclassed or class zero vessel + Company confirmation letter. Logbook must be
presented on the first day of the course as proof.
For participants entered on the scheme BEFORE 1st January 2015:
Documented minimum sea-going DP familiarisation (30 days) and completion of relevant
task section – and signing of relevant section by vessel master - onboard a certified DP
class / unclassed or class zero vessel + Company confirmation letter. Logbook must be
presented on the first day of the course as proof.
All participants must hold either a STCW Regulation II/1 ‐ II/2 ‐ II/3 Deck, Regulation III/1
– III /2 – III/3 – III/6 Engine and Regulation III/6 for ETOs. Certificate must be presented
on the first day of the course as proof.
Participants with alternative appropriate marine vocational qualifications (MVQs)* are
allowed participating on the course if hold a NI DPO request letter to enter in the DP
scheme approved NI.
*Marine Vocational Qualification: is a non-STCW Certificate of Competency issued by a
white list Maritime Administration for use in the administration’s local waters only.
Persons who have a DP Basic / induction course dated before January 1st 2012 can join the
course, if he/she completes DPO training within a 5 year period.
Duration
4 days.
Participants
Maximum 4 Students per instructor/Simulator.
Language
Course material is in English.
Course conducted in Danish or English.
Venue
Maersk Training Svendborg.
Page 15
DP Sea Time Reduction / NI Shuttle Tanker Course C
Purpose
For participants following the Nautical Institute Offshore scheme:
The purpose of the course is to reduce the period of DP watchkeeping required in order to
apply for a DP operator certificate by use of a full mission bridge simulator with full
redundant DP installed.
Successful completion of the course will reduce the required supervised DP sea time as
follows:
If DP education has been started before 1 January 2015:
5 days intensive simulator training = 42 DP sea time days. (Out of required 180 days)
If DP education has been started after 1 January 2015:
5 days intensive simulator training = 30 DP sea time days. (Out of required 60 days)
Following the DP Sea Time Reduction course any DPO trainee is required to do a minimum
of 30 days additional DP sea time before applying for a DP operator certificate.
For participants following the Nautical Institute Shuttle Tanker scheme:
The course is a prerequisite to complete the Nautical Institute shuttle tanker scheme. The
course is a Training course ‘C’ according to Nautical Institute rules and can be taken either
as phase 5, 7 or 9 in the shuttle tanker scheme.
Content
The course is designed to give the participant the opportunity to demonstrate knowledge
of:
• Operation of a DP System:
Analyse trends and other information.
Evaluate and respond to alarms.
• DP Operation:
Subsea construction operations.
Dive operations.
ROV operations.
OSV operations.
Cable lay.
Communication and teamwork skills.
Use of correct procedures and checklists.
• Emergency Procedures:
Communication and teamwork skills.
Objectives
Clear objectives are defined in NI Standards for the Sea Time Reduction Course (Standard:
DYNAMIC POSITIONING OPERATOR TRAINING SCHEME ACCREDITATION STANDARD,
January 2016 vers. 1.2). These objectives can be summarised as follows:
• Operation of a DP system:
Page 16
Demonstrate the ability to set up and operate the DP in different modes using
position reference systems, sensors and peripheral equipment associated with the DP
system, including manoeuvring in different modes.
• DP Operation:
Demonstrate the capability to plan and conduct any DP operation, including risk
assessment, contingency planning and assessment of vessel capability.
Demonstrate compliance with procedures and other relevant industry publications.
Evaluate the various information and warning as well as alarm messages
communicated to the operator.
• Emergency Procedures:
Recognise the conditions that will cause degraded operational status or emergency
status by means of warnings/alarms and FMEA.
Carry out procedures to stabilise the vessel position and heading following a variety
of system failures and take appropriate decisions and actions as to continuing or
abandoning the operation.
Assessment
There is no assessment on this course. The participants will be evaluated during the course
by the instructors, against objectives of the course, following The Nautical Institute
Requirements
Admission Requirements
For participants following the Nautical Institute Offshore scheme:
If DP education has been started before 1 January 2015:
Successful completion of the DP Simulator/Advanced course followed by 30 DP sea time
days logged in the Nautical Institute logbook.
If DP education has been started after 1 January 2015:
Successful completion of the DP Simulator/Advanced course.
For participants following the Nautical Institute Shuttle Tanker scheme:
Successful completion of the DP Simulator/Advanced course followed by 24 days sea time
as practical time on board AND participation in 2 complete offshore loading operations.
Duration
5 days.
Participants
Maximum 6.
(Ratio: 3 students per instructor and simulator)
Language
Course material is in English.
Course is conducted in Danish or English.
Venue
Maersk Training Svendborg.
Page 17
Chapter 2 - Glossary and Abbreviations
A & R winch
A & R wire
Abandon Dive
Abandonment & Recovery winch on a pipe laying vessel.
Abandonment & Recovery wire on a pipe laying vessel.
Equivalent to red alert when divers return to the bell immediately and bell is
brought back to the surface.
ADS
Atmospheric Diving Suit
Advisory Status
A status in between Green and yellow (degraded), defined in the ASOG or
WSOG
ARA
Acoustic Riser Angle system
ARP
Alternative Rotation Point
Artemis
Radio system used to measure vessels position. System operates using a
microwave frequency and measures the range and bearing of the vessel from a
fixed station that is generally installed on a platform.
ASOG
Activity Specific Operation Guideline
Back up DP
A physically separate DP control system that will be available in the event of a
total failure of the main DP control system.
Beacon
Free running device on seabed that generates acoustic pulses that are received
by the HPR system and used to establish the vessels position.
Blackout
Loss of all main electrical power.
BOP
Blowout Preventer
CALM buoys
Catenary Anchored Leg Moorings
CAM / CAMO
Critical Activity Mode / Critical Activity Mode of Operation
Capability Plot
A theoretical polar plot of the vessel’s capability for particular conditions of wind,
waves and current from different directions. These can be performed for different
thruster combinations and should be produced in accordance with IMCA document
CCTV
Closed Circuit Television.
CCW
Counter- clockwise
CG
Centre of Gravity
COS
Common Operator Station
CP
Controllable Pitch.
CW
Clockwise
DGPS
Differential GPS,a GPS supplied with a differential correction by one or more
reference stations.
DP
Dynamic Positioning: automatic control of vessel’s location with respect to one or
more fixed references.
DP Blackout
Loss of electrical power that prevents the DP control system operating.
DP Control System The part of the DP system that calculates position and provides thruster
commands.
DP Incident
A DP incident is a loss of position to the surprise of the
DP System
All equipment that supports automatic position control.
DPC
DP Controller
DPO
Operator of the DP Control System.
DPVOA
DP Vessel Owners Assosiation
DQI
Differential Quality Indicator
DSV
Diving support vessel: a vessel from which divers are deployed.
Duplex DP
DP control system with full redundancy and a smooth automatic changeover
between the two systems.
EBL
Electronic Bearing Line
ECR
Engine Control Room
ERA
Electrical Riser Angle system
Environmental Regularity Number
ERN
Page 19
ERO
ESD
ETO
FMEA
Footprint
Electronic Radio Officer
Emergency Shutdown and Disconnection for shuttletankers
Electrical Technical Officer
Failure Mode Effects Analysis
A graphic illustration of a set of real observations of a vessel’s DP station keeping
ability in particular environmental conditions.
FP
Fixed-pitch.
FPSO
Floating Production, Storage and Offloading vessels
GPS
Global Positioning System
GPS
Global Positioning System using satellites to establish a vessel’s position.
Green Alert
Normal Operational Status; adequate DP equipment is on line to meet the
required performance within the declared safe working limits.
HDOP
Horizontal Dilution Of Precision
HiPAP
High Precisition Acoustic Positioning system
HPR
Hydroacoustic Position Reference
HPR
Hydroacoustic Position Reference system.
I/O
Input/Output
Independent Joystick
A joystick that is independent of the DP control system.
Joystick
Positioning facility that uses a single lever for surge, sway and yaw control.
LBL
Long Base Line
Limit Alarms
Selectable values of position and heading excursion at which the operator wants
an alarm.
LTW
Light- weight Taut Wire
Manual Control
Use of thruster controls other than those associated with the DP Control System.
Microfix
Radio system used to measure a vessel’s position using range/range techniques.
System operates using a microwave frequency to measure the ranges of the
vessel to a number of fixed stations.
MLBEs
Mooring line buoyancy elements for monitoring purposes for shuttletankers.
MOB
Mobile transponder
MRU
Motion Reference Unit
NMD
Norwegian Maritime Directorate
Offshore Loading Terminals
OLT
Operator
Any member of the vessel’s compliment involved with DP equipment e.g. DP
operator, Master, Duty Engineer, Chief Engineer, Electrician, Taut Wire Operator,
Radio Operator.
OS
Operator Station
OT
Operator Terminal
PCB
Printed Circuit Board (an assembly of mounted electronic components that form
part of a system that can be quickly repaired by replacement).
Platform
Any structure that is fixed relative to the DP vessel.
Plough
Towed unit generally used to bury communications cable.
PMS
Power Management System
PRS
Position Reference System
PS
Process Station
Pseudo
Expression used when a position measurement system is interfaced to a DP
control system as another position reference.
RCVS
Remotely-controlled submersible camera vehicles.
Red Alert
DP Emergency Status: i.e. there is a loss of position, or position loss is inevitable.
Responder
A transponder except the interrogation is by an electronic pulse sent down a
cable. This is generally fitted to an ROV and interrogated down the ROV’s
umbilical.
RMS
Root Mean Square
ROV
Remotely Operated Vehicle
ROV
Remotely operated vehicle that is launched to operate subsea from a vessel.
Page 20
RPM
Revolutions Per Minute
Safe Working Limits The environmental limits that a vessel sets for safely working on DP taking into
account specified equipment failures.
SBC
Single Board Computer
SBC
Single board computer.
SBL
Short Base Line
SDP
Simrad Dynamic Positioning
Shallow Water
Less than 50m water depth.
SIMOPS
Simultaneous Operations
Simplex DP
DP control system with no redundancy.
SJS
Simrad Joystick System
Spar buoys
Large floating tower structures for offtake tanker loading
SPS
Simrad Planning Station
SSBL
Super Short Base Line
STC
Simrad Thruster Control
SVC
Simrad Vessel Control
Syledis
Radio system used to measure vessel’s position that uses range/range techniques
from fixed stations.
TAM / TAMO
Task Appropriate Mode / Task Appropriate Mode of Operation
Taut wire
A position reference using a tensioned wire to a seabed weight (vertical) or to a
fixed object nearby (horizontal).
THR
Thruster
Thruster
Any propulsion device used by the DP system.
TMS
Tether Management System
Transponder
Device on the seabed that responds to acoustic interrogation from the HPR on the
vessel. The timing and phase of the acoustic reply is used to calculate vessel
position.
Trencher
Subsea vehicle used for pipe or control line burial.
Triplex DP
A triple DP control system that is able to vote on all inputs and all outputs and
processors to identify a faulty unit.
Umbilical
Connection carrying life support and communication systems between a support
vessel and a diving bell, an ROV or similar device (also divers umbilical between
diver and bell).
UPS
Uninterruptible Power Supply
UPS
Uninterruptable Power Supply - unit to provide electricity continuously to DP
control system in the event of a blackout of the main ship’s power.
USBL
Ultra Short Base Line
UTC
Universal Time Coordinated
UTM
Universal Transverse Mercator
VRS
Vertical Reference Sensor
VRU
Vertical Reference Unit
WGS
World Geodetic System
WOP
Wheel Over Point
Worst Case Failure The worst case failure of a DP system is the failure that has been the basis of the
design and proved by the FMEA. In static terms this usually relates to a number of
thrusters and generators that are also used in consequence analysis.
WP
Waypoint
WSOG
Well Specific Operator Guideline
Yellow Alert
Degraded DP Operational Status for which the DP vessel has a pre-planned
response to prepare for the risks associated with a DP red alert.
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Chapter 3 - Introduction to DP
Background
Petroleum products play an important part in our modern civilisation, but even in Noah's time,
tar was used to stop leaks in boats and ships. Later mankind has discovered more and more
ways of taking advantage of these products.
Oil was first found near the Caspian Sea. The oil was discovered on land, but as time went on it
was found that these oil fields extended into the sea. As early as the beginning of the 18th
century a well was drilled about 30 m off the coast line near Baku. Even though this was not a
success, it was still the start of an era. In 1925 the first oil producing well was drilled in the
Caspian Sea.
Sicilian sailors 'fishing" for oil - Painting
of Johannes Stradanus (1523-1605)
The oil fields in California were also found to be extending into the sea, so the oil wells were
gradually moved into the sea here as well. These wells were connected to shore by piers, see
picture below. At first these piers or platforms were built of wood, but soon steel took over.
These piers could measure over 400 m.
Summerland, California 1902
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It did not take long before the piers were replaced with free-standing oil platforms in the
sea. The following shows the development from these piers to today's drilling platforms:
1869 The Americans, Thomas F. Rowland and Samuel Lewis respectively, took out a
patent on a jack-up platform and developed a project for a jack-up vessel.
1897 Oil drilling from a wooden drill tower, connected to shore by a pier in
Summerland, California.
1906 The coast of Summerland: 200 oil producing wells offshore.
1924 The first oil well in Lake Maracaibo, Northwest Venezuela.
1934 The first steel oil rig installed in the Caspian Sea, near Artem Island.
1947 Drilling in the Gulf of Mexico at a depth of 6 m. The platform could not be seen
from shore (Louisiana) except with binoculars.
1963 The jack-up platform Le Tourneau was constructed for drilling at a depth of 75 m.
1976 The Hondo Field platform was installed off South California at a depth of 260 m.
1978 The Cognac Field platform was installed off Mississippi at a depth of 312 m. Weight
around 59 000 ton steel.
A platform made of concrete was installed at the Ninian Field in the North Sea at
a depth of 138 m.
1988 The Bullwinkle, a jack-up platform, was installed in the Gulf of Mexico at a depth of
411 m (world record). Weight around 77 000 ton.
The installation of these platforms was expensive and it was even more expensive to
move them from place to place, so that test drilling for shorter periods was not interesting.
The restrictions they had with respect to water depth (normally 300 m) made it necessary
to look for other ways of extracting oil from the sea. The industry needed methods for
drilling in deep water, and an easier and less expensive way of moving the drilling activity
from place to place. This gradually led to the method of anchoring drilling vessels and
portable platforms. Several anchors or weights were used to keep the vessel/rig in place,
which at the same time minimised the movements.
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The following is a list of some of the offshore operations where anchoring systems have
been used:
1953 SUBMAREX, the first drilling vessel to use anchoring. This took place off the
coast of California at a depth of 120 m.
1954 The first drilling vessel in the Gulf of Mexico.
1962 The first semi-submersible drilling platform, C.P. BAKER, constructed in the
USA.
1970 Test drilling at a depth of 456 m from the drilling vessel WODECO 4.
1976 A world record in deep water drilling was set by the anchored vessel
DISCOVERER 534 at a depth of 1055 m off the coast of Thailand.
1987 New world record in deep water drilling set by DISCOVERER 534 at a depth
of 1985 m.
2002 Passed a depth of 2500 meters.
The anchoring systems have, however, their weak points. Elasticity in the anchoring
system, poor hydrodynamic damping, etc., expose the vessel or the rig to movements
made by waves, wind and current. In addition, drilling at such depths requires a lot of
equipment (winches, anchors, wires, etc.) with the result that vessels using this type of
system lose a great deal of their manoeuvring capability.
The cost and time taken to lay the spread of, maybe eight anchors, together with the
associated costs of the anchor-handling tugs, is very high. These costs start to escalate if
any of the anchors fails to hold when tensioned, and piggy-backing has to be resorted to.
(Piggy-backing is where a second anchor is laid behind the first dragging one, to back it
up). If the water is deep then the amount of ground tackle becomes great with
commensurate increases is weights of gear deployed, line lengths and power
requirements of tugs. The rig, once anchored, has a certain amount of movement due to
the flexibility inherent in the mooring spread, but there is a distinct lack of flexibility of the
manoeuvrability of the rig. If a position shift is needed, then some or all anchors may need
to be lifted and relaid. Small position changes may be made by means of winch spooling,
adjusting the line lengths, but there is a lack of precision in this. Likewise, heading
changes are limited. Other problems which may affect vessels/barges/rigs using spread
moorings and anchors concerns the hazards represented by any existing underwater
installation, such as pipelines. These hazards may exist in any water depth, of course, and
may influence the choice of positioning method even where mooring would be otherwise
ideal. In some fields there is a partial or total bar upon the use of anchors and moorings.
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The Mohole Project
The very first dynamic positioning system was used in 1961 in connection with the American
Mohole project. The purpose of this project was to drill into the so-called Moho layer, that is,
through the outer shell of the earth. To succeed in this the drilling was to be done where this
shell is at the thinnest, and that was where the great oceans are at the deepest. The depth was
around 4 500 m, and that was far too deep for the usual anchoring systems.
The problem was solved by installing 4 manoeuvring propellers/thrusters onboard the barge,
CUSS 1. The position in relation to the seabed was found by lowering a transmitter down to the
seabed which transmitted signals up to the barge (some form of hydro acoustic reference
system). The position in relation to the transmitter could be read on a display onboard the
barge. In addition, 4 buoys anchored around the vessel were used. These transmitted radio
signals to a radar onboard. By using different combinations of thrust and direction for the 4
propellers, it should be possible to keep the barge in position above the place of drilling. 9 th
March 1961 the CUSS 1 was able to maintain position by the help of dynamic positioning at a
depth of 948 m off La Jolla, California (picture below). Sometime later the vessel did 5 drillings
at a depth of 3 560 m, while maintaining position within a radius of 180m.
CUSS 1, the first vessel to be dynamically positioned in connection with the Mohole project in
1961.
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Overview
CUSS 1 was the first vessel which had some kind of DP system onboard. The system
functioned with manual control. But the manual control of the thrusters was a very complicated
function, and the idea of developing a control unit to take care of that function was born. Later
that year (1961) the Shell Oil Company, USA, launched the drilling vessel EUREKA. Very
soon equipment which automated the thruster commands was installed. In 1964 another
vessel, CALDRILL 1, was delivered to Caldrill Offshore Company, USA, with similar
equipment onboard. Both the EUREKA and CALDRILL projects were successful. EUREKA
drilled at a depth of 1 300 m with 6 m high waves and a wind of up to 21 m/s. CALDRILL could
drill at depths of maximum 2 000 m and was equipped with 4 manoeuvrable thrusters, each
with 300 hp. The position was found using two taut wire reference systems.
French engineers watched the American projects closely. France had interests in companies
laying pipelines in the Mediterranean, and dynamic positioning could make these operations
safer and more efficient. In 1963 the first dynamically positioned French vessels, namely
Salvor and Terebel, were laying pipelines in the Mediterranean.
A few years later the oil adventure started in the North Sea, and Norway and the UK became
interested in dynamic positioning. British GEC Electrical Projects Ltd equipped in 1974
WIMPEY SEALAB, an old cargo vessel converted to a drilling vessel, and in 1977 UNCLE
JOHN, a semi-submersible platform, with equipment similar to that which the Americans and
Frenchmen had named Dynamic Positioning (DP) System.
By the late 1970's, DP had become a well established technique. In 1980 the number of DP
capable vessels totalled about 65, while by 1985 the number had increased to about 150.
Today it is very rare that an offshore vessel is build without DP. It is interesting to note the
diversity of vessel types and functions using DP, and the way that, this has encompassed many
functions unrelated to the offshore oil and gas industries.
A list of DP functions would include the following:
-
exploration drilling (core sampling)
production drilling
diver support
pipelay (rigid and flexible pipe)
cable lay and repair
multi-role support vessels
accommodation or "flotel" units
hydrographic survey
pre- or post-operational survey
wreck survey, salvage and removal
dredging
rockdumping (pipeline protection)
subsea installation
cranebarge operations
well stimulation and workover
supply vessel operations
shuttle tanker operations
FPSO (floating production, storage and offloading vessels)
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-
heavy lift cargo vessels
cruiseliners
container vessels
mine countermeasures vessels
oceanographical research
seabed mining operations
rocket launch pad and support vessel
military support vessels (repair and maintenance support)
Some of the above categories relate to single vessels only, at present. The rocket launch facility
consists of the converted semisubmersible drilling rig "Ocean Odyssey" rebuilt to provide an
equatorial launch capability for satellites. An accompanying support vessel has been built, and
both are DP- capable.
Over the years, DP systems have become more sophisticated and complicated, not to mention
more reliable. Computer technology has developed beyond all recognition since its childhood
and modern systems make full use of the improvements. Position reference systems and other
peripherals are now more numerous and reliable. Redundancy is provided in the vessels
intended to conduct the higher-risk operations. This redundancy includes every element of the
DP system, not just the electronics, to the point where a modern DP vessel of the highest class
should maintain her position keeping capability subsequent to a total loss of all function in a
machinery space, or the bridge (or any other single compartment).
Over the years a number of manufacturers have been engaged in the design and supply of DP
systems. Some of the early systems were of US origin, such as Honeywell and AC Delco. Other
systems were of French origin (Alcatel and Thomson).
Current production is shared by Kongsberg in Norway (previously Simrad but originally
Kongsberg Albatross), with a smaller share of the market held by Marine Technologies, Navis,
Rolls Royce and Nautronix. The Nautronix manufacture stemmed from the American Honeywell
operation, the rights of which were sold to Nautronix of Australia in the early 1990's.
A few other manufacturers have produced systems on mainly a one-off basis. The Finnish
Hollming group produced a small number of DP systems, while a system was specially
developed by Vosper for the mine countermeasures vessels built by them for the Royal Navy
and the Navy of Saudi Arabia.
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Chapter 4 – Principles of DP
Dynamic Positioning can be defined as a system that automatically controls a vessel to
maintain her position and heading exclusively by means of active thrust. (definition)
SATELITE NAVIGATION
DIFFERENTIAL
CORRECTION
DP CONSOLE
COMPUTERS
GYRO
VRS
RADIUS
ARTEMIS
FANBEAM
RUDDERS AND
PROPELLLERS OR
AZIMUTH PROPELLERS TAUT
WIRE
THRUSTERS
HYDROACOUSTIC
POSITION
REFERENCE
By using the word "automatically" we exclude systems which are reliant upon a joystick or
other operator input in order to maintain control over the vessel, although manual control is
one of the functions of a DP system. In addition to controlling the vessel to maintain a given
(or "set point") position and heading, the system also caters for changes to position and/or
heading being implemented by a variety of means.
The DP/Vessel System
In simple terms, a DP system consists of a central processor linked to a number of position
reference and environment reference sensors. The ship is provided with sufficient power and
manoeuvrability by means of a variety of thrusters and propellers. The measured position of
the vessel is compared to the desired or set point position, the computers then generates
appropriate thruster commands to maintain or restore vessel position. Effects of wind forces
and other environmental forces are taken into account. A bridge control console allows the
operator to communicate with the system and vice versa, and vessel control to be effected.
It is essential to realise that the DP system does not simply comprise the computers and other
electronics that may be supplied by the DP system manufacturer. DP is a whole-ship system,
comprising those elements but including the vessels power plant, electrical system and
propulsionsystem. Also to be included in this concept is the human element. Vessel personnel
are an essential element of the system; those identified as "key" to the DP function must be
Page 29
fully competent. Competence is contributed to by means of individual's background,
experience and training, and competence is witnessed by means of personnel qualifications.
The "key" staff mentioned above includes the vessel's Master and watch keeping officers /
DPOs, her Chief and watch engineers, and the ETO or EROs.
Vessel Movement
The movement of a vessel may be described under six different classifications, known as the
SIX FREEDOMS OF MOVEMENT. Three of these are rotations, with the vessel turning about
various axes, while the other three are translations, or bodily movement in various directions.
PITCH
YAW
HEAVE
SURGE
SWAY
ROLL
The six freedoms of vessel movement
Surge – Along ship
Is described as a translation in a fore and aft direction, or fore-and-aft bodily movement of the
vessel. This movement is measured, monitored and controlled by the DP system.
Sway – Athwardship
Is described as a translation in an athwartships direction, or port-and-starboard bodily
movement. Like Surge, Sway is also measured, monitored and controlled by the DP system.
Yaw – Heading
Is described as a rotation about a vertical axis, or change of heading of the vessel. Yaw is the
third movement monitored and controlled by the DP system.
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Monitoring and control of the three freedoms of movement mentioned above constitute
Dynamic Positioning as defined earlier. Control of these three variables is dependent upon the
manoeuvrability of the vessel, provided by propellers, thrusters, etc. as will be described later.
Measurement of Surge and Sway requires an accurate position reference system, but
measurement of Yaw requires only a heading reference in the form of a gyro compass.
With no position reference, DP may automatically control the heading of the vessel while
vessel position remains under manual control (joystick for Surge and Sway).
Pitch
Is described as vessel rotation about the athwartships axis.
Roll
Is rotation about the fore-and-aft axis. Neither of these two movements can be controlled
(although Roll, may be dampened by active or passive stabilisation) but they must be measured
and monitored with precision. This is necessary for accurate position referencing. Pitch and Roll
are measured by means of a Vertical Reference Sensor or Vertical Reference Unit (VRS or
VRU).
Heave
is described as vessel translation in the vertical, or up-and-down movement of the vessel.
Heave is generally ignored by the DP system, although the more sophisticated VRU's will output
a value for Heave. This may be monitored by other ship systems where, for instance, heave
compensation is required for cranes or diving bell.
The DP console has MAN/JOYSTICK and AUTO buttons giving mode of DP Control. In AUTO,
all three (Surge, Sway and Yaw) movements are controlled automatically. In MAN/JOYSTICK,
control is affected manually by means of a joystick (Surge and Sway) and a "rotate" control
knob for manual control of Yaw. Additionally, in the Kongsberg systems, while in
MAN/JOYSTICK (manual control) three further buttons (SURGE, SWAY and YAW) allow the
operator to select automatic control of one or more movements individually. The operator may
thus select any combination of manual and automatic control at will e.g.
auto Yaw and Sway, manual Surge
auto Yaw and Surge, manual Sway
auto Surge, manual Sway and Yaw etc.
For example, when approaching station alongside a platform, the operator may make his
approach on Joystick control. Once in the vicinity of the platform and stabilised upon the
desired heading, the operator can engage auto Yaw control, leaving Surge and Sway under
manual control. Once position reference has been established, the Surge may be transferred to
auto control, then finally Sway. Now under fully automatic control, final adjustment to position
and heading may be made. While on station, the function of the DP is to maintain the vessel's
position and heading, counteracting any external forces such as wind and current that will
continually be trying to set the vessel away from the required position. In particular, rotation of
the vessel will be induced by wind forces upon asymmetric hull and superstructure
configurations. The DP must induce compensating surge, sway and yaw vectors in order to
restore and maintain position and heading.
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In addition to maintaining station and heading, the DP may be utilised to achieve automatic
change of position or heading, or both. The operator may choose a new position using a display
cursor, also choosing a speed. Once this is done he initiates the move, and the vessel takes up
the new position at the speed selected. Similarly, the operator may input a new heading. Upon
initiation, the vessel will rotate to the new heading while maintaining station. Automatic changes
of position and heading may be conducted simultaneously.
Positioning Principles
FINAL SETTLED
SITUATION
WIND
CURRENT
VESSEL
OFFSET BY
EXTERENAL
FORCES
The resultant vector will compensate for the Surge, Sway and Yaw vectors.
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Chapter 5 - The KONGSBERG DP System
Introduction
The Kongsberg Dynamic Positioning (DP) systems are computerised systems enabling the
automatic position and heading control of a vessel. Set-points for heading and position are
specified by the operator and are then processed by the DP system to provide control signals
to the vessel's thruster and main propeller systems. The DP system always allocates optimum
thrust to whichever propeller units are in use. Several configurations of DP system are
produced, covering the requirements for different levels of redundancy.
Block Diagram of a Kongsberg DP System
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NOTE! Text in capital letters in the following description refers to items in the block diagram
The Vessel Model
The DP system is based on a Vessel Model which contains a hydrodynamic description of
the vessel, including characteristics such as drag coefficients and mass data. This model
describes how the vessel reacts or moves as a function of the forces acting upon it.
The Vessel Model is provided with information describing the forces that are acting on the
vessel:
- A Wind Model uses a set of wind coefficients for various angles of attack to calculate the
WIND FORCE as a function of the wind speed and direction.
- A Thruster Model uses force/pitch/rpm characteristics to calculate the THRUSTER
FORCE according to the feedback signals from the thrusters/propellers.
Using the vessel characteristics and the applied forces, the Vessel Model calculates the
ESTIMATED SPEED and the ESTIMATED POSITION AND HEADING in each of the three
horizontal degrees of freedom - surge, sway and yaw. During sea trials, the Vessel Model is
tuned to optimise the description of the vessel characteristics.
Vessel Model Update
The Vessel Model can never be a completely accurate representation of the real vessel.
However, by using a technique known as Kalman Filtering, the model estimates of position
and heading are continuously updated with measured position information from positionreference systems and gyrocompasses.
The PREDICTED POSITION AND HEADING from the Vessel Model are compared with the
MEASURED POSITION AND HEADING to produce a POSITION AND HEADING
DIFFERENCE. Since these differences may be caused by noise in the measured values, they
are filtered before being used to update the Vessel Model.
Together, the Vessel Model and the Kalman filtering technique provide effective noise filtering
of the heading and position measurements and optimum combination of data from the
different reference systems.
If the reference system measurements are completely lost (position or heading dropout), there
is no immediate effect on the positioning capability of the system. The Vessel Model will
continue to generate position estimates even though there are no further model updates. This
"dead reckoning" positioning will initially be very accurate but will gradually deteriorate with
time.
Error Compensation Force
Even if appropriate Thuster/propeller forces are applied to counteract the effect of the
measured forces on the vessel, the vessel would still tend to move out of position due to
forces that are not measured directly, such as waves and sea current (together with any errors
in the modelled forces).
These additional forces acting on the vessel are calculated over a period of time according to
the filtered POSITION AND HEADING DIFFERENCE to produce an ERROR
COMPENSATION FORCE, which is added to the modelled forces to represent the total
EXTERNAL FORCES.
Page 34
The error compensation force is presented to the operator as being entirely due to sea current
since this is the main component.
Force Demand
The force demand that is required to keep the vessel at the required position is composed of
the following parts:
- The Force Demand for axes that are under automatic control
- The Force Demand for axes that are under manual control
- The Feed Forward
Force Demand for Axes under automatic Control
Consists of two parts:
-
A force demand that is proportional to the deviation between the estimated position and
heading and the position and heading setpoints
-
A force demand that is proportional to the deviation between the wanted and actual
speed
The POSITION AND HEADING SETPOINTS, specified by the operator, are compared with
the ESTIMATED POSITION AND HEADING from the Vessel Model. The differences are
multiplied by gain factors that are calculated and adjusted to optimize the station keeping
capability with minimum power consumption.
The wanted speed is compared with the ESTIMATED SPEED from the Vessel Model. If the
vessel is to maintain a stationary position, the wanted speed will be zero. This part of the force
demand therefore acts as a damping factor in order to reduce the vessel's speed to zero.
Force Demand for Axes under manual Control
When any of the axes are not under automatic control, you can use the joystick to manually
control the force exerted by the thrusters/propellers in those axes.
Feed Forward
In order to counteract changes in the external forces as soon as they are detected, rather than
first allowing the vessel to drift away from the required position, the calculated EXTERNAL
FORCES are fed forward as an additional force demand.
Thruster Allocation
The force demand in the surge and sway axes (the directional force demand), and in the yaw
axis (the rotational moment demand), are distributed as pitch and/or rpm setpoint signals to
each thruster/propeller.
The demand is distributed in such a way as to obtain the directional force and rotational
moment required for position and heading control, while also ensuring optimum
thruster/propeller use with minimum power consumption and minimum wear and tear on the
propulsion equipment.
If it is not possible to maintain both the rotational moment and the directional force demand
due to insufficient available thrust, priority is normally set to obtain the rotational moment
Page 35
demand (heading). If required, you can request that the priority is changed to maintain
position rather than heading.
If a thruster/propeller is out of service or deselected, the "lost" thrust is automatically
redistributed to the remaining thrusters/propellers.
Power Overload Control
The load on the main bus or on isolated bus sections is monitored, and power is reduced on
the connected thrusters/propellers by reducing the pitch/rpm demand if the estimated load
exceeds the nominal limit. The reduction value is shared between the connected
thrusters/propellers in such a way that the effect on the position and heading control is
minimised.
This function acts as an addition to the vessel's own Power Management System (PMS). The
power reduction criteria are set at lower overload levels than the load reduction initiated by the
PMS system.
The Extended Kalman Filter
The Extended Kalman Filter estimates the vessel’s heading, position and velocity in each of the
three degrees of freedom - surge, sway and yaw. It also incorporates algorithms for estimating
the effect of sea current and waves.
The Extended Kalman Filter uses a mathematical model of the vessel. The mathematical model
itself is never a 100% accurate representation of the real vessel. However, by using the
Extended Kalman filtering technique, the model can be continuously corrected.
The vessel’s heading and position are measured using the gyrocompasses and positionreference systems, and are used as input data to the DP system. These measurements are
compared with the predicted or estimated data produced by the mathematical model, and the
differences are then used to update the mathematical model to the actual situation.
The Controller
The controller calculates the resulting force to be exerted by the thrusters/propellers in order to
keep the vessel on position and heading (surge, sway and yaw).
In station-keeping operations, the DP Controller can be working in several of the following
modes, all with special characteristics:
•
•
•
High Precision control
Relaxed control
Green control
The High Precision control provides high accuracy station-keeping in any weather condition at
the expense of power consumption and exposure to wear and tear of machinery and thrusters.
The Relaxed control uses the thrusters more smoothly, at the expense of station-keeping
accuracy. However, this type of control cannot guarantee that the vessel will stay within its
operational area, and is only applicable for calm weather conditions.
The Green control uses very different control technology, called non-linear Model Predictive
Control, optimised for precise area keeping with minimum power consumption. The Green
control is applicable in all weather conditions.
The transition between DP controller modes is bumpless.
Page 36
High Precision and Relaxed Control
The controller consists of the following parts:
• Excursion Feedback
The deviation between the operator-specified position/heading setpoints and the actual
position/heading data, and similar deviations with respect to the vessel’s velocity/heading rate,
drive the excursion feedback. The differences are multiplied by gain factors giving a force
demand (restoring demand and damping demand) required to bring the vessel back to its
setpoint values while also slowing down its movements.
The main difference between High Precision and Relaxed control is the restoring characteristics
of the two controller types as indicated in the figure below
• Wind Feed-Forward
In order to counteract the wind forces as quickly as possible, the feed-forward concept is used.
This means that the DP system will not allow the vessel to drift away from the required position,
but counteracts the wind-induced forces as soon as they are detected.
• Current Feedback
The excursion feedback and wind feed-forward are not sufficient to bring the vessel back to the
desired setpoints due to unmeasured external forces (such as waves and current). The system
evaluates these forces over time, and calculates the force demand required to counteract them.
Green control
In the Green control mode, the system maintains the vessel within an allowed area with
minimum use of power.
The controller design consists of two main parts, each giving its contribution to the control:
•
The Environment Compensator is designed to compensate for the averaged
environmental forces. This demand will maintain the required position under averaged
conditions.
•
The Model Predictive Controller (MPC) uses a prediction (Position Predictor) of the
vessel movement as input for the control. The demand occurs during more drastic
changes in the external forces. When the operational boundaries are predicted to be
exceeded, the controller reacts to ensure that the vessel stays within the operational area
Page 37
Vessel under Green control in the operational area
The smooth control actions reduce wear and tear on mechanical parts of the power and thruster
system and reduce fuel consumption and greenhouse gases.
Due to its nature, this controller will not instantaneously react to sudden changes in external
forces, such as wind gust, unless the Position Predictor detects that actions must be taken
immediately. Unnecessary sudden use of thrust is therefore avoided.
The Position Predictor is made using the mathematical model of the vessel used in the External
Kalman Filter of the DP. The predicted position and heading are found step-by-step for the
whole prediction horizon (1 to 2 minutes).
The non-linear Model Predictive Controller is an online optimisation function, finding the best
compromise between using thrust and predicted overstepping of operational boundaries.
Page 38
K-Pos 11/21/31 system layout
K-Pos DP 11 Layout
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K-Pos DP 21 layout
Page 40
K-Pos DP 31 Layout
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Kongsberg IMO Equipment class 3 System
Page 42
Integrated Kongsberg System
Page 43
Kongsberg DP system main modes
This paragraph should be read in conjunction with the Kongsberg DP Operator Manuals
provided. For detailed instructions for the operation of the DP system, please refer to those
manuals.
The main modes of operation of a DP system are:
• Stand-by
•
Manual or joystick
•
Mixed Manual/Auto position and heading mode
•
Automatic position and heading
Other modes of operation include
• Autotrack
•
Follow Target
•
Auto pilot
•
Weathervane*
•
Auto-Approach*
*Functions are associated with Shuttle tanker operations.
On the Kongsberg DP panel are located pushbuttons for Stand-by, Manual/Joystick and
Auto. There are also buttons allowing selection of individual control axes, i.e. Surge, Sway
and Yaw.
In STAND BY mode the system is kept at a state of readiness. The system can be prepared for
operation in Stand By mode, but otherwise no operation is possible. While in Stand By mode,
sensors may be enabled, an Alternative Rotation Point may be selected and thrusters/propellers
may be enabled for use.
If in AUTO or MANUAL/JOYSTICK, then returning to STAND BY requires a double push on the
Stand By button. Reverting to "Stand By" causes the mathematical model to be lost.
In the MANUAL/JOYSTICK mode, the vessel is under the control of the Joystick for
manoeuvring. While operating in MANUAL/JOYSTICK mode, it is possible to select any one of
the three control axes (Surge, Sway or Yaw) for automatic control. Thus it is possible to operate
with Manual (joystick) control of Surge and Sway, with automatic control of Yaw (heading) by
pressing the "Yaw" button while in "Manual" mode. Likewise, the Operator may select automate
control of Surge and/or Sway. Further, it is possible to select two axes simultaneously for
automatic control (e.g. Yaw and Surge, leaving Sway on the joystick). Selection of all three axes
will cause the system to enter AUTO DP mode.
Important: In order to select Surge, or Sway, or both for automatic control, it is necessary to
enable a Position Reference system.
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Chapter 6 - The Marine Technologies DP System
The Marine Technologies DP system is composed of a number of different hardware modules
that communicate on a computer network. The number of modules used in the DP system
depends on the system class. A DP I configuration has no redundancy, whereas an IMO Class
II DYNPOS system is designed to be able to hold position in case of a single failure situation
based on a redundancy philosophy.
Both double- and triple redundancy philosophies are applied in the DP II design:
The double redundancy philosophy uses two different sources of the same information. When
these sources are compared it can be determined if one source is faulty but not necessarily
which one. In cases where it cannot be determined which source is faulty the operator has to
assess the sources and select the source to be used.
The triple redundancy philosophy uses three different sources of the same information. When
comparing these, failure is detected when the output from one source deviates from the other
two sources. This is also denoted 2 out of 3 voting (2oo3).
System Modules
The DYNPOS system consists of the following modules:
The power supply system (PS)
The fail-safe communication network (CN)
The sensor pack (SP)
The fail-safe control computer (CC)
The fail-safe operator station (OS)
The thruster controller (TC)
The block diagram in Figure 1 below shows the basic modules of the DYNPOS system. The
different modules and the interfaces between them are described in more detail in this section.
Figure 1: System Modules
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The Power Supply System
The components of the DYNPOS system are powered from two independent uninterruptable
power supplies (UPS). Each UPS is fed by the ship power system and will continue to feed the
DP system from its battery pack if the power source is lost. The battery pack is able to supply
power to the system for at least 30 minutes after loss of ship power.
The 110V AC power distribution is shown in Figure 2. The figure shows which components that
are fed by each UPS.
Figure 2: 110V AC Power Section
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A redundant supply system for 24 V power is also used. Important 24 V power consumers
(sensor- and thruster IO HW) have hardware logic to perform the switching from one
power source to another if one power source fails. The 24V power supply system is shown
in Figure 3:
Figure 3: 24 V Power Distribution
The Fail-safe Communication Network
Communication between the different modules is done by means of an Ethernet network. For a
DP I system a single network will be used whereas for DP II or higher a dual-, or fail-safe
communication network is used. All communication between the different system modules takes
place on both networks simultaneously. If a network failure results is broken communication
between two system nodes, the communication will continue uninterruptedly on the secondary
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network. A conceptual drawing of the fail-safe communication network is shown in Figure 4
below.
Figure 4: The fail-safe communication network
The Sensor Pack
The number of sensors in the DP sensor pack depends on the system class. For a class 1
system there is typically no redundancy in the sensors, whereas the sensor pack for a DP2
system is composed of a double or triple set of sensors in order to provide redundancy for all
measurements. If any single unit is to fail there is always a backup sensor. Failure of a unit is
detected by the CC and reported to the OS alarm system.
The sensors are interfaced to the DP system via the General Purpose IO Board HW module
and data is transmitted to the CC by the fail same communication network. For a DP2 system a
double set of IO hardware is employed to avoid loss of all sensors if one interface unit fails.
The DP2 sensor pack typically consists of:
Position reference systems (Normally three units)
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Gyro compasses (Normally 2 units)
Vertical reference sensors (Normally 2 units).
Wind sensors (Normally 2 units)
The Fail-safe Control Computer (CC)
The number of control computers (CC) in the MT DP system depends on the system class. A
class 1 system has one CC whereas a DP2 system uses three control computers (denoted
CC1, CC2 and CC3) running in parallel, meaning that all three units are performing the same
operations and running the same algorithms. In brief all CCs calculate the thruster command
signals (pitch command, rpm command, azimuth command) for all thrusters based on the same
sensor inputs.
For a class 2 system the output of the CC computers (thruster commands in surge, sway and
yaw) to the thruster controllers (TC) is constantly monitored by the TC. If one unit starts to
output commands that deviate from the two other units, it is voted out and the computer is
discarded. An alarm is issued to the operator to inform of the erroneous computer.
Some of the essential SW modules that constitute the control algorithm are:
• signal processing (sensor verification and voting)
• motion state estimator (Kalman Filter)
• reference model
• DP controller (thrust requirement algorithm)
• thruster allocation algorithm
• power overload prevention algorithm
Signal Processing
All sensors are checked for timeout in order to reject information from a sensor that has
stopped sending data. An alarm will be issued to notify the operator if a sensor times out.
A voting algorithm is also applied to all sensor data in order to detect and possibly identify
erroneous sensor data. Position and orientation information provided by reference systems
and gyros are compared to the “official” vessel position and orientation calculated by the
motion state estimator. If the sensor data exceeds a defined limit it will be considered
faulty. For reference systems a triple redundancy philosophy is typically used (for DP2) to
identify and reject the faulty unit. The same principle applies for Gyros if three units are
used instead of two. If only two sensors of the same kind are used the operator is asked to
choose which sensor to reject.
The Motion State Estimator
The motion state estimator employs the Kalman Filter algorithm to generate the system
state vector. The system state vector contains the “official” position/orientation and speed
of the vessel. The motion state estimator employs a mathematical model of the vessel.
The positions and velocities estimated by the state estimator are considered the “true”
states of the vessel and used as reference to determine the validity of the position
provided by the reference systems and heading provided by the gyrocompasses.
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Reference Model
The reference model generates the velocity references picked up by the controller to generate a
thrust command during a DP maneuver. A maneuver can be a position- or heading move
operation or a turn in a tracking operation.
The Controller (thrust requirement algorithm)
The controller algorithm calculates the forces to be applied (the force command vector) to keep
the vessel in the desired position based on the motion state vector and wind force estimates.
Thruster Allocation
The thruster allocation algorithm calculates the individual thruster commands for all available
thrusters based on the total thrust requirement values for surge, sway and yaw provided by the
controller. A thruster is considered available if it is selected for use by the operator and not
detected as faulty by the system (typically by providing a digital running and ready signal).
Power Overload Prevention
The power overload prevention algorithm compares allocated thruster force with available
power and reduces the thruster command if the allocated forces exceed the available power.
The objective is to be proactive and reduce the thruster command such that an overload
situation (shutdown of generator due to overload) will never occur in the first place. Normally the
max power the DP system is allowed to use is 95 % of the available power of the generators.
The Fail-safe Operator Station (OS)
The Operator Station (OS) provides the man-machine interface (MMI) that enables the operator
to monitor- and interact with the DP system. No critical DP calculations are performed by the
OS. For a DP2 system there will at least be two independent OS units working in parallel. It is
possible to operate the system from both units. If one of the units fails the operator can take
command on another unit and proceed monitoring- and interacting with the system.
The Thruster Controller (TC)
The thruster controller (TC) outputs digital and analog signals (DO, AO) to the thruster control
unit based on thruster allocation data received from the CC computer in command. It also
provides the feedback to the CC computers from digital- and analog signals received from the
thruster control unit. All communication between the TC and the CC units is done over the failsafe communication network.
A DP2 system will normally be equipped with one TC per thruster to provide the necessary
redundancy. If any one unit fails only one thruster will be un-accessible for the DP system. A
DP2 vessel is designed to be able to continue operation and maintaining necessary thrust
capacity if any one thruster fails.
The interface between the TC and the thruster/thruster controller unit is illustrated in Figure 5:
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In order for the DP system to use a thruster the following conditions must be met:
• The main mode switch is activated (MODE CONTROL ON BRIDGE)
• The thruster controller is running properly (Watch dog relay)
• The TC requests the thruster (Thruster select relay)
• The thruster is running properly.
The thrusters are designed in such a way that an electrically open input control signal or
an internal fault in the thruster will force the thruster to give zero thrust. Thruster output
signals like Ready and Running will then indicate that the thruster is not available.
In case of a faulty thruster controller the watch dog relay will let the thruster fall back to the
manual handles.
As mentioned previously the thruster controllers also have an additional role in monitoring
the control computers by comparing the data received from each one. If one CC unit
output data that deviate from that of the others, it will be considered faulty. The TC sends
the CC status information back to the DP system (to the CC) and the appropriate response
is handled from the OS.
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Operational Flow Chart
The system modules described in the previous chapter work together as shown in the block
diagram in Figure 6 below.
Figure 6: Operational Flow Chart
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Chapter 7 – Other Manufacturers of DP Systems
AutoNav IVCS 2000
AutoNav IVCS 2000 sets new industry standards for easy of use, accuracy and minimum
power consumption.
Positional accuracy of + / - 1 ft (30cm) is typically achieved in moderate sea conditions with
duty cycles of less than 1/2 of those previously obtained with current industry standard
products.
Offshore supply vessels
Cable / pipe laying vessels
Survey vessels
Rescue vessels
Drill ships
Diver support vessels
Research vessels
Fire fighting vessels
Ferries
Tankers and cargo vessels
Mine sweepers
Heavy lift vessels
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EasyDP 100
EasyDP 100 is a new generation station keeping system designed to meet the requirements
for : •
Supply and support vessels
Survey
Buoy laying
Yacht
Cruise liners and ferries
EasyDP is able to drive the entire
range of actuators :
•Azimut thrusters
•Voith-Schneider propellers
•Tunnel thrusters
•Classical propellers
•Waterjets propellers
Ideal for easy handling of ship at
low speed, station keeping work,
stand-by waiting periods
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Rolls-Royce - Icon
Icon Compact dynamic positioning (DP) systems from Rolls-Royce are an extension of
traditional joystick systems, providing dynamic positioning capability.
The DP range is designed to meet IMO specifications and requirements from the class of
societies. Products range from independent joystick systems and single IMO OP class 1
systems to sophisticated redundant systems satisfying the requirements of the IMO DP Class
2 and 3.
All positioning systems use common control platform architecture and are of modular design.
The systems are integrated with propulsion operation for maximum station capability and
utilise the latest controls and estimation technology.
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Wärtsilä – DPI, Dynamic Positioning Inc. - Platinum
Platinum Dynamic Positioning
The Platinum Dynamic Positioning (DP) System provides a simple and efficient solution for
vessel control, maneuvering and station keeping applications.
The redesigned UI offers intuitive operation, increasing safety and efficiency. Based on a
modular and scalable design concept, systems can be offered for virtually any platform in the
industry.
This system is designed to satisfy class notation requirements for Class 0, 1, 2 and 3, with
enhanced redundancy to meet the most demanding customer requirements.
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Chapter 8 – Planning, Setup and worksite approach
Operation planning
With any DP vessel operation, comprehensive planning is essential. The operational
requirements of the task in hand must be thoroughly discussed with the client, and a detailed
plan of the preferred sequence of events compiled. The plan must include the approach to the
worksite and setup, together with the positional requirements of the task itself. At all stages
there must be adequate contingency plans made allowing for escape manoeuvres under
degraded status.
Many factors must be taken into account when preparing the detailed planning. The DPO's
must be familiar with the details of the worksite and of the task in hand. In many operations
the vessel is simply providing a working platform for the client's operation, but it is essential
that the marine staffs are familiar with the detail of the operation, together with any possible
hazards.
Factors to be taken into account will include the following:
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•
•
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•
•
•
•
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•
•
•
•
•
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Position on the worksite, proximity of subsea, surface and overhead hazards.
Degree to which manoeuvrability or escape routes are impeded by fixed structure
location, or by any aspect of the operation.
Any operation-related external forces which will reduce the position-keeping capability
of the vessel (e.g. pipe tension).
Expected weather conditions on the worksite.
Predicted tidal rates and directions, and the reliability of these predictions.
Power of the vessel and her thruster configuration.
Depth of water on and around the worksite.
Equipment Class of the vessel, and that required for the operation.
Relating to the above, level of redundancy required and available.
Availability of position references, backup position references, and any factors which
might cause position references to become unavailable.
Any restrictions upon manoeuvring, or placing underwater hardware, that might
be enforced by the field operator.
Proximity of other vessels or barges at any stage of the operation, and the effects upon
the manoeuvrability of own vessel or the integrity of her position references.
Ability of own vessel to react to changes in weather or power status
Configuration for the Critical Activity Mode of Operation (CAMO) or, where appropriate
the Task Appropriate Mode (TAM). (see description later)
Preparation of the Activity Specific Operating Guidelines (ASOG), including onboard
discussion with all relevant stakeholders as part of the pre-project execution/ activity.
SIMOPS and marine vessel interaction and consequences arising from change of status
(Green to Blue, Yellow or Red).
Initiating “Positioning Standby”
•
(Positioning standby is a condition that is triggered during the execution of the Industrial
Mission when warranted. Positioning standby is initiated to bring all station keeping critical
elements (Equipment, People, and Processes) to a higher state of readiness, for a defined
period, with the objective of preventing a loss of position)
f the worksite is within an oilfield then during the approach to the worksite, contact will be
made with the platform OIM to obtain update information regarding the progress of the task.
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Traffic and ETA data will be exchanged, and any changes to the pre-determined plan
discussed. On arrival in the area, permission will be obtained to enter the 500m exclusion
zone around the installation, or similar compliance with local requirements and regulations.
This permission must be logged with date and time. The Work Permit will be obtained from the
Client.
The Bridge staff will obtain the latest meteorological reports and forecasts, relating these to
the operation in hand, particularly if weather conditions are expected to deteriorate or if any
stage of the operation is critically weather or sea-state dependent.
At this stage the DP system must be thoroughly checked, with a checklist to be completed.
Computers may need to be started up or reloaded. All position reference and other peripherals
will be inspected for readiness. Gyro compasses will be checked and a value determined for
compass error. A check also needs to be made on the magnetic compass, as this is
occasionally the only independent indication when two gyros disagree.
The engine room staff will need to be informed of the requirements regarding the level of
redundancy and MCR manning. They will have their own pre-DP checklist to complete. The
availability of generators must be discussed and agreed with the engineers.
Activity Operational Planning - CAMO / TAM / ASOG
Activity Operational Planning has been introduced to allow for a vessel to operate in different
modes based on the activity being carried out. This could be a DP class 3 vessel, operating in
DP class 2 mode during less critical activities.
Critical Activity Mode of Operation (CAMO)
Any DP vessel, including DP Class 2 and 3, can have the redundancy concept defeated if its
systems and equipment are not configured or operated in the correct way.
Critical Activity Mode of Operation (CAMO): This is generally a tabulated presentation of
how to configure the vessel’s DP system, including power generation and distribution,
propulsion and position reference systems, so that the DP system, as a whole is fault tolerant
and fault resistant. The CAMO table also sets out the operator actions should the required
configuration fail to be met. The term Safest Mode of operation (SMO) has been previously
used to describe CAMO.
The CAMO sets out the required equipment configurations and operational standards
necessary for the vessel to meet its maximum level of redundancy, functionality and
operation so that no single failure will exceed worst case failure. Every DP vessel/ rig has a
unique configuration (aka Critical Activity Mode Configuration) which must be determined
from a detailed understanding of the vessel’s DP FMEA and its operational characteristics.
The CAM configuration should be the default operational mode for a DP vessel when
conducting activities deemed or identified as critical.
A detailed review of the DP FMEA is done with a view to identify critical activity mode. It is
suggested that the results of this review are summarised in a vessel overview document.
Marine Technology Society Dynamic Positioning Committee –
The CAMO/TAM is presented in a tabulated format and shows two conditions (GREEN –
NORMAL) and (BLUE – ADVISORY) for each item, as follows:
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Task Appropriate Mode (TAM)
‘Task Appropriate Mode’ (TAM) is a risk based mode: Task Appropriate Mode is the
configuration that the vessel’s DP system may be set up and operated in, accepting that a
failure could result in effects exceeding the worst case failure such as blackout or loss of
position. This is a choice that is consciously made. This mode may be appropriate in
situations where it is determined that the risks associated with a loss of position are low and
will not result in damage to people, environment or equipment. The conditions under which
a DP Project and Construction Vessel may operate in TAM are defined and are usually
associated with operating well clear of the 500 m zone of floating or critical subsea assets
and where the consequences of a loss of position are evaluated and deemed acceptable.
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Activities where TAM is considered should have documented risk assessments carried out.
(Note - Task Appropriate Mode (TAM) in this context is not to be confused with Thruster
Assisted Mooring)
ASOG
Activity Specific Operating Guidelines (ASOG)
The ASOG differs from the CAM and TAM in that it relates specifically to a known location
and activity and, in the case of a DP drilling vessel/ rig, to a specific well operation. The
ASOG performs the same function as Well Specific Operating Guidelines (WSOG), which
relate to drilling activities. The ASOG differs from the CAM and TAM in that the ASOG sets
the operational, environmental and equipment performance limits for the DP vessel in relation
to the specific activity that it is undertaking, whereas the CAM and TAM may not be location
specific. An ASOG should be developed for every activity and location.
A central component in the ASOG is proven knowledge of black out recovery time.
Reference should also be made to relevant studies/analysis (if applicable) for an activity
being undertaken (e.g. analysis related to riser transfer operations, heavy lift operations,
permanent mooring connection operations).
The ASOG is presented in a tabulated format and shows four conditions (GREEN –
NORMAL), (BLUE – ADVISORY), (YELLOW – ACTIONABLE) and (RED – EMERGENCY),
as follows;
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Contingency planning
As a corollary of the above, it is important that the planning of the worksite approach includes
assessment of the various escape routes and options, and planning of otherwise unforeseen
contingencies. The worst contingency will be a power shortage caused by partial blackout. This
is the "worst case" failure mode and the diligent DPO will have taken such an event into
account, and will have prepared contingency escape options accordingly.
In all cases the DPO must keep in mind his least-power escape manoeuvre. The contingency
planning will also need to take into account the planned operation, as escape routes must also
exist after the initial approach.
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Risk Assessment
A formal Risk Assessment of the DP operation must always be conducted. Each company has
it’s own Risk Management System, which must be followed. The methods could vary
accordingly but how ever more or less formal they are, they should be well organised and
planned.
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ISO 8402:1995 / BS 4778 define risk management, which includes maritime risk assessment
as: “The process whereby decisions are made to accept a known or assessed risk and/or the
implementation of actions to reduce the consequences or probability of occurrence.”
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Identification of hazards
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Assessment of the risks concerned
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Application of controls to reduce the risks
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Monitoring of the effectiveness of the controls
The identification of hazards is most important since in determines the course of actions to be
followed thereafter. Observation of the activities helps in achieving perfect accuracy and
completeness which again can only be accomplished by a systematic process. For this it is
necessary to have professional training and instruction to assure its application in a thorough
and consistent manner. Also it is important to keep in mind that hazards must not be confused
with incidents whereas incidents must not denote consequences.
The marine risk assessment helps in evaluation of each hazard associated with the risks in
terms of the likelihood of harm and its potential aftermath. This assists in enabling the company
to imply priorities and exploit its scarce resources for greatest effect.
While settling with the application of controls, it is essential to take the frequency of the activity
into account so that a potential moderate risk may be more important to be addressed upon
than a rare but substantial risk.
The most relevant risks to monitor are:
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•
•
•
Health and safety issues of individuals involved directly or indirectly in the activity, or
those who may be otherwise affected
The environment
Property of the company and others
Risk assessment should be continual, flexible, reviewed regularly to improve safety and
preventing pollution. Since ‘risk’ is never a constant or concrete entity, the divergence of the
nature of perception and anticipation the level of danger from the risk undertaken is resolved by
experience, training and disposition. Human behaviour towards issues, general awareness, and
constant vigilance of those involved, all play a vital role in the organisation’s decision-making
process in the risk assessment in DP operations.
Worksite Approach
Initial DP Setup
Prior to the operation commencing, the vessel may be set up on DP outside the 500m exclusion
zone while a number of checks are made.
Any worksite approach must be made in a slow-but-sure manner with adequate planning,
proper completion of checklists, proper consideration of contingency plans and adequate time
for the building of the mathematical model.
At some point during the approach, transfer of control must be made from the navigation bridge
to the DP console location. In some vessels these two locations are adjacent but often the DP
system occupies a separate control room, e.g. the After Bridge in a DSV or ROV survey vessel,
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or the Bow House in a shuttle tanker. The vessel will normally be stopped well clear of any
obstructions, usually outside the 500m zone, and the transfer effected. A checklist should cover
transfer procedure. Items to be checked or tested include main engine/thruster control
functions, communications (external vhf/internal) radar and navigation aids, compass repeaters
and steering systems. In addition, checks must be made on specialist operational items, such
as the telemetry systems and ESD systems in a shuttle tanker. Thrusters and main propellers
must be "proved" by taking manual control and trying each thruster each way, checking
response and feedback. This step is important as, occasionally it happens that control of a
thruster will not transfer as selected.
At the outset of DP operations it will be necessary to inform various parties of the intention to
set up on DP. The Engine Room staff will have their own pre-DP checks and preparations,
and it may be necessary to discuss redundancy requirements with them. It will be necessary
to ensure that the engine room or MCR be continuously manned for many types of operation.
Depending upon the type of DP system installed it may be necessary to load or re-load the
computer or computers. Normally these are never switched "off', but left in a "stand-by" mode
when not on DP. This ensures that the systems do not suffer from humidity as the ventilation
fans run continuously.
The range of position references to be used will be decided. For an Equipment Class 1 vessel
a single position reference system (PRS) may be adequate, while for more critical function
work, three PRS are the standard. Which PRS to use will, of course, depend upon what is
available and the requirements of the task in hand. In the early stages of the operation a longrange PRS needs to be established, one that will provide uninterrupted cover during the
moves to the working location. This will usually be DGPS.
Once a PRS is established, then control of the manoeuvring may be transferred from the
manual thruster control panel to the DP panel or "desk". The control facilities for this will
consist of switches or buttons selecting "manual levers" or "DP" for each thruster, with
associated indicator lamps. Once control has been transferred, the operator must check that
the DP console is showing "ready" lamps for each thruster running, before selecting each
thruster into DP control by pressing "Enable" buttons for each.
Once thrusters are enabled on the console, they may again be proved, with the DP in manual
mode. The joystick and rotate control are operated, and the correct and expected response
observed from the individual thrusters.
The bridge team will be starting to fill out a "pre-DP" checklist at about this time. It is important
that this be done correctly and thoroughly. Checks will be undertaken on all DP and peripheral
equipment such as Gyros, VRS, Position References and Wind Sensors, together with the
power plant and thruster systems.
The vessel control may now be passed to the DP, usually by selecting the Auto Yaw function
first, stabilising the heading but leaving position control (Surge and Sway) under the Joystick.
In some DP systems this is known as "JSAH" (Joystick control with Automatic Heading). Next,
the system can be given the control of position by pressing the Auto Surge and Auto Sway
buttons. Before pressing either of these buttons it is important to reduce the movement in
each respect to as close to zero as possible, otherwise the system will issue instant large
thruster commands which may cause partial blackout.
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Once in full Auto DP, it is normal to allow some time for the vessel and system to settle down,
before continuing with the approach to the worksite. This time will allow the DPO to satisfy
himself that all systems are operating correctly, and that the vessel is maintaining a
satisfactory position and heading. The power management will be checked to ensure that
sufficient power is available, with sufficient power reserves in hand. Thruster outputs must
also be within acceptable limits. At this stage stopping one or more thrusters and noting the
effect on the position-keeping capability may test Thruster redundancy. For operations at
Equipment class 2 or 3 it will be necessary to stop all thrusters connected to one section of
busbar, ensuring that the vessel is able to maintain position subsequently.
After ten minutes or so the vessel should have settled to the current position, and the DP
mathematical model will have become established. The DPO will check that wind sensors are
indicating correct wind values and are agreeing with each other. It may happen that one wind
sensor is shadowed by structure. Comparison of values must be made and any discrepancy
accounted for. The most appropriate wind sensor should be selected for input to the DP. The
DPO must also determine that the value shown on-screen for the tide vector is approximately
that observed. Any large discrepancy here may indicate a separate fault within the system.
The vessel is now on Auto DP, perhaps 250 - 500m away from the working position. All
systems having been checked, it is now possible for the approach to the worksite to continue.
It is best that from now on all movements be made in Auto DP.
At this stage the DPO will need to deploy further position reference systems. For operations of
Equipment class 2 or 3 a minimum of three PRS must be deployed, preferably each of a
different operating principle. The choice of PRS will depend upon circumstances and
availability but it is advisable to have two PRS deployed when within 200m of fixed structure
and three PRS when within 100m. Vessel/Company/Client operating procedures may advise
more stringent requirements than this.
While locating into the final working position it is good practice to move the vessel in short
steps with a few minutes settle time between each move. The final 50m may be done in a
series of 10m moves with the last two or three moves being of no more than 5m. The speed of
the vessel must be carefully controlled at this stage. A typical approach velocity may be 0.25
m/s (0.5 knot) reduced progressively to 0.2m/s (0.4 knot) then 0.lm/s (0.2 knot) for the last few
moves. It is good seamanship to be on the cautious side for these stages of the operation.
During this period it is important that the DPO concentrates upon the DP system desk and
does nothing else. The other bridge watchkeeper should be carrying out all other bridge
functions associated with the approach and set-up such as handling all communications
monitoring all instrumentation other than the DP console and keeping a lookout.
Particular care must be taken when the working location is alongside a structure that is not
fixed. Examples may be semisub drilling rigs, crane barges, accommodation flotels, pipe
laying barges, offshore loading buoys, spar buoys or floating production units. Since these
installations are of necessity mobile it is necessary to assess the degree of movement
involved and adjust the planning accordingly. The minimum separation between the vessel
and such a structure must be set at a safe distance.
Account must also be taken of any underwater structure such as anchor/mooring lines. Not
only their charted or planned locations but where they actually are at the time of the operation.
Mooring line catenaries will depend upon line tensions. These must be checked before the
approach if any hazard is anticipated. The client should supply a catenary diagram with
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predicted line profiles for a variety of line tensions. Actual tensions should be checked along
with predictions due to weather, tidal and operating factors. The DPO must include in his
planning the required and expected clearances from all mooring or other lines from vessel
structure, downlines and probes. In this context the vessel must verify that the anchored unit
is not actually moving. Good permit-to-work procedures are necessary in such cases.
Final DP Setup
Once the vessel is in the final working position, then a number of tasks must be completed
before giving the go-ahead to commence the operation. A settling period of about thirty
minutes should be allowed, ensuring that the DP system has time to settle the vessel into
the location and build the mathematical model to its optimum state. During this time the
bridge watch keepers should complete the pre-operational checklist and verify that preoperational checklists are complete at other locations (e.g. the engine room staff will have
checklists of their own). Part of this check includes verifying that power generation levels
are adequate to cover existing and expected demands with the required levels of
redundancy and that thrusters and propulsion systems are operating correctly and within
acceptable limits.
Once the vessel is stabilised and settled to her working position the DPO should take careful
note of the magnetic compass heading corresponding to the gyro set point heading The DPO
should also note the mean thruster outputs, both azimuth and pitch (or rpm) for each propeller
while the vessel is in a static situation. These values will change over time as the wind, sea
state and tide change but information of this type is invaluable if all DP function is lost without
warning and manual manoeuvring has to be resorted to. These thrust settings are a good
"starting point" for the manual positioning of the vessel which may be necessary for some time
whilst the job is aborted prior to undertaking any escape manoeuvres.
The bridge team must be aware of the significant change in status once the go-ahead or
green light is given for the operation to commence. Irrespective of the type of operation, prior
to this moment the emergency contingency plan is one of safe escape from the location and
its hazards. Once, however, the "green light" is given, the contingency plan must allow for the
vessel to maintain position and heading under all circumstances until the task is aborted.
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Chapter 9 - Thrusters
Thruster Types
The thrusters provide the manoeuvring capability of the vessel. In general, three main types of
thruster are fitted in DP vessels; main propellers, tunnel thrusters and azimuth thrusters. Main
propellers, either single or twin screw, are provided in a similar fashion to conventional
vessels. In DP vessels where such main propulsion forms part of the DP function, propellers
are often of the Controllable Pitch (CP) type, running at constant rpm. This facilitates the use
of shaft driven alternators, as these could not be used if the shaft drive is not at constant rpm.
Page 69
PITCH CHANGE COMMAND AHEAD
PROP
BLADE
PUMP
CRANK RING
SPOOL VALVE
CRANK PIN
AHEAD
HYDRAULIC
FLUID
ACTUATOR
CYLINDER
ASTERN
HYDRAULIC
FLUID
ACTUATOR
PISTON
OIL
MANIFOLD
CROSSHEAD
PROP
BLADE
PUMP
CRANK RING
SPOOL VALVE
CRANK PIN
AHEAD
HYDRAULIC
FLUID
ACTUATOR
CYLINDER
ASTERN
HYDRAULIC
FLUID
ACTUATOR
PISTON
OIL
MANIFOLD
CROSSHEAD
Page 70
A more recent development is the installation of frequency control systems allowing variable
speed ac motors to be used in conjunction with fixed-pitch (FP) propellers. Conventional
rudders and steering gear usually accompany main propellers. Often the DP system does not
include rudder control; the auto pilot being disconnected and the rudder set amidships when in
DP mode. In many installations, however, the DP system includes rudder control.
In addition to main propellers, any DP vessel will employ a variety of thrusters for
manoeuvrability.
Typically a monohull Class 3 DP vessel will use six thrusters; three at the bow and three aft.
Forward thrusters tend to be tunnel mounted, operating athwartships. Here a cp or fp propeller
(or impeller) is mounted in a tunnel. Drive is from above using a bevel gearing, from an
electric motor or diesel engine. Pitching the blades port or starboard, or modulating the rpm
and direction produces thrust.
Usually two or three tunnel thrusters are fitted in the bow, with control applied identically to all.
The resultant turning moment applied to the vessel is most marked if the vessel does not have
appreciable headway - or sternway. Once the vessel is making way the effect of tunnel
thrusters drops off radically.
Page 71
Tunnel thrusters may be used at the stern also. These units consist of a cp or fp propeller
mounted in a short tunnel.
Azimuth thrusters
The unit projects beneath the bottom of the vessel and can be rotated to provide thrust in any
direction. Propeller drive is by bevel gearing from above. The whole unit may in some cases be
retracted into the hull.
Azimuth thrusters have the advantage that they can
provide thrust in any direction (compared with
tunnel thrusters) and are often used as main
propulsion. However, they are more troublesome to
locate satisfactorily. If fitted below the bottom of the
hull they increase the draught considerably and
need to be retractable, and to state the obvious, if
they are retracted in shallow water, their power is
not available.
Page 72
A wide employ Azimuth or Compass Thrusters. range of azimuth thrusters is available from a
number of manufacturers, ranging from 600kW to 7500kW (800 H.P. to 10,000 H.P.) with
propeller diameters ranging from around 2.0m to over 5.0m.
Another development in the thruster field is the podded propulsor. An example of this is the
Azipod, produced by Aquamaster of Finland. This thruster resembles an Azimuth thruster, but
the drive motor is contained within the pod, beneath the hull. This obviates the need for shaft
lines and bevel gearing. The propeller is a Fixed Pitch unit, with power supplied through
frequency convertors. Azimuth control over the full 360° is available. The propeller is
configured as a "pulling" unit, deflecting wake water past the pod that acts as the rudder.
Podded propulsion is currently being fitted in a variety of vessels, including a number of DPcapable ships.
A further type of thruster occasionally found in DP- capable vessels, is the Jet thruster,
manufactured by the Elliott-White company in the UK. These units consist of an impeller fitted
into a U-tube tunnel in the vessel's bottom. The discharged water passes through a deflector
element, giving it a horizontal vector, producing thrust. This unit is able to act as an azimuth
thruster, while being wholly contained within the hull. Generally, the thrust performance from
Jet thrusters is less than that obtained from Tunnel or Azimuth units.
Thruster failure modes
Thrusters are complex, and as such are vulnerable to a variety of failures. The variety of
failure modes is dependent upon the type of unit, i.e. a fixed pitch propeller has fewer failure
modes than a fixed pitch azimuth thruster. More failure possibilities are associated with cp
units, and many of these failures are only repairable in dry dock. This is expensive, and
thruster repairs cannot be properly tested out until the vessel is again afloat.
Any propeller is vulnerable to fouling by ropes and underwater objects. Particular damage
may be caused to seals. Any leaking seal will result in contamination of gearbox or actuator
oil, which will further result in mechanical damage and failure. Typically, a cp azimuth thruster
has six seals; one on each blade, one on the shaft, and one on the hull mounting. The DPO
must take every precaution to ensure that his propellers remain clear. A fouled propeller or
thruster immediately degrades or destroys the vessel's position-keeping capability.
If a thruster fails, the failure may be a pitch/revs freeze, or "fail-as-set", or it may fail to any
pitch/revs combination. It is important that the operator stops the thruster immediately, as a
runaway thruster can very quickly destabilise the vessel positioning capability. Many
propellers, particularly cp, have a "fail-safe" failure mode, usually relating to a loss of controlhydraulics situation. This fail-safe mode should be zero pitch, but it is important to realise that
other failure modes may leave the propeller failed to residual full pitch.
Page 73
Page 74
Chapter 10 – Environmental Sensors
Wind Sensors
Wind sensors are provided to give the system a continuously updated value for wind direction
and strength. Short-term variations in both must be compensated for if precise positioning is to
be achieved. Wind sensors or transmitting anemometers are fitted, usually in duplicate, to
provide feed-forward computer signals direct to the thrusters to compensate for wind induced
movement of the vessel from its set point position and heading.
Ultra Sonic Wind Sensor
A newer model is the Ultra Sonic Wind Sensor.
When the distant between the sending and the receiving element is known it is possible to
calculate the wind speed due to the measured delay or the speed up of the signal.
This provides the wanted 2 axis wind measurement.
The elimination of moving parts, together with a rugged stainless steel construction, means
that is maintenance free and requires no calibration on site. The heated head keeps the unit
free from ice and snow, providing continuous use in the most extreme weather conditions.
Page 75
The Wind Observer ll.
Wind sensor problems
Problems arise associated
with wind sensors due to
wind shadow of the sensor
element by ships structure
or from adjacent platform
structure. This will be
particularly pronounced if
the vessel is downwind of a
structure. It may happen
that the wind sensor at
masthead height is wind
shadowed by the platform
topside, while the bulk of
the vessel hull and
superstructure is less wind
shadowed by the lattice
structure of the platfom
jacket.
GALE
FORCE
WIND
WINDSENSOR
PLATFORM TOPSIDE
WINDSHADOW
VESSEL HULL AND SUPERSTRUCTURE
EXPOSED TO FULL FORCE OF WIND
THROUGH LEGS OF PLATFORM
Page 76
DOWNDRAFT FROM HELICOPTER
ROTOR BLADES MAY CAUSE
WINDSENSOR TO INPUT FALSE
HIGH VALUES THUS DESTABILISING
THE DP CAPABILITY
Other wind sensorproblems arise due to helicopter disturbance, particularly if the wind sensor
installation is close to the helideck. Helicopter downdraft can induce false momentary wind
sensor inputs, which are not representative of the wind forces acting upon the ship as a
whole. Since wind sensor input has an immediate feed forward effect, the result may be a
rapid drive-off as the DP attempts to compensate for apparent gusts.
Often two wind sensors are fitted, allowing the operator to select which input is likely to be the
most representative. The two sensors may be fitted at different heights on the mast, or at
opposite ends of an athwartships yard. In the latter case the operator would probably select
the windward sensor in order to avoid as much as possible disturbance caused by ships
structure. One suggested remedy to the problem of wind shadow from adjacent upwind
platform structure is to deselect the wind sensors from the DP altogether, however, caution
must be exercised here since there will be no wind feed forward available.
Page 77
WIND
WIND
WINDSENSOR
EXPOSED
FORCE OF WIND
EXAGGERATED
AT WINDSENSOR
PLATFORM
VESSEL HULL AND
SUPERSTRUCTURE
IN WINDSHADOW
STRUCTURE
WINDSENSOR EXPOSED
TO FULL FORCE OF WIND
VESSEL IN LEE OF
PLATFORM
EVERY GUST CAUSES
VESSEL TO DRIVE
TOWARD PLATFORM
AND LOSE HEADING
VESSEL
EXCURSION
Problems will arise when changes in wind speed and/or direction occur, resulting in
deterioration in positioning of the vessel. It must also be remembered that while all wind
sensors are deselected the processor will continue to use the model values for the wind, i.e.
the values last recorded. If, when a wind sensor is reselected into the system, there are
different wind values obtaining (which is likely) then a temporary loss of position control may
be expected.
LOADING
TERMINAL
Windsensor located here may
suffer turbulence from the
loading tower
Better Location for
the windsensor
Page 78
In vessels where the helideck is adjacent to the wind sensor location, or where helicopter wind
sensor disturbance has been observed, it is common to deselect wind sensor input during
helicopter movements. In some cases this will necessitate temporary suspension of the
operation in progress.
Shuttle tankers in Weather Wane mode can also have turbulence in lee of the loading tower ,
FPSO etc.
WINDSENSOR LOCATION
WINDSENSORS LOCATED
IN POSITIONS 1 AND 2
WILL GIVE INPUT
DISTORTED BY
TURBULENCE
FROM
STRUCTURE
LOCATIONS 3 AND 4
ARE BETTER BUT INPUT
MAY NEED TO BE SCALED
DOWN DUE TO ALTITUDE
EXAGGERATING THE
WIND STRENGTH
3
1
4
2
Page 79
Tide or Current
A vector is shown on the PosPlot display pages indicating the set and rate of the tide. It is
important to realise that this is not a measured value; there is no tide sensor deployed, but a
derived vector determined from a summation of differences between predicted positions and
measured positions. This displayed vector will not necessarily be accurate, as it will contain
elements of residual errors within the system.
In modern DP systems, a facility is provided allowing a more rapid determination of the current
value; this facility is called "Quick Model Update". When selected, this facility allows the
operator to accelerate the process of building the mathematical model, and thus the current
vector, for a limited period of time. This facility might be used to advantage immediately after the
turn of the tide. If the tide has turned quickly, the DP system may become less stable due to the
model value of the tide still relating to the old value. Applying the Quick Current Update facility
may bring about a rapid improvement in the system response. However, it is important to realise
that the accuracy of the current determination will be in inverse proportion to the length of time
involved in the model update period. Use of the Quick Model facility may result in larger errors
showing in the tidal indications.
Roll & Pitch
It is necessary to provide an input to the processor regarding vessel attitude, i.e. angles of roll
and pitch, on a continuous basis. Several of the Position Reference systems function by
measuring angles relative to the shipboard sensor element. Roll and pitch of the vessel will
introduce errors into these angle inputs that will translate into position errors. By providing the
system with constant roll and pitch angle data, the position reference input data may be
corrected to the true vertical. Roll and Pitch information is provided from a Vertical Reference
Unit or Sensor (VRU or VRS).
Page 80
From Kongsberg Seatex two types of vertical reference sensor are available. The simplest
type yields angular information regarding roll and pitch. A more complex type utilises more
accelerometers to yield values for vessel Heave (vertical bodily movement) also. Although a
consideration of heave is not essential to the DP function, this output can be utilised
elsewhere, in heave compensation for diving bells or crane hook, or in fire monitor
stabilisation.
Page 81
Chapter 11 – Position Reference Systems
Notes for use of Position Reference Systems
Extracts of
IMO, INTERNATIONAL MARITIME ORGANISATION MSC/Circ. 645
GUIDELINES FOR VESSELS WITH DYNAMIC POSITIONING SYSTEMS
3.4.3 Position reference systems
1. Position reference systems should be selected with due consideration to operational
requirements, both with regard to restrictions caused by the manner of deployment and
expected performance in working situation.
2. For equipment classes 2 and 3, at least three position reference systems should be
installed and simultaneously available to the DP-control system during operation.
3. When two or more position reference systems are required, they should not all be
of the same type, but based on different principles and suitable for the operating
conditions.
4. The position reference systems should produce data with adequate accuracy for the
intended DP-operation.
5. The performance of position reference systems should be monitored and warnings
provided when the signals from the position reference systems are either incorrect or
substantially degraded.
6. For equipment class 3, at least one of the position reference systems should be
connected directly to the back-up control system and separated by A-60 class divisions
from the other position reference systems.
Page 83
The Hydro-Acoustic Position Reference System
A number of manufacturers are involved in the production of HPR equipment. We will confine
our remarks mainly to the Kongsberg systems. The principle of position measurement involves
communication at hydroacoustic frequencies between a hull-mounted transducer and one or
more seabed located transponders.
The system is based upon the Supershort baseline (SSBL) principle with all acoustic
transmit/receive elements mounted in a single transducer unit. Alternatively, Long baseline
acoustics may be used.
In the SSBL system, an interrogating pulse is transmitted from the transducer. This pulse is
received by the transponder on the seabed, which is triggered to reply. The transmitted reply
is received at the transducer. The transmit/receive time delay is proportional to slant range.
The hull mounted transducer is able, by means of its supershort baseline configuration, to
determine the angles of the incoming reply with respect to the vertical, both longitudinal and
athwartships. These angles and range define the position of the ship relative to that of the
transponder. A number of corrections are automatically made. The angles must be
compensated for values of roll and pitch (from the VRS). The determined position will be the
position of the transducer, and offsets will be allowed for to give the position of the Centre of
Rotation (CR) of the vessel. If the Centre of Rotation is changed, or if a different transducer is
used, then the offsets within the DP system automatically change to provide correct coordinates
Page 84
The basic elements of an HPR system are shown above. Frequencies used are in the 20-40
KHz range. The Kongsberg HPR 400 system uses a discrete range of channels of
transponder communication, each channel has two designated interrogation frequencies, and
one reply frequency. Position reference may take place from a single transponder laid on the
sea bed. Greater reliability and accuracy can be obtained from using more transponders. Any
number of transponders can be used provided they operate upon different channels, the
limitation being the number of channels the HPR system installed is capable of working with
(normally 5).
A similar HPR system, the Nautronix ATS uses transducers and transponders as described
above, but greater reliability in acoustic communication is obtained from interrogating the
transponder by means of a "chirp". The chirp is a multi-frequency interrogation signal, which
will have greater ability to survive attenuation or noise pollution in difficult acoustic conditions.
The HPR system is very versatile; it is not simply used for position reference of DP vessels. It
is also used for position monitoring of seabed or subsea vehicles, also for marking for
relocation of subsea features, e.g. wellheads, pipelines etc. It may also be used for buoy and
valve control functions for offshore loading operations, and for the location and monitoring of
the position of a diver or diving bell.
SSBL HPR Principles
Transducers are fitted to the bottom of the
vessel in the form of a probe, able to extend
approximately four metres below the shell
plating. Accordingly they are provided with
means to extend and retract (electric motordriven Hull Unit) with controls both local and
remote (at the HPR display unit). A ships
bottom sea chest or gate valve is provided to
ensure the watertight integrity of the vessel
when the probe is withdrawn into the hull.
The probe is normally raised when the HPR
is not in use, particularly when the vessel is
underway.
The positioning is by "Ultra Short Baseline
Interferometry". The transceiver emits an
acoustic "ping" at the channel-designated
frequencies. This "ping" is projected by the
transducer down to the transponder on the
sea bed. The transponder emits a reply at
the designated frequency; this reply is
received by the transducer. Slant range is
determined from the delay time while the
transducer head is able to determine the direction from which the reply originated.
A number of Transducer types are available; the Standard unit and the Tracking type, also the
more accurate HiPAP type. Often a vessel is fitted with more than one transducer. Each unit is
able to operate in several different beamwidth modes. The
SSBL Narrow beam has a scope of about 45° while Wide beam has about 160°. An
intermediate "Medium" beamwidth of 110° is also available. The HiPAP uses a directional
Page 85
narrow beam of 10° within a 180° coverage area. The Tracking transducer has the facility of
being able to direct its narrow beam in azimuth and elevation (or depression) angles so as to
follow a selected transponder. This gives greater ranges on a particular transponder, and also
helps prevent interference from other sources. If the angle between the vertical and the
acoustic path to the transponder exceeds 45°, then better results will be obtained if the HPR
system is operated in "Fixed Depth" mode, or if a depth sensing transponder is used.
The operator is able to select the interrogation rate of the system. A high rate of interrogation
is used where the vessel or transponder may be moving, or where three or less transponders
are active, and ranges are relatively short (up to about 250 metres). Typical high interrogation
rates may be 1.0 sec intervals, or 2.0 sec or 4.0 sec. Low interrogation rates with intervals of
8.0 sec to 40.0 sec may be used where the position situation is fairly static, where long ranges
to the transponder dictate lower rates, or where it is necessary to save transponder battery
capacity. Under normal circumstances, the operator would select "wide" beam operations until
transponder communication is established, then he may transfer to "narrow". If transponder
communication is lost then the system automatically reverts to the "wide" mode. Some
systems also operate in a "medium" beam mode.
A typical CRT display runs on Windows NT, and has a number of facilities. It is able to portray
the ship in the display centre either Relative or True. Relative may display ships head up, or in
the case of the HPR forming part of a DP console fitted into the after bridge of a vessel, hence
aftfacing, Relative mode would then display stern-up. True display, when selected, will put
North at the top of the screen, but this necessitates a gyro feed. Display range in metres may
be selected by the operator.
When in use, the
display shows the
vessel at screen
centre, and
deployed
transponders in
their appropriate
locations. Each
transponder has
a symbol which
appears on the
CRT
accompanied by
a table of
positional coordinates. The
co-ordinate mode
may be selected
by the operator
as Polar, in which
case the coordinates
displayed would be Range, Bearing and depth (horizontal range in metres, relative bearing in
degrees, depth in metres). The operator may alternatively select Cartesian co-ordinates such
that the three values displayed represent x, y and z offsets (distance starboard in metres,
Page 86
distance forward in metres, depth in metres). The operator also has the facility of placing a
selected transponder at the display centre.
Once communication with one or more fixed transponders has been established, the position
data may be fed through to the DP system by means of a suitable interface, for position
reference. Again, compensation is made to allow for the offset distances between the
transducer sensor and the rotation centre of the vessel. If more than one transponder is in
use, then the DP pooling logic is able to resolve the best positional data from each. It may
happen that one (or more) of the transponders selected is not in a fixed position, e. g. one is
mounted upon an ROV. Since the DP must ignore position-reference indications from this
transponder the operator is able to "mobile select" that transponder. Thus, it will appear on
HPR and DP displays such that the operator can monitor its position, but its input will be
ignored by the DP as regards position measurement.
Long Baseline Systems
Another system; the long baseline system, uses an
array of three or more transponders laid on the
seabed in the vicinity of the worksite. One transducer
upon the vessel interrogates the transponder array,
but instead of measuring range and angular
information, ranges only are measured. Position
reference is obtained from range geometry from the
transponder locations. This principal provides greater
accuracy than the super short baseline (SSBL)
system since the baseline length is not limited by ship
dimensions, but it is reliant upon two-way acoustic
communication. This system is dependant upon the
vessel calibrating the positions of the individual
transponders forming the seabed array, before use.
One current system from Kongsberg, their HPR 418
is able to operate both in Super-short baseline mode,
with one or more transponders, or in Long baseline
mode using three or more transponders for greater
accuracy. With the Long Baseline (LBL) system, it is
necessary for the array of transponders to be laid and
then calibrated. This calibration is done by allowing
each transponder in turn to interrogate all the others
in the array, in turn. If, at the same time, the vessel
has a DGPS or other geographically referenced
positioning system, then the transponder array may also be geographically calibrated. Long
Baseline systems are more suitable for deep water operation, where SSBL systems would
suffer from long information update
rates. Accuracy is also superior to the SSBL method. LBL systems have a typical accuracy of
0.2% - 0.4% of water depth.
Page 87
Short Baseline System
The Honeywell/Nautronix short baseline system uses a
single acoustic Beacon placed on the seabed, transmitting
continuously. On board ship the acoustic pulses are
received by a number (usually four) of passive
hydrophones placed at different locations. Since the
physical locations of the hull mounted hydrophones are
known, then the times of arrival of individual pulses at each
hydrophone can be compared, and the time differences will
define the direction and distance of the pinger from the
vessel.
Transponders
A number of different types of transponder are available for different functions and situations.
The Kongsberg MPT transponder may be configured for SSBL or LBL positioning, array
calibration, and with telemetry commands. These commands include sensor readings for
depth, temperature, inclination and heading, plus acoustic release. It is also possible to switch
transponder channel by telemetry command from the surface. The SPT transponder is used in
SSBL positioning, while the RPT transponder is used for ROV or Tow fish positioning.
A standard transponder may be secured by bracket to subsea structure, or it may be moored
to the seabed. The recommended method of mooring is to fit the transponder with a divinycell
float, and attach it to a 150kg sinker by means of 1-2 metres of mooring chain. The unit is then
deployed onto the seabed by means of a light wire line, either from the deck of the ship or
from a small boat, with the wire buoyed off, or simply left slack.
Another method of deploying a transponder is to secure it into a purpose built tripod which is
then lowered to the seabed on a wire rope. Once located, sufficient slack is paid out to
accommodate subsequent vessel movement. Deployment and recovery is by a winch and
davit arrangement.
Other transponder functions are available for use in special circumstances. An Acoustic
Release Transponder may be laid upon the seabed without a down-line. Operating normally
the transponder provides position reference. After use the transponder can be recovered by
Page 88
acoustic command, releasing it from the seabed. The transponder floats to the surface and
can be recovered by boat or from the ship.
An Inclinometer transponder facility provides information not only for position reference but
also regarding angular attitude. A unit of this type may be used in subsea installation, or in
drilling. In the latter case the transponder would be secured to the lower flex joint of the
marine riser. In a DP drillship information upon flex-joint angle is used to position the vessel,
the object being to maintain the riser vertical at the seabed in the face of constantly varying
tides.
A standard transponder may be approximately 1 metre in height by 120mm in diameter. For
small vehicles, such as RCVS (remotely-controlled submersible camera vehicles) miniature
transponders are available, of much smaller dimensions. Transponders such as these can
also be carried by a diver.
For ROV positioning, more reliable communications may be obtained by using a Responder
instead of a transponder. A responder is used where a hard-line umbilical is available.
Vehicles of this type are usually very noisy, providing poor acoustic conditions for HPR Using
a Responder eliminates one through-water journey for the acoustics. A spare umbilical
channel is used to trigger the responder.
The foregoing notes apply to the Kongsberg HPR system. A UK company, Sonardyne,
manufactures and markets a similar SSBL system with common channels to the Kongsberg
system. Thus Sonardyne and Kongsberg transponders are compatible, but carry different
identification numbers. A further USBL system, the Nautronix ATS has already been referred
to.
Factors affecting acoustic positioning
HPR is a versatile and widely used position reference for DP and other purposes. It does,
however, suffer from some limitations.
Typical factors affecting HPR are:
•Noise
•Transmission losses
•Reflection of signals
•Ray bending caused by variations in sound velocity
Operationally, noise and aeration are the main problems. Acoustic (noise) interference may
originate from the ship's machinery or thrusters, from other vessels or nearby installations, or
from any sub-sea operation in progress (e.g. drilling or ROV work).
Aeration from any cause will result in a loss of signal. Common causes of aeration are
thrusters (vessel or ROV), mud or stone dumping, divers' breathing gases or rough seas.
Careful positioning of transponders by the operators can obviate many of these anticipated
problems. Other problems result from acoustic refraction, particularly at long horizontal ranges
from the transponder, caused by temperature or density layers in the water. Further problems
arise in shallow water, where vessel and thruster noise may be a major problem, and also
where vertical separation between transducer and transponder is limited.
Page 89
System accuracy is dependant upon a number of factors, some of which cannot be allowed for
such as water temperature. In general, accuracy can be taken to be between 1% and 3% of
slant range for SSBL systems, and 0.2% to 0.4% of water depth for LBL systems. Horizontal
range should not exceed water depth for best accuracy; at larger horizontal ranges than this
there is an increased risk of loss of signal due to refraction.
Page 90
The Artemis Microwave Position Reference System
In this system, position reference is obtained by means of communication at 9 GHz (Xband, 3cm wavelength) radio waves, or microwaves. The system described is the Dutch
ARTEMIS system and involves two stations; one located on board the DP vessel itself and
the other at some fixed location ashore, or upon a platform or FPSO installation.
The position reference is in the form of range and bearing. The station on board ship is
referred to as the 'Mobile' station, while that ashore is the 'Fixed' station. Position data is
thus obtained as the range and bearing of the Mobile station with reference to the Fixed
station. Each station consists of an Operating Unit and an Antenna Unit.
The principle is simple. The two antennae automatically train so as to face each other at all
times when a c.w. microwave link is established. The Mobile station transmits a signal,
which is received by the Fixed station and retransmitted as a reply. The time lapse between
Mobile station transmission and reply reception is proportional to range between antennae.
The azimuth or bearing is measured at the Fixed station and is transmitted, encoded, as
part of the reply. The signal used is a very brief interruption or occultation in a continuous
wave transmission; this interruption being detected at the Fixed station and a similar reply
interruption initiated. For accuracy, successive displayed ranges are obtained by averaging
a great number of observed time lapses (one thousand or ten thousand, depending upon
function selected).
Position reference is thus obtained in a continous, reliable, accurate manner utilising only
one external (Fixed) station. Once a microwave link is established then voice
communications are possible using handsets. Position reference can be obtained whenever
there is a line-of-sight between the Fixed and Mobile stations which gives the system
greater operating ranges than BPR or Tautwire. Typical maximum range is about 30km, but
for DP purposes 5km is a more realistic figure. At ranges greater than this the bearing
Page 91
accuracy of about 0.03 degrees leads to a deterioration in positional accuracy inadequate
for DP purposes. Range accuracy of around 0.5 metres is obtained. Range and bearing
obtained are then passed into the DP system by means of a suitable interface and
corrections are applied for vessel roll and pitch values (which cause the antenna to move)
and for antenna offsets relative to the rotation centre locations.
The Artemis Fixed station is often set up permanently, or as a temporary installation upon a
fixed or floating production platform or similar. Before use the Fixed station must be
calibrated for bearing. To do this it is necessary to obtain a visual reference direction - to
identify some fixed object nearby (another platform perhaps) to which the bearing can be
obtained by reference to terrestial co-ordinates. A small telescope is shipped into a
mounting on the Fixed station antenna, and the antenna trained by hand until the reference
object lies on the cross-wire in the telescope. The antenna is thus trained onto the
reference bearing, which bearing is entered into the unit display. The reference bearing
may be referred to True North or any local grid North. Subsequent to such calibration the
telescope can be unshipped and bearing readout for any antenna direction should follow
correctly.
The Fixed station may be a temporary installation, placed upon a platform for one particular
job of limited duration. In this case it is necessary for the equipment to be installed, taking
care that the antenna will cover the sea area of the DP vessel's worksite, also that the
antenna can rotate without fouling platform structure. It is also necessary to ensure that the
antenna position will not interfere with other platform functions. A power supply must be
obtained, preferably with a redundant battery back-up supply, and platform staff will need to
be instructed in the operation of the system. If the vessel is to work all round the platform
then two or maybe more Fixed stations will need to be installed to cover the 360 degrees.
The elevations of the Fixed station antennae must be compatible with that of the Mobile
station, since, with only a 22 degrees vertical beamwidth it is possible, at close ranges, for
the signals to be lost if Fixed and mobile antennae are at different heights.
Page 92
Artemis Operation
Pair number
0
1
2
3
Mobile Station
9200 MHz
9300 MHz
9230 MHz
9270 MHz
Fixed Unit*
9230 MHz
9270 MHz
9200 MHz
9300 MHz
The Artemis has 4 frq. Pair for multi user operation.
The Mobile and Fix stations are operated via the keyboard and display of the operating
panels. Once a link has been established between the two stations, the complete system
can be controlled from one station, generally the Mobile station.
Operator interaction between the keyboard and the display of the operating panel is via a
menu structure. This is basically the same for both the Mobile and Fix station.
1
2
3
<MAIN MENU>
OPERATE
MODIFY
MONITOR
MOB
WAKE UP
FIX
WAKE UP
Main menu Mobile
station
4
5
6
1
2
3
<MAIN MENU>
OPERATE
MODIFY
MONITOR
FIX
WAKE UP
MOBILE
CONFIG
4
5
6
Main Menu Fix station
From the main menu a selection can be made from the six categories by keying in the
number adjacent to the category.
OPERATE
is selected to display the position parameters distance and azimuth.
MODIFY
is selected to read and/or change station parameters, location parameters,
autosearch scan angles, clock, operating mode, communication parameters and voice
channel volume.
MONITOR is selected to monitor supply voltages, mixer crystal currents, Automatic Gain
Control, tuning and servo signal readings, temperature of wave guide and distance
measuring circuit.
WAKE-UP is selected to define the time and date the station is to switch itself on, this
function is normally not used.
FIX (if Mobile station)
is selected at the Mobile station to remotely access the Fix menu
structure to read and/or modify Fix station parameters.
MOBILE (if Fix station)
is selected at the Fix station to remotely access the Mobile menu
structure to read and/or modify Mobile station parameters.
CONFIG
is selected to configure an ABU before it is put into operation for the first time.
Page 93
ANTENNA INDICATOR DISPLAY
The antenna indicator display shows:
a)
b)
c)
The ANTENNA DIRECTION marker, which indicates the direction of the antenna
with respect to the chosen reference direction. When the EOP is connected to the
Mobile station, the marker indicates the relative antenna bearing. When connected to
the Fix station, the marker indicates the azimuth.
The MAIN BEAM marker, if signal containing the correct address code is received
and the antenna is locked to the counter station.
The AUTOSEARCH SCAN SECTOR marker, if the autosearch operating mode is
selected and an autosearch scan sector is set. The unshaded area indicates the
actual scan sector.
ARTEMIS OPERATIONAL
From the viewpoint of the DP operator, the major advantage of Artemis compared with many
other PRS is its range. When approaching the worksite the Artemis link can be established
when a mile or more away, and provided line-of-sight is maintained then position reference is
available immediately DP is engaged. However, there are drawbacks. Interference can be
experienced from radar transmissions from other vessels (or from ones own!) at 3cm
frequency. Also, loss of signal will occur due to any line of sight break, such as a vessel
passing through the beam, or personnel on the platform working in the vicinity of the Fixed
antenna. Precipitation may also cause loss of signal. An operational disadvantage is the lack
of control over the Fixed station. Being sited upon a remote platform it is not of immediate
access to ships staff and is subject to interference from unauthorised platform staff.
occassionally for example, the batteries are "borrowed" for another purpose and not replaced
Page 94
ARTEMIS DIP ZONES
PRODUCT
800
H1 x H2
H1 =
Mobile
antenna
height
H2 =
Fixed
antenna
height
6000
10000
14000
18000
22000
26000
30000
26000
30000
700
600
500
DIP
ZONES
400
300
6000
10000
14000
18000
22000
DISTANCE (metres)
At certain ranges, mutual interference will cause loss of signal. These are known as "Dip
zones". A table of Dip zones is provided, entered with elevations of Fixed and Mobile
antennae, and yielding Dip zone ranges.
ARTEMIS BEACON
BASE
POSITION
ARTEMIS
BEACON
ARTEMIS
MOBILE
ANTENNA
ARTEMIS MICROWAVE
LINK
THE BEACON IS SIMPLY
A TRANSPONDER. NO
BEARING DATA TRANSMITTED
OFFSHORE LOADING
TERMINAL WITH
ROTATING TURNTABLE
BEARING MEASURED
AT MOBILE ANTENNA
SHUTTLE TANKER
DURING APPROACH
TELEMETRY LINK ALLOWS TURNTABLE AZIMUTH TO BE
TRANSMITTED TO THE VESSEL SUCH THAT BEACON OFFSET
CAN BE COMPENSATED FOR, CORRECTING THE RANGE TO
THE BASE LOCATION
Under some circumstances it may not be possible to establish an Artemis Fixed Station.
This may be where the fixed station location is not positively "fixed" in position. An example
of this situation is an offshore spar buoy used for shuttle tanker loading. Increasingly such
tankers are fitted with DP for positioning during the loading operation, and Artemis is used
Page 95
for position reference. The spar buoy, however is mobile to a certain extent and may be
able to rotate. In this case the Fixed Station is replaced by an Artemis Beacon. The Beacon
is a transponder with a broad beam antenna. The antenna itself is fixed. The system is
used in exactly the same manner as the standard Artemis, a microwave link may be
established without personnel assistance on the installation. This is essential since many
Offshore Loading Terminals using Artemis beacons are unmanned. Range data is obtained
in the same way as with a Fixed station with identical accuracy. Bearing is obtained at the
Mobile station end, using the Mobile antenna direction and gyro heading reference. Bearing
accuracy is thus reduced, being only as good as the gyro compass - perhaps 0.25 to 0.5
degrees. For this reason it is suitable for shorter range operations.
Page 96
The Taut Wire position reference system
Taut Wire differs from other position reference systems by being chiefly mechanical in
principle, not reliant upon radio or acoustic transmissions.
15.1 Taut Wire system principle
CENTRE OF
ROTATION
x
y
D
D
CENTRE OF
ROTATION
DEPRESSOR
WEIGHT
Y
Y = D tan
+
X
y
X = D tan
+
x
A Taut Wire system consists of a constant tension winch unit fitted on deck, with a boom or
'A'-frame projecting over the side of the vessel. Wire from the winch drum passes over a
sheave at the end of the boom, through a sensor head, and terminating in a depressor weight
on the seabed. Position reference is obtained from measurements of wire angle and water
depth, the position of the vessel being defined relative to the location of the stationary
depressor weight.
Page 97
Bandak Mk. VIII
A typical Taut Wire system is the Bandak Light Weight Taut Wire Mark VIII. In this system
the depressor weight has a mass of 350 kg, while 500m of 5mm wire is provided. Maximum
wire angle is 30 degrees to the vertical in any direction, and the maximum water depth for
use is 350m. The motor and drive unit is mounted on deck at the side of the ship, with the
system control panel adjacent.
The deployment method uses an ‘A’ frame, which stows in the vertical position. To deploy,
the ‘A’ frame is lowered to the horizontal, projecting overside.
The weight is then lowered to the seabed. Once at the seabed the system automatically puts
the winch into tension or "mooring" mode. At this moment the wire length deployed is read by
the system. Subsequent movements of the vessel are accommodated by the spooling of the
winch, while wire angles to the vertical, both in the longitudinal (fore-and-aft) and in the
athwartships plane are continuously monitored by the sensor units at the end of the boom.
Taut wire signals are fed back to the DP through a suitable interface and can be accepted by
the operator at the DP console.
The system continually corrects input data for values of roll and pitch such that wire angles
are relative to the true vertical instead of the local ship vertical. Corrections are also applied
to allow for the offset distance of the position of the sensor head relative to the Centre of
Rotation of the vessel.
Frequently, the Taut Wire system is fitted with remote controls such that the DP operator may
deploy the boom and weight from the bridge. Often, however, this facility is not used unless
the operator can actually see the system he is deploying; it is not a particularly safe practice
to remotely operate the Taut Wire without being able to see what is happening.
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A Taut Wire system is particularly robust and reliable, together with being accurate.
Maintenance is necessary but this is under the control of the vessel operators - there is no
remote and inaccessible equipment involved. Spare wires and depressor weights can be
carried and the system may be regularly oiled and serviced to schedule. A worn, stranded or
frayed wire can be replaced. In constant use the wire may wear at the same spot all the time.
To overcome this problem the wire can be unshipped from the weight and cropped back
approximately ten metres every couple of weeks or so. This brings a new portion of wire
onto the sheaves when working and also freshens the connection to the depressor weight another point of possible failure. It is important to log the remaining wire length at each
"crop".
Taut Wire range limitations
MOONPOOL TAUT WIRE
NO BILGE KEEL LIMIT
BUT VERY LIMITED
RANGES IN SHALLOW
WATER
SHORTER RANGE
IN SHALLOW WATER
BILGE
KEEL
LIMIT
LONGER RANGE IN
DEEPER WATER
One drawback of the system is its limited operating range, due to the 30-degree wire angle
limit. This limit is imposed due to the increasing risk of dragging the weight at larger angles. A
dragging weight could, of course, lead to immediate position errors from the system. The
operating range is thus dependent upon water depth; the deeper the water the greater the
range of cover. Another limit is imposed by the bilge keel of the vessel impinging upon the
wire before the 30-degree angle is reached.
Often a vessel is fitted with two Taut Wire systems, one on each side, allowing the operator
to select the most advantageous system for the circumstances. If the vessel is working close
alongside a platform, with divers or ROV deployed into the platform, then the Taut Wire on
the side away from the platform will be used. This keeps the wire away from the operation
and also allows the vessel to deploy the Taut Wire before making her final move onto the
worksite without the wire coming onto the bilge keel.
In some vessels the Taut Wire system is fitted at the forward extremity - right on the bow of
the vessel. One problem associated with this location is the large accelerations experienced
Page 99
in moderate to rough seas due to vessel pitching. Occasionally system dropout will result
from the winch spooling rate being unable to cope with the pitching.
Tautwire deployment
TAUTWIRE DEPLOYED
ON SIDE AWAY FROM
PLATFORM OR STRUCTURE
THE TAUT WIRE SHOULD BE DEPLOYED
WITH A POSITIVE OUTWARD ANGLE
SUCH THAT POSITION REFERENCE FROM
IT IS MAINTAINED DURING THE EARLY
STAGES OF A MOVE AWAY
TW ANGLE
LIMIT
IF THE TAUT WIRE IS DEPLOYED PLUMB
TW
ANGLE
LIMIT
VERTICAL
WITH THE VESSEL ON THE WORKING
POSITION, THEN ANY MOVE AWAY WILL
RESULT IN TAUTWIRE DROPOUT AS THE
WIRE CONTACTS THE BILGEKEEL
DEPRESSOR
WEIGHT
When using Taut Wire as a position reference, it is necessary to plan with care the position
for deployment of the depressor weight. The seabed can be a cluttered place and often a
field operator will stipulate that there must be nothing placed on the seabed within a certain
distance (perhaps ten or twenty metres) of pipelines, control lines or other seabed
installations. In addition to this constraint the operator would aim to place the weight in a
position so that the final working position of the vessel does not result in the wire angle
almost at its 30-degree limit. This would prevent the vessel moving in one direction. In
practice, a working limit of 20 degrees is recommended.
A further drawback of the Taut Wire system is that there is no geographical reference for
position. It is never known with certainty exactly where, in terms of geographical co-ordinates,
the depressor weight landed. Since position reference is from the weight then exact coordinates for the ship are not available as they would be if using a UTM-referenced position
reference such as DGPS.
Page 100
Effects of strong tides on Taut Wire
VESSEL
OFFSET
POSITION OF VESSEL
AND WIRE WITH NO TIDE
TIDE
EFFECTS EXAGGERATED
FOR ILLUSTRATION
THE WIRE MAY ALSO
OSCILLATE OR VIBRATE
IN STRONG TIDES
WIRE BENDING
DUE TO TIDE
An other drawback of the Taut Wire system is that the current / tide can bend the wire and
subsequently make an error in the positioning. The wire may also oscillate or vibrate in storng
tide.
The Taut Wire requires a continuous power supply and it uses a fairly large amount of power
when compared with other position references. As such it is not connected to the UPS
system powering the remainder of the DP but is wired directly into the main switchboard.
Several other types of Taut Wire system are in use.
The Taut Wire Mk 8-22 from Bandak has a deep-water capability down to 500m. This unit is
provided with a computer, modelling the curve in the wire caused by tidal forces. Typical
accuracy is around 10m, deteriorating to 20 - 30m at 500m depth.
Page 101
An other model from Bandak is the Mk 12.
This model does not have the hydraulic weight catcher but use depressor weight with a
special shape.
Page 102
For some types of vessel a platform type of Taut Wire can be used. This may be fitted in a
bridge or cellar deck of a semi-submersible vessel where access to the sea is available
directly beneath the Taut Wire location platform. While similar to the type already described,
this unit dispenses with the extending telescopic boom. Instead the weight simply lowers
directly away from its housing structure. Often, this type of Taut Wire is housed in a
deckhouse structure, so it is protected from the elements when in use and at other times.
Page 103
For some vessels a Moonpool Taut Wire is suited. This unit is mounted inboard with
a depressor weight deployed through the bottom of the vessel through a small moonpool or
wet well. The sensor head is incorporated into an elevator unit that is lowered from the
tweendeck stowage level down to the keel level. The depressor weight and wire are then
lowered onwards from there. Hydraulic accumulator provides movement compensation and
positional data is obtained and processed in exactly the same way as in the types previously
described. The function of a Moonpool Taut Wire is to enable a DP vessel to operate in
surface ice conditions. It also has the advantage of obviating the bilge keel angular limit. A
disadvantage arises, with this type of installation, in shallow water, since the sensor head is
that much closer to the seabed than with a deck mounted unit. This further reduces the
horizontal scope of the system, already limited in shallow water.
Page 104
Another style of taut wire is the Horizontal or Surface Taut Wire system. This unit gives
position reference relative to a fixed structure. The wire is passed across to the platform
adjacent and secured. The geometry is different to the Vertical Taut Wire but principles are
the same. No boom is needed, instead the sensor is located atop a short vertical tower.
Range is limited to about 100 m wire length but the wire is wholly "in view" unlike the Vertical
Taut Wire system.
Page 105
Taut wire display page Kongsberg DP
Page 106
Fanbeam System
Fanbeam is an auto tracking laser system for one or a number of targets. Used with DP, only
one target is tracked at a time.
Scanner unit
Control unit
The system is made up of 4 parts:
• Scanning head
• Control unit
• Display
• Power supply
Power unit
Display unit
The scanning head is placed on the top
viewing. The laser has two lenses, one
the right-hand lens when facing forward
must be kept clean. When the laser is
will face astern.
The laser can be tilted vertically so that
point towards the target. A telescope is
the laser for this use. The laser is
Class 1 laser, the same as the laser used
in a CD player.
of the mast to achieve full 360°
for transmitting and one for receiving,
being the transmitter. The lenses
parked, the lenses
the centre axis will
mounted on top of
classified
as
a
Targets
Page 107
The targets must be equipped with reflectors, preferably retro prisms that return incoming
light inside ±30° of the centre of the prism. Reflective tape, “Diamond Grade White Reflective
Tape” is recommended, can be used at short range only because the quality of the return
signals is poor. The longer the distance, the more prisms must be used, stacked on top of
each other.
•
•
•
For distances up to 1 km three prisms should be used.
For distances over 1 km six prisms should be used.
To make a target visible from all angles the prisms should be formed as a circle consisting
of 8 prisms and the circles stacked on top of each other.
Range Measurements
The laser measures range with great accuracy.
Theoretical maximum range = 30.000 m
Theoretical minimum range = 3,75 m
The laser beam:
20° vertical fan shape (± 10°) from the centre axis of the lens.
The fan shape is made by the lens.
Due to loss, this fan will be reduced to approx. 6° (± 3°) at 2 km.
2 cm horizontal beam. Laser is parallel light beams.
Bearing Measurements
The scanner contains a motor which rotates the laser in the horizontal axis and an encoder
which measures the bearing. To find the bearing the system has to find the target centre.
This is achieved through the laser scanning the target in a specified sector. Based on the
reflected light the target’s horizontal centre of gravity is calculated. The system will
automatically adjust the centre of the sector to the target’s centre of gravity.
The sector has to be large enough for the system to be able to follow the target if the vessel
suddenly changes heading, and the effects of pitch and roll.
The laser’s horizontal bearings are measured with an optic encoder with a resolution of 0,01°.
This makes the accuracy better than 0,02°. At a distance of 1 km the inaccuracy will be 35
cm.
The system gives bearing and range to target relative to the vessel. To find the vessel’s
position the DP system must know the vessel’s heading. The accuracy of the measurements
of an ordinary gyro is approx. 0,5°. At a distance of 1 km the inaccuracy will be 8,8 m, and
thus making the gyro the limiting factor for the system.
In addition, the scanning head’s position in relation to the vessel’s centre of gravity must be
corrected for pitch and roll by VRS signals.
Operation
The operation is simple:
1. Search for targets
2. Select a target
3. Park the laser after use
Page 108
Use the arrows to go between the different commands, and select command by pressing
the ENT button. On older versions with a plasma display, the command line refers to
buttons just underneath the display, or function keys (F keys) on a keyboard when
using a PC.
Main Menu
The main menu consists of 4 commands:
If you suspect that you have changed something that should not have been changed, switch
the system off and on, the values will then be reset.
•
1. Setup
•
- used for initial setup as data out put, Alarms Colours and Vessel Outline
Page 109
•
•
3. Park
2. Track
- used when searching and selecting target
- used when the laser returns to parked position
4. The COMMAND menu used for different command functions
Page 110
Fanbeam Maintenance
Control and wash scanning head regularly. Wash with a mild detergent. Clean lenses using a
proprietary lens cloth. The connectors to be greased with silicon grease.
Every 6. month the shaft seal has to be lubricated. Apply a grease gun to the nipple under
the laser until grease is just seen appearing from the upper lip. Use lithium based
multipurpose grease.
If greasing is neglected, the shaft seal has to be replaced. The result might also be ingress of
water with resulting problems.
Page 111
CyScan system
CyScan is a high performance, laser based, location tracking system specifically engineered to
aid the automatic positioning of vessels in marine applications. It can serve as the primary or
back-up location device and is applicable to virtually any type of vessel.
CyScan comprises a scanning sensor system and associated PC based data processing
software. The system is usually permanently installed on a vessel as an integrated part of its
navigation or dynamic positioning system.
The scanner takes range and bearing measurements to pre-positioned passive retro-reflective
targets with high precision. The target(s) are typically located in the vicinity of a required
docking station or on another vessel. The measurements are used to determine the vessel’s
position and heading in relation to the target location. Readings can automatically feed into the
vessel’s navigation control system via an industry standard data interface.
Typical applications include dynamic positioning, docking control, shuttle mooring , tanker
berthing, diver / ROV support and hydrographic surveying.
Page 112
RadaScan system – FMCW principle
RadaScan is an advanced position reference system for use in DP and other vessel control
applications. It has been specifically designed for the offshore sector with a usable range out to
1000m under all weather conditions.
Its typical measurement accuracy is 0.05%.
The system comprises an FMCW (Frequency Modulated Continuous Wave) radar operating in
the 9.25 GHz maritime radiolocation band. This picks up reflections from pre-mounted retroreflective targets. These are non-powered and introduce a unique code into the reflected signal
allowing realiable identification of the targets by digital signal processing. A network interface
provides a convenient connection to any DP system.
Page 113
RADius system – FMCW principle
RADius is a relative positioning system developed for operation in harsh and demanding
environment utilising radar technology.
New technology
RADius represents a new way of utilising FMCW (Frequency Modulated Continuous Wave)
radar technology in short range and direction monitoring. The system is primarily designed for
use in harsh and demanding offshore environments where GPS may be inaccurate, and the
need for an accurate, independent and available system is crucial. A typical application for
RADius is to control or assist the dynamic positioning of a vessel next to a platform or in
alongside operations.
Applications
As a stand-alone system, or interfaced with dynamic positioning or integrated bridge systems,
RADius provides an extremely accurate and responsive aid to loading and unloading
procedures for operations in all weather conditions. Docking and oil loading procedures are also
applications that can take advantage of the RADius system.
Page 114
Transponder #1
Transponder #3
distance
azimuth
Transponder #2
Interrogator
The RADius system measures distance and angle to the transponders. It can operate on one
transponder alone or up to five simultaneously.
Transmitted signal
time
Reflected signal
time
Transponder
Distance = ½ • Fb/Fs • Ts • c
Velocity = ½ • D • L
Fb – beat frequency
RADius measures range based on FMCW (Frequency Modulated Continuous Wave) principle It
sweeps the frequency transmitted and mix the transmitted signal with the received signal. The
transmitted signal has changed frequency during the flight time of the received signaled and it is
the difference between the transmitted frequency and the received frequency that is measured.
This frequency difference is proportional to the distance to the transponder.
Page 115
Transponder
Patch
Path 11
d
α
Path 22
Patch
φd = s •
s=
d
*s
in(
α)
2π
λ
, Phase difference, patch 1 and 2
RADius determines angle to target by utilization of several receiver antennas. The principle is
then based on measuring the phase of the carrier simultaneously on the different antenna
elements shown in the picture. When the signal source is right in front of the antennas, the
carrier will be equal on all the elements showing that the angle to the transponder is 0. When
the transponder is off center of the antennas, the radio beam will hit the antennas different. The
difference is then used to determine the angle to the transponder.
Page 116
GNSS(Global Navigation Satellite System) - Overview
GNSS (Global Navigation Satellite System) is a satellite system that is used to pinpoint the
geographic location of a user's receiver anywhere in the world. Two GNSS systems are
currently in operation: the United States' Global Positioning System (NAVSTAR or GPS) and
the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third,
Europe's Galileo, is being developed for future use. Each of the GNSS systems employs a
constellation of orbiting satellites working in conjunction with a network of ground stations.
All GNSS systems can be divided into three basic segments:
Space segment (satellites)
Control segment (control stations)
User segment (Receivers)
This will be discussed in the following.
GPS History
The GPS System was created and realized by the American Department of Defense (DOD) and
was originally based on and run with 24 satellites (21 satellites being required and 3 satellites
as replacement). Nowadays, about 30 active satellites orbit the earth in a distance of 20200 km.
GPS satellites transmit signals which enable the exact location of a GPS receiver, if it is
positioned on the surface of the earth, in the earth atmosphere or in a low orbit. GPS is being
used in aviation, nautical navigation and for the orientation ashore. Further it is used in land
surveying and other applications where the determination of the exact position is required. The
GPS signal can be used without a fee by any person in possession of a GPS receiver. The only
prerequisite is an unobstructed view of the satellites (or rather of the sky).
The correct name of the system is NAVSTAR (Navigation System for Timing and Ranging), but
commonly it is referred to as GPS (Global Positioning System).
1973
Decision to develop a satellite navigation system based on the systems
TRANSIT, TIMATION und 621B of the U.S. Air Force and the U.S.
Navy.
1974 - 1979
System tests
1977
First receiver tests are performed even before the first satellites are
stationed in the orbit. Transmitters are installed on the earth’s surface
called Pseudolites (Pseudo satellites)
1978 - 1985
A total of 11 Block I satellites are launched in this period.
1980
Launching of the first Block I satellite carrying sensors to detect atomic
explosions. This satellite is meant to control the abidance of the
agreement of 1963 between the USA and the Soviet Union to refrain
from any nuclear tests on the earth, submarine or in space.
1979
Decision to expand the GPS system. Thereupon the resources are
considerably shortened and the program is restructured. At first only 18
Page 117
satellites should be operated. 1988 the number of satellites is again
raised to 24, as the functionality is not satisfying with only 18 satellites.
1980- 1982
The financial situation of the project is critical, as the usefulness of the
system is questioned again and again by the sponsors.
1983
When a civilian airplane of the Korean Airline (Flight 007) was shot
down after it had gone lost over Sovjet territory, it was decided to allow
the civilian use of the GPS system.
1986
The accident of the space shuttle "Challenger" means a drawback for
the GPS program, as the space shuttles were supposed to transport
Block II GPS satellites to their orbit. Finally the operators of the
program revert to the Delta rockets intended for the transportation in
the first place.
1989
The first Block II satellite was installed and activated.
1990 - 1991
Temporal deactivation of the selective availability (SA) during the Gulf
war. In this period civil receivers should be used as not enough military
receivers were available. On July 01, 1991 SA is activated again.
08.12.1993
The Initial Operational Capability (IOC) is announced. In the same year
it is also definitely decided to authorize the world wide civilian use free
of charge.
March 1994
The last Block II satellite completes the satellite constellation.
17.07.1995
Full Operational Capability (FOC) is announced.
01.05.2000
Final deactivation of the selective availability and therefore
improvement of the accuracy for civilian users from about 100 m to
20 m.
20.03.2004
Launching of the 50st GPS satellite.
25.09.2005
Lauch of the first IIR-M GPS-satellite. This new type supports the new
military M-signal and the second civil signal L2C.
GPS System Description
Space Segment
The space segment consists of at least 24 satellites.
Number of Satellites
Number of orbital planes
Satellites pr. Plane
Orbital inclination
Orbital Radius
Orbital Period
Repeat ground track
24 + spares
6
4
55 deg.
20.180 km
11 h 58m
23 h 56m
Page 118
The first of the satellites was brought to its orbit as early as 1978. During the years the satellites
became more and more sophisticated and meanwhile five different types of these satellites exist
(Block I, Block II, Block IIA, Block IIR(M) and Block IIF).
Block I Satellites
From 1978 to 1985 11 Block I satellites were launched from California, each having a weight of
845 kg. None of those still operates today. The lifespan was supposed to be
4.5 years, but all of them exceeded this lifespan with about 5 years. The oldest
of the satellites, in the beginning designed as prototype for the testing of the
system, has been operating for 13 years.
All signals of the Block I satellites were accessible for civil users.
GPS-Block I Satellite
Block II Satellites
GPS-Block IIF Satellite
GPS-Block IIA Satellite
Block II satellites weigh more than 1500 kg and have a wingspan of approximately 5.1 m. The
first of these satellites were launched in 1988 from Cape Canaveral. They are constructed for a
service life of 7.5 years. In 1990 the first Block IIA satellite (A for "advanced") was launched. A
total of 9 Block II and 18 Block IIA satellites were launched till September 1996. Although the
satellites are still in six different orbits, each with the same angle to the equator, the newer
Block IIA satellites have a slightly different constellation in space.
Block IIRs began replacing older Block II/IIAs in July 1997. There are currently thirteen Block IIR
satellites on orbit, the last being a modernized IIR (IIR-M). The Block IIR satellites boast
dramatic improvements over the previous blocks. They also have reprogrammable satellite
processors enabling problem fixes and upgrades in flight.
In September 2005, the first satellite of a new generation (IIR-M, Replacement-Modernized) was
successfully launched. The satellites of this type have the capacity to implement a second civil
signal (L2C) and a new military signal with a new code (M-code on L1M and L2M). The satellite
now weighs 2 tons and costs $ 75 million.
Initially, the satellites of the Block IIR generation should be brought to their orbit in groups of
three by space shuttles. But after the challenger catastrophe in 1986 it was decided to take the
satellites to the orbit in pairs with Delta rockets.
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Paradoxically, the first two satellites
launched with a Delta rocket were lost
when the rocket had to be destroyed due to
a malfunction shortly after the lift off. This
was the first malfunction ever of a rocket of
this type.
Block II satellites have a couple of further
features which are not related to the GPS
system. For example they are equipped
with sensors capable of detecting atomic
explosions.
The fourth generation (Block IIF) is planned
to provide a third frequency for civil use
(L5), allowing position determinations with
even higher precision.
Status
Status of the GPS system is best followed from the official web site for the Glonass system;
https://www.glonass-iac.ru/en/GPS/
Timing
Timing is essential for the system to work. Therefore each satellite is equipped with two
rubidium and two cesium atomic clocks. From the base frequency of the atomic clocks
(10.23 MHz) all other frequencies that are required for the GPS-satellite are derived. The newer
satellites of Block IIR and IIR-M are equipped with three rubidium atomic clocks. Their extreme
precision of ± 1 second in 1 million years is absolutely necessary for the functioning of the
system.
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Satellite Orbits
The satellites orbit the earth with a speed of 3.9 km per
second and have a circulation time of 12 h sidereal time,
corresponding to 11 h 58 min earth time. This means
that the same satellite reaches a certain position about 4
minutes earlier each day. The mean distance from the
middle of the earth is 26560 km. With a mean earth
radius of 6360 km, the height of the orbits is then about
20200 km. Orbits in this height are referred to as MEO –
medium earth orbit. In comparison, geostationary
satellites like Inmarsat – satellites orbit the earth at
42300 km, which is about twice the distance of GPS
satellites.
The satellites are arranged on 6 planes, each of them
containing at least 4 slots where satellites can be
Orbits of the GPS-Satellites
arranged equidistantly. Today, typically more than 24
satellites orbit the earth, improving the availability of the system.
The inclination angle of the planes towards the equator is 55°, the planes are rotated in the
equatorial plane by 60° against each other. This means that the orbits range from 55° north to
55° degrees south. (Block I satellites had an inclination of 63° against the equator.)
By this arrangement of the orbits it is avoided that too many
satellites are to often over the north and south pole (like it was
the case in the TRANSIT system, where the satellites ran on
polar orbits).
However the orbits run far enough to the north and south to
guarantee GPS availability in Polar Regions. Furthermore this
arrangement leads to a rather stable constellation, as orbit
disturbing factors like solar winds and gravitation fields have
about the same influence on all of the satellites.
Inclination of the orbital planes The number and constellation of satellites guarantees that the
signals of at least four satellites can be received at any time all
over the world.
The closer you get to the poles, the lower over the horizon the satellites are located. They can
still be received very well, but in no case they are directly above. This may lead to a - typically
insignificant - loss of the precision of the position determination. This effect, caused by the
geometry of the satellite arrangement, happens from time to time on any spot of the earth
surface and can be forecasted.
Control Segment (Monitor Stations)
The GPS-System is controlled by the US Army. The “master control station” (Schriever Air
Force Base) and five additional monitoring stations (on Hawaii, Cape Canaveral, Ascension
Islands, Diego Garcia and Kawajalein) are set up for monitoring the satellites.
During August and September 2005, six more monitor stations of the NGA (National
Geospatial-Intelligence Agency) were added to the grid. Now, every satellite can be seen from
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at least two monitor stations. This allows calculating more precise orbits and ephemeris’s data.
For the end user, a better position precision can be expected from this. In the near future, five
more NGA station will be added so that every satellite can be seen by at least three monitor
stations. This improves integrity monitoring of the satellites and thus the whole system.
Position of the monitor stations and the master control (Note Cape Canaveral not shown)
The passive monitor stations are GPS receivers
which track all satellites in their range and collect
data of the satellite signals. The raw data are
then sent to the mater control station where the
data are processed. The stations on
Ascension Islands, Diego Garcia and Kwajalein
are also transmitting stations for correction data.
Satellite-tracking-station on Hawaii
The "master control station" is located on the Schriever Air
Force Base (formerly Falcon AFB), about 20 km south of
Colorado Springs.
Here the data from the monitor stations are processed 24 h
a day in real time. As results, information about orbits and
clocks of the satellites are obtained. Doing this, possible
malfunctions can quickly be detected.
Additionally, from the raw data new ephemeris data are
calculated. Once to twice a day, these data and other
commands are sent back to the satellites via the transmitting antennas on Ascension Islands,
Diego Garcia or Kwajalein. Block IIR satellites are capable of exchanging data with other
satellites and can correct their orbit data on their own. In theory they only need a contact to a
ground station every 180 days.
Schriever AFB, Colorado
User Segment (GPS-Receiver)
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The user segment consists of the GPS receiver situated at the user.
Seatex DPS 700
Transmitted GPS Signals
The GPS signal is quite complex and offers the possibility of determining the following
parameters: one-way (passive) position determination, exact distance and direction
determination (Doppler effect), transmission of navigation information, simultaneous receiving of
several satellite signals, provision of corrections for ionospheric delay of signals and
insusceptibility against interferences and multi path effects. In order to fulfil all these
requirements, the signal structure described below was developed.
Each GPS satellite transmits two carrier signals, designated as L1 and L2.
The L1 frequency (1575.42 MHz, wavelength 19.05 cm).
The L1 frequency carries the C/A code (see below). In addition to the C/A code
navigational information is modulated into the L1 signal. The information contains data
like satellite orbits, clock corrections and other system parameters (information about the
status of the satellites). These data are constantly transmitted by each satellite. From
these data receiver gets its date, the approximate time and the position of the satellites.
The L2 frequency (1227.60 MHz, wavelength 24.45 cm)
The L2 frequency carries the P code and is only used by receivers which are designed
for PPS (precision positioning code). Mostly this can be found in military receivers.
Modulation of the carrier signals
The L1 and L2 frequencies are the carrier signals, which are modulated by phase modulation as
follows:
C/A and P (Y) Code
The carrier phases are modulated by three different codes:
1. The C/A code (coarse acquisition).
The C/A code is the base for all civil GPS receivers.
The C/A code is a pseudo random code (PRN) which looks like a random code but is
clearly defined for each satellite. It is repeated every millisecond.
Pseudo Randum Numbers (PRNs)
Each satellite have a unique number and the satellites are identified by the receiver by
means of PRN-numbers. GPS satellites are numbered from 1 – 32. These PRN-numbers
of the satellites appear on the satellite view screens of many GPS receivers.
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2. The P code (p = precise). It modulates the L1 as well as the L2 carrier frequency and is
for special users. (Military). For protection against interfering signals transmitted by an
possible enemy, the P-code can be transmitted encrypted.
3. Y-code During this anti-spoofing (AS) mode the P-code is encrypted in a Y-code. The
encrypted code needs a special AS-module for each receiving channel and is only
accessible for authorized personnel in possession of a special key.
The P- and Y-code are the base for the precise (military) position determination. Since January
31, 1994 the AS-system is operating continiously and the P-code is only transmitted as Y-code.
Position Determination with GPS.
In a considerably simplified approach, each satellite is sending out signals with the following
content: I am satellite X, my position is Y and this information was sent at time Z. In addition to
its own position, each satellite sends data about the position of other satellites. These orbit data
(ephemeris and almanac data) are stored by the GPS receiver for later calculations.
For the determination of its position on earth, the GPS receiver compares the time when the
signal was sent by the satellite with the time the signal was received. As explained earlier, each
satellite transmits a pseudo random code (PRN) which is known to the receiver. The receiver
can now compare the PRN in the receiver memory with the PRN it just received.
The following graph shows two identical codes. The colored rectangles symbolize binary 1,
white gaps symbolize 0. The violet rectangles are the signal from the satellite; the orange
rectangles are the signal from the receiver. Now it is determined how “far” the signals have to be
shifted until they are aligned. The distance corresponds to a time – the runtime of the signal
from the satellite to the receiver. By means of this runtime the distance between the satellite
and receiver can be calculated.
Comparison of two signals. Top: shifted; Bottom: aligned
Knowing the position of the satellite in space and receiving data from other satellites, the
present position can be calculated by trilateration (meaning the determination of a distance from
three points). This means that at least three satellites are required to determine the position of
the GPS receiver on the earth surface. The calculation of a position from 3 satellite signals is
called 2D-position fix (two-dimensional position determination). It is only two dimensional
because the receiver has to assume that it is located on the earth surface (on a plane twodimensional surface). By means of four or more satellites, an absolute position in a three
dimensional space can be determined. A 3D-position fix also gives the height above the earth
surface as a result.
In the following an explanation is given, how the position determination by GPS works. For
simplification, in the first step we assume that the earth is a two-dimensional disk. This allows us
to do some understandable sketches for illustration. The principle can then be transferred to the
model of a three-dimensional globe.
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In the example on the left, the time needed by a signal
to travel from the first of two satellites to the receiver
was determined to be 4 s. (In reality this value is far
too high. As the signals travel with the speed of light
(299 792 458,0 m/s), the actual time span for signals
from the satellite to the receiver lies in the range of
0.07 s.)
Based on this information, we state that the receiver is
positioned somewhere on a circle with a radius of 4 s
around the first satellite (left circle).
If we perform the same procedure with a second
Position determination with two satellites satellite (right circle), we get two points of intersection.
(in a 2-dimensional world)
On one of the two points the receiver must be
situated. Now we have used two satellites. But the
process is called trilateration, not dilateration so don't we need a third satellite? We may use a
third satellite but we could also assume that the receiver is located somewhere close to the
earth's surface and not deep in space, so we can neglect point B and know that the receiver
must be found on point A. The area in the picture above which shaded grey is the region in
which GPS signals are supposed to be “realistic”. Positions outside this area are discarded, so
is point B.
This assumption replaces the third satellite which would in theory be required for the process of
trilateration. In this example a position is obtained from only two satellites.
So we just need a third satellite for a third dimension and that's it? Well, in principle yes. But…
The problem lies in the determination of the exact runtime of signals. As explained above,
satellites impose a sort of time stamp on each transmitted data package. We know that all
clocks of satellites are absolutely precise (they are atomic clocks after all) but the problem is the
clock in our GPS receiver. Atomic clocks being to expensive, our GPS receivers are based on
conventional quartz clocks which are comparatively inaccurate. What does this mean in
practice?
Let's stick to our example and
suppose the clock in our receiver is
0.5 seconds early compared to the
clock in the satellite. The runtime of
the signal seems to be 0.5 s longer
than it actually is. This leads to the
assumption that we are on point B
instead of point A. The circles that
intersect in point B are called
pseudoranges. They are called
“pseudo” as long as no correction of
the synchronisation errors (bias) of
the clocks has been performed.
2D position determination with 2 satellites and clock error
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Depending on the accuracy of the clock in the GPS receiver, the determined position will be
more or less wrong. For the practice of GPS based navigation this would mean that no
determined position can ever be of any use, as the runtimes of the signals are so short, that any
clock error has an overwhelming influence on the result.
A clock error of 1/100 second, which is difficult to imagine but quite common from car races or
skiing races, would in GPS navigation lead to a mistake in the position of about 3000 km. To
achieve an accuracy of 10 m of the position, the runtime of the signal must be precise to
0.00000003 seconds. As atomic clocks are no option in GPS receivers, the problem is solved in
another and quite elegant way:
If a third satellite is taken into account for the calculation of
the position, another intersection point is obtained: in case
that all clocks are absolutely precise, point A would be
obtained, corresponding to the actual position of the
receiver.
In case of the receiver clock being 0.5 s early, the three
intersection points B are obtained. In this case the clock
error stands out immediately. If now the time of the receiver
clock is shifted until the three intersection points B merge to
A, the clock error is corrected and the receiver clock is
synchronized with the atomic clocks in the satellites.
The GPS receiver can now be regarded as an atomic clock
itself. The distances to the satellites, formerly regarded as
pseudoranges, now corresponds to the actual distances and
the determined position is accurate.
2D position determination with 3
satellites and corrected clock
In case of the example – a two dimensional disc world – we
error
therefore need three satellites for an unequivocal
determination of our position. In the real world which has one additional dimension (the height),
we would need a fourth satellite.
Well, then why is it always said that three satellites are enough?
In practice you get a two-dimensional position determination (2D-fix) with three satellites. The
position is bound to be located on the earth's surface. The fourth satellite is the geocenter; the
distance to the “fourth satellite” corresponds to 6360 km (the radius of the globe). Therewith the
fourth satellite necessary for the calculation is given, but the calculation is restricted to locations
on the earth surface. However the earth is not a perfect sphere. The surface of the earth in this
case means the earth geoid, corresponding to sea level. If the receiver is located on a
mountain, the determined position again is afflicted with an inaccuracy, as the runtime of the
satellite signals is wrong.
Conclusion:
3 satellites solution
=
4 (or more) satellites solution =
2D position,
Solution: Latitude, longitude and time
3D position –
Solution: Latitude, longitude, height and time
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Carrier Phase
Carrier phase is another processing technique that gathers data via a carrier phase receiver,
which uses the radio signal (carrier signal) to calculate positions. The carrier signal, which has a
much higher frequency than the pseudo-random code, is more accurate than using the pseudorandom code alone. The pseudo-random code narrows the reference then the carrier code
narrows the reference even more. After differential correction, this processing technique results
in sub-meter accuracy.
Carrier phase follows the same general concept as described earlier, but uses the satellite's
carrier as its signal, not the messages contained within. The improvement possible using this
signal is potentially very high if one assume a 1% accuracy in locking.
For instance, the GPS C/A signal broadcast in the L1 signal changes phase at 1.023 MHz, but
the L1 carrier itself is 1575.42 MHz, over a thousand times faster. This corresponds to a 1%
accuracy of 19 cm using the L1 signal, and 24 cm using the lower frequency L2 signal.
The difficulty in making a carrier phase system is properly aligning the signals. The navigation
signals are deliberately encoded in order to allow them to be aligned easily, whereas every
cycle of the carrier is similar to every other. This makes it extremely difficult to know if you have
properly aligned the signals, or are "off by one" and thus introducing an error of 20 cm or a
larger multiple of 20 cm. This integer ambiguity problem can be addressed to some degree with
sophisticated statistical methods that compare the measurements from the C/A signals and by
comparing the resulting ranges between multiple satellites. Generally this means that the
receivers locks on to the code in order to find the correct.
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Glonass History
Glonass (Global'naya Navigatsionnaya Sputnikovaya Sistema or in english: GLObal NAvigation
Satellite System) is a radio-based satellite navigation system, developed by the former Soviet
Union and now operated for the Russian government by the Russian Space Forces. Its United
States' counterpart is the Global Positioning System (GPS).
Development on the GLONASS began in 1976, with a goal of global coverage by 1991. From
1982 through April 1991, the Soviet Union successfully launched a total of 43 GLONASSrelated satellites plus five test satellites. In 1991, twelve functional GLONASS satellites in two
planes were available; enough to allow limited usage of the system.
Following the disintegration of the Soviet Union in 1991, continued development of GLONASS
was undertaken by the Russian Federation. It was declared operational on September 24, 1993
by then-president Boris Yeltsin, however the constellation was not completed until December
1995.
In the six years following completion, Russia was unable to maintain the system. By April of
2002, this resulted in only eight satellites remaining in operation, which rendered the system
almost useless as a global navigation aid.
With GLONASS falling rapidly into disrepair, a special-purpose federal program named "Global
Navigation System" was undertaken by the Russian government. According to it, the GLONASS
system was to be restored to fully deployed status (i.e. with 24 satellites in orbit and the global
continuous coverage) by 2011.
The New York Times reported that Russia had committed to accelerated launches, with eight
satellites scheduled to be orbited in 2007 and a goal of reaching global coverage in 2009. As of
April 2007, the number of operational satellite had increased to twelve, in addition to one new
satellite in its commissioning phase.
Cooperation with the Indian government
In January, 2004 the Russian Space Agency (RSA) announced a joint venture deal with India's
space agency, the Indian Space Research Organization, where-in the two government agencies
would collaborate to restore the system to constant coverage of Russian and Indian territory by
2008 with 18 satellites, and be fully operational with all 24 satellites by 2010.
Discussions with United States government
Following the December 2006 meeting in Moscow of the GPS-GLONASS Interoperability and
Compatibility Working Group (WG-1), an announcement appeared on both US and Russian
government websites stating both sides had made significant progress in understanding the
benefit to the user community of changing GLONASS to a signal pattern that is in common with
GPS and Galileo.
On May 18th 2007, Russian president Vladimir Putin signed a decree officially providing open
access to the civilian navigation signals of the GLONASS system, to Russian and foreign
consumers, free of charge and without limitations.
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Glonass System Description
Space segment
Fully deployed the GLONASS system would consist of 24
operating satellites within three orbital planes spaced
longitudinally by 120 degrees. Each satellite plane is
nominally inclined to the equator by 64.8° a somewhat
higher inclination compared to GPS thus making the
coverage better for higher latitude sites. An illustration of this
effect can be found in the below figures, where the
GLONASS satellites help to reduce the gap in the sky.
Polar plot of GPS satellite sky tracks as
seen from latitude 57.4 N for a period of
24 hours The midpoint of the plot is
equivalent to the zenith direction with the
elevation decreasing to zero at the outer
edges of the figure. Due north is upwards
in the figure.
Polar plot of GLONASS+GPS satellite sky
tracks as seen from latitude 57.4 N , for a
period of 24 hours
Number of Satellites
Number of orbital planes
Satellites pr. Plane
Orbital inclination
Orbital Radius
Orbital Period
Repeat ground track
24 + spares
3
8
64.8 deg.
19.130 km
11 h 15m
8 days*
* Due to the constellation, a fully operational Glonass system will (on same location) show a
satellite in the same direction and altitude every 24h.
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Each satellite is identified by its slot number, which defines the orbital plane and its location
within the plane. The 1st orbital plane has slot numbers 1…8, the 2nd orbital plane - slots
9…16, and the 3rd orbital plane - slots 17…24.
None of the first launched Glonass satellites are in use today. Since the decision to focus on the
repair of the Glonass system, there have been sent up several new generation Glonass
satellites have been launched:
December 2004: 3 satellites, December 2005: 3 satellites, December 2006: 3 satellites.
The 2nd generation Glonass satellites (Glonass-M) satellite have better signal characteristics, a
2nd civilian signal, as well as a longer design life (7-8 years instead of the current 3 years).
Glonass-K satellites is the 3rd generation Glonass satellites that was put into service. These
satellites will have increased characteristics, a 3rd civilian signal and improved lifespan (10-12
years)
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Status
Status of the Glonass constellation can be followed at the official web site:
https://www.glonass-iac.ru/en/GLONASS/
Timing
Just as the GPS system, timing is essential for the system to work. GLONASS time is based on
an atomic time scale similar to GPS and Glonass satellites carries atomic clocks. This time
scale is UTC as maintained by Russia (UTC (SU)). There is a difference in the time tags of GPS
and Glonass satellites.
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Ground Segment
The ground control segment of GLONASS is entirely located within former Soviet Union
territory. The Ground Control Center and Time Standards is located in Moscow and the
telemetry and tracking stations are in St. Petersburg, Ternopol, Eniseisk, Komsomolsk-naAmure. The fact that all ground stations are within the former Sovjet Union can lead to update of
satellites being “old” when not in sight from a ground station. This problem will be less and less,
with new generation satellites, being able to autonomous operations without being in contact
with a ground station.
User Segment (Glonass-Receiver)
The user segment consists of the Glonass receiver situated at the user.
Seatex DPS 700
(DPS200 only)
Transmitted Glonass Signals and position determination
Just as the GPS signal, the Glonass transmits signals on 2 frequencies, called L1 and L2. There
is a difference in the frequencies from the GPS. As the GPS, several signals are transmitted on
the frequencies: The SPS, standard positioning service and the PPS, precise positioning
service together with navigation signals. There is a difference in the way the satellites are
recognized by the receiver. Glonass uses so called FDMA - Frequency Division Multiple Access
– system. This means that the codes of the satellites are the same for each satellite, but the
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frequency is different for each satellite, giving the receiver the information of which satellite is
sending the signal. For GPS the military P-code is encrypted to a so called Y-code a method
known as AS or antispoofing. This is not done for GLONASS although the Russian authorities
advise not to use the military code for navigational purposes due to military reasons.
The determination of the position is working on the same principles as the GPS system.
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Galileo History
Galileo will be Europe’s own global navigation satellite system, providing a highly accurate,
guaranteed global positioning service under civilian control. It will be inter-operable with GPS
and GLONASS, the two other global satellite navigation systems.
The European Union and European Space Agency agreed on March 2002 to introduce their
own alternative to GPS, called the Galileo positioning system. The total cost of the system was
expected to be about GBP £2.4 billion. The first experimental satellite was launched on 28
December 2005. Galileo is expected to be compatible with the modernized GPS system that will
be operational by after 2012. The receivers will be able to combine the signals from both
Galileo, Glonass and GPS satellites to greatly increase the accuracy. At present (2007) there is
some uncertainty regarding the future funding of the project. The time span of a fully operational
system goes beyond 2010 and maybe even later.
Gallileo System Description
Space segment
The Galileo space segment will comprise thirty satellites in a Walker constellation with three
orbital planes at 56° nominal inclination. Each plane will contain nine operational satellites,
equally spaced, 40° apart plus one inactive spare satellite to replace any of the operational
satellites in case of failures. The spare, nonoperational satellite in each orbit plane, ensures
that in case of failure the constellation can be
repaired quickly by moving the spare to replace
the failed satellite. This could be done in a
matter of days, rather than waiting for a new
launch to be arranged which could take many
months.
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The orbit altitude of 23 222 km results in a repeat a constellation repeat cycle of 9 days 23
hours 20 min. during which each satellite has completed seventeen revolutions.
Number of Satellites
Number of orbital planes
Satellites pr. Plane
Orbital inclination
Orbital Radius
Orbital Period
Repeat ground track
27 + 3 spares
3
9 (10 with spares)
56 deg.
23.222 km
14 h 05m
9 days 23 hours 20 min.
Control segment
The core of the Galileo ground segment will be the two control centres. Each control centre will
manage “control” functions supported by a dedicated Ground Control Segment (GCS) and
“mission” functions, supported by a dedicated Ground Mission Segment (GMS). The GCS will
handle spacecraft housekeeping and constellation maintenance while the GMS will handle
navigation system control.
The GCS will use a global network of nominally five TTC stations to communicate with each
satellite on a scheme combining regular, scheduled contacts, long-term test campaigns and
contingency contacts.
The TTC Stations will be large, with 13-metre antennas operating in the 2 GHz Space
Operations frequency bands. During normal operations, spread-spectrum modulation (similar to
that used for TDRSS and ARTEMIS data relay applications) will be used, to provide robust,
interference free operation. However, when the navigation system of a satellite is not in
operation (during launch and early orbit operations or during a contingency) use of the common
standard TTC modulation will allow non-ESA TTC stations to be used.
Mission control segment
The Galileo Mission Segment (GMS) will use a global network of nominally thirty Galileo
Sensor Stations (GSS) to monitor the navigation signals of all satellites on a continuous basis,
through a comprehensive communications network using commercial satellites as well as cable
connections in which each link will be duplicated for redundancy. The prime element of the GSS
is the Reference Receiver.
The GMS communicates with the Galileo satellites through a global network of Mission Up-Link
Stations (ULS), installed at five sites, each of which will host a number of 3-metre antennas.
ULSs will operate in the 5 GHz Radionavigation Satellite (Earth-to-space) band.
The GMS will use the GSS network in two independent ways. The first is the Orbitography
Determination and Time Synchronisation (OD&TS) function, which will provide batch processing
every ten minutes of all the observations of all satellites over an extended period and calculates
the precise orbit and clock offset of each satellite, including a forecast of predicted variations
(“SISA”, Signal-in-Space Accuracy) valid for the next hours. The results of these computations
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for each satellite will be up-loaded into that satellite nominally every 100 minutes using a
scheduled contact via a Mission Up-link Station.
User Segment (Receiver)
The user segment consists of the receiver situated at the user.
Transmitted Signals and position determination
Galileo receivers receive the signals broadcast by the Galileo satellites and process them to
compute position. Through this processing, the receivers extract measurements giving an
indication of the distance from the user to the satellite.
They also decode the Galileo navigation data, which contain fundamental pieces of information
for computing the user position such as the position of the satellites or the satellite clock errors
as determined by the Galileo ground segment and up-linked regularly to the Galileo
constellation.
The frequencies used by the satellites are within the 1.1 to 1.6 GHz band; a range of
frequencies particularly well suited for mobile navigation and communication services.
Each Galileo satellite will broadcast 10 different navigation signals making it possible for Galileo
to offer the open (OS), safety-of-life (SOL), commercial (CS) and public regulated services
(PRS).
A distinction is made between signals containing navigation data (the data channels) and
signals carrying no data (pilot channels). In the diagram this is highlighted by plotting the data
channels and the pilot channels in orthogonal planes, a means of indicating that the signals of
the data and pilot channels are shifted by 90 degrees in phase, which allows for their separation
in the receivers.
Galileo signals have codes
All the satellites transmit at the same frequency, that is, the Galileo signal at L1 is broadcast at
1575.42 MHz from any satellite. To allow the receivers to distinguish which satellites the signals
are coming from and to allow the receivers to measure the time it took the signal to travel from
the satellite to the receiver (the basic measurements used for position determination), a code is
added to the signal. This code is different for each satellite and its design is one of the many
arts involved in making a good satellite navigation system.
A long code would allow tracking of very faint signals, such as those received inside a building,
but it would be very difficult to acquire because a receiver acquires the signal by searching for
the delay of the received code and for a long code there are more possibilities than for a shorter
code.
Shorter codes are good for achieving fast acquisition but may lead to false acquisitions of
satellites when a receiver confuses the signal between two satellites. This is because the
capability of a receiver to distinguish between two different codes is inversely proportional to the
length of the codes.
The code is like a key. When a receiver attempts to acquire a satellite signal, it compares the
code of the received signal with a local replica stored on the receiver. When the two codes
match, the receiver channel is open, otherwise the receiver tries with another code
corresponding to another satellite till it succeeds.
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However, not all the elements of the code need to match precisely and it is sufficient if the two
codes are close enough. This means that the more complex the keys are, that is, the longer the
codes, the more difficult it is to confuse keys, and therefore to mix up satellites.
Why so many different Galileo signals ?
In the end, a good engineering compromise has been found, but since those compromises may
not satisfy all type of users (an indoor, static user may like long codes, while outdoor, fastmoving users would prefer short codes), the solution has been to provide alternative codes with
different characteristics on the various Galileo signals. This is one of the reasons why there are
so many signals.
In GPS only one signal is available, which does not allow the kind of optimisation performed for
Galileo. This will be overcome with the GPS modernization programme where more GPS
signals will be made available.
Another reason for having so many signals in Galileo is to allow the receiver to estimate the
ionospheric delay error. This error is due to the delay that the navigation signals suffer when
they travel through the ionosphere. This delay, makes the distance from the satellite to the user,
as measured by the receiver, appear longer than it actually is and if not corrected would lead to
large positioning errors. Fortunately, this delay is proportional to the frequency of the signal,
with lower frequency signals experiencing a longer delay than higher frequency signals.
Therefore, by combining measurements to the same satellite at two different frequencies it is
possible to produce another measurement where the ionospheric error has been cancelled out.
This cancellation becomes more effective as the separation between the two frequencies
increases. This is the reason why Galileo services are generally realized using pairs of signals.
The open services are realized by using the signals at L1, E5a and E5b, whether data or pilot.
Several combinations are also possible, such as a dual frequency service based on using L1
and E5a (for best ionospheric error cancellation) or single frequency services (at L1, E5a, E5b
or E5a and E5b together) in which case the ionospheric error is removed using a model, and
even triple frequency services using all the signal together (L1, E5a and E5b), which can be
exploited for very precise centimetric applications.
The safety of life services are based on the measurements obtained from the open signal and
use the integrity data carried in special messages designated for this purposed within the open
signals. The safety of life service is like a data channel within the open signals.
The Commercial service is realized with two additional signals in the 1278.75 MHz band plus
also the capability to include commercial data within the open signals.
The Public Regulated Service is realized by two signals in the 1575.42 MHz and the 1278.75
MHz band. The signals are encrypted allowing the implementation of an effective access control
scheme.
Finally, the distinctive shape of the spectrum of the signals is due to the special modulation
adopted for Galileo. This modulation has been adopted to avoid interference with other satellite
navigation systems within the same band, which is indeed the case of GPS at L1. The
modulation adopted is the so called BOC(1,1), meaning Binary Offset Carrier of rate (1,1). This
kind of modulation allows GPS and Galileo signals to occupy the same frequency while avoiding
mutual interference. This makes building receivers that use both GPS and Galileo simpler
because GPS and Galileo use the same frequency.
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DARPS (Differential Absolute and Relative Positioning System)
Some DP functions require the positioning of a vessel relative to a moving, rather than fixed,
position. An example of this is the operation of a DP shuttle tanker tandem loading via a bow
loading hose from the stern of a floating production vessel (FPSO). The FPSO may be turretmoored and in a continuous weathervane mode. As well as the FPSO having a certain amount
of positional wander, the stern of the FPSO describes the arc of a circle providing a complex
positioning problem for the shuttle tanker.
The Seatex DARPS system (Differential Absolute and Relative Positioning System) is
configured to handle this problem. The storage tanker or FPSO uses DGPS to monitor its
absolute position using the Network DGPS. This enables the shuttle tanker to determine both its
Absolute and its Relative position. For the measurement of relative position differential
corrections are not used, as the errors are the same for the shuttle tanker as they are for the
FPSO. A DARPS transponder is placed on the point of reference (FPSO) and telemeters
received GPS data to the UHF transceiver aboard the shuttle tanker. A computer aboard the
shuttle tanker then makes a comparison between the GPS position of the transponder and the
GPS position of the tanker, deriving a range/bearing vector which may be input to the DP
system as position reference.
When the shuttle tanker is in the approach mode it will initiate the UHF comms with its
transceiver, activating the FPSO DARPS transponder. This link has a range of some 2 - 3 km.
and remains active until loading completes and the tanker moves off, when the system can be
switched off.
RELATIVE GPS
FPSO USING THRUSTER
ASSISTED MOORING ON A
WEATHERVANE HEADING
SHUTTLE
TANKER
THE SHUTTLE TANKER RECEIVES
GPS DATA DIRECTLY ANDTELEMETERED
FROM THE FPSO, THUS RELATIVE POSITION
IS DETERMINED
20.6
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Sources of Errors in GNSS systems
Selective Availability
The most relevant factor for the inaccuracy of the GPS system is no longer an issue. On May 2,
2000 5:05 am (MEZ) the so-called selective availability (SA) was turned off. Selective availability
is an artificial falsification of the time in the L1 signal transmitted by the satellite. For civil GPS
receivers that leads to a less accurate position determination (fluctuation of about 50 m during a
few minutes). Additionally the ephemeris data are transmitted with lower accuracy, meaning that
the transmitted satellite positions do not comply with the actual positions. In this way an
inaccuracy of the position of 50 – 150 m can be achieved for several hours. While in times of
selective availability the position determination with civil receivers had an accuracy of
approximately 100 m, nowadays 20 m or even less is usual. Especially the determination of
heights has improved considerably from the deactivation of SA (having been more or less
useless before).
The reasons for SA were safety concerns. For example terrorists should not be provided with
the possibility of locating important buildings with homemade remote control weapons.
Paradoxically, during the first gulf war in 1990, SA had to be deactivated partially, as not
enough military receivers were available for the American troops. 10000 civil receivers were
acquired (Magellan and Trimble instruments), making a very precise orientation possible in a
desert with no landmarks.
Meanwhile SA is permanently deactivated due to the broad distribution and world wide use of
the GPS system.
The following two graphs show the improvement of position determination after deactivation of
SA. The edge length of the diagrams is 200 m, the data were collected on May 1, 2000 and May
3, 2000 over a period of 24 h each. While with SA 95 % of all points are located within a radius
of 45 m, without SA 95 % of all points are within a radius of 6.3 m.
Plot of the position determination with and without SA
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"Satellite geometry"
Another factor influencing the accuracy of the position determination is the "satellite geometry".
Simplified, satellite geometry describes the position of the satellites to each other from the view
of the receiver.
If a receiver sees 4 satellites and all are arranged for example in the north-west, this leads to a
“bad” geometry. In the worst case, no position determination is possible at all, when all distance
determinations point to the same direction. Even if a position is determined, the error of the
positions may be up to 100 – 150 m. If, on the other hand, the 4 satellites are well distributed
over the whole firmament the determined position will be much more accurate. Let’s assume the
satellites are positioned in the north, east, south and west in 90° steps. Distances can then be
measured in four different directions, reflecting a “good“ satellite geometry.
The following graph shows this for the two-dimensional case.
If the two satellites are in an advantageous position,
from the view of the receiver they can be seen in an
angle of approximately 90° to each other. The
possible positions are marked by the grey circles
(Errors on position). The point of intersection A of the
two circles is a rather small, more or less quadratic
field (blue), the determined position will be rather
accurate.
If the satellites are more or less positioned in one line from the view
of the receiver, the plane of intersection of possible positions is
considerably larger and elongated- The determination of the position
is less accurate.
DOP – Dilution of Precision
To indicate the quality of the satellite geometry, the DOP values (dilution of precision) are used.
Based on which factors are used for the calculation of the DOP values, different variants are
distinguished:
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GDOP (Geometric Dilution Of Precision); Overall-accuracy; 3D-coordinates and time
PDOP (Positional Dilution Of Precision) ; Position accuracy; 3D-coordinates
HDOP (Horizontal Dilution Of Precision); horizontal accuracy; 2D-coordinates
VDOP (Vertical Dilution Of Precision); vertical accuracy; height
TDOP (Time Dilution Of Precision); time accuracy; time
HDOP-values below 4 are good, above 8 bad. HDOP values become worse if the received
satellites are high on the firmament. VDOP values on the other hand become worse the closer
the satellites are to the horizon and PDOP values are best if one satellite is positions vertically
above and three are evenly distributed close to the horizon.
Satellite Orbits
Although the satellites are positioned in very precise orbits, slight shifts of the orbits are possible
due to gravitation forces. Sun and moon have a weak influence on the orbits. The orbit data are
controlled and corrected regularly and are sent to the receivers in the package of ephemeris
data. Therefore the influence on the correctness of the position determination is rather low, the
resulting error being not more than 2 m.
Multipath effect
The multipath effect is caused by reflection of satellite signals (radio waves) on objects. It was
the same effect that causes ghost images on television using antennae on the roof.
For GPS signals this effect mainly appears close to platforms (reflections) or with low satellites
reflecting signal onboard the vessel. The reflected signal takes more time to reach the receiver
than the direct signal. The resulting error typically lies in the range of a few meters. . If the
reflected signal is very strong, the GPS receiver might lose lock on the satellite.
Multipath is difficult to detect and sometime hard to avoid.
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Atmospheric effects
One of the biggest sources of inaccuracy is the
reduced speed of the signal in the troposphere and
ionosphere. While radio signals travel with the
velocity of light in the outer space, the speed in the
ionosphere and troposphere is slower.
In the ionosphere in a height of 80 – 400 km a large
number of electrons and positive charged ions are
formed by the ionizing force of the sun. The layers
of electrons and ions in the ionosphere refract the
Influenced propagation of radio waves through the earth's atmosphere
electromagnetic waves from the satellites, resulting
in an elongated runtime of the signals.
These errors are mostly corrected by the receiver by calculations. The receivers have a build in
model of the standard ionospheric conditions, giving possibility to calculate the delay of the radio
signal for standard conditions. Theses variations are taken into account for all calculations of
positions. However single frequency receivers are not capable of correcting unforeseen runtime
changes, for example by strong solar winds.
The effect of ionospheric activity is very much depending on the solar activity. The solar activity
goes in cycles of 11-12 years.
Signals with lower frequencies are slowed down more than signals with higher frequencies. If
the signals of higher and lower frequencies which reach a receiver are analysed with regard to
their differing time of arrival, the ionospheric runtime elongation can be calculated. Dual GPS
receivers use the signals of both frequencies (L1 and L2) which are influenced in different ways
by the ionosphere and are able to eliminate another inaccuracy by calculation. Dual frequency
receivers can to a large extend eliminate the effect of ionospheric activity.
Due to the phenomenon known as Ionospheric scintillations, the effect of the ionosphere can
vary locally. Irregularly structured ionospheric regions can cause diffraction and scattering of
trans-ionospheric radio signals. When received at an antenna, these signals present random
temporal fluctuations in both amplitude and phase. This is known as ionospheric scintillation.
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Ionospheric scintillations can cause the receivers to loose track – or lock - of satellites. The L2
frequency is more vulnerable to scintillations due to the techniques used.
The tropospheric effect is a further factor elongating the runtime of electromagnetic waves by
refraction. The reasons for the refraction are different concentrations of water vapour in the
troposphere, caused by different weather conditions. The error caused that way is smaller than
the ionospheric error, but can not be eliminated by calculation. It can only be approximated by a
general calculation model.
The following two graphs visualize the ionospheric error. The left data were collected with a
one-frequency receiver without ionospheric correction, the right data were collected with a twofrequency receiver with ionospheric correction. Both diagrams have approximately the same
scale (Left: latitude -15 m to +10 m, longitude -10 m to +20 m, Right: latitude -12 m to +8 m,
longitude -10 m to +20 m). The right graph clearly shows less outliers.
Clock inaccuracies and rounding errors
Despite the synchronization of the receiver clock with the satellite time during the position
determination, the remaining inaccuracy of the time still leads to an error of about 2 m in the
position determination. Rounding and calculation errors of the receiver sum up approximately to
1 m.
Relativistic effects
The following section shall not provide a comprehensive explanation of the theory of relativity. In
the normal life we are quite unaware of the omnipresence of the theory of relativity. However it
has an influence on many processes, among them is the proper functioning of the GPS system.
This influence will be explained shortly in the following.
As we already learned, the time is a relevant factor in GPS navigation and must be accurate to
20 - 30 nanoseconds to ensure the necessary accuracy. Therefore the fast movement of the
satellites themselves (nearly 12000 km/h) must be considered.
Whoever already dealt with the theory of relativity knows that time runs slower during very fast
movements. For satellites moving with a speed of 3874 m/s, clocks run slower when viewed
from earth. This relativistic time dilation leads to an inaccuracy of time of approximately
7,2 microseconds per day (1 microsecond = 10-6 seconds).
The theory of relativity also says that time moves the slower the stronger the field of gravitation
is. For an observer on the earth surface the clock on board of a satellite is running faster (as the
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satellite in 20000 km height is exposed to a much weaker field of gravitation than the observer).
And this second effect is six times stronger than the time dilation explained above.
Altogether, the clocks of the satellites seem to run a little faster. The shift of time to the observer
on earth would be about 38 milliseconds per day and would make up for an total error of
approximately 10 km per day. In order that those error do not have to be corrected constantly,
the clocks of the satellites were set to 10.229999995453 Mhz instead of 10.23 Mhz but they are
operated as if they had 10.23 MHz. By this trick the relativistic effects are compensated once
and for all.
There is another relativistic effect, which is not considered for normal position determinations by
GPS. It is called Sagnac-Effect and is caused by the movement of the observer on the earth
surface, who also moves with a velocity of up to 500 m/s (at the equator) due to the rotation of
the globe. The influence of this effect is very small and complicate to calculate as it depends on
the directions of the movement. Therefore it is only considered in special cases.
Sum of errors
The errors of the GPS system are summarized in the following table. The individual values are
no constant values, but are subject to variances. All numbers are approximate values.
Ionospheric effects
± 5 meter
Shifts in the satellite orbits
± 2.5 meter
Clock errors of the satellites' clocks
± 2 meter
Multipath effect
± 1 meter
Tropospheric effects
± 0.5 meter
Calculation- und rounding errors
± 1 meter
Altogether this sums up to an error of ± 15 meters. With the SA still activated, the error was in
the range of ± 100 Meter.
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Differential GNSS
Differential GPS (DGPS) or Glonass (DGlonass) systems are a way of correcting the received
signal of the GNSS satellite in order to improve accuracy.
The way it works includes 2 receivers, namely the one we have at the user end and one placed
at a known point, normally a shore.
As the receiver ashore - known as the reference station - have information of its own position
and are receiving signals from a number of satellites, the position error at the reference station
can be determined. This is calculated in to a correction for each satellite.
Observed Position:
Co-ordinates [Lat, lon,
determined from GPS
Surveyed Position:
Antenna
[Lat, lon,
This correction is then send to the user by various means:
— Maritime radio beacons (the IALA beacons)
— HF, UHF or VHF radio systems
— Satellite-based systems (Geo stationary Inmarsat & Spotbeam satellites)
GPS sats. Providing positions to the Reference station (RS) AND User (vessel)
Calculated correction for EACH satellite is transmitted by various means to the user and processed
at the user end, optimizing the position.
It is of utmost importance to understand, that the satellites used to send the corrections to the
user are NOT GNSS satellites, but geostationary satellites in much higher orbits.
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On board the vessel (user end), the corrections are processed and used on the same satellite
and in that way optimizing the position. It is only possible to correct satellites which are visible to
both the reference station and the user.
Errors introduced to the GPS signals are often similar over large geographical areas. The DGPS
principle therefore assumes that major errors like:
•
•
Satellite clock and ephemeris errors
Disturbances in the ionosphere and troposphere
are of the same magnitude at both the reference station and the mobile station. The corrections,
when processed and incorporated into measurements in the user GPS receiver, virtually
eliminate errors common to both the DGPS reference station and the on-board GPS receiver.
This is normally the case when the distance between user and reference station is relatively
small.
Distance between Reference station and user is small. Effected by same errors
If the distance between user and reference station increases, the errors at the reference station
and the user end is not the same. This is referred to as “spatial decorrelation” Spatial
decorrelation can be described as a gradual increase in error as User/reference station distance
increases. This error is due to differences in the path of the radio signal from the satellite to the
reference station and user receivers. When the signal paths to reference station and user are
not identical, the resulting differences show up as position errors that cannot be removed by
differential correction (because this error is not common to base and rover). Usually such
differences are the result of variation in the Earth's ionosphere and troposphere.
Distance between reference station and user is big. Effected by different ionosphere and having different errors
Due to the phenomenon known as Ionospheric scintillations, the effect of the ionosphere can
vary locally. Irregularly sturctured ionospheric regions can cause diffraction and scattering of
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trans-ionospheric radio signals. When received at an antenna, these signals present random
temporal fluctuations in both amplitude and phase. This is known as ionospheric scintillation.
Some reference stations around the equator have a dual receiver and are transmitting the error
due to ionosphere to the user. When the user has a dual receiver, this error can be calculated
onboard and the information from the reference station used to improve accuracy even more.
Dual receivers and dual corrections improves accuracy, but does not totally removes the effects
of scintillations.
FUGRO
On board DP vessels these corrections are bought from companies specialized in optaining and
distributing corrections. One of these companies are Fugro Seastar.
Fugro Seastar AS is situated in Oslo, Norway and is part of Fugro Norway AS.
Fugro Norway AS is owned by Fugro N.V in The Netherlands, a multi-national group with
approx. 7000 employees and over 250 offices in about 60 countries.
DGNSS Infrastructure Management and Support
Fugro operates a global DGNSS network.
The network includes:
100 reference stations,
an extensive data communication network and
3 manned network control centres (NCC)
The reference stations are fitted with high quality GNSS
(GPS/GLONASS) receivers and a data communications link.
The differential correction data generated is gathered centrally
at the NCC's, and then broadcast to the mobile users using 10
different communication satellites.
These include the 4 Inmarsat operational satellites for use
with conventional Inmarsat communications equipment and
high power channels on a further 6 satellites.
The main advantage of the high-power data broadcast is support for small omni-directional
antennae systems.
Fugro Seastar AS has management and operational responsibility for Fugro's DGNSS
infrastructure in Europe, Africa and Middle East.
The reference stations upload correction information to 3 Network control centers for quality
control, and these stations transmits the corrections to the satellites. The signals are decoded at
the user end with a special decoder – A demodulator. The decoded signal – the correction – is
send to the receiver and used for differential correction.
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SBAS - Satellite Based Augmentation System
A Satellite Based Augmentation System (SBAS) is a system that supports wide-area or regional
augmentation through the use of additional satellite-broadcast messages. Such systems are
commonly composed of multiple ground stations, located at accurately-surveyed points. The
ground stations take measurements of one or more of the GNSS satellites, the satellite signals,
or other environmental factors which may impact the signal received by the users. Using these
measurements, information messages are created and sent to one or more satellites for
broadcast to the end users.
The two main SBAS systems at present (2007) are WAAS (Wide Area Augmentation System)
operated by USA and EGNOS (European Geostationary Navigation Overlay Service) operated
by the European ESA, but more systems will be available eventually, giving a more or less
global coverage of corrections.
SBAS corrections do not need extra antennas or extra demodulators to be used by the receiver.
The only thing needed is a SBAS prepared receiver.
The area covered by WAAS/EGNOS/MSAS is expected to be bigger then the original plan due
to many countries will participate ( e.g. China, India) or there will be installed Reference Station
other places in the world ( e.g. Rio de Janeiro , Angola, Panama )
In the United States of America, the Federal Aviation Administration has taken the lead for
developing its Wide Area Augmentation System or WAAS. The WAAS signal was made
available for non-aviation users in 2000. It currently delivers accuracies of one meter horizontal
and two meters vertical and supports aviation precision approach performance.
An Initial Operational Capability (IOC) for aviation use started in July 2003 and its Full
Operational Capability (FOC) is planned for the end of 2007.
Japan is developing an SBAS founded on its Multi-function Transport Satellite (MTSAT) called
the MTSAT Satellite Augmentation System or MSAS. The first phase based on single
geostationary satellite coverage. Initial phase of MSAS were commissioned September 2007.
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While the second phase is based on dual geostationary satellite coverage. We expect MSAS to
deliver a Non Precision Approach capability, and this could be enhanced to provide precision
approach performances
Canada’s SBAS (known as CWAAS) strategy is based on an extension of the US WAAS
coverage by deploying a network of reference stations and linking these to the US WAAS
master control stations.
The first international Wide Area Reference Station (WRS) was installed in Gander, New
Foundland, Canada in June 2005.
India’s SBAS, GAGAN (GPS and GEO Augmented Navigation), is being co-ordinated by the
Indian Space Research Organisation and the Airports Authority of India. They are planning for
an initial operational capability in 2006/7.
The People’s Republic of China is deploying its Satellite Navigation Augmentation System
(SNAS). There is also a high level of interest in Brazil and the African continent.
There are currently (2006) 10 test stations in Africa – with two more planned for South Africa
and Madagascar.
More international installations will be installed in Mexico City, Merida, and Puerto Vallarta,
Mexico; and at Goose Bay, Canada.
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How does SBAS work
In principle all SBAS systems are working the same way. The following is based on the WAAS
system, but other systems have the same specifications.
Similar to a beacon transmitter the WAAS system collects data from strategically placed and
well characterized ground locations. However, unlike the standard DGPS a WAAS system
cannot directly send corrections to the pseudorange data.
This is because the unit has no idea where you are. Instead it attacks the problem by
addressing the individual sources of error and sending corrections for each one of them. The
biggest component of error is due to ionospheric delays followed by clock errors and ephemeris
errors. In addition another significant error source was troposphere errors.
To attack these error sources the WAAS system sends clock corrections, ephemeris
corrections, and ionospheric corrections. It cannot compute tropospheric corrections due to the
localized nature of this error but it does remove the tropospheric error component from the data
it computes so that the local receiver can apply its own corrections based on an atmospheric
model that is based on the current sky location of the SV.
The ground stations do not send the data directly to a WAAS satellite for re-transmission but
rather to a master ground station that analyzes the input and computes the full detailed error
information.
This fully correct data is then sent to the GEO satellite to be sent to the GPS receiver. Note that
there is no correction done by the GEO satellite; the master ground station will even correct for
errors produced by the GEO satellite itself. There are redundant master stations as well for
system wide integrity.
Clock errors can change rapidly so this data is update every minute if required, ephemeris
errors and ionosphere errors don't change nearly so fast so they are only updated every 2
minutes and can be generally be considered valid for up to 3 times that period of time.
Even this time is very conservative in practice. Clock and ephemeris data is specific to a
satellite but ionospheric errors are specific to your location therefore they must be sent
separately. After receiving raw data from each of the ground station the master station divides
the country into a grid and then builds ionospheric correction information on a per grid location
basis from the data received from each reporting station. It is this grid location that is used by
the GPS receiver to determine the applicable ionospheric corrections. In addition the master
station determines the validity of the data it receives and can indicate invalid data within 6
seconds to the GPS receiver.
Of the forms of error correction supported by WAAS only the ionospheric data requires
knowledge of the receiver position. Clock and ephemeris data is available for any receiver even
if it is currently located outside the area covered by ionospheric correction data. In addition
system integrity data can be used outside this area. The designers of WAAS developed a grid
system of correction data that permits a receiver to use the data it needs for this correction.
Here is what the grid system looks like.
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The master ground station computes a correction for ionospheric data for each of the points on
the grid based on field data it gets from the other ground units. Of course it is very unlikely that
there will be data for all of the points on the grid over the entire earth so the GPS receiver
downloads a grid mask that is part of the almanac data that tells it where to expect corrections.
The mask is divided into bands as shown on the drawing and each band contains 201 grid
points (except the last one which has 200). A single bit is used to represent the availability of
data on each grid point so the entire band can be contained in a single packet of data. Each geo
satellite can have data for up to a maximum of three or four bands but may have less.
The master station will generate correction data or each of the grid points tagged in the mask. A
receiver will locate its position relative to 4 grid points and interpolate the data from those 4
points based on their relative distance. If only 3 points have data then the receiver will compute
a triangle of the 3 points and if it is inside the triangle it will use the 3 points to interpolate its
correction. Otherwise, the use of any of the grid points for correction is undefined. It seems
however that most GPS implementations will use the data from even a single point if it is "close"
to the current location.
In addition to correction data the WAAS system places a high degree of importance to system
integrity. Each ground station and the master station has independent sources of critical data
and can determine if an SV is out of calibration. Bad data can be identified and relayed to the
receiver within about 6 seconds. The geosynchronus satellites that retransmit the data can even
be used as regular GPS satellites as part of the regular GPS solution since they also relay
regular satellite ephemeris data for themselves. This data like all of the other data, is generated
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at the master station and can be turned off independent from the regular WAAS corrections if
the satellite drifts too far.
All of the data for a region is loaded into the WAAS SV, so only one is required to receive
everything. A second provides redundancy.
The geostationary satellites do provide a signals very similar to that of the GPS-satellites and on
the same frequency. Therefore these satellites may be used for position calculation and
additionally, the correction data sent out can be used to improve accuracy for position
calculation with all GPS satellites.
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Chapter 12 – Positioning Weighting
It is of the outmost importance to the DP function to have:
•
•
•
Reliable Continuous Accurate - position reference systems.
Reliable :
Reliability is of vital importance, to operations where the loss of position reference can course
risk to life and property.
Continuous :
A typical DP system requires positioning update once per second.
Accuracy :
Normal navigation systems in common use for navigation purpose do not always provide the
necessary accuracy for DP operation
The DP vessels Position Reference Systems (PRS) are specifically designed and provided for
the DP operation purpose and normally independent of the vessel's normal navigation
equipment.
Position-reference systems Handling
Following types of Position Reference Systems are in common used in DP vessels:
Artemis - Hydroacoustic Position Reference - Taut Wire - Differential GNSS –
Fanbeam / CyScan / SpotTrack - RADius / RadaScan (FMCW principle)
SATELLITE NAVIGATION
DIFFERENTIAL
CORRECTION
RADIUS
ARTEMIS
FANBEAM
TAUT
WIRE
HYDROACOUSTIC
POSITION
REFERENCE
The different position reference media
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Early DP systems (c.1970's) did not utilise this pooling technique, and reliance at any time was
upon one PRS only. Such a system may have been connected to two or more PRS, indeed, the
operator may have two or more PRS activated and running, but the DP system could only
accept one PRS input, operator selected. If that PRS failed, it was up to the operator to detect
the failure, deselect the errant PRS, and engage an alternative. That process may have taken
some time and could result in a considerable positional discontinuity.
Each of these systems operates separately and independently of the DP system, and feed
information to the DP by means of an interface. The DP system can handle the different PRS
input, and by pooling / weighting the positions the DP obtain an accurate position. This process,
is a function of the mathematical filtering the system.
A modern DP system is able to pool position-reference data from two or more position-reference
systems. If only one position reference system is enabled into the DP then it is simply
calibrated, filtered and used, but if two or more are available, then some form of pooling is
required.
In the DP systems the pooling is reliant upon Weighted averaging. Various methods of weighted
averaging are possible. Weighting may be manually achieved, or automatic. If automatic
weighting principles are used, the basis for the weighting may be Variance. With Variancebased weighting, the weighting value will depend upon the spread or jitters exhibited of the
positional data from each position-reference system, or it may be determined from the offsets
observed between successive measurements compared with the DP model position.
A weighting system based upon Variance-based principle may suffer problems. For instance, a
very low value for Variance (thus high weighting) may result from a position-reference system,
which is frozen, and has become a "perfect" position- reference system. Further, the data
update rate must be taken into account, since a position-reference system with a high update
frequency may appear to have a higher apparent Variance than one with a slow update.
Kalman Filtering of Position Data
The mathematical technique of Kalman filtering is to provide a method of combining
measurements of data from different sources in a statistically optimum manner. The
requirement of integrating two or more position-reference systems inputs in a DP system is an
example of the use of Kalman filtering.
In any DP system two principal factors must be combined. One of these is the software model of
the vessel position. This is determined from knowledge of the previous position and of the
forces acting on the vessel. The other factor is the position measured from the positionreference systems. These two, model and measured positions, are combined to determine a
best estimate of the vessel position. This estimated position is then used to modify the model.
The weighting within the Kalman filter upon model position and measured positions will depend
upon the expected performance of the position-reference system. If the position-reference
system is "noisy", i.e. the variance is large, then greater weight should be placed upon the
model. If the position-reference systems are accurate, then greater weighting should be
allocated on them.
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The following description of the weighting between different position-reference systems and
different tests relates to the Kongsberg’s K-Pos systems.
Position reference system Selection and Monitoring
In the K-Pos systems you can enable and disable position-reference systems by using the
buttons in the Sensors button group.
Each button has a status lamp that shows the status of the reference system:
Lamp Off:
Disabled
Lamp flashing:
Either enabled and calibrating, or enabled and calibrated but no longer
accepted
Lamp on:
Enabled and accepted (acceptable position measurements are received)
Alternatively the position reference systems can be enabled and disabled from the Reference
System dialog box. In newer Software this dialog box also provides an option to only monitor a
position reference system. The monitored system will not influence the DP model (zero weight).
Establishing Reference Origin
Several different position-reference systems are normally used with the K-Pos systems. Each
position-reference system provides measurements relative to a known reference point specific
for that reference system (e.g. GPS reference point is 0.0° N – 0.0° E). The reference point of
the first position-reference system that is selected and accepted for use by the K-Pos systems,
becomes the reference origin. This position becomes the origin in an internal Cartesian coordinate system to which all position information is calibrated according to. All computed
position information is defined by this co-ordinate system.
When the first position-reference system is selected, the K-Pos systems system takes the first
position measurement and uses this to establish reference origin. The position reference
system that defines the reference origin is marked with an asterisk on the Refsys view. On the
Posplot view, (if within the range of the view), the position of the reference origin will be
indicated by a small circle around an asterisk. As long as no other reference system is selected,
the position of the reference origin will be calibrated and continuously established in the centre
of all measurements. It is therefore recommended to wait a couple of minutes before enabling
other reference systems, to allow an accurate calibration of the reference origin. It should be
avoided to enable other position-reference system immediately after the first one. The vessel
should be kept at low speed during this calibration.
When the second position-reference system is selected, the calibration of the reference origin
stops, and the coordinate system is locked to the established reference origin.
Note that the position-reference system that becomes the reference origin is not treated as
being better or more reliable than any other position-reference system. It concerns only the
location of the reference origin. You should select the reference origin that is most appropriate
to your operational requirements. If your operation require positioning on absolute positions it is
recommended to choose the most accurate and reliable system as the reference origin.
Changing the Reference Origin
The first position-reference system that is selected remains as the reference origin until you
deselect all position-reference systems and select a new system as the first one.
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Calibration of other position reference systems
In newer Software all new position-reference systems that are enabled will be calibrated relative
to the position of the DP model. The calibration of the new position-reference system is finished
when the position co-ordinates have settled to a defined precision. The acceptance limit for
calibration (limit for the accepted variance) is derived from the “Expected Accuracy” for the
reference system. This is a value usually set to 2.24m as default. This value can be changed in
the Reference System Set-up dialog box. This value also influence levels for reference system
tests and alarms.
Messages, warnings and Alarms
There are many messages related to position-reference systems, which could be reported to
the operator. Not all will be mentioned and explained in this chapter. All messages are
explained in the Help menu on the K-Pos systems. This is some of the alarms, warnings and
messages that may be given related to position-reference system handling in the K-Pos
systems:
Calibration Error
No Reference System Active
Position Dropout
Reference High Noise
Reference High Offset
Reference High Variance
Reference Median Deviation
Reference Median Rejected
Reference Prediction Error
This is in addition to all the alarms, warnings and messages that are related to a specific
position- reference system.
Tests on position measurements
There are several tests performed on the different position-reference system used in the K-Pos
systems. The purpose of all the tests is to verify whether the position measurements are outside
acceptable limits or not. If the measurements are outside the limits the position-reference
system could be rejected by the and K-Pos systems.
The K-Pos systems reports when the defined limits are exceeded.
The following tests are carried out by the K-Pos systems:
Freeze test
Variance test inclusive spike detection
Prediction test
Divergence (Bias) test
Median test
Timeout / Failure in input - alarms
Before any tests are performed the signal received from the PRS is checked by the K-Pos
systems. Error situations that are detected and reported could be:
Timeout (i.e. data are completely lost)
Telegram checksum failure
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Serial line failures
Status failures (such as no differential correction for GPS)
Examples of messages are:
GPS INVALID
HPR NOT OK
ARTEMIS SYSTEM TELEGRAM ERROR
ARTEMIS COMMUNICATION ERROR
Freeze test
If a position-reference system has an internal error causing the same measurements to be
continuously sent to the Vessel Model, the K-Pos systems could, if no precautions were taken,
mistake the data for good and stable measurements.
To prevent this position-reference system to be used, a freeze test is implemented. This test
rejects the position-reference system if the variance between 4 subsequent measurements from
the same position-reference system is approximately equal to zero.
The system will provide the following alarm:
REFERENCE POSITION FROZEN
The freeze test is disabled for some position-reference systems (usually GPS and/or Artemis)
due to the resolution in the data from these position-reference systems.
Standard Deviation and the Variance test including spike detection
For any position-reference system Kalman filtering technique is used. A circle is placed around
a representative sample of position returns. The size of the circle relates to the variance or
spread, in meters, of the samples of position measurements. The radius of the circle thus
corresponds to where 67% of all the raw historical measurements is within. The value for the
radius of the circle is called the “Standard Deviation”, SD.
As mentioned under calibration of position-reference systems, the acceptance criteria for
calibration are derived from the “Expected Accuracy” for the reference system. This is a value
usually set to 2.24m as default. This value can be changed in the Reference System Set-up
dialog box. This value also influence levels for reference system tests and alarms.
GPS and HPR provide an estimate of the accuracy/variance of each position fix. The estimate is
in the position telegram from the GPS to the DP if configured to do so, and in the HPR it is
automatically forwarded to the DP system.
For the Artemis and Fanbeam the estimate is based on a-priori information about the accuracy.
The Variance is defined as (SD)².
In the Variance test the variance of the raw measurements (Raw Var) is compared with the
estimated variance (Estimated Var) for the specific position-reference system:
If the Raw Var < Estimated Var ⇒ Combined Var = (Estimated Var + Raw Var)/ 2
If the Estimated Var < Raw Var ⇒ Combined Var = Raw Var
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This means that if the Raw variance is more accurate than the Estimated variance, the
Combined variance, which the K-Pos systems will use in the Variance test, is the sum of the
two variances divided by two.
If the Estimated variance is more accurate than the Raw variance, the K-Pos systems will use
the Raw variance as the Combined variance in the Variance test.
When the Combined variance is larger than 2.45 x Expected Accuracy (value in the Reference
System Set-up dialog box.) The system will provide the following warning:
REFERENCE HIGH NOISE
The position-reference system is not rejected.
When the Combined variance is larger than 3.0 x Expected Accuracy (value in the Reference
System Set-up dialog box.) The system will provide the following warning:
REFERENCE HIGH VARIANCE
The position-reference system is rejected.
Prediction test
The prediction test detects sudden jumps in the measured values and immediately rejects those
measurements that lie outside the limit. The test will also sooner or later reject a system that is
drifting away from the other position-reference systems as the measurements are compared
with the Vessel Model.
The limit for the Prediction test for each position-reference system is derived from the calculated
Standard Deviation of each position-reference system. The limit is not constant, but varies with
the Standard deviation of the position-reference system and the position of the Vessel Model.
The prediction error limit of the most accurate position-reference system at any time, called the
Minimum Prediction Error Limit, is displayed on the Refsys view. The limit can be observed as a
numerical value and as an unbroken grey circle with radius equal to the value.
The limit of the prediction test is derived from the Standard Deviation. With continuously “noisy”
measurements the Standard Deviation of that position-reference system will increase, and thus
also the prediction test limit. The aim of the prediction test is to avoid validation of “noisy”
positions. Therefore the prediction test limit is limited upwards.
The prediction limit is also limited downwards, in order to avoid a very low prediction limit when
the Standard Deviation is low. This is to avoid that perfectly acceptable measurements are
rejected.
The operator can change the minimum and maximum values of the prediction limit under the
Validation page in the Reference System dialog box.
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Choosing Acceptance Limit Narrow, relates to prediction test limit between 5 and 15 m.
Choosing Acceptance Limit Normal, also relates to prediction test limit between 5 and 15 m.
Choosing Acceptance Limit Wide, relates to prediction test limit between 8 and 25 m.
Even though the minimum and maximum values for Low and Normal is equal, there is a
difference in how the prediction test limit is computed, meaning that choosing Low allows
smaller jumps in the measurements than choosing Normal. For instance transitory position
dropouts due to rough sea will be allowed when choosing Normal.
High Limit is default when sailing in the modes Auto Track High Speed and Autopilot/ Autosail.
Note that not all vessels have the opportunity to change the Acceptance Limits. Then the limit is
set to Normal as default value.
If positions are rejected by the Prediction test, the system will provide the following warning:
REFERENCE PREDICTION ERROR
Slow drift tests
There are two slow drift tests added in the K-Pos systems. The Divergence/Bias Test used for
two or more position-reference system active, and the Median Test used for three or more
position-reference system activated.
Divergence test (Bias Test)
The divergence test has been introduced in order to give an early warning of potential drift on
position-reference systems. The Divergence Test is activated automatically when there are two
or more position-reference systems calibrated and online in the K-Pos systems The Divergence
test detects when measurements from one position-reference system drift from the others.
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The same test as for the Prediction test is used, but the actual limit is scaled to a lower value.
The test gives a warning before the system is rejected by the prediction test, if the drift speed is
lower than 1.0 to 1.5 m per minute.
The system will provide the following warning:
REFERENCE HIGH OFFSET
The drifting system is not rejected by the divergence test. If the drifting continues and the
system is not deselected, the position-reference system will be rejected by the Prediction test.
Median Test
For redundant operation a DP vessel will usually (where possible) use three or more positionreference systems, allowing the DP system to apply Voting logic to the measurements. Voting
could involve taking the average value, or the Median value of the three or more inputs. The
Median is used, not the average, since if averaging was adopted, the inclusion of data from the
erroneous system would pollute the average value. The good systems would then show
excessive offsets, which might result in them being rejected also.
The median test can be activated when three or more position-reference system is online in the
K-Pos systems. When measurements from one position-reference system differ from the others,
the median test detects this and if selected (by the Operator) it will give the Operator a warning
and the position-reference system will be rejected.
The offsets from the Median value for each position-reference system are examined and
checked against a reject limit, called the Median Test Limit. The Median Test Limit is set to 80%
of the Minimum Prediction Error Limit.
The Median Test limit is displayed on the Refsys view. The limit can be observed as a numerical
value and as an unbroken blue circle with radius equal to the value. A position-reference system
exceeding the Median Test Limit will be rejected if “Warning and reject” is selected under the
Validation page in the Reference System dialog box. The Median Test can be set to either “Off”,
“Warning” or “Warning and reject”.
The operator can switch on/off the median test in the Reference System dialog box:
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The system will provide the following warning if “Warning” is selected:
REFERENCE MEDIAN DEVIATION
The system will provide the following warning if “Warning and Reject” is selected:
REFERENCE MEDIAN REJECTED
Considerations regarding Median test
In some situation such as a combination of two DGPS’s and one HPR with both GPS’s drifting
in the same direction, the selection of “Warning and Reject”, could result in the HPR to be
rejected. The operator should consider the situation and in this situation the operator should
have activated only “Warning” for the median test. Then the system would have notified the
operator and the operator could have investigated the problem by using the Refsys view.
Refsys view
In the Kongsberg K-pos series equipment, a position reference display page, Refsys view,
gives a graphic presentations of position-reference system data. The information is letter and
colour- coded for each position-reference system.
The centre of the plot is the present Vessel Model position.
The Standard Deviation is shown as a broken circle for each position-reference system.
The Minimum Prediction Error Limit is shown as an unbroken grey circle centred upon the
display centre. A numerical value of the Minimum Prediction Error Limit is also displayed.
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The Median Test Limit can be observed as a numerical value and as an unbroken blue circle
centred on the median position, with radius equal to the value. The radius is defined as 80% of
the Minimum Prediction Error Limit.
For each position-reference system, the capital letter with no circle around it represents the last
raw data measurement for this system.
For each position-reference system, the small inner circle with a capital letter inside represents
the filtered position for this system.
By clicking on one of the position-reference systems names in the Refsys view, small crosses,
“+”, will be displayed, representing a one-minute trace of the raw data for the selected positionreference system. The name of the selected position-reference system will be shown in the
“Show Raw History” field, in this case the A *GPS-1.
The position-reference system that is providing the Reference Origin is marked with an asterisk,
in this case the A *GPS-1.
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For more detailed description of all the options in the Refsys view, it is referred to the K-Pos
Systems Operator Manual. The weighting of position-reference systems is described below.
Weighting
Each position-reference system is assigned a Weighting value. The Weighting values are
shown in the Refsys view. The different weightings are based on the calculated variance for
each position-reference system. In this way, the system is able to place more emphasis on the
position-reference systems that are providing the most accurate measurements. The higher the
system’s variance, the lower its weighting factor. Most emphasis is placed by the K-Pos
systems on the position-reference system with the highest weighting factor. The earlier
mentioned Combined Variance is used in the calculation of weighting factors.
Raw position measurements are filtered in the way that the new filtered position is equal to nine
times the old filtered position plus the new measurement, divided by ten.
The statistical mix of two or more position-reference systems, in order to provide the calculation
of the vessel position, can simplified be explained in this manner:
Three position-reference systems are enabled: Artemis, HPR and Taut Wire.
In this example the Artemis is exposed to some noise. The LTW is very steady, while the HPR
system is subject to very much noise and is close to the Minimum Prediction Error Limit.
For illustration we look separately at Northings:
PRS System
Northing
Weight
Product
No_____________________________________________________
1
Art
-22.0m
0.3
= - 6.6
2
LTW
-20.0m
0.7
= -14.0
0.0
3
HPR
-27.0m
0.0
=
________________________________________________________
Weight sum:
1.0
= -20.6
The weighting values always total 1.0.
Thus, from the above we can see that the noisy measurements from the HPR are not affecting
the final position, and that the position is dependant upon measurements from both Artemis and
Taut Wire, with a bias towards the more accurate system.
Operator Considerations
If two position-reference systems are used, one good and one poor, then it is possible for the
relative weightings to be 0.99 and 0.01. Under these circumstances the poor reference could
frequently be rejected. There is no link between accuracy and reliability. If the Taut Wire is
represented with the weighting of 0.99 and the HPR weighted 0.01, it could happen that the
depressor weight of the Taut Wire then starts to slowly drag through soft mud on the seabed.
The DP system knows only that the relative calibration is no longer correct, thus the system with
the lower weighting could be rejected - in this case, the HPR. Thus, with two position-reference
systems only, there is a danger that a good position-reference system could be rejected while a
poor or erroneous one could be retained and used for positioning. This is a good argument for
the use of three position-reference systems in any operation where positioning is vital or critical.
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It must be mentioned that when using HPR as a position-reference system the DP system will
treat each transponder as a separate position-reference system, each with it's own weighting.
The DPO, however, must treat HPR as one position-reference system only. This is because the
same transducer is used to position on all transponders. However more transponders will give
more reliability than just one, since you may loose contact with one transponder while remaining
contact with another one. The positioning is also calmer with more than one transponder, with
less thruster use. A drilling rig often uses two HPR transponders located on the wellhead. One
of these will be active, with the other one ready for use if necessary. It should be considered to
use both simultaneously.
If two separate and independent HPR systems are in use, each interrogating different
transponders on the seabed, it could be treated as two position-reference systems.
Even if three position-reference
systems are used, with Median test on,
it is possible to defeat the redundancy
in the system through poor working
practice. One common practice was to
deploy the Taut Wire, and locate a
HPR transponder on the depressor
weight before lowering. This gives two
position-reference systems on the
same downline, which is most
convenient. The third positionreference system may be the Artemis
system. The DP accepts all three
position-reference systems in the
normal way, giving three steady lights
on the console. Then the Taut Wire weight starts to drag; the transponder goes with it, and the
position-reference system that is rejected is the Artemis - the only good one! The DP believes
the vessel is on location, with good HPR and Taut Wire measurements. The reality is that the
ship is drifting off.
The DPO should use caution in his choice of position-reference systems. For any critical
operation it should be considered to utilise more than two position-reference systems. Two
position-reference systems are not adequate, since there will arise the question as to which one
has failed when contradictory reference data is received from the two systems. Three systems
will give more security against this possibility, especially if the DP system is programmed to
apply a Median test.
Where three position-reference system are required, the DPO should choose systems, which
have differing principles, e.g. HPR, DGPS and Fanbeam; i.e. one acoustic underwater system,
one radio/satellite system and one optical laser system. This reduces the probability of
Common-mode failure, where one event may result in the failure of multiple position-reference
systems. Common-mode failure is more likely to occur in situations where the choice of
position-reference system has included two or more of the same systems, i.e. two taut wires
and one Artemis. Even though the taut wires are separately located and powered through
independent protected supplies, it is possible for a vessel movement to cause both taut wires to
drop out of angular limits together, leaving the vessel with one position-reference system only.
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Likewise, a violent roll may cause the spool-rate of the winches to be exceeded and (both) taut
wires to break.
Despite the above comments, the DPO may be obliged to use a less-than satisfactory
combination of position-reference systems simply because a better option is not available. In
these circumstances it is necessary that great care is taken in the deployment and operation of
the available position-reference systems, to ensure that they are not jeopardised for any
foreseeable reason.
Some operations require three position-reference systems. Any reduction in position-reference
system input will result in the operation being suspended. It should therefore be considered to
use four position-reference systems as the norm. This may seem a little excessive but there is a
logical reason. If we consider a deep water drilling operation with riser connected, working fully
redundant with three position-reference systems. The rig's procedures will demand a riser
disconnection for any degradation within the positioning capability. This disconnection
represents a considerable cost in terms of lost time. With three position-reference systems,
degraded status is obtained if one of the three is lost. If, however, four position-reference
systems were deployed, then the loss of one of them leaves the vessel operational. There is no
lost time, and slightly less urgency in getting the fourth position-reference system back on-line.
Position Information in the Kongsberg DP systems
Position data may be received from position-reference systems in a number of different forms:
Global systems, such as DGPS giving data in Lat./Long format.
Global systems, such as DGPS giving data in UTM format
Local references such as Taut Wire and HPR giving position data in local Cartesian (XY) coordinates with reference to a local reference-origin.
PRS dialog boxes
A number of dialog boxes allows the Operator to set up the required parameters for the
handling and display of position data. The following dialog boxes relates to position-reference
system handling in the K-Pos systems:
Reference System Set-up
Navigation
Position
The Reference System Set-up dialog box allows the Operator to specify, for individual positionreference systems, the input datum, false Northing and Easting values required, fixed values of
offset between antenna or sensor from the vessel’s CG, and any fixed offset values which may
be applied to the reference system origin.
The Navigation dialog box allows the Operator to specify among other things the datum and
UTM zone used internally by the controller. This is to normalise global reference systems, which
may use different datum. This is necessary when using anything other than local reference
systems.
The Position dialog box within the Display Units menu, where the Operator may select the
presentation datum of position information. The Operator may also select different presentations
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of position format (e.g. Geographic - Lat./Long, UTM or Cartesian XY - N/E offsets from a local
Origin).
For more detailed information on the handling and display of position information, it is referred
to the K-Pos system’s Operator Manual.
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Marine Technologies Reference Systems
A reference system is a sensor that provides measurements of vessel position. Since there is a
bit more to the usage of reference systems than the other sensors, a dedicated chapter has
been devoted to this subject.
Enabling/Disabling Reference Systems
The dialog used to enable/disable reference systems is similar to the Sensor dialog. It can be
accessed from the Sensor menu on the menu bar or by clicking on the reference system symbol
in the lower left corner of the screen. The status of the different reference systems will be
indicated in the dialog. The dialog is shown in Figure 71.
A reference system can also be enabled and disabled by pushing the different buttons
representing the reference systems on the upper panel.
Figure 71 Navigator/Reference System Dialog Box
The local reference systems (Fanbeam, CyScan etc.) will always be calibrated before they
become enabled, due to the fact that they do not know the global position (Lat, Lon) of the
vessel. These systems will be calibrated in accordance with the “official” vessel position, as
estimated by the vessel state estimator. The state estimator is not active in Standby mode, and
therefore the local systems will not be calibrated unless the vessel is in Joystick or DP mode. To
make sure that the vessel state estimator is updated by global position data, at least one global
system (GPS) should be enabled before entering Joystick or DP mode.
Reference System Philosophies
As for the heading sensors, two different philosophies are applied to the usage of reference
systems. Which philosophy to use is either pre-set for a specific DP system or made available
for the operator to select (in Reference System Setup dialog, as described later in this chapter).
The user interface for reference systems on the OS (e.g. the dialog in Figure 71) will change
according to the selected philosophy. The different philosophies are described below.
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Primary/Secondary/Auxiliary
When using this philosophy the vessel model is only updated by the reference system that is
currently “In Use”. The other reference systems, if enabled, will still be used in the voting
algorithm, but the “official” position will be estimated based on reference system “In Use” only.
The reference system “In Use” will always be the reference system selected by the operator as
“primary”. If the primary reference system fails, the secondary system will become the one “In
Use”. If the secondary is “In Use” and the primary reference system again becomes available,
the operator has to manually transfer control back to the primary system. This can be
accomplished by opening the reference system dialog and pushing the apply button with the
desired reference systems set to primary and secondary.
Navigator Weighting
When using the navigator weighting philosophy all reference systems will be taken into account
when estimating the “official” vessel position. Each reference system will be assigned a weight
based on its accuracy. The weight of a reference system will determine how much influence it
has on the official position. From the refsys view dropdown menu it is possible to open a dialog
showing the relative weight (in %) of the different systems. This dialog is shown in Figure 72. In
this example all four reference systems are equally accurate.
Figure 72: Navigator Weight dialog
Note:
When using the “primary, secondary” approach it is an underlying assumption that the operator
is actively involved to make smart choices of reference systems to be used as primary,
secondary and auxiliary. Failing to do so can have serious consequences for the safe operation
of the vessel. As an example, when staying close to a rig it would often be considered a smart
choice to select a laser based local reference system as the primary system, as these are
normally more accurate and due to the fact that close proximity to the rig can result in blocking
of satellites and loss of correctional data for GPS systems.
Reference System Setup
From the Sensor menu on the menu bar the operator can open the Reference System Setup
dialog to make changes in settings for the reference systems. The dialog is shown in Figure 73.
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Figure 73: Reference System Setup Dialog
The reference system location refers to the location of the antenna/sensor on the vessel. The
position has its reference to the stern and port side of the vessel. The height of the reference
system gives the distance from the waterline with a negative value as the positive z-axis is
pointing downward. During commissioning of a vessel the location of the reference system
antennas is always configured as part of the systems settings. However, it is possible that
antenna locations at some point are changed without updating the system settings of the
vessel. In that case the incorrect location of the antenna will typically be reflected in the refsys
view, with a large distance between the reference system symbols. The location of the antenna
should be verified with these settings and service personnel should be notified if it is incorrect.
From the dialog the DPO can select the limits for different criteria used to trigger alarms for poor
DGPS conditions. The quality indicators used for a GPS are the number of satellites is sees, the
HDOP value and the number of seconds passed since it last received differential corrections
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(for a DGPS). The DPO can also choose what action the system should take when any of these
limits are exceeded. By default the system will reject a reference system under poor conditions.
The DPO can however temporarily override these settings and set the system to only issue an
alarm when ant of the criteria are unsatisfying. These settings are only temporary and will be
reset to default if system is rebooted.
From the reference system setup dialog the operator can also select the filter parameter to be
used for the reference systems. The meaning of this parameter is described in the coming
chapter.
Reference System Filtering
From the reference system setup dialog the operator can select the filter parameter to be used
for the reference systems. Just as for the other active sensors (Gyro, Wind, VRU), this
parameter determines how much the system should filter the incoming measurements from the
reference systems. High filtration (slider to the right) means that the system will rely heavily on
the mathematical model of the vessel as opposed to heavy reliance on the measurements. High
filtration will give a more smooth, calm operation of the vessel as wave induced motion of the
vessel will be prevented from entering the feedback loop. On the other hand, low filtration will
typically result in a more “stressed” vessel with more thruster activity.
Sonardyne Setup
Form the sonardyne setup the DPO can change the limit of time out for the signal from the
sonardyne system. This is the limit in seconds since last measurement received from the
system to it is detected faulty. Also the noise value can be changed. This value indicates how
noisy the signal is. If the signal is noisy this value can be increased. This will allow the system to
go further away from the other without rejecting it, but it also means that it is not so trusted. It
will be given a lower weight in a weight solution.
Differences in Reference System data
A pre-set acceptance limit for the reference systems will determine how big a deviation between
two or more reference systems that is necessary before a system is considered invalid. If only
two referenced systems are available and the difference between these exceeds the predefined
limit, a dialog will pop up and prompt the DPO to choose which system to use (see Figure 74).
When selection is made, this reference system is used, and the other is disabled. If three or
more systems are used the DP system will determine which system is invalid and vote this out,
based on the distances between all systems enabled. In that case an alarm will indicate to the
operator that the reference system is voted out.
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Figure 74: System Select Dialog Box
Loss of Position data (Dead Reckoning)
If the DP system looses input from a PRS an alarm will be issued to inform the DPO that the
reference system is lost, and the status of the PRS will be set to unavailable. If all PRS are lost
an alarm will indicate to the DPO that all reference systems are lost. In that case the vessel will
go into a mode called Dead Reckoning (DR), where the DP system is solely running on the
internal vessel model. The DP system should be able to hold position reasonably well for a few
minutes, long enough for the DPO to respond to the situation.
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Chapter 13 – The UTM Coordinate system
Universal Transverse Mercator
The Universal Transverse Mercator (UTM) projection is used extensively for survey and other
offshore work. Much DP related navigation will be based on the UTM system, so a description
is given here.
UTM is a grid system based upon Northings and Eastings, in metres. The intention is to
reduce the distortion present in the traditional Mercator projection, based upon Latitude,
Longitude and True North. UTM is, like Mercator, a cylindrical projection, but in UTM the axis
of the cylinder runs along the plane of the equator, the line of contact between the cylinder
and the sphere is thus a meridian and its anti-meridian.
Obviously a single cylindrical projection of this type cannot be used to chart the whole terrestrial
surface, and if the difference in longitude between the contact meridian and the charted area
were great, then distortion would be great also. The useful scope of the projection consists of a
zone 6 degrees of longitude in width, centred upon the contact meridian, known as the
CENTRAL MERIDIAN. Within this zone distortions are minimal. Zones are numbered from the
180° meridian eastward. Thus zone 1 spans 180 ° to 174° West, with the Central Meridian on
177° West. The North Sea is mostly within zone 31 (Greenwich to 6 ° East, Central Meridian 3 °
East).
Within a particular zone, the Northings and Eastings (in metres) are arranged to increase in a
Northward and an Eastward direction, respectively, irrespective of position upon the globe.
For Northings the datum is the equator, with Northern hemisphere Northings having a value of
Page 173
zero on the equator, and increasing northwards. For the Southern hemisphere, a false
Northing of 10,000,000 is established on the equator, with Northing values decreasing from
this as one moves south. This resolves the problem
of requiring positive values increasing northwards
throughout.
For Eastings, a value of zero is found on the
Central Meridian, however, this would result in
negative values to the West of the Central
Meridian. To overcome this problem a False
Easting value of 500,000 is established on the
central meridian, with Easting values increasing in
an easterly direction. This allows the whole zone to
be covered by positive Easting values.
Because UTM is a grid system there is no
convergence of meridians and the map graticule is
a true 90 degree square grid. There is thus a
difference in direction between Grid North and
True North, and this difference itself will vary
across the area. The difference will be zero on the
Central Meridian. For DP work it is important that
this difference is a known value, and also whether
Artemis fixed stations are calibrated to True or
Grid bearings. For short range work, or relative,
long range work there is no problem, but if a
worksite location is being established by a DP
vessel at a long range from an Artemis reference
origin, using Artemis co-ordinates, then
considerable positional error can appear.
It is important to realise that UTM co-ordinates from one zone (i.e. based upon a particular
central meridian) will not in any way match up with co-ordinates for the same location based
upon another central meridian. When planning a task it is necessary to check that all worksite
diagrams and plans supplied are drawn to the same projection and central meridian datum.
POSITION INFORMATION
Position data may be received from position-reference systems in a number of different forms:
Global systems, such as DGPS giving data in Lat/Long format. The applicable
datum must be known (e.g. WGS84, ED87 etc.)
Global systems, such as DGPS giving data in UTM format
Local references such as Taut Wire and HPR giving position data in local cartesian
(x,y) co-ordinates with reference to a local reference-origin.
Page 174
Meridian Convergence = Grid North – True North
For high precision navigation the meridian convergence can be taken into account
.
Page 175
UNIVERSAL TRANSVERS MERCATOR ZONE TABLE
WEST OF GREENWICH MERIDIAN
U.T.M Zone
Central Meridian
LONGITUDINAL BORDERS
From
To
EAST OF GREENWICH MERIDIAN
U.T.M Zone
Central Meridian
LONGITUDINAL BORDERS
From
To
30
3
deg. West
0
deg. West
6
deg. West
31
3
deg. East
0
deg. East
6
deg. East
29
9
deg. West
6
deg. West
12
deg. West
32
9
deg. East
6
deg. East
12
deg. East
28
15
deg. West
12
deg. West
18
deg. West
33
15
deg. East
12
deg. East
18
deg. East
27
21
deg. West
18
deg. West
24
deg. West
34
21
deg. East
18
deg. East
24
deg. East
26
27
deg. West
24
deg. West
30
deg. West
35
27
deg. East
24
deg. East
30
deg. East
25
33
deg. West
30
deg. West
36
deg. West
36
33
deg. East
30
deg. East
36
deg. East
24
39
deg. West
36
deg. West
42
deg. West
37
39
deg. East
36
deg. East
42
deg. East
23
45
deg. West
42
deg. West
48
deg. West
38
45
deg. East
42
deg. East
48
deg. East
22
51
deg. West
48
deg. West
54
deg. West
39
51
deg. East
48
deg. East
54
deg. East
21
57
deg. West
54
deg. West
60
deg. West
40
57
deg. East
54
deg. East
60
deg. East
20
63
deg. West
60
deg. West
66
deg. West
41
63
deg. East
60
deg. East
66
deg. East
19
69
deg. West
66
deg. West
72
deg. West
42
69
deg. East
66
deg. East
72
deg. East
18
75
deg. West
72
deg. West
78
deg. West
43
75
deg. East
72
deg. East
78
deg. East
17
81
deg. West
78
deg. West
84
deg. West
44
81
deg. East
78
deg. East
84
deg. East
16
87
deg. West
84
deg. West
90
deg. West
45
87
deg. East
84
deg. East
90
deg. East
15
93
deg. West
90
deg. West
96
deg. West
46
93
deg. East
90
deg. East
96
deg. East
14
99
deg. West
96
deg. West
102
deg. West
47
99
deg. East
96
deg. East
102
deg. East
13
105
deg. West
102
deg. West
108
deg. West
48
105
deg. East
102
deg. East
108
deg. East
12
111
deg. West
108
deg. West
114
deg. West
49
111
deg. East
108
deg. East
114
deg. East
11
117
deg. West
114
deg. West
120
deg. West
50
117
deg. East
114
deg. East
120
deg. East
10
123
deg. West
120
deg. West
126
deg. West
51
123
deg. East
120
deg. East
126
deg. East
9
129
deg. West
126
deg. West
132
deg. West
52
129
deg. East
126
deg. East
132
deg. East
8
135
deg. West
132
deg. West
138
deg. West
53
135
deg. East
132
deg. East
138
deg. East
7
141
deg. West
138
deg. West
144
deg. West
54
141
deg. East
138
deg. East
144
deg. East
6
147
deg. West
144
deg. West
150
deg. West
55
147
deg. East
144
deg. East
150
deg. East
5
153
deg. West
150
deg. West
156
deg. West
56
153
deg. East
150
deg. East
156
deg. East
4
159
deg. West
156
deg. West
162
deg. West
57
159
deg. East
156
deg. East
162
deg. East
3
165
deg. West
162
deg. West
168
deg. West
58
165
deg. East
162
deg. East
168
deg. East
2
171
deg. West
168
deg. West
174
deg. West
59
171
deg. East
168
deg. East
174
deg. East
1
177
deg. West
174
deg. West
180
deg. West
60
177
deg. East
174
deg. East
180
deg. East
Page 176
Page 177
Chapter 14 – DP Operations
Pipelay Operations
Dedicated lay barges or pipe laying vessels have laid the majority of offshore oil and gas
pipelines. The commonest method of installation is the on-board construction of the pipeline
by means of sequential welding of sections of the pipe string. The vessel moves forward as
the pipeline descends to the seabed in an S-curve, partly supported by a Stinger, or support
gantry overhanging the stem of the lay vessel. Other methods of pipe laying are shown
below, J-lay, used for deep water operations, and Reel-lay, or Drum-lay, where the pipe is
preconstructed at a shore factory and reeled aboard the vessel.
Pipe is constructed in a linear pipe fabrication facility called the "Firing Line". Pipe is
brought up from hold storage and is prepared for fabrication and welding. Often, 12 metre
pipe lengths are welded into 24 metre double joints prior to arrival at the firing line. In the
firing line, a number of stages of welding take place, both externally and internally within the
pipeline. Each operation is conducted at a "station". Further stations conduct X-ray and
NDT testing on the welded joints, anti-corrosion coating, and weight coating if necessary.
Each station is equipped with a button controlling indicator lamps. When all the buttons, at
each station have been pressed, a "green line" shows on the DP bridge, and the DPO
initiates a move ahead a distance equivalent to the joint-length. Once the move ahead has
been completed, the firing-line operations continue.
It is essential that tension be maintained on the pipeline. At the back end of the firing line,
the pipe is controlled by a number of pipe tensioners. These consist of sets of caterpillar
tracks clamping the pipe, either top-and-bottom, or side located. The tensioners control the
Page 179
movement of the pipe, maintaining a set tension on the pipe string. The pipe is supported
aft of the firing line by the stinger, which is an open lattice gantry extending beyond the
stem of the vessel, sloping downwards. The stinger contains a number of sets of support
rollers adjusted and positioned to support the pipe in the area known as the "overbend".
This is the area of greatest stress on the pipe, and the area most vulnerable to buckling
damage. The tension on the pipe helps to reduce the likelihood of pipe buckle at this point.
The pipe takes up a catenary profile, or "sagbend" between the end of the stinger and the
seabed. The set tension is to ensure a smooth transition from the unsupported sagbend to
the touchdown point on the seabed. If tension is lost, then damage will occur at the
touchdown area, and the pipe will have to be recovered for repair. It can be seen that pipe
tension is an all-important factor in the lay operation.
Pipe tension values are communicated to the DP system by means of loadcells
incorporated in the tensioners. The DP system is continually working with this external
force, using thruster power to maintain tension. In adverse sea states, the tensioners are
working hard to maintain tensions on the pipe string within set criteria, and the DP system
must also play its part in this tension control.
Page 180
S-lay Operations
A pipe laying operation may begin with a lay-down adjacent to a fixed platform. It may
happen that the pipeline end is simply being laid down within a specified 'box' or area on
the seabed, or it might be necessary to carry out a pull-in to a J-tube or similar. If the
operation involves a J-tube pull-in, the J-tube will have already been installed on the
platform, and a pull-in wire will be rigged through it. A pull-in winch will be fitted on the
platform to handle the job. The vessel will position itself in the correct location, lined up with
and the correct distance from the J-tube. The end of the pipe is fitted with a pull-head, to
which can be shackled the pull-wire, which is passed across from the platform by
messenger or crane hook. The platform pull-in winch takes the load onto the pipe
tensioners, and the pipe end moves out from the vessel down to the J-tube. Once the
mating is complete, the vessel can start to move ahead, laying pipe as she goes.
During the pipe laying operation, the vessel will be moving ahead under DP control in steps
equal to the joint length, often 24 metres. It is vital that these moves are conducted
precisely, rapidly, and with no overshoot and consequent back-up. The DPO must be
provided with effective position reference at all times. Some of the surface, and sub-sea
references are not suitable due to the distances travelled by the vessel, and the limited
ranges available. Dual DGPS is a common facility, backed up by two Taut Wires.
If it should be necessary to abandon the lay operation due to adverse weather, then the
procedure is to use the A & R (Abandonment & Recovery) winch. A temporary lay-down
head is welded to the end of the pipe, and to this is attached the A & R wire. This is passed
down the stinger maintaining tension with the A & R winch just as if it was pipe being laid.
The end of the pipe may be laid on the seabed, and the A & R wire slacked off. The vessel
may then weathervane and ride out the storm with the A & R wire still attached but kept
slack, or alternatively the wire may be buoyed-off. When the weather abates, the procedure
is reversed, the pipe recovered to the stinger, tensioners re-engaged. The lay-down head
can be burnt off and the lay operations continue.
Page 181
J-lay Operations
If the water depth is great, unacceptable stresses and strain levels are imposed on the pipe
during the overbend stage. This can be avoided by using the J-lay technique. In many
operations, the stinger is configured as a tower, angled between the vertical, and up to 20
degrees from the vertical. Pipe lengths are pre jointed into triple or quadruple joints before
being raised to the vertical for welding onto the pipe string.
Reel-lay Operations
This type of operation varies from those described in that the pipe string is prefabricated in
one length at a shore-based factory. The vessel loads the pipeline straight from the factory,
spooling it onto its reel or carousel. The lay ramp is used to guide the pipe onto the drum,
and if the pipe is a rigid steel construction it is pre-radiused onto the reel. This configuration
is commonly used for the lay of flexible pipelines. In these cases the lay spread may be
mobilised onto any suitable vessel for the duration of the contract.
Page 182
Seabed Tractors and Trenchers
Trenchers are used for carrying out pipeline bury operations. One such is the "Digging
Donald" operated by the vessel "Trenchsetter". This vehicle weighs around 140 tonnes,
depending upon how it is configured, and is handled by a gantry over the stern of the
"Trenchsetter". A typical operation involves burying a pipeline previously laid. The pipeline
may be a 20" diameter rigid steel pipe, required to be buried to a depth of 1.5 metres. The
"Trenchsetter" will position herself with her stem over the pipeline, heading 90° across the
pipeline direction. The trencher is prepared and deployed over the stern and lowered to just
above the seabed. Trencher deployment is aided by heave-compensation on the handling
system. The trencher carries cameras, lights and sonar facilities, and can be rotated on its
suspension hooks. Once the trencher is correctly lined up with the pipeline, the position
may be accurately adjusted by extending or retracting the trencher gantry. When location
has been confirmed, the trencher is lowered onto the seabed over the pipeline. The weight
of the trencher remains mostly on the gantry, being controlled by heave-compensation gear
on the gantry. Only about 40 tonnes of the weight should rest on the seabed. The position
reference of the vessel is now transferred to the Trim-Cube sensors on the trencher support
wires, which remain vertical. The position of the vessel is now being controlled by the
movements of the trencher, with the Trim-cube feeding back wire angle data to the DP
system. The DP, in turn, is correcting the position of the vessel in order to keep the trencher
wires vertical. The DP system would be configured with the centre-of-rotation located on
the trencher. Heading can thus be adjusted according to the environment or any other
constraints.
A typical operation of this type would have the trencher vehicle configured for trenching for
a run of one or two kilometres, after which the vehicle would be recovered and reconfigured
with back-hoe blades for a second pass over the same ground to complete the bury
operation. The handling frame that carries the trencher can also be configured to install
protection mattresses, and to lift and clear boulders from the seabed prior to mattress
installation.
Page 183
Rockdumping operations
A small fleet of vessels exists for the purpose of dumping rock on the seabed for a variety
of purposes. They range from large bulk-carrier style vessels, able to carry out precision
bury operations using fall pipes, to smaller deck-loading vessels mainly used for erosion
rectification projects.
The commonest types of vessels are the fall pipe rock dumping vessels. Of mini bulk-carrier
construction, they are fitted with self-unloading hopper and conveyor facilities. A fall pipe
derrick is located at one side of the vessel, fed with stone from a hopper. At the bottom end
of the fall pipe is an ROV built into the fall pipe structure. The ROV has an aperture through
the middle, through which the rock falls, onto the seabed. The ROV is heave-compensated,
and is motorised with thrusters enabling precision positional control. Facilities contained
aboard the ROV may include optical TV cameras, sidescan sonar, lights, and seabed pipe
tracker unit, acoustic transponder and responders, and depth meters.
For a linear operation, such as the protection of a specified length of pipeline, a three-pass
sequence may be adopted. The vessel tracks along a line allowing a rock lay-down
alongside the pipeline. At the end of that pass, the vessel will be traversed a few metres to
the other side of the pipeline; she will then backtrack to the "start" position, laying down
rock on the opposite side of the pipeline to the previous pass. The third pass will be centred
over the pipeline to fill in between the previous two passes. This should completely cover
the pipeline. A comprehensive survey spread of tracking and recording equipment allows
the whole operation to be monitored and controlled from the ROV Control and Survey
shack.
Obviously, the vessel will be under the control of the DP system during all rock dumping
operations. A commonly used feature is the Auto Track function, allowing the vessel to
track accurately along a line defined from pre-set way points. The Track Offset facility
allows the DPO the ability to adjust the tracking by any desired amount to allow for any
mismatch between the listed co-ordinates of the pipeline target and its actual position. The
ROV can be offset itself a limited distance from the vertical, depending on the water depth.
Another function for vessels of this type is to carry out bury operations for pipeline
crossovers, or to bury a pipeline prior to the installation of a crossing line.
Page 184
Dredging operations
Most new dredgers, of whatever type or function, will feature DP capability, the precision
positioning available from DP is an insurance against expensive mistakes. Most dredgers
are of the trailing suction type, and the vessel will move along parallel tracks. The tracks
must be close together in order to provide continuity, but overlap between tracks must be
minimised.
DREDGING
In a DP system optimised for dredger operations, the system's functions measure the
dredging forces, suction pipe elevation and azimuth, and automatically compensates for the
draghead forces. In addition, the DP system handles failure in draghead force
measurements in order to avoid loss of positional control and subsequent damage to the
dragheads.
The Draghead Position Control function, in combination with the Low Speed Auto Track
function allows the operator to specify precisely the track followed by the draghead.
Effectively, this places the vessel Centre of Rotation on the draghead, even though the
position of the draghead may not be fixed relative to the hull. Sensors attached to the
suction pipes provide the system with angular data allowing the determination of the
position of the draghead relative to the vessel at all times. Tension measurements allow the
dredging forces to be directly compensated by the DP system. The tension measurements
allow the DP to calculate the horizontal force, its direction and turning moment. This is to
prevent any possibility of astern movement of the dragheads, which would result in
damage.
Dedicated display pages on the DP system allow the DPO to monitor all vital information
such as draghead speed and forces, vessel speed, heading, position and cross-track error
relative to the way points.
Page 185
Cable lay and repair operations
All modern cable-lay vessels are DP-capable. Cable may be laid by plough or crawler
vehicle. In the case of the former the vessel tows the plough by hawser, and the plough
trenches, lays and covers the cable all in one operation. Plough hawser tension can be fed
back to the DP system for direct compensation. The vessel may use Follow-Sub or Auto
Track techniques as appropriate. If the cable to be laid is a short one (i.e. between two
offshore platforms) then the cable may be handled by a seabed crawler vehicle.
Cable repair operations may involve lengthy search patterns being conducted, then a
period on DP in deep water away from land while the cable is repaired. The use of DGPS
as a position reference is valuable. DP also serves to position the vessel when carrying out
a shore-end connection. Since this is often done in shallow water there may be a problem
obtaining position reference. Again, DGPS serves as a valuable PRS here.
Page 186
Dive support operations
M.S.V. CHALLENGER
DIVING DIAGRAM
SCALE - 1:500
CHALLENGER
CHALLENGER
LOWESTOFT
MOONPOOL
No 3
Thruster
22
24
AIR
DIVING
STATION
10
No 5
Thruster
27
METRES
BELOW
DWL
30
20
MAXIMUM MOONPOOL
UMBILICAL EXTENSION
DEPTH
metres
10
20
30
40
50
UMBILICAL
metres
17
19
24
31
40
26
28
33
40
49
No 3
Thruster
running
No 3
Thruster
stopped
29
MAXIMUM CAGE/WET BELL
UMBILICAL EXTENSION
37
36
DEPTH
metres
30
44
45
40
53
50
10
20
30
40
50
UMBILICAL
metres
22
25
32
39
48
28
31
38
45
54
No 5
Thruster
running
No 5
Thruster
stopped
DP/SIM/D/C/008.01
Page 187
There are three modes of underwater operation. Up to a depth of 50m the technique is
"air diving", i.e. the diver's breathing gas is compressed air. The diver may be deployed
from a basket over the side of the vessel, or from a wet-bell, or from a mini-bell. The latter
two methods represent increased safety to the diver in the most hazardous diving zone.
One requirement of the air-diving set-up, from a DP vessel, is that the amount of umbilical
the diver may be given, measured from the tending point (basket or bell) must be no more
than 5m less than the distance to the nearest thrusters. This is to ensure that the diver
cannot be drawn into a thruster.
Below 50m the diver must be deployed from a diving bell and his breathing gas will be a
helium/oxygen mix (Heliox). The diving bell maintains the diver at the pressure of the
working depth, and mates with a hyperbaric complex on board the vessel. The divers live in
this hyperbaric chamber, also maintained at the pressure of the working depth, for up to 28
days, travelling "to work" in the diving bell. This technique is known as "saturation diving".
The bell is usually deployed through the moonpool, an open well in the centre of the vessel.
A typical "bell run" would consist of three divers (two swimmers and a bell-man) operating
for an eight hour shift. The swimmers are provided with all gas, hot water for heating, and
communications through umbilicals connected to the bell and ultimately to the vessel.
Emergency supplies of breathing gas are carried on the bell, and each diver has a bale-out
bottle on his back - for dire emergencies only. The bell-man remains in the bell to tend to
the swimmer's umbilical and to assist in an emergency.
At present the practical limit for bell diving is about 400 - 450m. At greater depths than this,
ROV or a diver in an Atmospheric Diving Suit (ADS) must do the work. ROVs (Remotely
Operated Vehicles), or unmanned submersibles may be very sophisticated units able to
operate a wide variety of tooling, sensors and other instrumentation.
Atmospheric Diving Suits are used to place a diver in very deep water. The suit is a
pressure vessel, with air at atmospheric pressure inside. The diver is able to operate claws,
manipulators, tooling etc., to carry out a wide variety of operations. He is not subjected to
the pressure of the working depth so has no breathing gas and decompression problems.
Several different types of ADS are in use; some, like the "Newt Suit" have legs so the diver
can walk, others are fitted with thrusters allowing the diver to "fly".
Many DSV operations entail positioning the vessel close to a platform structure, enabling
the divers to access the structure, wellheads etc., for inspection and maintenance. In some
vessels it is possible to operate two diving "spreads" simultaneously. Each diving operation
is under the control of a dive supervisor, operating from the dive control cabin. The dive
supervisor has all the communications and bell control to his fingertips, and he is able to
start or stop a dive at any time. The safety of the divers is his responsibility. The dive
supervisor for each shift is under the overall control of the Dive Superintendent. There will
be a dive status/alert system in force, with green, flashing amber and flashing red status'
and alarms. Green status is 'normal'; dive able to proceed. Flashing Amber alarm indicates
a degraded status; divers to return to the bell and obtain a seal. Flashing Red alarm
indicates a dive emergency status resulting from a vessel loss of position; divers to return
to the bell and be recovered to the surface.
Before engaging in diving operations from a DP DSV, the DPO must ensure that all his
systems are functioning correctly with full redundancy available in all areas. The vessel
Page 188
must be stabilised on her location, and must have had at least 30 minutes to settle and
build the mathematical model into the system. All pre- DP and pre-dive checklists must be
complete and the status board marked up. Warnings should be promulgated in the vessel
and on the platform to inform that diving operations are taking place. In many cases the
diving operation will need to comply with the permit-to-work system. The bridge team need
to appraise themselves of the number of divers in the water, their location, depth and work
situation, particularly whether they are working on an open seabed or in a tightly constricted
site, also the number of the emergency transponder on the bell, and that on the ROV.
All diving operations are hazardous but some more so than others. Shallow water air diving
from DP vessels is particularly difficult; the proximity of the thrusters, the amount of noise
from thrusters and the strength of tide all providing problems which must be overcome.
Sometimes diving operations may only take place at slack water, when the tidal rate is less
than about 0.9 knot. Under these circumstances the in-water time is precious and the DP
vessel must be fully prepared for each dive window.
Diving is particularly hazardous in the vicinity of underwater mooring lines and anchor
chains. Sometimes an anchored pipe lay barge is moving slowly ahead under the
control of its anchor wires. Because these wires are moving it is not possible for a DSV to
set up within the catenary due to the hazard of fouling the bell cables on the mooring lines.
It is necessary for the barge to stop before divers can be deployed on the stinger. A further
hazard to divers working from DP DSVs is the presence of the Taut Wire position reference.
It is possible for the diver to pass clear of the wire on his way out to the worksite, and to
return on the wrong side of it. This would prevent the diver from returning to the bell.
Usually this problem is averted by the deployment of the Taut Wire system on the side of
the vessel away from the worksite, but otherwise it may be possible to mark the wire with a
light-stick.
Page 189
Another particularly hazardous technique is Habitat diving. A Habitat is a compartment
placed over a seabed operation, like an inverted shoebox. Gas is pumped into the habitat
until the interior is de-watered and dry. The divers enter the habitat, either by locking the
bell directly onto it, or from the water, accessing from underneath. The habitat may be
located on a joint in a pipeline needing welding. However, the operation is totally dependent
upon the presence of the support vessel above. If this is a DP DSV, and she has a run-off
then the habitat may be dragged with consequent major survivability problems for the
divers.
Page 190
Survey and rov support vessels
Vessels of this type may perform a multitude of tasks from hydrographic survey, wreck
investigation, underwater recovery, site survey pre-lay (pipe or cable), installation
inspection and maintenance. Although the task itself may indicate a relatively nonhazardous job, it must be remembered that the location itself may imply considerable
hazard. A location up-tide of a fixed installation, or in the "blow-on" position may be
unsuitable for a non-redundant vessel. An ROV may be deployed direct from a gantry or
`A' frame at the side or stern of the vessel, or from a Tether Management System
incorporating a cage or garage. If deployment is directly overside then great care must be
taken to ensure that the umbilical does not foul the thrusters or propellers. Communication
with the vehicle will generally be via acoustic responder, with the interrogation signal
travelling down the umbilical. This is more secure than using transponders, as the vehicle
is usually acoustically noisy. At least one transponder should be located on the vehicle in
addition to the responder or the responder should have an emergency battery, in case the
vehicle becomes lost due to a parted umbilical.
ROV TETHER MANAGEMENT SYSTEM
ROV DECK UNIT
HOIST
CABLE
TETHER
ROV GARAGE
WITH TETHER
REEL
ROV
ROV CARRIES
RESPONDE
Page 191
The Follow Target mode enables the vessel to automatically follow a moving target and
keeps the vessel within a “position window” relative to the target. In Multi Target mode it is
also possible to keep the vessel heading within a “heading window”.
The moving target must be equipped with a mobile reference transponder or laser
reflector in order for the SDP system to monitor its relative position. If, for example, the
moving target is a Remotely Operated Vehicle, then the vessel must be equipped with a
Hydro-acoustic Position Reference (HPR) system in order for the SDP system to monitor
its relative position.
In addition to a mobile reference transponder on the target, an additional fixed positionreference system (such as DGPS) is required. Alternatively a fixed transponder (deployed
on the seabed) can be be used.
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Shuttle tanker
Offtake or shuttle tankers have made use of DP capabilities since 1981. The requirement
arises out of the problems associated with the export of crude oil from offshore fields. Many
fields export via pipelines but this option is an expensive one.
If circumstances do not allow the provision of a pipeline, then another export method must
be found. The commonest method is to provide a system of offshore storage for oil, and
one or more tankers to transport the oil to a destination. The problems arise when
considering loading arrangements in the more environmentally exposed locations in the
North Sea and elsewhere. In bad weather, a tanker moored to a buoy or tower arrangement
will impose heavy loads on the mooring lines and terminal structure.
SURFING
YAWING
FISHTAILING
COMBINED WITH HEAVY
SWELLS PLACE EXCESSIVE
LOADINGS UPON HAWSER
AND TOWER STRUCTURE
In a mooring system, the vessel movements iving rise to problems are fishtailing, yawing,
surfing and heave motions. With the exception of the heave component, these vessel
movements may be stabilised or controlled by the use of DP techniques. Accordingly, tankers
intended to load at Offshore Loading Terminals (OLTs) will be fitted with systems very similar
to those in any other DP-capable vessel, but configured specifically for the offshore loading
function.
The installations, which need to support offtake tanker loading, vary from field to field.
Some loading terminals consist of CALM buoys (Catenary Anchored Leg Moorings). Other
installations are Spar buoys, which are large floating tower structures moored by a spread
of mooring lines. Spar buoys usually carry a rotating turntable at the top to handle vessel
moorings and hose handling equipment. Examples of Spar installations include the
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Kittiwake Spar in the North Sea. Another configuration is the Articulated Tower, which is
connected to a seabed located gravity base by means of a universal joint. Yet another
configuration is the rigid tower, which like the Spar and Articulated configurations, exports
through rotating turntable heads. The F3 OLT in the Dutch sector is an example of a rigid
tower export terminal.
OFFSHORE LOADING TERMINAL CONFIGURATIONS
ARTICULATED
TOWER
CALM BUOY
SUBMERGED
TURRET
LOADING
TURRET
DOCKING
PORT
FIXED
TOWER
SPAR
UKOLS
LOADING
BUOY
HOSE
A facility which does not have any surface hardware shows greater levels of vessel safety
than the above arrangements. The UKOLS facility in the Statfjord field has a long loading
hose connected to a mid-water buoy. The buoy is positively buoyant and is moored at a
fixed depth, above a gravity base housing the Pipeline End Manifold (PLEM). Vessels using
this facility have no need for a mooring hawser; the only connections to the buoy consist of
the hose. Hawserless systems are common nowadays, being used in a variety of OLT
configurations. A more recent development is the Turret Loading system, where the loading
connections are located in a neutrally-buoyant turret. The turret is mated into a docking port
built into the forebody of the vessel, and carries the flow line connections to the vessel.
Once locked into position, the vessel is able to weathervane around the turret.
Shuttle tanker arrangements
Broadly speaking, shuttle tanker operations may be divided into four groups; systems with
hawser moorings, hawserless systems, STL systems and those vessels configured to load
directly from FPSO installations (Floating Production, Storage and Offtake vessels).
The earliest offshore loading arrangements feature hawser moorings as part of the system.
The hawser is regarded as an insurance against failure of the DP capability. The mooring
arrangement consists of a heavy mooring hawser, comprising a nylon grommet, secured to
the OLT at its standing end, with a short length of chain next to the stopper plug. The chain
is provided as an anti-chafe measure where the mooring runs through the fairlead or roller.
Page 194
Located on the foredeck is the stopper with hydraulic, remotely controlled latching for the
hawser plug. Immediately abaft the stopper is the traction winch, which handles the heavy
messenger. The messenger is spooled onto a take-up storage drum after coming off the
traction winch. The messenger is streamed out from the OLT prior to the arrival of the
vessel by work boat, or by the previous tanker on the terminal laying it out in the required
direction. The messenger is buoyant, but is provided with buoys and lights for visibility.
Recovery of the messenger may be assisted by the work boat, or may be direct from the
bow of the tanker. Once the messenger has been recovered, the vessel is manoeuvred
towards the OLT location, picking up slack in the messenger all the time. During this stage
the vessel control may be under the DP system, or may still be in Manual control. If the DP
system is being used at this stage, then preparations will have been made beforehand,
transferring control to the DP console, acquiring thrusters and references and proving all
correct by means of checklist procedures.
Some vessels have all control elements located in the forebridge, with a transfer procedure
laid down for taking control forward from the main bridge. In other vessels, the DP system
is located in the mainbridge.
The DPO will select the correct OLT location in the DP system menu ("Select buoy"). This
allows the DP system access to the details of the OLT being approached. Each OLT has
different criteria regarding position co-ordinates, position circle and alarm limit
specifications, Position reference availability and offset values, and movement
characteristics of the OLT.
OLT APPROACH AND POSITION CIRCLES
All radii values
shown are for
illustration only
OUTER LIMIT at 74m
(ESD 3)
INNER LIMIT at 64m
(ESD 1)
INNER ALARM at 37m
(ESD 3)
VESSEL IN AUTO-APPROACH
ON WEATHERVANE HEADING
OLT
POSITION CIRCLE at 48m
POSITIONING CIRCLE DEFINED
BY OPERATOR - BETWEEN 44m AND 52m
SETPOINT RADIUS CIRCLE ADJUSTED BY DPO
TO EFFECT APPROACH IN 'AUTO APPROACH' MODE
The vessel may be placed into DP control for the whole of the approach operation, using
the "Auto-Approach" facility. In this case the heading of the vessel is set to the
Weathervane function, and the DPO inputs the radius of the set point circle. This circle will
be gradually decreased in radius by the DPO, in steps to allow the vessel to approach the
Page 195
OLT under control. The Set Point Circle is the locus of the bow reference point of the
vessel. The position on that circle that the vessel is being controlled towards is determined
by the weathervane heading, keeping the OLT location directly ahead. The speed of the
vessel is under the control of the DPO, so by using these facilities a totally controlled
approach to the OLT location may be made.
Once the hawser is latched into the stopper and the vessel is located onto the Set Point
Circle of the designated radius for that OLT location, the system may be transferred to the
"Mooring" mode. In this mode, the position of the vessel is constrained to the defined set
point circle, on the calculated weathervane heading. Loadcells in the bow stopper assembly
measure the hawser tensions and feed this data back to the DP system for automatic force
compensation. This is an important function, since if these measurements were not
available, then hawser tension would be interpreted by the DP system as a current, which
would adversely affect the calculation of the weathervane heading. Any error in the
calculation of weathervane heading could lead to an unplanned breakaway from the OLT.
The same considerations apply to the wind sensor input to the DP system. If the wind
values input is significantly in error, due perhaps to wind shadow from the OLT itself, then
the weathervane heading may be incorrectly calculated.
Once the hawser has been stoppered, the hose is connected. Usually, the hose is
connected to the end of the hawser, and is handled automatically by hydraulic actuators to
locate it into and secure it to the hose coupler.
ARTEMIS
BEACON
LOADING
TERMINAL
LOADING
HOSE
ARTEMIS
MOBILE
ANTENNA
MOTION CHARACTERISTICS OF BOTH
VESSEL AND TERMINAL CONTAINED
IN MATHEMATICAL MODEL
FOREBRIDGE
MOORING
HAWSER
HPR TRANSDUCER
TUNNEL
THRUSTERS
TUNNEL
THRUSTER
MAIN PROP
(C.P. CONSTANT
R.P.M.)
RUDDER
AMIDSHIPS
OR ACTIVE
HPR TRANSPONDERS
VESSEL IN CONTINUOUS
"AUTO HEADING SELECT"
DP SYSTEM LOCATED IN
FOREBRIDGE WITH SLAVE
SYSTEM ON MAIN BRIDGE
HAWSER TENSION MEASURED BY
STOPPER LOADCELLS AND FED
TO DP SYSTEM FOR COMPENSATION
AS AN EXTERNAL FORCE
DESIRED HAWSER TENSION
SET BY DPO USING THE PROPELLER
BIAS INPUT CONTROL. DP SYSTEM
ADJUSTS SURGE VALUE TO MAINTAIN
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The DPO may adjust the surge value of the vessel position by inputting a new value of Set
Point Circle radius. With the system in "Mooring" mode, there is a preferred radius, with a
small amount of leeway for the DPO to make adjustment, i.e. Designated radius 44m, with
a DPO input adjustment of 44 - 49m. In bad weather conditions, the separation distance to
the OLT may thus be increased marginally. Alternatively, the adjustment may be by a
function known as "Propeller Bias", in which the DPO is able to input plus or minus values
in tonnes, which are changes required to the hawser tension. The DP system will then
automatically adjust the surge (i.e. set point circle radius) in order to achieve the input
tension value.
The vessel must be ready to break off the operation at any time and get underway. To this
end, three emergency status are defined and alarmed; ESD1, 2 and 3. ESD stands for
Emergency Shutdown and Disconnection. The vessel and field operating handbooks will
specify the criteria under which any ESD status is raised, and the accompanying actions.
Often the OLT is unmanned, and the vessel initiates ESD functions by means of a simple
bridge-mounted selector switch. Local (on-board) controls carry out the necessary ESD
functions in the vessel, while off-vessel functions are carried out by telemetry control. In
general ESD1 results in the export pumping being stopped and key valves being closed in
readiness for a breakaway. ESD2 initiates the hose being uncoupled; also a drencher
system will envelop the bow area with water spray reducing the risk of explosion or fire from
any sparks that could be struck. ESD3 initiates the unlatching of the hawser. Vessels
operating without hawser connections will not have an ESD3 position on the switch
Hawserless OLT arrangements
WATER DEPTH - 42m
NO HAWSER
REMOTE POWER ROTATION ON
POSITION REFERENCES - DGPS, ARTEMIS & HPR
ARTEMIS BEACON
ALL DP AND CONTROL
EQUIPMENT PLACED
ARTEMIS ANTENNA
IN BOW BRIDGE HOUSE
CARDISSA
LOADING HOSE
TWIN TUNNEL
THRUSTER
HPR TRANSDUCER
STERN TUNNEL
THRUSTE
MAIN
PROPELLE
AND
HPR TRANSPONDER
The installation described above is for a vessel using a mooring hawser. In fact, the hawser
is not only a redundant element, it provides added complications. Its tension values need to
be measured and directly compensated for. The function of the hawser is mainly that of
positioning insurance.
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More modem OLT installations are designed as hawserless. The only physical connection
to the OLT is the hose, and this is kept slack at all times. This simplifies the business of
approach and connection, and the DP system does not have the complication of having to
deal with the external force represented by hawser tension
While on DP loading operations, the DPO will monitor the turntable azimuth and compare it
with the heading of the vessel. If there is significant misalignment, he can rotate the
turntable by telemetry, by using a "left/right" control in the bow house. If the misalignment
exceeds 20° then an automatic ESD is initiated.
Submerged turret loading operations
A small number of DP-capable shuttle tankers are designed for DSL (Direct Shuttle
Loading) operations from a submerged turret. Fields configured for this type of operation
includes the Harding and Heidrun fields in the Norwegian sector of the North Sea.
In the Submerged Turret Loading system (STL), a submerged buoy is secured to the
seabed by means of a catenary mooring system. The buoy is connected to the loading
pipeline from the production platform, via the Pipeline End Manifold (PLEM) adjacent the
buoy location on the seabed. The buoy will be located by means of an acoustic transducer
installed in the forepart of the vessel, and a transponder on the buoy. These are linked to
the HPR system interfaced as position reference to the DP system on the bridge. Once the
messenger and turret
POSITION REFERENCE
FROM HPR & DGPS
RETRIEVAL LINE
RECOVERED BY
TANKER. MESSENGER
LED TO LOADING CONE
TURRET LOCATED AND
DOCKED INTO CONE.
VESSEL INTO THRUSTER
ASSISTED MOORING
WITH AUTOMATIC
WEATHERVANING
EXPORT RISER
LOADING CONE
SUBMERGED TURRET
MOORING LINES
TRANSPONDER
PIPELINE END MANIFOLD (PLEM)
Page 198
wire have been recovered, the buoy will be raised up until it is locked into a mating cone in the
double bottom space forward. Once locked, the loading connections can be secured and
loadings commence. The vessel is able to rotate around the turret without placing any
positional loads on it. All position and heading control is from the DP system and the
thrusters/propeller.
Position reference is obtained from DGPS and LBL acoustics. The buoys are located by
means of USBL acoustics, as are the MLBEs (mooring line buoyancy elements) for monitoring
purposes.
FPSO unit operation
Floating Production, Storage and Offtake units (FPSOs) consist of a floating unit, either shipshape, or semisubmersible, containing all the facilities for producing crude oil, and sometimes
gas.
A number of FPSO units are of the monohull configuration, with Turret Mooring facilities. With
this arrangement, the FPSO is positioned by means of an array of anchors. The mooring lines
are handled by the turret, which is a large circular centre-section of the vessel's hull located
amidships or forward of amidships. The FPSO is thus able to weathervane around the turret
maintaining her heading into the weather conditions and sea state. Arrangements vary from
installation to installation. Some FPSOs rely totally on their mooring spread for positioning, with
heading control effected simply by allowing natural weathervaning. In a number of FPSOs,
however, a heading-assist function is provided by thrusters. An added complication is the
limited range of heading change available. It may be possible to allow heading to change a
maximum value (e.g. 270°) either side of the base heading. This is especially the case where
the FPSO is handling more than one riser.
Loading Operations From FPSO Units
TURRET-MOORED
FLOATING
PRODUCTION
VESSEL
FPV USING
THRUSTER-ASSISTED
MOORING ON A
WEATHERVANE
HEADING
SHUTTLE
TANKER
RISERS
MOORING
Page 199
With any FPSO/offtake tanker operation, the tanker will experience a more severe positioning
problem than with more conventional operations. A number of factors contribute to this
problem, not the least of which is the harsh environment on some of the more exposed FPSO
locations
In addition to problems associated with the environment, offtake tankers will have other
difficulties in maintaining station on a moving target. The offtake vessel must keep position
within a circle defined by the length of the loading hose. Most of the FPSO/tanker operations
are hawser-assisted.
The base position is the hose terminal point on the stern of the FPSO, and the tanker must
maintain position on a circle around this base position, with the expected minimum and
maximum warning and alarm criteria established. But, the stern of the FPSO is not a fixed
location. The mooring and positioning system in the FPSO allows a degree of movement,
especially in deep water. Further, the FPSO may be continually weathervaning, so the stem
may be moving laterally, relative to the tanker. The base location for this movement is the
turret axis, and since this is located forward of amidships, or even at the bow, the stem may
exhibit considerable and rapid movement. Surge and fishtail movements may result in up to
30m of movement in the reference position in the FPSO. The most critical stage is often when
an almost fully loaded shuttle tanker is working with an almost empty FPSO.
The vessel's positioning strategy will depend upon the characteristics of the FPSO
configuration. The set point position circle is set at a defined radius to suit the loading
arrangement. The set point circle is centred upon the reference position, which is the hose
boom end on the FPSO. This position itself is subject to considerable movement, but the use
of a relative position reference should reduce problems in this area. The required position on
the set point circle is determined by the heading of the vessel. This heading may be a
calculated weathervane heading, calculated by the DP system in the tanker without reference
to the FPSO, or it may be the FPSO heading obtained by means of telemetry. In the latter
case the two vessels should maintain the same heading at all times. Further limitations arise
due to the maximum allowable angular offsets between the alignment of the hose and the
heading/position of the tanker.
For positioning, vessels of this type will use a relative GPS (DARPS) position reference as the
prime positioning aid, yielding position information reduced to range/bearing data from the
FPSO terminal location. The other preferred position reference is Artemis, with an Artemis
fixed station located on the FPSO, the mobile station located on the tanker. A weak link
identified in these arrangements relates to the FPSO gyro. This heading data is transmitted to
the tanker via the DARPS uhf link, meaning that the operation of the DARPS and input
heading are both reliant upon the one uhf link; if this link is lost the whole positioning strategy
is jeopardised.
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Offshore support vessel operation
Supply Vessels alongside FPSO/Rig
By using both the a laser/radar system and the DGPS/-Glonass system there can be a
disagreement between PRS’s due to the FPSO/Rig is moving in her chains.
The laser/radar system will tell the DP to follow the FPSO/Rig and on the other hand the
DGPS/-Glonass system will tell the DP to stay in global grid.
If the OSV only are using one target in a laser/radar system there is a risk that the OSV and
the FPSO can have a collision when the FPSO is changing heading due to the weather.
Multi targeting
The Follow Target mode with Multi Targeting enables a DP vessel to follow a moving structure
or vessel equipped with two or more mobile transponders. The moving structure can be
followed both with respect to position and heading.
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Riser angle monitoring
Riser Angle Monitoring (RAM) allows optimised vessel position during drilling operations. The
various functions are described in the following subsections.
Text in capitals refers to items in Figure 1.
The BOP is a fixed structure on the seabed. The angle between the centre line of the BOP
and a vertical line is called the BOP ANGLE (also known as the riser reference angle). This
angle is either measured by sensors attached to the BOP or specified manually.
The riser, which is assumed to be a rigid structure, is connected to the vessel at the UPPER
FLEXJOINT, and to the BOP at the LOWER FLEXJOINT. The RISER ANGLE, at the lower
flexjoint, between the riser and a vertical line, is measured by sensors attached to the riser.
The RISER DIFFERENCE ANGLE, at the lower flexjoint, between the riser and the centre line
of the BOP, should be kept to a minimum to reduce mechanical wear. This angle is zero when
the vessel is located at the ADVISED RISER POSITION.
The sensors may be either Electrical Riser Angle systems (ERA) or Acoustic Riser Angle
systems (ARA).
Page 202
The monitoring is based on measurements of the RISER DIFFERENCE ANGLE. To provide
redundancy in the angle measurements, there are usually at least two riser and two BOP
sensors. The riser sensors and BOP sensors are generally configured in pairs, each pair
being mounted in corresponding monitoring positions on the riser and BOP. The RISER
DIFFERENCE ANGLE, and therefore the ADVISED RISER POSITION, is
computed for each riser/BOP sensor pair. Each sensor pair is given a name (such as Yellow
and Blue).
The BOP and riser sensors usually provide measurements in twodifferent coordinate systems.
Both coordinate systems have their origin at the lower flexjoint, with the Z axis vertically
downwards, but the X and Y axes may be rotated.
For each BOP sensor, you must specify the orientation of the sensor coordinate system in
relation to North (unless the sensor measurements incorporate a compass measurement).
For each riser sensor, you must specify the orientation of the sensor coordinate system in
relation to the coordinate system of the corresponding BOP sensor or the manually-entered
BOP position.
The sensors may provide measurements in the following forms:
Inclination (angle from the vertical) in the X direction, and inclination in the Y direction.
Inclination in a specified direction (bearing) in relation to the X axis.
To allow for inaccuracies in the installation of the sensors, you can specify bias values for the
angle measurements.
Position Advisory function
The Position Advisory function uses the riser angle measurements to calculate the vessel
position that will bring the RISER DIFFERENCE ANGLE to zero.
The Position Advisory function gives one zero-angle position for each interfaced and enabled
angle sensor set, and also a composite position which is calculated using the average of the
accepted sensors selected for composite zero-angle-position calculation.
Emergency Joystick Advisory (EJA) function
The Emergency Joystick Advisory function (EJA) can be activated in Auto Position (during
position dropout) and Joystick modes. The EJA provides a graphical indication on a relative
Posplot view, of the direction in which joystick thrust must be applied to move the vessel
towards the advised position. This
position is calculated by the Position Advisory function.
Hold Flexjoint Angle (HFA) function
When this function is active, the system will try to position the vessel to retain the lower
flexjoint angle as it was when the function was entered. Based on measured riser angles and
estimated riser angle response to vessel movements, the HFA function will adjust the vessel’s
position setpoint to maintain the lower flexjoint angle.
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Chapter 15 – Power Systems
Power generation, Power management and distribution systems
As we have mentioned before, power requirements in DP vessels are often much higher than in
conventional ships. The commonest type of power installation is diesel-electric, with all thruster,
power and services supplied from electric motors. Direct-drive diesel is not suitable due to the
uncertainties in starting and stopping. It is, however, possible to apply direct-drive diesel to a
controllable pitch thruster, with all thrust control obtained from blade pitch angle. A few DP
vessels are equipped with individual direct-drive diesels coupled to each thruster, the propeller
in each case being c.p.
Diesel engines
Diesels are frequently used to provide power to turn propeller shafts, often through clutches
and couplings, or to drive electrical alternators in diesel-electric installations. In vessels where
main diesels are used to drive propeller shafts, auxiliary diesel engines are provided to
generate electrical power, otherwise the vessel may utilise Shaft Alternators for the generation
of electrical power. Diesels usually run on Diesel Oil or Gas oil. In general, DP vessels do not
run on Heavy fuel oil.
Thruster motors are usually constant rpm a.c. motors driving c.p. propellers, with all thrust
control provided by the pitch control. A more recent development is the synchro-motor drive,
in which a synchro-converter provides a variable frequency voltage supply to the a.c. motor.
Varying the frequency has the effect of varying the r.p.m. of the motor, thus this type of drive
is suitable for DP installations with fixed-pitch propellers. This type of has installation virtually
superceded the cp propeller. Almost all new-build DP vessels having a diesel-electric power
plant feature fixed-pitch propellers and variable-frequency drive.
In a diesel-electric installation, a number of generators provide power to a switchboard on a
"power station" basis. Typically the voltage generated in a diesel-electric installation is hightension, e.g. 6 kV or 6.6kV, although some large drillships run at voltages up to 12 kV. Main
and auxiliary switchboards run at 440V or 240V with power fed from the HV switchboard via
transformers. The generators are driven by diesel engines, each of which should be provided
with independent services such as fuel, cooling and lubrication. The provision of a number of
generators lends itself to the principle of redundancy. Failure of one generator will leave a
number of others on-line, and normal margins of working should ensure that loss of one
generator does not result in an emergency status. The number of generators running can be
changed to match the power requirements. In favourable conditions, fuel can be conserved
by shutting down generators, keeping them ready for instant start-up upon deteriorating
conditions.
Switchboards
Beside the direct driven propellers all power generated on board has to go through a
switchboard. In order to provide redundancy there are at least two main switchboards and one
emergency switchboard.
Different equipment is connected to either one of the main switchboards and normally in a
way so as to provide greatest redundancy, e.g. Bowthruster 1 on switchboard No. 1 (BUS 1)
and Bowthruster 2 on switchboard No. 2 (BUS 2). The same consideration gapplies with for
radar 1 and radar 2, and again fuel pump 1 and fuel pump 2, etc.
Page 205
For each equipment there are a fuse protecting the switchboard if there is a short-circuit
somewhere in the system, like the power line to the deck crane. The fuse will then cut off the
power on this line and also avoiding loss of power to the place with the short-circuit.
Between the two main switchboards BUS 1 and BUS 2 there is a big switch, the BUS Tie
which enables the two switchboards to be linked together. In DP operations this Tie switch is
normally open (Not switched), and the ship can still have power on one switchboard if there is
a problem on the other. For Equipment Class 3 operations, it is a requirement that the
switchboards be isolated, i.e. the Bus Tie be open. For Equipment Class 2 operations, the Bus
Tie may be open or closed.
In DP operations which are deemed not to be safety-critical the switchboards may be linked
together with the Bus Tie closed, and with only one generator running the ship can still run all
equipment, whether connected to Bus No. l or No. 2.
In DP operations a situation might arise where there are three bow thrusters. Nos 1 and 3 are
connected to Bus 1, while bow thruster No 2 is connected to Bus 2. If Bow thruster No 1 OR 3
fails, the vessel may still have sufficient forward thrust and redundancy allowing the operation
to continue. If, however, bow thruster No 2 was to fail, the vessel is no longer redundant. The
subsequent loss of Bus 1 would leave the vessel with no thrust capability forward. The DP
operator has to decide whether he can continue the operation or not, and under these
circumstances he must decide to abort the operation.
Many vessels utilise combined diesel direct drive and electric drive. Arrangements vary, but
often there will be twin screws, controllable pitch in a conventional arrangement, together with
a variety of thrusters. The screws will be driven at constant r.p.m. by one or more diesels, also
driving shaft alternators providing electrical power onto the switchboard. Additional electrical
power is provided by diesel alternators. Thrusters are electrically powered, or driven by their
own independent diesels.
A common (simplified) setup for direct driven vessel with shaft generators.
Power management
Central to the concept of safe operation and redundancy is the monitoring of available power.
For the purposes of safety, DP operations should not be carried out when power demand
Page 206
exceeds 80% of power available, or when output from any individual thruster exceeds 80% of
its maximum thrust. In general, there should be sufficient power available to provide current
demand with a spinning reserve equivalent to one generator. This protects against critical
situations arising as a result of the loss of one generator. In cases where one or more
generators are stopped and on stand-by, "auto start" facilities are provided to automatically
start and bring on-line those generators at some pre-set limit of available power. Alarms are
provided within the DP system when the power limits are reached and exceeded.
Power Management Systems (PMS) are intended to ensure that critical power shortages or
blackout situations are avoided. A simple power management arrangement is a form of
Blackout Prevention, ensuring that circuits are tripped off the switchboard under overload
conditions. More complex systems contain a number of levels of load shedding. At a
predetermined load value "start-blocking" will be initiated on large motors. This is to ensure that
a blackout is not inadvertently tripped by the action of starting a motor when there are
insufficient reserves. The starting current on a motor is far in excess of its full-load running
current. Load shedding will occur as power reserves dwindle. Circuits will be dropped off the
board in reverse order of importance. The DPO needs to be familiar with the load shedding
routine, as he needs to know what reserves he has available after a power shortage problem. In
some vessels, the power management is poorly organised such that, after tripping, the thrusters
are still in the system but the thruster control system has tripped out. Thus the thrusters are still
running but cannot be controlled!
For operations under Equipment Classes 2 and 3, the level of redundancy required is such that
the power available for position keeping should be sufficient to maintain position subsequent to
worst case switchboard failure, i.e. the loss of one complete section of switchboard and the
generators supplying it. Vessels of Equipment Class 2 may have busbar sections connected by
bus tie breakers, but these breakers must separate automatically upon overload or short circuit
failure within one section. Vessels of Equipment Class 3 must operate with bus tie breakers
open, with each section of busbar isolated from the remainder. As technology develops, more
and more systems are intended for operations with a closed bus bar, but the industry has not
yet fully accepted class 3 operations with a closed bus bar.
It is essential that the various switchboard protection devices, intended to prevent switchboard
failure resulting from overload, over-volt or reverse-current conditions, are properly maintained
and regularly tested.
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Electric system single line diagram from Maersk Achiever
Page 209
Gener ator #4
Gener ator #3
Gener ator #2
Gener ator #1
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
Thruster #4
Thruster #3
Thruster #2
Thrus ter #1
Thrus ter #5
Thruster #6
Thrus ter #7
Thrus ter #8
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
11 kW
Ge nerator #5
Ge nerator #6
Ge nerator #7
Generator #8
Simplified electric system single line diagram from DSS 21 semi sub
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Chapter 16 – The UPS system
UPS (Uninterruptable Power Supply)
Part of the power system to the DP system is the UPS which is a battery pack supplying
continently power to vital equipment such as DP Control unit, Position Reference Systems and
Environmental Sensors.
As the power is limited e.g. the LTW can not run on the UPS but DGPS, HPR etc. have less
consumption and is therefore connected to the UPS .
Due to the required redundancy in class 2 and 3 vessel the different DP Control unit / PRS /
Sensors are connected to different UPS.
The electrical power onboard are generated on a number of generators and switching between
them on the switchboard can cause minor disruption in the power, and a few seconds without
the right voltage can result in one of the system mentioned above can drop out.
Here will the UPS provide a steady current to connected equipment.
Be aware of that the UPS is build for a certain power consumption and when changing
equipment or adding new equipment which exceed the power the UPS is build for can damage
the UPS, and cutting the power to all connected equipment.
DNV requirement for UPS is that the battery installed for each UPS is to be able to provide
nominal output power for 30 minutes after loss of charger.
DNV requirement for numbers of UPS
AUTS (0)
0
AUT (1)
1
AUTR (2)
2
AUTRO (3)
2 + 1 in separate compartment
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Chapter 17 – Shallow / Deep Water and Tides
Shallow water
DP-capable vessels experience a number of problems when operating in shallow waters.
Depending on vessel function (e.g. diver support, pipe laying, cable laying, crane barge, etc.)
these problems may hamper or stop the vessel from working.
Positon reference systems
Any subsea position reference system is vulnerable to problems in shallow water.
A Taut Wire system will have limited range due to the relatively short vertical side of the triangle
and the small (about 35 degrees) maximum wire angle available.
A DP cable laying vessel making the shore end connection may only have a few metres under
the keel. Other position reference systems may not be available, and a Taut Wire has a
maximum range under these circumstances of only two or three metres.
MOONPOOL TAUT WIRE
SHORTER RANGE
IN SHALLOW
WATER
NO BILGE KEEL LIMIT BUT
VERY LIMITED RANGES
IN SHALLOW WATER
BILGE
KEEL
LIMIT
LONGER RANGE
IN
DEEPER WATER
Since this is roughly the limit of the position-keeping capability of the vessel, the Taut Wire
sensor will soon reach its angular limit, at which point the DP system will reject it as a position
reference.
The bilge keel limit makes this problem worse in a conventional monohull vessel. A moonpool
Taut Wire may be even more limited since, although avoiding the bilge keel limit, the wire
sensor and origin point is closer to the seabed, being located at keel level.
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Underwater acoustic position reference systems (HPR) are also particularly vulnerable in
shallow water. When it is considered that the transducer stick out some 4 to 5 metres below the
keel, and the transponder is located at up to 5 metres above the seabed, then the vertical
separation between the two becomes considerably less than the water depth. Since HPR range
is largely dependent upon vertical separation between transducer and transponder, then this will
be adversely affected in shallow water. Also there will be proportionally more noise and
interference from the vessel and its operation in shallow water.
The above factors are worsened if the shallow water area experiences strong tide, as is
frequently the case. A strong tide will cause the Taut Wire to bow, resulting in inaccuracy.
This, in itself, is will get worseif quantities of kelp are allowed to foul the wire, again, frequently
the case in shallow water. In strong tides the vessel's thrusters will be working harder,
providing more noise and air bubbles to further reduce the reliability of the HPR signals.
For the reasons given above, DP operations such as diver support, requiring full redundancy,
should use at least one surface position reference system within the spread of (minimum
three) systems in use, particularly if the divers are working inside a jacket or other structure.
This is less important if the diver is working in an open-water location, where a spread of, say
two Taut Wires and an HPR is satisfactory. The DP operator should have a further position
reference system available and ready to deploy if one of the existing systems becomes
unusable due to any shallow water-related problem.
In very shallow water there is, of course, the possibility of the HPR transducer head boring
into the sand, but hopefully we can discount this as a practical problem!
Position keeping capability
In shallow water it may be found that the vessel will not "settle" to her position as readily as
she would in deeper water. This will certainly be a problem if the tide is strong, since the
thrusters will be working harder to compensate for the tidal flow. The vessel will be working
closer to the limits of its environmental capability, with consequent reductions in reserve
power and thrust availability.
In strong tides there will be an element of added mass to the vessel due to the drag factor of
the moving water. This added mass factor itself will reduce the position keeping capability of
the vessel, and there may also be effects of "squat" caused by the close proximity of the
seabed. This squat results from the reduction in pressure of the water beneath the hull, due to
the water velocity, with a consequent increase in draught and a possible further loss of
position keeping ability.
In shallow water, the tide will often turn very quickly. Since DP systems deduce the tide from
the vessel estimator, or mathematical model, then a rapid change in tidal conditions will result
in deterioration in position keeping. In a diving or ROV support vessel this may cause the work
to be temporarily abandoned during the period of the turn of the tide. In the southern North
Sea and elsewhere, tidal conditions are such that diving and ROV operations can only take
place for a limited period around slack water, and this interruption to this already limited diving
window may cause serious delays.
Many locations have tidal conditions only permitting operations from DP vessels around neap
tide days.
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E
A
B
G
D
Summery:
A. Excessive thruster noise causing interference with acoustic reference.
B. Reduced vertical separation between transducer and transponder reduces the working
range of HPR systems.
C. Strong tides and greater turbulence result in poor acoustic conditions.
D. Strong tides causing thrusters to work harder with less power in reserve.
E. Strong tides causing use of more generators with less power in reserve.
F. Strong tides causing thrusters to work harder generating more noise giving worse signal /
noise ration.
G. Limited horizontal range of LTW.
As a result of these problems the positioning-keeping capability of the vessel is often
degraded in shallow waters.
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Deep water
Numerous DP operations take place in deep water. Many of the Brazil and Gulf of Mexico
oilfields are in deep water areas, as are the more recently developing fields west of Shetland
in the UK sector and several locations in West Africa. Problems in these areas relate to the
difficulty in obtaining suitable reliable position references, and in maintaining position in strong
tides.
In deeper water, subsea position references may be unreliable or inaccurate. Taut wire
systems are useful up to depths of 300-500m; at greater depths problems of deteriorating
accuracy appear. Specialised taut wire systems are designed to work down as deep as
2000m, but here the accuracy will be poor due both to the angular resolution at such depths,
and to wire bending in strong tides. Bending of the wire will be more pronounced in areas
where tide shear causes a number of different tide vectors to affect the wire profile at different
depths.
Other problems will affect hydroacoustic position references, and specialist high-power
transducers and seabed beacons may be used. The choice of acoustic PRS in deep water
should be made on the basis of suitability. Long Baseline systems provide high accuracy but a
low update rate ( up to 10 seconds ), while SSBL and Short baseline systems have lower
accuracy, especially in deep waters, although SBL systems have faster update rates. The use
of an acoustic system in combination with a dual DGPS is common, however, it is possible for
the dual DGPS to be lost, leaving a vessel reliant upon a possibly low accuracy acoustic
system. Similarly, if dual DGPS is interfaced together with an LBL system, a position jump
caused by the DGPS systems both changing constellation simultaneously may not be
detected by the LBL acoustics for a number of seconds, due to the low update rate of LBL
systems. A more satisfactory solution may be to use dual DGPS in combination with a dual
acoustic system. The dual acoustic system may be a combination of SSBL and LBL.
It may be considered preferable to avoid altogether the use of subsea references, and to use
a combination of DGPS and other radio references instead. In deeper waters, of course, there
is less likelihood of there being a nearby platform on which to base reference system. On the
other hand, because of the depth, it is often the case that pinpoints positional accuracy is not
vital; if the water is a mile deep and a search operation for a crashed aircraft is being
undertaken, the vessel has considerable positional leeway before the operation is affected.
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Solitons
The occurrence of Solitons in areas worked by DP-capable vessels is a cause for concern, as
they can be a major factor in the destabilisation of position-keeping. Solitons are described as
localised nonlinear internal waves in the ocean, i.e. waves that propagate beneath the ocean's
surface along the boundary layer between water of greater and lesser density. This
description does not mean much to the average ships' officer, but a better description may be
that of localised areas of high rates of current, occurring at frequent irregular intervals, and
lasting for short time periods.
SURFACE
THERMOCLI
NE
DIRECTION
OF
MOVEMENT
SEABED
SOLITON
A very interesting study of Solitons was presented by Captain Sanjai Jatar, of the Offshore
Support Vessel "CSO Venturer", His experiences were based on observations during projects
carried out in the South China Sea. The occurrence of Solitons was of a major concern.
The characteristics of Solitons are listed:
-
-
The waves radiate from a small region
They travel substantial distances
Swift currents and areas having bathymetry like sills on the seabed are required for their
generation
They travel in packets, with the leading waves being of maximum intensity
They are influenced by tidal variations, sea currents and thermal structure of the water
column
They travel at approximate velocities of 2 to 4 knots
Solitons in shallower waters further along the direction of travel were observed frequently
but did not appear to have the same speed of travel, or effect on the positioning of vessels.
This was possibly due to waves travelling from deeper water to shallower water
Solitons are observed as distinct bands of choppy water with short, steep, randomly
oriented waves
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-
They can be detected and identified in light sea conditions on a 3cm radar up to ranges of
3 to 4 n.m.
They are stronger in intensity during Spring Tides
In a particular area they always approach from approximately the same direction
The effects of the passage of a Soliton on a DP vessel are a sudden surge of the vessel
causing excursions of position and heading. There is insufficient time for the current to update
within the system's mathematical model. The vessel may remain unstable in terms of position
and heading for some time after, overshooting and hunting the setpoint values. At the same
time, unreliable results will be obtained from subsea position references. Both tautwire and
HPR systems will be degraded due to velocity of surface and subsea water, and additional
noise and aeration (air bubbles) from thrusters. Excursions can be as much as 100 metres or
more. There is a more pronounced effect on semisubmersible vessels due to their greater
draughts.
If operating in areas where Solitons are known to be prevalent, then there are a number of
precautions that may be taken by DPOs. A careful watch should be kept on 3cm radar which
might give early warning of the onset of a Soliton. Careful watch also on Doppler Log, to give
early indication of change in vessel speed. Position reference should be obtained from
surfaceorientated systems as far as possible. Additional generators should be kept online to
compensate for rapid power needs with the onset of a Soliton. If possible the vessel heading
should be altered into the direction from which the Soliton is approaching. With the detection
of a Soliton the diving operation should be abandoned and the bell recovered, or other steps
taken to mitigate the effects of a position/heading excursion. ROV operations should be
terminated and the ROV recovered or landed on the seabed. Care must be taken when
handling suspended loads underwater, crane operations, downlines, etc.
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Chapter 18 – Rule and Regulations
Disclaimer
In this chapter you will find documents and extracts of documents from the
different Rules and Regulations and other DP related issues.
These documents will not give a full understanding of the mentioned regulations
Please refer to the original documents
Page 221
Redundancy rules and regulations offshore safety
For many DP operations, the provision of redundancy is not required, and is not cost effective.
The function of redundancy is to provide greater system reliability in the face of component
failures, by means of the provision of back-up facilities, or an over-capacity of systems.
The requirement for redundancy will be highlighted by a Risk Assessment undertaken for the
task in hand. Risk analysis is a technique required within the Safety Case for all offshore
installations, and must include ship operations within the installation's 500m. exclusion zone.
Depending upon the consequences of a loss of vessel control, the required level of redundancy
is specified.
The main functions of system redundancy are; (i) to prevent a catastrophic failure of the system
resulting in a collision with the installation, and, (ii) to allow the vessel to maintain position in a
degraded status subsequent to a loss of any single part of the DP system. Note that the word
'system' includes all components involved in the positioning of the vessel, including thrusters,
generators, switchboards as well as the DP electronic fit.
For all DP vessels, all failure modes and their effects should be considered in a formal FMEA
(Failure Mode Effects Analysis) study. The modes that should be considered are the sudden
loss of major items of equipment, the sudden or sequential loss of several items of equipment
with a common link, and various control instability failures. Faults that can be hidden until
another fault occurs should also be considered. Also to be considered are the methods of
detection and isolation of the fault mentioned. Operator responses to the types of failure
considered should be reflected in the vessel's operations manual. The FMEA should consider
likely operational scenarios of the vessel, such as shallow water, high tidal stream rates and
limited provision of position reference. Some of the Classification Societies require the vessel to
maintain a FMEA document as part of the DP Class notation.
Levels of redundancy for a range of different types of DP vessel are given in the "Guidelines for
the design and operation of Dynamically Positioned vessels" published by IMCA. Further
guidance is given in an IMO document entitled "Guidelines for vessels with Dynamic Positioning
systems" which is an internationally recognised and agreed document, applicable to new
vessels, (constructed on or after 1st July 1994).
How much redundancy is required in a DP system? The location in which a DP vessel is
allowed to work and the scope of the work it is going to carry out should be governed by the
amount of redundancy the vessel has in its DP system. This was originally addressed by the
NMD (Norwegian Maritime Directorate) and IMO and led to the introduction of “Consequence
Classes” and “Equipment Classes”.
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Consequence classes
The NMD used to group the “consequence” of failure into four classes:
• Consequence Class 0 operations, which are operations where loss of position
keeping capability is not considered to endanger human life or cause damage;
• Consequence Class 1 operations, which are operations where damage or pollution of
small consequence may occur in case of failure of the positioning capability;
• Consequence Class 2 operations, which are operations where failure of the
positioning capability may cause pollution or damage with large economic
consequence, or personnel injury; and
• Consequence Class 3 operations, which are operations where loss of position
keeping capability will probably cause loss of life, severe pollution and damage with
major economic consequences.
IMO, Equipment Classes
Instead of ‘consequence classes’, The IMO Guidelines specify three Equipment Classes, Class
1, 2 and 3. Class 1 includes none redundant vessels. Class 2 vessels are those that will not
suffer a loss of position as a result of a single fault or failure in any active component or system.
Class 3 vessels are those that will not suffer a loss of position as a result of any single failure,
including all components in one fire subdivision, and all components in one watertight
compartment, from fire or flooding. The guidelines detail the level of redundancy in the various
elements of the DP installation for each equipment class, and outlines the FSVAD or Flag State
Verification and Acceptance Document which states the equipment class complied with for the
vessel.
Provision of redundancy arrangements may take a number of forms. The simplest level is to
merely duplicate an element, with a manual selection of one or other components. This may
be the case with windsensors where the operator may choose port or starboard windsensor
selected into the system. This is acceptable since there is no possibility of a catastrophic
failure pending the loss of windsensor input. The DP is able to function quite happily without
windsensor input for a short period of time providing there are no large variations in wind
direction and strength.
The above arrangement is not suitable for more vital elements of equipment. When
considering the control system computer a more comprehensive arrangement is required. In
many installations, two complete identical computers are fitted, both working in an operating
mode but only one of which is "on-line" and the other on stand-by. Upon failure of the online
computer, the stand-by unit takes over. Arrangements are made for each computer to monitor
the other to ensure that both contain identical data. This allows for a true "bumpless transfer"
as far as the operator is concerned, and complies with the requirements for Equipment Class
2, DP(AA) and Dynpos (AUTR).
Another approach to the problem is to provide triple redundancy or "triplex" operation. This is
the arrangement seen in the Kongsberg K-Pos 31 system where three computers are
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provided, all operational and on-line. Three computers in a triad perform exactly the same job
operating on exactly the same data from sensors etc. If one of the computers fails this is
automatically detected and isolated. The voting logic of the system allows the malfunctioning
unit to be identified The advantage of this configuration is that the operation can continue
subsequent to a failure since there are two operational units remaining giving a measure of
redundancy. This also gives rise to the facility of "hot repair" to the malfunctioning unit. The
triplex philosophy is carried through the whole DP system so that all major sensors are
triplicated (VRS, Gyro, etc). Whilst increasing the level of reliability of the system it must be
pointed out that triplicating systems is more expensive than duplicating them so there is
usually a cost penalty.
This arrangement does not, of itself, fulfil the requirements of DP(AAA) or Dynpos AUTRO
classifications for Equipment Class 3. In Class 3 vessels, a separate control computer must
be fitted in a remote location to back up the main, duplex or triplex, computers.
Redundancy of position reference systems is broadly achieved by using two or three PRS
together. As mentioned previously, a modern DP system is able to pool position reference
data and achieve a "best fix" from several different sources. If three PRS are used together
then the loss of one of them is not catastrophic. The operator should not subject himself to
common mode failure by deploying two PRS of the same type since it is then possible for both
units to fail simultaneously due to the same cause; for instance, two taut wire PRS can fail
together if the vessel rolls violently such that the winches cannot match the accelerations. In
all cases the PRS should be safeguarded against loss of power supply.
Redundancy of power supply arrangements is a more complex subject. It is necessary to
provide sufficient power, whether diesel-electrical plant is installed or direct-drive diesel, such
that the vessels operational capability can be maintained subsequent to the failure of any
single power unit. Power management arrangements must be provided so that when
consumption of power reaches the level of power available then non-essential loads are shed
in reverse order of their importance. In a diesel-electric installation with a number of diesel
alternators providing power, then a "spinning reserve" must be maintained of the equivalent of
at least one alternator capacity. Power distribution arrangements must be made such that no
single fault within the switchboard, cabling and distribution network can prevent sufficient
thruster supply to maintain vessel position and heading. The electrical arrangement of main
and auxiliary busbars is normally sufficiently versatile to allow power to be maintained despite
a considerable amount of failure within the system. The DP system itself is supplied from an
Uninterruptible Power Supply (UPS) which is redundant within itself, takes power from two
separate busbars and also has a 30 minutes battery back-up. The UPS, it must be stressed,
only supplies the DP system (console, computers, reference systems) and not thrusters.
The provision of thrusters and propellers for the vessel must also take on board the need
for redundancy. In simple terms this means that the vessel must have sufficient thruster
capability so as to be able to remain on station and heading subsequent to losing thrust
from any one thruster. Fully redundant DP vessels generally are fitted with a minimum of
six thrusters. The system has alarm functions set at 80% of thruster output. This warns the
DP operator that he is reaching the point at which he would be short of thrust if for any
reason one thruster were lost. The situation becomes more complicated again when
considering vessels having a direct-drive diesel element to the manoeuvring arrangements.
Consider the case where twin diesels are coupled to c.p propellers, with electrical power
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generated from shaft alternators, additional power being provided from separate diesel
alternators. A single fault could immobilise one main diesel, stopping that propeller and
leaving the vessel deficient in electrical power since that shaft alternator capacity has also
been lost. This is a "worst case" and must be allowed for in the documented capability
levels for the ship.
For Equipment Classes 2 & 3, the arrangements must be such that the vessel will remain on
station on DP after loss of all thrusters supplied from one section of the switchboard. If the
vessel is working to Equipment Class 3 the divided power system should be located in
separate watertight spaces with A-60 fire subdivision.
It must be realised that the best arrangements possible regarding redundancy cannot achieve
total reliability. Redundancy arrangements can be negated by the physical location of
equipment. In multi-computer systems the two, three or four computers are usually located in
the same compartment. It is common to see both gyro compasses and VRSs side by side,
likewise the two elements of the UPS are usually located adjacent each other. System cabling
is often grouped together in a common trunking. This arrangement is prone to fire, explosion
or flood damage. Likewise, a fire on the bridge which destroys the bridge console is likely to
knock out the DP in its entirety. Similarly, the best redundancy arrangements cannot allow for
multiple simultaneous failures. It is well known that "things always go wrong in threes" and this
is especially true shipboard, with a harsh environment. Severe weather conditions with water
flying about can result in many failures in a short time.
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Overview of authorities
International Maritime Organization (IMO)
Marine Safety Comitte (MSC) has issued guidelines based on input from the industry:
• MSC/Circ.645, 6 juni 1994, Guidelines for Vessels with Dynamic Positioning
Systems based on IMCA M113
• MSC/Circ. 738 Guidelines For Dynamic Positioning System (Dp) Operator
Training And Experience based On IMCA M117
The Nautical Institute
Administer a DP Training scheme
Offshore Service Vessel Dynamic Positioning Authority (OSVDPA)
Administer a DP Training Scheme
International Marine Contractors Association (IMCA)
Issues operational and competence guidelines for DP and other offshore work.
Marine Technology Society – DP committee
Issues operational and competence guidelines for DP and other offshore work.
Guidelines for Offshore Marine Operations (G-OMO)
Guidelines for Offshore Marine Operations is a guideline covering general offshore operations
DNV/GL
- Administer a DP Training Scheme based on DNV/GL standards.
- Issues Rules for classification of ships and MODU’s with notation for vessels with dynamic
positioning systems.
- Issues several other documents as OS (Offshore Standards) or RP Recommended
Practices
Lloyd's Register of Shipping
- Issues Rules for classification of ships and MODU’s with notation for vessels with dynamic
positioning systems.
ABS - American Bureau of Shipping
- Issues Rules for classification of ships and MODU’s with notation for vessels with dynamic
positioning systems.
Other classification societies also have notations for DP systems
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Client Requirements
Clients will normally have requirements for DP equipment and also competences of DP
operators included in contracts for the vessels. Below is an example of the requirement from
Petrobraz, for Drilling vessel/semi on DP received November 2005. This has been freely
Translated by Maersk.
4. DYNAMIC POSITIONING SYSTEM
4.1
System of Controllers with estimate algorithm and predictive type control (Kalman filters)
and triple configuration (three computers) for the controllers and consoles, including video
monitor, keyboards joysticks. It shall be accepted a double configuration (two main controllers),
as long as the Unit is provided with a back-up DP controller system completely independent
(including “software”), separated from the main controllers.…
4.2
Systems for Reference of Position (SRP) configured in number, type and degree of such
technological updating that guarantee reliability and accuracy in the whole range of contractual
water depth. In addition to this:
4.2.1 Minimum of two (02) hydro acoustic systems which shall feed the DP controllers
simultaneously, as system for reference of position totally independent……
4.2.2 A minimum of two (02) systems of positioning by satellites, independent, with minimum
accuracy of three (03) meters. At least one of the primary receivers must have capacity to
receive GPS dual frequency (L1/L2) signals, besides a minimum of one (01) GLONASS
receiver. Each system must possess double redundancy in the system of reception of
differential signal, in the following way: two (02) different systems via satellite (example:
“Inmarsat” and “Spot Beam”) and two (02) different systems via radio (distinct frequencies and
redundant broadcasting stations, with range reaching all the operational scenario of the Unit –
examples: IALA-MRB and UHF)……
4.3. Systems for Measurement of the Drilling Riser Angle:
4.3.1. Two independent systems type ERA (“Electric Riser Angle”), with data transmission via
pods of the BOP and connected simultaneously to the DP System….
4.3.2.1. The transponders ARA must posses as accessory function, the possibility of
reconfiguration (via submarine telemetry) for hydro acoustic positioning transponders, operating
in one of the SSBL/USBL/SBL mode.
4.4. Sensor Systems constituted of, a minimum of, 03 gyroscopic compasses, 03 VRU’S
(Vertical Reference Units) or MRU´S (Movement Reference Units) and 03 anemometers. ….
4.5
System of Continuous Supply of Power (“UPS´S” of the DP): a minimum of three (03),
feeding the Controllers, Systems of Reference of Position and Sensors, with redundancy in the
feeding by distinct bus circuits and with changer over and alarm, by-pass, in a shared way and
batteries with capacity for a minimum of 30 minutes of operation in case of cut of the primary
supply of power…..
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4.7. Print Outs systems of the screens of the DP controllers and acoustic systems, in a color
printer (connected to the UPS).
4.8. The Alarm Printers of the DP system must possess redundancy (a minimum of two in
parallel, being one in operation and the other one in stand by mode, with automatic change-over
in case of failure or manual with alarm, both connected to the UPS).
4.10. System of Alarms of the DP status: the Unit must possess visual and sound alarm
indicators of the dynamic positioning system [green (optional)/yellow/red], controlled manually
by the DP operator from the bridge/and or DP room(s) (it shall not be accepted automatic
alarms activated directly by the DP controllers).
4.13. The Unit must comply with the recommendations of the IMO defined by the MSC/CIRC
645, from 06/06/1994, “Guidelines for Vessels with Dynamic Positioning Systems”, in addition to
the requirements of the Classification Society to which is subordinated.
4.14. The Dynamic Positioning System must have been submitted to a preliminary reliable
analysis type FMEA (Failure Mode and Effects Analysis) carried out in a recent period (one year
maximum prior to the beginning of the Contract) by institution recognized by the IMO…..
4.15. Petrobras shall carry out regular annual inspections and tests on the Dynamic Positioning
System, besides eventual tests in case of upgrade or in the occurrence of grave incidents
related to the DP.
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Chapter 19 – Failure Mode Effect Analysis (FMEA)
FMEA is a highly important part of the delivery of a DP system. In MSC/Circ. 645 there is a
requirement for testing of the DP system, which includes the FMEA and annual trial process.
Below paper gives a short explanation of the FMEA. The FMEA process has developed since
the very beginning and is continuously developing with technology. It is recommended to follow
the developments in the subject on ongoing basis.
FAILURE MODES AND EFFECTS FOR DPS
By
Captain C.A. Jenman, B.Sc, MRINA, MRIN, MNI Director, Global Maritime Ltd
There is an increase in the number of diving support vessels operating offshore and there is a
number of such vessels nearing completion at several yards. In both the Norwegian and
British sectors of the North Sea diving operations from DSVs have been influenced by the
guidelines issued jointly by the Department of Energy and the Norwegian Petroleum
Directorate. One of the recommendations of the guidelines is that a failure mode and effect
analysis (FMEA) should be carried out for the main components of the Dynamic Positioning
System.
This recommendation has been followed by some operators and owners, sometimes for
technical reasons, sometimes for safety or political reasons, and sometimes just for
commercial reasons. The results of such work have been as mixed as the reasons for them
being commissioned. The reason for this is simple; for high-quality results the engineers
carrying out the analysis must have an overall understanding of the vessel to a depth almost
equal to the designers of the individual systems. In addition, they must have a clear idea how
the vessel will be put to work operationally. Such capabilities are rare. The analysis however
can be greatly simplified if clear objectives and a methodical approach is adopted.
What is an FMEA?
For the want of a nail the shoe was lost
For want of a shoe the horse was lost
For want of a horse the rider was lost
For want of a rider the battle was lost.
This sums up and explains what FMEA is all about. The problem facing the engineers
carrying out an FMEA of a sophisticated vessel is to identify the "nail" that is critical, prove
that it is critical and show how the "criticality" can be reduced or avoided.
Every piece of equipment can fail. Every system can fail. In an FMEA the various ways in
which a piece of equipment or system can fail have to be considered to see which failure
modes, if any have critical effects. To those familiar with fault tree analysis this will seem to be
that kind of analysis under a different name. It is true that they are very close. The difference
is that a fault tree approach is very much equipment orientated whereas an FMEA must cover
equipment failures but then put them into operational perspective to give the effects. Generally
only critical effects are really of interest and operators and owners are quick to act if they can
be shown to be critical.
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What is a critical failure?
In offshore terms a critical failure is a red alert. In diving terms it means a rapid return to the
diving bell and bell recovery. In dynamic positioning terms it is a loss of the ability adequately
to control vessel position and/or heading. Of course the word adequately is vague, but it
depends on the diving operation, the water depth and the nature of the failure. A "drive off" of
Sm could be more critical than a "drift off' of 10 m.
A yellow alert, although not immediately critical to safety, is commercially critical because the
sub-sea work cannot continue and much diving work is carried out on a lumpsum basis.
Generally a DP yellow alert implies loss of redundancy or a reduction in power margin such
that a red alert is imminent or could occur from a simple single failure or trip. In these
situations the diver(s) return to the bell; often the bell is recovered to await a diving "green" to
resume. A significant loss in diving time is very costly, especially if there are many divers
employed in the work.
Where are critical failures found?
There are several key areas where, in the author's experience, potential critical situations and
failures are found. A few are highlighted below.
Each DSV is designed with the intent of being able to maintain position whenever it is
reasonably possible for divers to work. For most monohulls restriction on heading for higher
sea states can be a limitation, but this limitation in comparison to a semi-submersible is well
reflected in costs. Many DP DSVs also have a fire-fighting capability and thus the power
generation is usually more than adequate for all the thrusters.
The problems usually come in ensuring the thrusters can develop their full thrust while on DP.
Reductions in effective thrust come from poor flow or thruster interference or from poor pitch
control i.e., full pitch is not achieved although ordered by the computer or joystick.
Inadequate thrust can also be the result of equipment not being available, generally thrusters
or generators. When routine maintenance is considered at the design stage a reduction in
availability of the vessel for drydocking, repairs and inspections is costed. When the vessel is in
service a greater availability is often expected.
If one generator is unavailable because of routine maintenance at sea or because of a failure,
one side of the high voltage board is likely to have inadequate power in the event of an earth
fault. An earth fault on a high-voltage switchboard is an unlikely event, but the guidelines state
that no single fault should cause a catastrophic failure. This is where the risk of a critical failure
needs to be put in perspective.
Mechanical failures tend to give experienced operators many warnings. Electrical failures tend
to give no warning. At the design stage many discussions on effective electrical supply
redundancy, result in complicated power distributions to avoid a common supply to critical
equipments. The designer's intent is not always achieved. The designer's intent is also
sometimes modified on board for other reasons and redundancy is lost. These failure modes
and effects are generally easy to find and simple to test.
The more difficult problem is the identification of all hidden failures which, when combined with
a simple single failure, cause a critical situation. An example is where there are two supplies
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with a changeover relay. If the normal supply fails the relay operates and effectively ensures
continuous supply. This changeover relay could be unused for years and sit in a failed condition
so that when called upon the changeover does not happen. Loss of one or all thrusters could
result.
Sometimes great thought has been put into ensuring that fully redundant duplicate supplies are
provided to a piece of critical equipment only for both to be terminated together on the same
pair of pins, such that one loose termination gives the effect that all the duplication was meant
to avoid.
Most DP DSVs have several reference sensors both for position, and attitude (roll, pitch and
azimuth). Usually the vertical reference units are duplicated, as are the gyro compasses. This
does not mean that they are secure. It is possible for one to fail such that the operator is unable
to know which is in error. It is also common that these duplicated units are positioned next to
each other in a small compartment (often an electrical switch room); thus mechanical damage
or fire results in the loss of both.
A vessel with several position reference systems can also be quickly reduced to only one in
some circumstances. For example, in shallow water or near an offshore structure some acoustic
positioning systems are not suitable, a vertical taut wire is restricted and has to be picked up
every few meters and an Artemis can be shielded, switched off at the fixed station or be
interfered with.
There are other analyses that can be carried out on DP DSVs and it is worth mentioning some
of them.
Hazard analysis - In many ways this analysis follows on from an FMEA. Situations and failure
modes that are a hazard cab be quantified and, for example, the probability of a DVS colliding
with a fixed structure be determined for various tasks close to the platform.
Reliability analysis - The mean time between failures for various equipments and systems is
useful when determining the level of redundancy that is cost effective. It is also essential to
have reliability data to calculate the probability of various hazards occurring. In practice, when
the design philosophy is that no single failure should cause a critical situation, cost effective
redundancy studies on the systems associated with dynamic positioning are not carried out.
Availability analysis - The availability of a system is the probability of the system being able to
perform its intended task. Availability combines reliability, mean time to repair and maintenance
downtime, it is ideally what is required for a vessel to quote a lumpsum price. To achieve a
realistic assessment vessel availability has to be combined with the probability of various
environmental conditions existing. This leads into downtime or operability analysis.
An FMEA is a useful exercise to operators and owners of DP DSVs if carried out by capable
engineers. Other studies can also be beneficial but the real question in the author's opinion is
not being asked.
The effects that these studies are designed to avoid are:
1.
Death or injury to a diver.
2.
Collision with a fixed platform.
3.
Prolonged yellow alerts from equipment failure.
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The diver wants to know that the risk of death or injury is very low or negligible. The owner (or
operator) of the DSV and the operator (and partners) of the platform wants to know the risk of
collision. The operator of the DSV wants to know the risk of downtime. These are the answers
that consultants should strive to give the offshore industry.
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Chapter 20 - Human Factors in the DP operation
Human factors in the DP industry have had a high focus for many years. With the STCW 2010
amendments, this is believed to increase further. DP manufacturers are constantly seeking the
best user interface, based on thorough research. This research can again be used for the
benefit of the entire DP operation. Below are 2 projects reproduced as example of the research
in human factors in the DP industry.
Project 1
PILOT PROJECT
By
J. M. Hughes Global Maritime March 1996
Background
The role of the human being in dynamic positioning (DP) has never bee taken for granted by the
industry. Since the very early days of DP in offshore oil operations some 25 years ago it has
been recognised that for vessels to operate successfully so close to offshore structures in
adverse weather conditions and for such long periods requires the successful integration of a
number of complex inputs, viz.,
• in the design, construction and reliability of DP equipment,
• in the management and the operation of DP vessels and
• in the role of human beings in maintaining and operating DP vessels
Historical Development
Design. Construction and Reliability
In the early years a disproportionately large number of DP incidents were caused principally by
hardware failure. Although figures are not available for the period before 1980 there is little
doubt that inadequate control systems and equipment contributed to the majority of run-offs,
platform collisions and subsequent accident. At that time the industry was on a steep learning
curve and was, metaphorically, flying by the "seat of its pants". It was not until the 1980s that
serious efforts were made to monitor and introduce effective controls in the industry. Statistics
compiled for the Dept of Energy Diving Inspectorate for the period 1980-1988 identified
equipment failure as the main cause in 54% of all DP incidents.
The troublesome incidence of equipment failure did bring about improvement measures and
led, for example, to the widespread recognition and implementation of the concept of
engineering redundancy in the industry. Previously, many DP systems, despite apparent
sophistication and technical complexity, were characterised by a large number of single point
failures. Complex systems are of little value if the failure of one small component can result in a
major breakdown and lead to a major incident. Design and technical improvements resulted in
the introduction of back-up hardware, i.e. propulsion, power generation and distribution, back-up
control systems, fail-safe systems, etc. The concept of redundancy is now central to DP
operations, where, the higher degree of operational risk requires a similarly high degree of DP
system and operational security, which, in turn, is attained by operating to a higher level of
system redundancy.
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In addition, a greater understanding of DP systems has been gained via system specific
studies, such as Failure Modes and Effects Analyses, (FMEAs), which identify critical
components and methods of introducing greater reliability and robustness.
There is little doubt that in the past 25 years, the quality and reliability of hardware and, indeed,
software have improved, not least because of the resources that have been directed towards
achieving that improvement. Designers, manufacturers, industry associations and classification
societies have all contributed.
Statistics for the period 1989/90 compiled by the DPVOA of DP incidents on diving vessels
indicates that equipment failure, including thrusters, electrical, references and computers
accounted for 36% of the total. Although the statistical bases for the two periods are not
identical, nevertheless and unquestionably, there have been improvements.
Management and Operation
Similarly, considerable resources have been invested in developing the management and
operation of DP vessels. DP vessel owners and operators themselves have been particularly
active in this area. Many have adopted management systems and have developed operational
and project skills that would have been considered improbable some years ago.
Client groups have also played a part. They are understandably anxious to protect their offshore
investments, which they see as being exposed to considerable risk every time that a DP vessel
is going about its legitimate business only a few metres away from their platform. Clients have
tended to choose the vehicle of DP audits as the means of assessing the capabilities of DP
vessels and thereby, limiting their exposure to risk. The use of DP audits in this way has not
been a universally popular nor effective method of achieving the desired objectives.
Perhaps the greatest single influence in establishing acceptable standards for DP operations
was the implementation of a joint Norwegian / British document, "The Guidelines for the
Specification and Operation of DP Vessels". The first edition was published in 1980 and was
superseded soon after by an update in 1983. These comprehensive guidelines set standards
not only for the provision of equipment but also, importantly, for the operation of equipment, the
establishment of operating limits, assessment of DP capability, etc. In addition it is in this
document that the fast attempts are made at addressing the people factor. Since their
introduction many years ago these guidelines have served as an authoritative base for the
development of many other codes and guidelines.
The DPVOA has been the most energetic body in this area, providing the industry with
numerous publications, standards and studies in aspects of equipment and safe DP operations.
For example, the DPVOA "Guidelines for the Design and Operation of DP Vessels, 1989",
provided the industry with the most comprehensive and authoritative set of standards so far. In
addition the DPVOA has published authoritative equipment specific studies, including, studies
on position reference systems, thruster failures, switchboards, etc.
Human Element
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The role of the human being, in particular the DP Operators, has always had a high profile in the
industry. The emphasis has followed traditional lines of providing standards for training and
qualification and then providing the means through which these standards can be achieved.
For example, at the start of the 1980s a DP Operator training scheme was set up in association
with shore based training establishments. The initiative for this came from the industry itself and
the Nautical Institute. As a result there are now various centres that provide DP Operator
training and issue appropriate certificates.
Also, a DP Operator log book system was established some years ago, in which an operator's
experience, including types of DP operation, types of DP systems, length of service, etc. are all
recorded. This is modelled on diver's log books and aircraft pilot logs. It has also resulted in the
establishment of DP Operator training courses, both onshore and shipboard.
The industry's approach is set out in one of the DPVOA's recent publications, entitled, "Training
and Experience of Key DP Personnel". This provides training standards and qualifications
required for various positions within the DP industry.
One other area where the human element appears is in the investigation of DP incidents, the
results of which are regularly published and distributed to the industry. Analyses of DP incidents
in the years 1980 to 1993, indicate that approximately 50% are caused in some way by human
error. This statistic prompted the DPVOA to undertake a pilot study into the human factors
aspects of DP operations. The timing of this initiative by the DPVOA coincided with a trend
throughout the industrialised world to direct more resources towards understanding people and
analysing human behaviour within an industrial context. The result has been an almost
exponential growth in influence of such people-related disciplines as occupational psychology
and the pseudo science of human factors.
In choosing to start off a project on human factors the DPVOA decided to steer clear of the
expert view. In keeping the initial part of the project "in-house", the DPVOA was following a
strategy that that had served it well in the past, i.e. that of the industry's own people looking at
its own problems, identifying particular areas of concern, defining the extent of the problems
and establishing appropriate strategies and plans.
Human Factors Pilot Project
The impetus that brought about the human factors project was that the NMD formally asked the
DPVOA what they had done to reduce the number of human errors in DP accident causation.
Although the quoted percentage levels of human error are low when compared with some other
industries, at approximately 50%, it is still a significant number.
Literature Review and Identification of Problem Areas
The stated objectives of the pilot project were, broadly, to identify human factors issues that
have an effect on the safety of DP operation, identify problem areas and establish whether
further investigative work was necessary.
The project was carried out over a period of a few weeks in the Autumn of 1994. It consisted
initially of a review of relevant human factors work in similar areas and assessment of the
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implications for DP operations. The following studies and references were considered as
particularly relevant, since they provided a maritime perspective.
Human Element in Shipping Casualties - Tavistock Institute of Human Research
Safer Ships Cleaner Seas - Lord Donaldson's Report on the "Braer" Grounding
Fatigue and Stress at Sea - Medical Research Council - Prof LD Brown
Accidents and Loss Prevention at Sea - Nautical Institute Conference, 1993
Following the review of the references five specific areas of interest were identified as being of
particular significance to DP operations. They formed the basis of the project. They were;
•
•
•
•
•
Man/machine interface
DP management issues
Training and competence
Identification of stressors
Aspects of human behaviour
In selecting the above five areas the project personnel were extending the expected original
scope away from purely an examination of the activities of DP Operators standing or sitting at a
DP console. In doing so they were acknowledging that the real scope for human factors extends
across the whole spectrum of vessel and company operations and incorporates human
participation in design construction, management and organisation as much as operational or
hands-on errors.
The resources available to the project were limited and did not permit detailed studies of the
selected topics. To achieve validity and relevance within the resource constraints it was decided
to concentrate on identifying and exploring the responses of DP Operators to the selected
topics and to review their responses objectively. There are obvious pitfalls in carrying out a
study in this way, because the project could become a channel for the expression of the
subjective views of a particular group of people, thereby losing impartiality. The method of
overcoming that pitfall was to ensure consistent standards were maintained in the conduct of
the work and, especially, in the method of data collecting.
Method of Data Collection
In order to link the areas for consideration with the real world of DP operations it was necessary
to undertake an objective and relevant data collecting exercise. Three different methods were
considered, vim., observation, questionnaire, interview. The first, observation (fly on the wall),
i.e. watching DP operators in action over a period, was discounted principally because the time
available did not permit enough observation time on a wide enough range of vessels. There are
also obvious disadvantages in this method. When under observation, human beings are liable
to adjust their behaviour to something other than normality, giving a false picture of what they
would normally do under other circumstances.
A combination of questionnaire and interview was chosen. The questions were divided into the
five selected areas. Half of the questionnaires were completed by DP Operators on their own.
The other half were used in interview sessions. The questionnaire consisted of 108 questions,
comprising open and closed questions, some of which required specific Yes/No type answers,
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others required the participants to rank answers in order of preference, while others were open
ended to encourage expansive answers.
Statistical Range
Given the time and resources available it was clearly not possible to obtain data from every DP
vessel or every DP Operator. Out of a total of fifty DPVOA member vessels, four were selected
as being representative of the industry as a whole in operational range and complexity. Two
were dive support and two were specialist subsea and topside support vessels. Three were UK
manned and one Dutch. Three were mono-hull and one semisubmersible. The DP Operators on
two of the vessels were employed directly by the vessel operator and on the other two they
were employed through manning agencies. In total fourteen DP Operators were involved in the
project.
The statistical sample of vessels was 8% of the total number operated by DPVOA members and
3% of DP Operators. Absolute confidentiality was a precondition of the exercise. The identity of
the vessels, owners and DP Operators have not been disclosed and have remained anonymous
throughout.
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Project 2
Introduction to Human Factors Studies
A large amount of research work has been carried out into human factors. Some of which have
relevance to the world of DP operations. The following two examples provide a flavour of the
range of opinion and diversity of approach.
1. Analysis of Human Actions
In particular, the work of noted psychologists, (Prof. Reason and others), has considered human
error in terms of three levels of action, i.e. skill based actions, rule based actions and knowledge
based actions. According to their studies, it should be possible to determine the role of the
human being in accident analysis and allocate it to one of these three actions. Throughout the
course of the project the behavioural patterns of DP Operators were considered against these
three elements of skills, rules and knowledge.
Skill Based Actions
In a skill based action, an action is pre-learnt and repeated, automatically, in pursuit of an
objective. As far as application of the human mind is concerned a skill based action should be
quite simple. Skill based actions are characterised as being capable of copy, repetition and,
possibly, automation. There are many skill based actions in the world of DP operations from the
general acquired skills of operating the DP console to the operation of a particular piece of
equipment, such as DGPS, Artemis or even the remote operation of the taut wire unit. When
doing accident investigation a failure to follow a skill based rule is normally readily identified and
the fault pinned on the operator.
Rule Based Actions
In a rule based action, there are standardised or customary procedures for doing certain things,
The procedures are consciously followed without considering possible alternatives. In the
context of DP operations these procedures could refer to the DP setting up procedures or, in
particular, the procedures adopted when the vessel is moving under DP control, possibly with a
diver down. It is generally accepted that DP vessels are particularly vulnerable at these times.
Failure to comply with the rule based action can lead to failure to control the vessel and
subsequent DP incident. There is normally little difficulty in establishing rule based violations in
the course of carrying out an accident investigation.
Knowledge Based Actions
In knowledge based actions, the role of the human being is more complex. In knowledge based
actions the human being is required to exercise his powers of discretion and to carry out
problem solving, planning and choosing between alternative courses of action. Knowledge
based problems can not be resolved simply by resorting to a skill based solution or following a
set procedure. Despite widespread attempts to turn most actions into the exercise of a skill or
the compliance with a procedure, we still have not reached the stage where we can dispense
with the rational human being, who is required to assess a situation and choose an appropriate
course of action from a number of alternatives while still observing his acquired skills and
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complying with relevant procedures. Human error in this context is still an identifiable factor. It is
often expressed in the following manner, "You shouldn't have done it that way, you should have
done it this way." I call this the Harry Enfield treatment" or, more conventionally, it can be
expressed as the benefit of hindsight. Knowledge based errors that lead to DP incidents can be
identified by accident analysis techniques, however, this requires a considerable amount of
knowledge and understanding by the analyst.
2. Human Error in Perspective
Recent studies of ship casualties carried out by Dr Hans Payer of Germanischer Lloyd
considered human error against a background of other factors, in particular organisational and
management. This approach moves away from analysis of the human being in isolation and
requires human error to be put into context. In terms of accident causation, this requires the
vertical causation line to be examined as well as the horizontal causation line.
Dr Payer recognises that human beings are intuitively familiar with human error as a principal
cause of accidents. In other words there is a tendency to look to the human fast and then to
work out from there. This is expressed in the culture of blame that is so often associated with
accident causation. This culture of blame is reinforced by society's way of dealing with the
consequences of accidents, for instance, in this country and elsewhere there is a long and
established tradition of adversarial court actions which is consciously set up to foster the blame
culture, where one side blames the other. In accident cases in Scotland, e.g. the defender tries
to place all the blame on the hapless injured party and the pursuer tries the opposite. In
analysing accident reports that come through the system and which support this blame culture,
it is evident that little attempt is made to seek out the real causes. It is often more a case of
using the accident investigation in defending one's own position or promoting one's own
interests.
It is a fact that most severe accidents are caused by a catastrophic combination of errors, both
human and organisational. Seldom are all the factors exposed in the normal investigation
process.
Human Error
A typical range of human error types are given below:
Fatigue
Negligence
Ignorance
Greed
Folly
Wishful thinking
Mischief
Laziness
Alcohol/drugs
Lack of seriousness
Misjudgement
Sloppiness
Physical Limits
Boredom
Inadequate training
The potential for human error is intensified in times of stress and panic. Optimum performance
requires a certain amount of pressure and even stress. But excessive pressure causes anxiety
and hostility. In any case the normal thinking and decision making processes are impaired
under too much pressure. Slackness, on the other hand also leads to too low performance. Both
extremes can contribute significantly to human error.
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Once a problem starts, the normal human objective is to return as soon as possible to the
normal condition, before it reaches a critical level of no return. The typical reaction of man to a
crisis is subdivided into three psychological ranges:
perceiving
thinking
action
The danger level can be reached by lack of sufficient reaction time, errors in perception,
thinking and action. Such errors will either lengthen the time or increase the magnitude of the
danger build up. After a problem starts there will be some kind of warning, either by observation
or unintended changes in operation or by warning systems. After the warning is noticed, the
source of the mishap has to be recognised. The thinking period ranges from the identification of
the problem to the decision process for the best course of action to take, based on information
available, experience of the operator, etc. Finally the corrective action is started usually
following a plan so that the system is returned to normal operation as soon as possible.
Otherwise the problem may escalate to a dangerous state, possibly leading to an accident.
Organisational Errors
Many accidents, in particular those that are tragic in nature, such as the Piper Alpha, Herald of
Free Enterprise, and those that have been subject to impartial scrutiny invariably go beyond the
narrow confines of examining the role of the human being and investigate organisational
factors. Collections of individuals as well as individuals in organisations contribute to accident
situations. One prime cause is that individuals as well as organisations are prepared to take
calculated risks. Many failures, however, can be tracked back to organisational malfunctions
that result in errors and bad decisions.
A representative range of organisational failures is listed below:
Time pressure
Language problems
Cost / Profit Incentives
Morale
Rules and Regulations Management style
Incentives
Communication
Pressure of production
All of the above factors have a possible negative effect on organisational reliability. This is an
extremely complex issue and involves the interaction of human beings at many levels. One
classic example in the world of DP operations is when the client applies pressure on the DP
management team to continue operations in marginal conditions. Another example from the
world of shipping is the Herald of Free Enterprise, where the company management
commitment to safety was ineffective, almost to the point of criminal negligence. Further
examples of organisational failures were exposed in the Piper Alpha disaster, where, not only
were the platform operators at fault but also the regulatory authority, DEn, whose organisation
and the exercise of its responsibilities in respect of safety inspection was considered by Lord
Cullen as being superficial and ineffectual.
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Conclusion
The data collected in the pilot project gave a practical dimension to much of the human factors
work carried out by Reason and Payer, in particular some of the organisational perspectives
identified by Payer. The following results can be seen in the context of the human factors theory
as well as providing the industry with a set of practical recommendations.
SPECIFIC RESULTS
1. DP Consoles
Physical overload of DP consoles is a potential problem. The growth of peripheral equipment
around DP consoles can result in practical problems. For example, the siting of an HPR unit on
one vessel required the DP Operators to overreach in order to operate the equipment. Normally
this would not be of particular significance, however, on this vessel, when fully operational, it
was necessary to operate the HPR every time the vessel moved and that was every two to
three minutes. The combination of the frequency of repetitive vessel moves and the physical
movements required by the DP Operators introduced a small yet perceptible risk of stumbling or
tripping and a failure to keep up with operational requirements. There is little doubt that, had a
DP incident occurred as a result of the DP Operator failing to complete the required
manipulation on time, then the investigation of the incident would have identified human error as
one of the contributing factors.
Other examples existed of some consoles that were so overloaded with equipment and
monitors that the DP Operator's view of the adjacent platform or the working deck was
obscured. On other vessels, additional equipment was located in potentially dangerous
situations, with one example being related of a VHF set and removable hand held receiver
installed directly above the DP console, so that if the hand receiver fell from the unit it would fall
directly onto the push buttons on the console. This arrangement was certainly convenient for
the DP Operator, not having to reach very far to answer the VHF, but less than secure from the
point of view of maintaining the integrity of the DP console. A DP incident caused by the hand
held receiver slipping out of the grasp of the DP Operator would invariably identify human error
as being of causal significance.
Recommendation
Companies to carry out assessments of the equipment overload at each DP console and
assess the suitability of the control console environment, looking particularly at the
consequences of typical movement or manipulative errors, that happen from time to time.
2. Work Overload and Under Stimulation
The study identified that multi-tasking had become a feature of the work of DP Operators. This
imposition of other responsibilities has the potential to result in work overload, particularly in the
senior ranks. There was considerable amount of anecdotal evidence to support this. One senior
officer is quoted as saying that his extra duties were excessive, resulting in work overload both
when on duty at the DP console at other times. Such extra duties are many and varied and
include responsibility for communications, shipboard administration, stability and ballast control,
planned maintenance, safety administration, project and client liaison.
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For most DP Operators, however, work overload is not a permanent problem. On the contrary,
on many occasions DP Operators are under utilised and are mentally and physically under
stimulated. This is characterised by long hours of enforced attendance at the DP console.
These periods of calm and inactivity provide ideal conditions for lapses in concentration. Many
DP Operators expressed the sentiment that their role was frequently downgraded to computer
watching and interrupted by short periods of excessive demand. One DP Operator referred to
such periods of excessive demand as being, "the madhouse hours" and are characterised by
significant increases in stress levels.
During periods of calm and little activity, when the DP Operators are not required to make
vessel movements and when they are not actively involved in the conduct of the vessel's
operation, there is possibility that the DP Operators can become mentally isolated from subsea
or vessel activities. On such occasions the information and involvement of the DP Operators is
at a minimum. It is possible for the diving control room to neglect the DP control area and for
such critical information as the location of the diver to be kept from the DP Operators. This is a
potentially dangerous phenomenon.
Overload and under utilisation are recognised features of the work of a DP Operators. DP
incident analyses should establish the workload of the DP Operator at the time and then
ascertain whether it was a contributory factor.
Recommendation
Companies to assess the workload on DP Operators, identifying occasions of overload and
under utilisation and assessing the consequences for the safety of DP operations.
3. Temporary Handover Arrangements
In general shift handovers were found to be carried out in a thorough and effective manner
throughout the sample. However, the same was not necessarily the case for short term
handovers. Regardless of the circumstances and the duration of the handover it is necessary
that they are carried out thoroughly so that the oncoming DP Operator is made fully aware of
all relevant information. Serious DP incidents have occurred during temporary handovers,
when the stand-in DP Operator was not made fully aware of the circumstances or of the status
of the DP operation.
Introducing a new person into the DP operating situation, even for a short period, can create
instability and disrupt the teamwork that can be built up by the team of onboard DP Operators.
This can be of particular relevance on occasions when the Master or a person of equivalent
stature takes over temporarily from the on watch DP Operator to allow him to carry out
another task. These occasions can provide an uneasy environment, especially for junior DP
Operators, since mistakenly perhaps, some feel that their performance is being subjected to
scrutiny.
Recommendation
Companies to assess DP Operator hand over procedures and to ensure that they are
implemented effectively, establishing tighter procedures as necessary. This could be done by
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documenting the methods for controlling handovers. There are precedents in other industrial
environments, where proper controls are required, such as in the transfer of responsibility in a
Permit to Work system. This is frequently done by use of a checklist.
4. Distracting Presence of Non-DP Personnel
DP control areas are not normally segregated effectively. They are often considered as a
convenient meeting area for personnel with no direct role in the DP operation of the vessel.
On some ships the DP control area was freely accessible to other personnel. The following
comments were made; 'There is continual distraction", "There are plenty of casual visitors",
"The DP bridge is not an effectively closed of area", "Distraction is rarely absent" and "There
is just too much noise ". This has the potential of distracting DP Operators, yet at the same
time the presence of other personnel can have a stimulating effect on some DP Operators.
Some DP Operators are more inclined to be distracted than others.
It is vitally important that the DP control area offers an appropriate working environment. This
should be one which is dedicated exclusively to the promotion of safe DP operations and is
effectively free from all other distractions and influences. Analyses of accidents should
consider these aspects and establish whether there is any causal relationship.
Examples from other similar control environments indicate that this matter has been
considered and measures taken to reduce or eliminate distractions. Similar environments
include, diving control areas, airline cockpits and even passenger coaches.
Recommendation
Assess the arrangements on DP vessels for isolating outside distractions. Industry wide
guidelines should be established, in much the same way as coach drivers and airline pilots
are protected from external influences.
5. Inadequate Manuals / Documentation
Peripheral equipment, in particular, is frequently supplied with inadequate documentary
support, i.e. operations instructions, failure modes, etc., and it is frequently not user friendly. It
is also widely recognised that training and familiarisation with new equipment are frequently
inadequate. This has a particularly important human factors dimension, since, the ability to
carry out the responsibilities and duties as a DP Operator are largely dependent on ensuring
that personnel are provided with the necessary information and knowledge of systems. This is
not always the case.
Recommendation
Companies to assess the arrangements on DP vessels as to the adequacy of documentary
support. Where deficiencies exist appropriate corrective actions should be taken.
6. Risk Taking
There is supporting evidence that the margins of operational safety as required by the various
guidelines are being exceeded. Such guidelines include the industry standard 1983 NPD/DoE
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guidelines, the DPVOA guidelines for the operation of DP vessels and the equipment
requirements of the classification societies. It appears that the most frequent abuses are in the
operation of position reference systems. The reasons for the failure to comply with accepted
standards of operation are many and various. In some cases it is a question of wilful
acceptance of the risks involved and in others there is ignorance of the requirement. Risk taking
was also seen as expressing individualistic tendencies. The syndrome of "I know best" was
certainly not commonplace but it was detected. Some were more liable to disregard the rules
than others. The project established that there appear to be links between risk takers and
certain personality traits, including bravado, over-confidence, showing off, disinterest and
lethargy.
However, some DP Operators expressed irritation at the question and assumed an air of
righteous indignation at the underlying suggestion, asserting that, "standards of professionalism
are too high to allow risks to be taking while I am on watch."
The analysis of DP incidents should consider risk taking as a potential causal factor.
Recommendation
Companies should assess the frequency and the potential consequences of risk taking and
where necessary, to implement stricter enforcement procedures. In addition, companies should
consider the psychological suitability of DP Operators.
7. DP Incident Reporting
Current DP incident reporting and communications procedures are based on a system set up by
the DPVOA, where all member companies are encouraged to report a DP incident on an
appropriate form and to forward the information, in confidence, to the secretariat of the DPVOA,
where the incidents are analysed, conclusions drawn ands results published. In order to build
up a comprehensive data base of incidents, it is important that all DP incidents are reported, for,
failure to do so deprives the industry and the DP Operators with valuable information on
failures. There, is evidence, drawn from the survey, that some DP incidents remain unreported,
even within the vessel. There is also evidence to suggest that the published reports are not
distributed to every DP vessel.
The quality of the analysis of DP incidents is influenced by the quality and quantity of
information that is provided by the company and the ship on the DP incident report form. There
is evidence that some incident reports are intentionally slanted to provide a sanitised version of
events, thus introducing inaccurate and misleading elements into the analysis.
This potential to mislead is clearly an element that has its roots in human factors. It appears that
there is a defensive mechanism at work that influences people to resort to doctoring tactics, the
consequences of which are eventually to deceive the end users of the scheme, i.e. incident
analysts and DP Operators to whom the information is of most use since they are able to learn
from DP failures elsewhere. This defensive mechanism is in part influenced by the competitive
market conditions that persist in the world of offshore DP vessel contracting, where DP incidents
can reflect poorly on the DP vessel and the company and can lead to contractual difficulties with
the client group.
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Recommendation
Companies to review the ways in which they implement the DPVOA incident reporting scheme,
in terms of the quality and content of information that is relevant to the incident and in the
distribution of the published reports. In addition, a climate of openness should be encouraged
where errors, incidents and failures are shared between all individuals involved.
8. DP Operator Training and Competence
The survey revealed that a considerable number of DP Operators interviewed considered that
the off- vessel DP Operator training did not fully equip them for real life DP operations. Initial
training was too general. Comparisons were made with other similar occupations, e.g. that of an
airline pilot. Pilots are required to undergo regular land-based simulator training and
competence assessment procedures. Application of similar schemes in the DP industry would
be of immeasurable value in improving awareness and assisting in raising standards of DP
Operator performance.
There was a strong view in favour of using DP simulator training to stretch DP Operators to the
limit, since to a large extent they are not put under sufficient pressurise in the training
environment. DP Operators should be trained for emergencies and should have to undergo
simulator programmes that provide extraordinary and emergency situations. It is evident from
the survey that the most experienced DP Operators have gained most of their experience of
coping with emergencies from real life emergencies. However, more recently trained DP
Operators do not have the same level of experience, nor in many cases are they likely to.
Improvements in the DP associated equipment and systems reliability have resulted in fewer DP
incidents, with fewer opportunities to learn from real life situations.
Training is generally considered to be one of the most powerful tools in ensuring the
competency of DP Operators to deal with routine and extraordinary actions and is directly linked
with the human factor element
Recommendation
The industry should assess the adequacy of current DP Operator training requirements and
standards, comparing them with similar occupations, make changes as necessary. Competency
standards and competency assessments should be established and implemented throughout
the DP industry.
9. Knowledge Base - Vessel Specific
All DP Operators should have an adequate knowledge of the vessel, on which they are serving.
It became apparent in the course of the project that procedures for ensuring adequate vessel
specific knowledge are haphazard with some DP Operators clearly having insufficient
knowledge of their current vessel. In one case DP Operators had no knowledge at all of their
vessel's FMEA. There are many reasons for carrying out analyses of DP failure modes and
consequences, one of the principal being to provide the DP Operators themselves with detailed
information on the specific idiosyncrasies of each vessel's DP system. No two systems are the
same. The consequences of failures can not always be relied upon to be repeated from vessel
to vessel.
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Recommendation
Companies to review the vessel specific procedures for ensuring that DP Operators are
adequately knowledgeable about the vessel on which they are serving and to implement
changes in management practices and standards as required.
10. Geographical Differences
On the evidence of the survey, safety standards and expectations of performance are not
applied consistently across the world. In particular, standards and expectations vary according
to geographical location. The survey identified that there is considerable room for abuse of
safety procedures in certain parts of the world, especially those where external controls such as
those exercised by clients and national authority are less intrusive. Temptations to relax safety
constraints is offset by the implementation of acceptable company specific standards and the
professionalism of the DP Operators themselves.
Recommendation
Review the role of national governments, IMO and clients and provide acceptable and
consistent standards and expectations throughout the world.
11. Fatigue
There is clear evidence to suggest that the performance of DP Operators is affected by fatigue.
Principal causes are sleep related, noise fatigue, operational stress, particularly when in bad
weather or operating close to the limits of acceptable safety margins. All of these features can
affect the ability of the DP Operator to perform his functions properly.
Sleep related problems figure highly as the most significant cause of fatigue, especially night
shift work. The shift pattern of 12 hours on followed by 12 hours off appears now to be the
normal for DP vessels. The advantage of this pattern is that it normally gives DP Operators the
opportunity to have a long spell off duty, during which they can pursue other interests, carry out
some form of recreational activity and get adequate sleep and rest, however, the down side is
the length of time on duty. This is especially significant in night shift working, i.e. 1800hrs to
0600hrs or 0000hrs to 1200hrs, where proper sleep is not always attained during the off-duty
hours and where the on-duty time covers the period when the body and the mind would rather
be asleep. The analyses of DP incidents should cover this aspect. Attempts should be made in
the analysis to identify evidence to link time of day or fatigue with the DP incident.
One of the customary ways of overcoming sleep related fatigue problems is to ensure that there
are two on watch. On most DP vessels this is a requirement. DP diving vessels have operated
the two man DP watch for many years and it is now enshrined in all DP diving operating
guidelines. Increasingly, other DP vessels have also adopted this system. The two man watch
has not been introduced specifically to overcome this problem yet, with two on watch it is
expected that one will be able to monitor the other for signs of fatigue and will- respond
accordingly if it is detected. However, this does not provide a cure for problems of this type and
a fatigued DP Operator is just as likely to "nod off' while in control of the DP console at 2 o'clock
in the morning on a two man watch as on a single man watch.
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Fatigue related symptoms also arise when DP Operators are experiencing under stimulation,
regardless of the time of day. The survey provided clear evidence to show that boredom, lack of
vigilance and intrusive thoughts can all combine to undermine DP Operator performance.
The question of fatigue should be considered in the analyses of all DP incidents.
Recommendation
Carry out further survey of incidence of fatigue on board DP vessels, assessing frequency and
potential consequences. This can be done on a company or vessel specific basis.
12. Irresponsible Behavioural Patterns
Although this was not considered by the DP Operators involved in the project as being of great
significance, the fact is that most had direct experience of other DP Operators acting in a
potentially dangerous manner, e.g. by violating rules, procedures and by taking risks. In line
with expectations no DP Operators admitted behaving in an irresponsible manner themselves.
Had anybody done so, it would have revealed an upright honesty bordering on insanity.
However, in the analyses of DP incidents
it is important to ensure that this aspect is considered thoroughly and with absolute impartiality.
Recommendation
Assess the extent of the problem and take appropriate remedial measures, including
which, companies should consider means of assessing the suitability of DP Operators before
appointment and during service.
13. Employment Conditions
The overriding opinion is that the professionalism of DP Operators is of a high standard and that
having accepted the professional responsibilities that go with the post, there is little likelihood
that standards will be undermined by matters such as employment conditions.
However, there was minority view that there is a correlation between terms and conditions of
employment, job security, etc. and attitudes of the DP personnel that could have safety
implications. Where morale is low, safety standards and general standards of operation can
suffer. In analysing the answers from the target group it is self apparent that the level of morale
is made up of a large number of variables. It is notoriously difficult to measure and it is very
easy to confuse morale with other variables. However, the survey did provide evidence to
indicate that where employment conditions are unfavourable and uncertain it is unlikely that
morale among DP Operators will be high.
The following quotes give an indication of the responses, - "Reduction in terms and conditions
could lead to attitude problems with the job " - "Lack of motivation could result, " - "Pay
peanuts and you get monkeys ". - "More stress means less safety".
Recommendation
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Companies should assess the methods they adopt for influencing the morale on board DP
vessels and should assess the practical operational benefits from steps taken to improve
morale.
14. Status of DP Operators in the Industry
In the survey it was identified that there are not enough opportunities for DP Operators to
contribute to the development of the industry. Their wealth of knowledge and experience are
generally underutilised.
Where efforts have been made to integrate DP Operators more positively into the management
of the DP operation and in project planning and management there appear to have been
favourable benefits. The company management of one of the DP vessels involved in the project
had made considerable efforts to give DP Operators a prominent role and status, e.g. by
involving them in project planning and management. This had resulted in a particularly positive
attitude being displayed by the DP Operators, with high levels of job satisfaction and associated
feelings of well-being as far as the onshore - offshore relationships were concerned. They were
generally more satisfied.
In the majority of cases, however, where the management style did not take such a positive line,
it was evident that, generally, DP Operators were less inclined to show positive attitudes
towards the company and were predisposed to express a more mercenary and utilitarian
attitude towards their work.
The relevance of this division, in terms of human factors, is that companies that take steps to
motivate their people in a positive way are promoting self esteem, job satisfaction and,
eventually improved performances and productive capacity.
Recommendation
Companies and the industry organisations should provide opportunities for DP Operators to
contribute to the development of the industry as a whole, such as on technical, operational and
training matters. Companies should also take heed of the obvious benefits that are associated
with positive management styles, where real efforts are made to help develop human
performance. This can be done by providing an appropriate management framework.
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Chapter 21- DP incidents
DP incidents happens every day. It is of vital importance for the continues improvement process
in every company, that all incidents are reported and handled properly for learning the lessons,
avoiding future incident and improving performance. IMCA has developed an incident reporting
scheme, which is widely used as template by many companies in the industry. The DP reporting
scheme can be found on IMCA web page. IMCA is also issuing quarterly and annual reports of
DP incidents in order for the entire DP industry to learn and improve.
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IMCA DP Event bulletin2016:
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Classic DP incident
(www.Dynamic-Positioning.com)
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Chapter 22 – Read more, Various links
IMO:
www.IMO.org
The Nautical Institute:
www.nautinst.org
OSVDPA:
www.osvdpa.org
IMCA:
www.IMCA-int.com
Marine Technology Society – DP Committee:
www.dynamic-positioning.com
DNV/GL:
www.dnvgl.com
Lloyds:
www.Lloyds.com
American Bureau of Shipping (ABS):
www.Eagle.org
IACS (other classification Societies):
http://www.iacs.org.uk/Explained/members.aspx
Guidelines for Offshore Operations (G-OMO):
www.g-omo.info
Kongsberg:
www.km.kongsberg.com
Marine Technologies:
www.Marine-Technologies.com
Rolls Royce Icon
http://www.rolls-royce.com/products-and-services/marine/product-finder/automation-andcontrol/positioning-and-manoeuvering/icon-dynamic-positioning-control.aspx#section-productsearch
Wärtsilä
www.wartsila.com/dp/products/platinum-dynamic-positioning
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Chapter 23 – Drawings
On the next pages you will find examples of drawings from various DP systems and vessels.
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Dan Swift – cable layout
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NB 190 – cable layout
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NB 190 – Cable Layout C-Joy system
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NB 190 – Cable Layout Position Reference Systems
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NB 190 – Cable layout DP Alert system
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NB190 – Single Line Diagram Power Set Up
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Kongsberg – Power and wiring for panel
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Kongsberg – Network Structure 2xOS and DPC2
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Kongsberg – Principle block diagram DPC 2
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Kongsberg – Principle Block Diagram I/O signals DPC2
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Kongsberg – Power and wiring diagram DPC2
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Kongsberg – Power and wiring diagram RBUS A and B section
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Kongsberg – 3 position change over switch system
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Kongsberg –cC-1 principle block diagram and I/O
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