DNVGL-RP-F115
Edition September 2019
Pre-commissioning of submarine pipelines
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RECOMMENDED PRACTICE
DNV GL recommended practices contain sound engineering practice and guidance.
©
DNV GL AS September 2019
Any comments may be sent by e-mail to rules@dnvgl.com
This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this
document. The use of this document by others than DNV GL is at the user's sole risk. DNV GL does not accept any liability or responsibility
for loss or damages resulting from any use of this document.
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FOREWORD
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Changes - current
CHANGES – CURRENT
This document is a republished version of the October 2018 amendment of the first edition of DNVGLRP-F115 (September 2016). No changes have been made to the content of this document.
Amendments October 2018
References to legacy DNV documents have been updated to reflect the new DNV GL portfolio.
Editorial corrections
In addition to the above stated changes, editorial corrections may have been made.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Page 3
The partners of the JIP were:
—
—
—
—
—
—
—
Dong Energy
Baker Hughes
Halliburton Pipeline & Process Services
Technip
Subsea 7
Nord Stream AG
Statoil
—
—
—
—
—
—
INPEX
Saipem
ConocoPhillips
IKM Testing
Atlas Copco
Wood Group Subsea
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Pre-commissioning of submarine pipelines
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Page 4
Changes - current
This recommended practice is one of a series resulting from a joint industry project (JIP) led by DNV GL to
reduce cost and increase predictability without compromising quality and safety in international offshore
development projects by recommending standardized parameter for selected items.
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Acknowledgements
Acknowledgements................................................................................. 4
Section 1 Introduction............................................................................................ 7
1.1 General............................................................................................. 7
1.2 Objective...........................................................................................7
1.3 Scope and application.......................................................................7
1.4 Limitations........................................................................................ 7
1.5 Structure of this recommended practice...........................................8
1.6 Referenced standards....................................................................... 8
1.7 Definitions.........................................................................................9
1.8 Units............................................................................................... 13
Section 2 Pre-commissioning philosophy.............................................................. 14
2.1 General........................................................................................... 14
2.2 Pipeline system design................................................................... 14
2.3 Flooding, cleaning and gauging...................................................... 15
2.4 Pressure testing..............................................................................15
2.5 Dewatering and drying................................................................... 15
Section 3 Flooding, cleaning and gauging.............................................................17
3.1 Flooding.......................................................................................... 17
3.2 Cleaning.......................................................................................... 19
3.3 Gauging...........................................................................................21
Section 4 System pressure test............................................................................ 24
4.1 Objectives....................................................................................... 24
4.2 Pressure test operation.................................................................. 25
4.3 Acceptance criteria......................................................................... 29
Section 5 Dewatering, drying, inerting and product filling....................................31
5.1 Dewatering..................................................................................... 31
5.2 Drying............................................................................................. 34
5.3 Inerting...........................................................................................38
Section 6 Miscellaneous issues............................................................................ 40
6.1 Operational principles..................................................................... 40
6.2 Documentation................................................................................41
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Contents
Changes – current.................................................................................................. 3
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CONTENTS
6.5 Temporary pre-commissioning equipment...................................... 46
6.6 Pre-commissioning fluid................................................................. 48
6.7 Corrosion assessment and water treatment................................... 49
6.8 Other considerations/special systems............................................ 52
Section 7 References.............................................................................................54
7.1 References...................................................................................... 54
Appendix A Checklist for flooding, cleaning and gauging......................................55
A.1 Checklist for flooding, cleaning and gauging.................................. 55
Appendix B Checklist for system pressure testing................................................ 57
B.1 Checklist for system pressure testing........................................... 57
Appendix C Checklist for system test pressure calculation................................... 60
C.1 Checklist for system test pressure calculation................................ 60
Appendix D Checklist for dewatering, drying and inerting.................................... 65
D.1 Checklist for dewatering, drying and inerting.................................65
Changes – historic................................................................................................ 67
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Contents
6.4 Pig traps (launchers and receivers) and test heads........................ 46
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6.3 Pigs and pig trains......................................................................... 43
1.1 General
The pre-commissioning phase is an important and essential part of any pipeline project, normally consisting
of the following activities:
Figure 1-1 Pre-commissioning operations
Note that some pipelines do not require drying/inerting (e.g. water injection, production and MEG lines).
Most pipeline codes have some requirements to gauging and pressure testing. However, this is the first
recommended practice covering pre-commissioning operations specifically as a whole, e.g. taking into
consideration overall design, system arrangements, management and planning, functional requirements and
execution principles for pre-commissioning operations of a pipeline system.
1.2 Objective
The objective of this document is to provide guidance on safe, reliable and effective planning, design and
execution of pre-commissioning activities for submarine pipeline systems.
1.3 Scope and application
This recommended practice is intended to be used as supplement to DNVGL-ST-F101, i.e. submarine pipeline
systems. The philosophy in this report applies to new pipeline systems as well as existing pipeline systems
e.g. subject to repairs.
1.4 Limitations
The following general limitations apply:
— Flexible pipe systems are not covered by this recommended practice; however flexible pipe section,
e.g. flexible tails, or flexible risers, may be pre-commissioned along with the rest of the pipeline system
according to this recommended practice.
The following operations – and the sequence of these – are commonly defined as commissioning operations
and not further considered in this recommended practice:
— Product filling; introduction of product into the pipeline. Pending degree of pre-commissioning operation,
product filling may involve use of pigs as barriers for separation.
— Decommissioning; temporarily or permanently taking an operating line out of service. Pending system
status and purpose of decommissioning, this will normally include use of pigs for removal of the product
from the pipeline system.
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SECTION 1 INTRODUCTION
Sometimes the dewatering pig train may be driven by the product and then dewatering will be part of the commissioning.
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1.5 Structure of this recommended practice
This recommended practice is structured to constitute the basis for a pre-commissioning manual (project
specific).
The following operations – and the sequence of these - are commonly defined as standard for precommissioning:
1)
2)
3)
4)
5)
6)
Flooding; flooding the pipeline with the pre-commissioning fluid, normally by use of separation pig(s) in
order to remove air and/or in combination with cleaning. See [3.1].
Cleaning; removal of internal debris from the pipeline following installation. Normally performed by
running one or several pigs through the pipeline. See [3.2].
Gauging; verification of the pipeline’s cross section profile. Normally performed by running one or more
pigs with a metallic plate of specified size through the length of the pipeline. Alternative to gauging
plates are instrumented gauging tools/calliper pigs. See [3.3].
System pressure testing; verify that the pipeline system is free from gross errors and leakages. Normally
performed with water or other incompressible media. See Sec.4.
Dewatering; removal of the pre-commissioning fluid. Normally performed by running one or more pigs as
separation/barrier between various media. See [5.1].
Drying and inerting; removal of residual pre-commissioning fluid and lowering the dew point to
acceptable level. Normally most relevant for gas/condensate systems and normally involves nitrogen/dry
air purging or vacuum drying, alternatively MEG swabbing can be considered. See [5.2] and [5.3].
Guidance note:
Note that the above operations may be performed in a different order or in combination, and that one or more operations could be
omitted.
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1.6 Referenced standards
1.6.1 Relationship to other standards
The following standards include requirements which, through reference in the text constitute requirements of
this document.
References are either defined as normative or informative. Normative references in this document are
indispensable for its application. Informative references provide additional information intended to assist the
understanding or use of the document.
Guidance note:
Normative references are typically referred to as ‘testing shall be performed in accordance with ISO xxx’, while informative
references are typically referred to as ‘testing may be performed in accordance with ISO xxx or ISO yyyy’, or ‘recommended
practice for testing is given in DNVGL-RP-F xxx’.
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In case of conflict between requirements of this document and a referenced DNV GL service documents,
the requirements of the document with the latest revision date shall prevail. Any conflict is intended to be
removed in next revision of that document.
Where reference is made to standards other than DNV GL service documents, the valid revision should be
taken as the revision which was current at the date of issue of this standard.
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Guidance note:
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1.6.2 DNV GL standards
The latest revision of the following document applies.
Document code
Title
DNVGL-ST-F101
Submarine pipeline systems
1.6.3 DNV GL recommended practices
The latest revision of the following documents applies:
Document code
Title
DNVGL-RP-A203
Technology qualification
DNVGL-RP-F109
On-bottom stability design of submarine pipelines
DNVGL-RP-F113
Pipeline subsea repair
DNVGL-RP-N101
Risk management in marine and subsea operations
1.6.4 Other standards
Document code
Title
API 5L
Specification for Line Pipe
API 17B
Recommended Practice for Flexible Pipe
ASME B31.4
Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids
ASME B31.8
Gas Transmission & Distribution Piping System
ISO 13623
International Standard, Petroleum and natural gas industries – Pipeline transportation systems
ISO 8573
Compressed air - Part 1: Contaminants and purity classes
ISO 9001
Quality management systems – Requirements
1.7 Definitions
1.7.1 Definitions
Table 1-1 Definition of terms
Term
Description
acceptance criteria
specified indicators or measures providing an acceptable level and that are used in assessing
the ability of a component, structure or system to perform its intended function
battery limit
the interface between the pipeline or pipeline system and adjacent facilities not defined as
parts of the pipeline or pipeline system
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Pre-commissioning of submarine pipelines
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Description
components
bends, fittings, flanges, valves, mechanical connectors, isolation joints, anchor flange, buckle
arrestor, pig traps, clamps, couplings etc.
debris
the undesirable particles and objects present in the pipeline such as rust, loosely adhered mill
scale, weld splatter, construction objects, sand, rubble, litter etc.
free flooding
flooding operation performed without the use of pumps, regardless of the use of pig or not
golden welds
critical welds e.g. tie-in welds that will not be subject to pressure testing, where 100%
ultrasonic testing, 100% radiographic testing, and 100% magnetic particle testing or 100%
liquid penetrant testing of non- ferromagnetic materials shall be performed, see DNVGL-STF101 Sec.10 for more details
gross error
a gross error is referred to as a significant error or damage present in the system that normally
should have been discovered by quality control activities prior to or during pipeline installation
Damages caused during ploughing, other trenching methods or third party damages e.g.
anchor, anchor chains or wires are all to be considered as gross errors. HISC issues and other
defects (e.g. creep mechanisms causing failure after a number of hours) are also referred to as
gross errors.
hold period
the time the pressure is kept within the system after stabilisation is confirmed
incidental pressure
the maximum internal pressure the pipeline or pipeline section is designed to withstand during
any incidental operating situation (including dynamic effects)
leak test
test to detect leakages in the pipeline system or local connections
mill pressure test
the hydrostatic strength test of each joint performed at the mill
pipeline system
pipeline with compressor or pump stations, pressure control stations, flow control stations,
metering, tankage, supervisory control and data acquisition system (SCADA), safety systems,
pig launchers and receivers (if relevant), corrosion protection systems, and any other
equipment, facility or building used in transportation of fluids
product
fluid transported in pipeline
safety class
a concept adopted to classify the significance of the pipeline system with respect to
consequence of failure, see DNVGL-ST-F101 Sec.2
stabilisation period
from the time the test pressure has been reached and until the commencement of the hold
period
strength test
the strength test shall reflect the highest utilisation the part will be exposed to during its entire
life
For linepipe see definition of m pressure test.
system pressure test
final test of the complete submarine pipeline system to reveal gross errors or leaks
submarine pipeline
system
please see definition in DNVGL-ST-F101 Sec.1
treated water
shall be understood as water which has been treated with additives
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Term
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1.7.2 Abbreviations
Table 1-2 Definition of abbreviations
Term
Definition
CRA
corrosion resistant alloy
FCG
flooding, cleaning, and gauging
FEED
front end engineering design
HAZOP
hazard and operability study
HISC
hydrogen induced stress cracking
HSE
health, safety and environment
ID
pipeline inner diameter
ILT
in-line tee
MEG
methyl ethylene glycol
MIC
microbiologically influenced corrosion
PLEM
pipe line end manifold
PPE
personal protection equipment
QHSE
quality, health, safety and environment
RP
recommended practice
SCADA
supervisory control and data acquisition system
SJA
safe job analysis
SMYS
specified minimum yield stress
SRB
sulphate reducing bacteria
UV
ultra violet
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1.7.3 Symbols
Table 1-3 Definition of symbols
Symbol
SI-Unit
Definition
[1/K]
coefficient of volumetric thermal expansion of pipe wall material
-
pressure test ratio, see DNVGL-ST-F101 Table 5-8 (typically 1.05)
[1/K]
coefficient of volumetric thermal expansion factor of test medium
[J/kgK]
heat capacity of test medium
[-]
discharge coefficient
[m]
inner diameter of pipe
[m]
outer diameter of pipe
[Pa]
Young’s Modulus
2
[m/s ]
gravity constant = 9.81
[Pa]
bulk modulus of test medium
[m]
pipeline length
[kg]
mass of test medium at end of test hold period, at (
[kg]
mass of test medium at start of test hold period, at (
[kg/kmol]
air molecular weight (= 28.97)
[-]
Poisson ratio = 0.3
[-]
incidental to design pressure ratio
[Pa]
pressure outside pipe
[Pa]
internal design pressure
[Pa]
incidental pressure
[Pa]
local incidental pressure
[Pa]
local test pressure
[Pa]
required local test pressure
[Pa]
governing system test pressure (considering actual test medium density)
[Pa]
differential pressure
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)
)
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SI-Unit
3
Definition
[m /s]
volume rate
[J/kmole-K]
universal gas constant = 8314.5 J/kmole-K
3
[kg/m ]
density of test medium
[s]
time
[s]
test hold period
[°C]
temperature
[°C]
temperature variation
2
[W/m °C]
total heat transfer coefficient for steel wall, insulation, coating and surroundings
3
actual pipe volume at pressure
[m ]
3
pipe volume with zero overpressure
[m]
water depth
[m]
pipe wall thickness
[-]
volume fraction of air before pressurizing
[-]
volume fraction of air at start of test hold period
[-]
compressibility of gas
[m ]
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Symbol
and temperature
1.7.4 Verbal forms
Table 1-4 Definition of verbal forms
Term
Definition
shall
verbal form used to indicate requirements strictly to be followed in order to conform to the document
should
verbal form used to indicate that among several possibilities one is recommended as particularly suitable,
without mentioning or excluding others, or that a certain course of action is preferred but not necessarily
required
may
verbal form used to indicate a course of action permissible within the limits of the document
1.8 Units
SI units are recommended.
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SECTION 2 PRE-COMMISSIONING PHILOSOPHY
2.1 General
The pre-commissioning philosophy shall be established considering as a minimum the following:
— pipeline system configuration
— pipeline characteristics:
— product
— diameter, features and length
— seabed topography
— pre-commissioning system design:
— overall pre-commissioning installation/pre-commissioning sequence
— pre-commissioning battery limits
— component spread requirements:
— compressors and pumps
— pigs and pig trains
— pig launchers and receivers
— pre-commissioning fluids
— space requirements
— vessel requirements
— access to pipeline ends or on intermediate structures (ILTs, PLEMs ....)
— outline procedures including:
—
—
—
—
flooding, cleaning, and gauging - method and acceptance criteria
system pressure testing - method and acceptance criteria
dewatering, drying and inerting - method and acceptance criteria
contingency plan
— special system evaluations:
—
—
—
—
—
—
—
pre-commissioning fluid, chemical treatment and filtration
filling direction, water disposal
codes break (onshore/offshore)
multi-diameter pipeline, connect to live pipeline
material considerations (seawater, exposure limitations)
winterisation/freeze protection
subsea connectors flooding caps, pressure caps and their associated tooling
— quality and HSE considerations
— planning, schedule and engineering considerations
— documentation requirements.
2.2 Pipeline system design
The pipeline system design should address pre-commissioning and commissioning operations. These issues
should be considered at the earliest stage during FEED phase.
System design should allow for access and connection of relevant equipment for performing necessary precommissioning and commissioning work.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Page 14
— provision and location of permanent/temporary pig traps, where applicable
— venting, isolation and draining arrangements relative to pig launch/receipt, where applicable, pressure
testing and future tie-in
— pre-commissioning fluid injection (such as dead leg flushing)
— possibility for proper removal of air during filling with pre-commissioning fluid
— pigging direction(s), where applicable
— piggable item list covering, where applicable:
— minimum allowable bend radius
— minimum distance between bends, Tees, Wyes and other components
— maximum allowable internal diameter change with tapering requirements when/if change in inner
diameter is required
— internal coating evaluations in relation to effects on dewatering/drying and types of allowed/
recommended pigs to be used (gas lines)
— subsea connection and flushing/purging of relevant piping/volumes (operational pigging)
— fluid diversion in case of subsea pig launcher/receiver used during operational life (kicker and bypass
lines)
— system component specific design requirement, such as subsea connectors cladding or ID matching the
pipe ID, full bore valves and valve seats capable of withstanding the differential pressure generated by
the water column and test pressure, valves seats capable of withstanding void in case vacuum drying is
foreseen, etc.
2.3 Flooding, cleaning and gauging
A philosophy for flooding, cleaning and gauging shall be established considering pre-commissioning fluid
source location and quality, need for filtering and additives, the direction of flooding, free flooding or using
pump spread or a combination, see checklist in App.A.
Vessel requirements for flooding, pig tracking, pig retrieval shall be established, in case of vessel operations.
Special cleaning requirements through all phases of the project, e.g. internal sand blasting, use of end
protectors, handling procedures, and inspection need to be established.
Gauging technique for multi diameter pipeline systems shall be decided.
Confirmation of acceptance criteria shall be ensured.
Contingency possibilities shall be evaluated.
2.4 Pressure testing
A philosophy for pressure testing shall be established considering individual component testing, hold periods,
codes and code breaks, test sections and golden welds.
Vessel requirements for pressure testing and leak detection shall be established, in case of offshore
operations. Contingency possibilities shall be evaluated. See checklists in App.B and App.C.
2.5 Dewatering and drying
A dewatering and drying philosophy shall be established considering all relevant parameters, see checklist in
App.D.
Requirements to inerting and preservation shall be established.
Figure 2-1 illustrates the decision making process of dewatering and drying operations in general.
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The design should as a minimum include for:
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Figure 2-1 Dewatering and drying flowchart
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Recommended practice — DNVGL-RP-F115. Edition September 2019
3.1 Flooding
3.1.1 Objectives
The objective of flooding is to prepare for cleaning, gauging, tie-in and pressure testing of the pipeline
system. The flooding, cleaning and gauging operations may be combined.
Guidance note:
Flooding may also be required in order to stabilise the pipeline on the seabed and increase the submerged weight prior to
trenching.
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3.1.2 Flooding operation
3.1.2.1 Flooding technique selection
Flooding may be performed either by pumping or free flooding.
The use of pig(s) as an interface between the pre-commissioning fluid and empty pipeline is the preferred
method for flooding, see [3.1.2.4]. This interface, when propelled in a steady and controlled manner will
minimise residual air and assist with removal of any construction debris present.
3.1.2.2 Fluids
Requirements and recommendations to quality of pre-commissioning fluids are given in [6.6] and [6.7].
Pre-commissioning fluid should be sampled and analysed, in due time before the operation, to verify if
additives are required and/or to determine the appropriate additive treatment strength.
Samples of the pre-commissioning fluid should be taken during flooding operation, where practicable, to
document actual properties.
When pre-commissioning operations are performed from environmentally sensitive area, specific
considerations should be taken to minimize as much as possible the impact of such fluids in case of pipeline
failure.
3.1.2.3 Flooding direction
Flooding direction should preferably be from the deep end towards the shallow end of the pipeline to
minimize residual air content and to limit the risk of excessive pig speed.
If this is not possible – given system or operational constraints - due considerations should be made with
respect to any need to ‘back-pressure’ the pipeline with compressed air or by partially or completely preflooding the line before the pig run (as a measure for pig speed control).
In addition, the direction may be dictated by other aspects associated with the pipeline design, e.g. wyes,
non-return valves.
Special care should be taken in case of pressurised air systems. Allowable pressures should be kept
significantly below the systems design pressure.
3.1.2.4 Flooding with pigs
Use of separation pig(s) as interface between the pre-commissioning fluid and empty pipeline is the preferred
method for flooding a pipeline. This is in order to:
— Create a defined interface between the filling fluid and air within the pipeline.
— Ensure that any air bypassing the initial filling pig is captured in the slugs of fluid between the following
pigs in the pig train.
— Remove debris from the pipeline and to keep loose debris in suspension so that it can be discharged from
the receiving end of the line (pig receiver). See [3.2].
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Pre-commissioning of submarine pipelines
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SECTION 3 FLOODING, CLEANING AND GAUGING
— pig launchers and receivers must be designed to accommodate the required number of pigs including
expected debris and accommodate the required flow rates of the operation
— pig design
— pig speed and pig separation control, see [6.3.3] and [6.3.2]
— suitably sized filling equipment
— volume of fluid supply reservoir
— need for pre-flooding.
Adequate sized filters/strainers shall be used to limit the ingress of debris, see [6.6].
3.1.2.5 Free flooding
A 'free flooding' operation is performed without the use of pumps. The differential pressure between pipeline
internal and external pressure at seabed is the driving force. This flooding is done with or without use of
pigs. Free flooding operations are normally followed by further flooding operations by use of pumps (at either
topside or subsea) and pigs to remove residual air, cleaning and gauging.
Guidance note:
If the pipeline requires to be re-pigged after free flooding, the accumulative effects of entrapped air at local high points should be
taken into account when selecting the pumping equipment and effect on pigs.
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Considerations should be given to sizing inlet piping to limit water velocity.
Adequately sized filters/strainers should be used to limit the ingress of particles and organic material. Free
flooding generates a risk of ingress of unknown quantity of solids such as sand and organic materials into the
pipeline. To minimise the above risk, water supply should be taken at a suitable distance above the seabed.
If the free-flooded fluid does not contain any chemical treatment, consideration to exposure time to
untreated water and entrained debris should be given.
Guidance note:
In some cases, a pig may be launched during free flooding operations to ensure the interface between the fill water and the empty
pipeline is maintained. This is usually carried out when the topography of the pipeline is ‘undulating’.
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3.1.3 Special considerations
For pipelines located in areas with extreme topography or riser sections, excessive pig speeds resulting
from steep inclination of various sections may be expected. Gradients greater than 1:10 in conjunction with
elevation changes in the order of tens of metres will generally result in unstable pig speeds.
The detailed flooding flow behaviour of the pipeline should be analysed to determine if the pig speed
instability is detrimental to the pigging operation, otherwise the following methods may be considered:
—
—
—
—
—
pumping water in front of the pig train
free flooding
air packing the pipeline to an appropriate pressure prior to pigging
fill against a controlled discharge to allow pressure to build up in front of the leading pig
use of high friction pigs.
In smaller diameter (<0.152 m (6")) pipelines, it is advisable that filling is carried out without the use of
pigs, although solid cast or foam pigs may be evaluated if a barrier in front of the filling fluid is required. See
[3.1.2] and [6.3.2].
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The following should be considered:
For requirements to the pre-commissioning fluid, see [6.6].
For requirements for additives, see [6.7].
For temporary equipment, see [6.5].
3.1.5 Acceptance criteria
Flooding is considered acceptable when:
— pipeline filled with pre-commissioning fluid in accordance with the requirements and successful arrival
of all pigs
— successful air inclusions check (to be performed during system pressure test).
3.2 Cleaning
3.2.1 Objective
Cleaning as part of pre-commissioning operations is usually conducted in order to ensure that debris
is removed together with any foreign objects which may have been deposited into the pipeline during
construction activities.
The objective of cleaning operation will be to:
— Ensure the product in the pipeline is not contaminated.
— Allow effective use of product corrosion inhibitors during operations, hence if the internal pipe surface is
corroded these product inhibitors cannot come into contact with the surface they are intended to protect.
— Ensure flow-efficiency. This particularly applies to longer pipelines where flow-effects due to debris and
internal roughness are of larger importance.
— Prevent damage to permanent equipment, e.g. valves.
3.2.2 Cleaning operation
3.2.2.1 Cleaning technique selection
Pipeline cleaning may be performed by the following methods or in combination:
—
—
—
—
pigging
high velocity water flushing
high velocity air flushing
gel cleaning.
3.2.2.2 Pigging
On larger diameter (≥0.152 m (6")) pipelines the most common and preferred cleaning technique during
pre-commissioning operations is to use bi-directional pigs, with or without brushes. In most cases these pigs
also have magnets attached to pick up material i.e. welding wire, bolts, nuts, pig brush bristles, etc. The
more pigs that are passed through a pipeline, the better the cleaning effect will be.
Metal brush pigs shall generally not be used in CRA pipelines in order to avoid iron inclusions that will lead to
unsightly staining or initiate pitting corrosion. Brushes and pigs should be made from a suitably compatible
chrome nickel alloy or from non-metallic materials. In many cases aggressive mechanical brush cleaning
of CRA’s is not advised as this will disrupt the 'passive' layer on the material. As this material is particularly
susceptible to contamination by carbon steel the use of magnetic pigs for removal of ferrous materials is
advised.
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3.1.4 Technical requirements
In internally coated pipelines, brushes (metallic) shall not be used. However, nylon brushes may be proposed
if verified to be suitable for the type of internal coating.
If a pipeline is expected to have much debris, bypass ports may be included in the pigs.
3.2.2.3 High velocity water flushing
In smaller diameter pipelines (<0.152 m (6")) where pigs are not used, or found unfeasible, cleaning may be
carried out by ‘high velocity’ water flushing.
Selection of required flushing rate (velocity) should consider:
—
—
—
—
—
—
pipeline diameter
pipeline length
pipeline internal roughness
pipeline design/operation pressure
viscosity of flushing medium (to verify level of turbulent flow)
available flushing spread capacity (flow/pressure).
There is no common industry accepted velocity range for ‘high velocity’ flushing, however, a minimum
velocity of 1 m/s is commonly used.
Pressure/flow plots should be generated from the operation for subsequent correlation and comparison with
calculated pressure/flow profile.
Debris flushed from the pipeline should be collected and analysed to confirm effectiveness of the cleaning,
where feasible and if required.
3.2.2.4 High velocity air flushing
High velocity air flushing is generally limited to short, smaller diameter pipelines. 'Oil free' compressed air is
run through the pipeline and debris blown out at one end into a ‘knock out’ drum. There is a safety issue with
this type of cleaning due to the high energy expansion of the air used.
Effectiveness of air flushing to transport debris/particles increases with increasing pressure (air density)
and velocity. Due to the lower air density compared to water, a higher flushing velocity is normally required
compared to water flushing. Minimum air flushing velocity required transporting debris/particles of a certain
size and characteristics in a horizontal pipe section may be estimated based on available transport models.
An upper limit on air flushing velocity should be specified considering risk of erosion and damage to system
due to direct debris impact.
3.2.2.5 Gel cleaning
Gel cleaning may be used where a significant amount of debris is expected and there is a high risk that a
brush/cleaning pig may get stuck due to build-up of debris in front of the pig.
Gel enhances the efficiency of the pigs as they:
— seal the pig
— prevent build-up of debris in front of the pig by keeping the debris in suspension
— further lubricates the pig which reduces wear.
A trailing pig (foam/sealing pig) behind the gel would normally be recommended in order to have a defined
interface to pump against during gel cleaning operations.
Guidance note:
A ‘pick-up’ gel is used to keep debris in suspension and away from the pigs. A ‘sealing gel’ at either end of the gel pig train
protects the ‘pick-up’ gel from the propelling medium which may be liquid, product, air or nitrogen.
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Metal brush pigs may be used in flexible pipelines with internal metal carcass provided they are compatible
with the carcass material. Metallic brushes shall not be used in flexible pipe sections without an internal
metal carcass.
If it is required to run pigs through permanent mainline valves, consideration to valve protection/pre-cleaning
before pigging shall be given.
Debris from the pipeline may block or contaminate small bore pipe work and instrument branches.
Consideration shall be given to blanking or removing these branches/items.
Design of the pig receiver and discharge pipe work should consider preventing blockage by providing
adequate space in front of the pigs for any debris pushed by the pigs.
If using gel as a part of the pre-commissioning gel intruding into small bore piping and/or gel not sufficiently
removed and being transported into the process systems may cause problems and needs to be considered.
3.2.4 Acceptance criteria
The standard acceptance criteria is to reduce the ‘debris’ in a pipeline to a level that will not affect pipeline
performance, receiving facilities and its components during operation. The acceptance criteria shall be
defined or agreed by the operator.
The preferred method for determining pipeline cleanliness shall be evaluated and selected for the individual
pipeline system. This could be based on the following:
— Weight of debris in front of last pig and between intermediate pigs (trend of recovered debris).
— Disk condition for the recovered pigs.
— Analysis of water discharged from the pipeline in front of, between pigs, and after the pig train.
Note that the first and last of these options may not be feasible for subsea pipeline with both ends subsea.
3.3 Gauging
3.3.1 Purpose
The purpose of gauging is to confirm that the bore is of an acceptable minimum diameter, suitable for
transportation of the product and the future running of pigs.
Gauging shall also be performed to indicate that no larger damage/dents or excessive ovalisation/local
buckling has been introduced during installation of the pipeline.
Gauging should be undertaken upon completion of pipeline system installation and of all operations likely to
induce impact on the pipeline, e.g. trenching, backfilling, pipeline lifting and lowering etc.
3.3.2 Gauging operation
3.3.2.1 Pigging with gauge plate
Gauge plates only provide information on the maximum deviation in pipeline cross section. If the gauge plate
is damaged during its run through the pipeline it normally gives no indication of the following:
— Location damage occurred.
— Number of damaged areas.
— Type of obstruction that caused the damage, i.e. pipe dent, debris, bends or valves (gauge plate on
several pigs may aid in deciding if the damages are due to debris).
Gauge plates are only suitable for gauging the minimum diameter in multi-internal diameter pipelines. In
such case a calliper pig will be required.
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3.2.3 Special considerations
3.3.2.3 Calliper pig
These can be mechanical lever callipers, or high standoff electronic measuring devices. Depending upon
design they will give minimum and maximum ID based on points on the clock face around the pipe. The more
advanced electronic devices, in conjunction with an odometer will give a bore map of the entire pipe.
Some electronic gauging pigs are bi-directional.
Specific care shall be taken in the design of the electronic housing and sleeping mode should be
recommended to save the battery life.
Guidance note:
Calliper pigs are sometimes used during pre-commissioning, in addition to regular gauging, to perform a baseline survey of the
complete pipeline system to be used as reference for later inspection runs. Later inspections should then use equivalent pig
configuration.
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Bi-directional calliper pig should be run if:
— Damaged gauge plates require further investigation.
— The pipeline system has variations in nominal ID and it is required to prove the ID of the larger ID
sections.
— It is required to prove that the pipeline system is free of damages, dents or buckles beyond what can be
detected by the selected gauge plate size.
— It is required to document that no excessive ovalisation has taken place.
3.3.3 Technical requirements
3.3.3.1 Gauge plate material and fabrication
The following should be used as basis for the design and fabrication of gauge plates:
—
—
—
—
—
Aluminium construction, between 3 mm and 10 mm thick.
Chamfered 45° x half plate thickness on leading edge.
Slotted, with 8-12 slots (depending on pig size).
Gauge plate size should comply with the requirements given in [3.3.5].
Size (OD) should be stamped on face of plate and visible when installed on pig.
3.3.3.2 Gauge plate location
It is good practice to have gauge plates on several of the pigs in the flooding, cleaning and gauging pig train.
As a minimum one gauge plate should be fitted on the last pig of the pig train to reduce the risk of damage
to the plate by debris.
The gauge plate shall be mounted on the pig in such a way that it is not damaged by contact with pipe wall in
bends or with preceding discs during running or during pig insertion/retrieval, and normally on the rear disc
package, close to the guiding disc.
For pigs that need to be back loaded, precautions to prevent interference/contact to the gauge plate from
guide and seal discs need to be considered in the pig design.
3.3.3.3 Calliper pig
Calliper pig shall comply with technical requirements stated in DNVGL-ST-F101 Sec.10. The reporting
specification of a calliper pig should be equal or better than a gauge plate.
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3.3.2.2 Instrumented gauge plate
An instrumented gauge plate gives an indication of whether a gauge plate is damaged or not via a signal that
is activated if the gauge plate is damaged. Thus it indicates the gauge plate condition prior to removal of the
gauge pig from the pipeline. The signal may also aid in localization of an indicated damage.
3.3.4.1 Multi diameter pipelines
For gauging pipelines with variable internal diameters, 'intelligent' gauging tool (bi-directional calliper pig)
should be used.
Gauge plates are ineffective on multi-diameter pipelines as they have a fixed size based on the smallest pipe
internal diameter and therefore the larger diameter pipeline sections cannot be checked. The gauge plate
proves the internal bore of the pipeline at a given minimum internal diameter and ‘passage’ of a gauge plate
may be used to ensure passage of a future calliper pig.
3.3.4.2 Expansion pools
Pigging onshore, i.e. pull-through of gauge plate(s), may not sufficiently confirm pigging ability of the spools
after installation as these types of measurement 'tools' always will be positioned in the lower part of the pipe
during the pull-through operation. Use of a 'wheel trolley' or centralising discs as part of the gauging device
may solve this problem.
3.3.5 Acceptance criteria
A gauge plate is acceptable if no deflection of the plate is observed.
If several gauge plates are used at least one of the gauge plates shall be acceptable.
The gauge plate may show minor abrasions and ‘nicks’ on the plate edges. Any such defects shall be
analysed and subject to operator’s acceptance.
A damaged gauge plate is indicative of a reduction in pipeline bore either by debris, damage to the pipeline
or partially closed valve. To conduct an investigation the following considerations would be required:
—
—
—
—
—
—
Occurrence of high pressure spikes on the pressure recording from the pig run.
Calculation of pig position at high pressure spikes.
Likely areas of concern on pipeline i.e. component location, changes in pipe diameter.
Levels of debris discharged.
Past history of pipeline installation.
The gauging shall document that the minimum ID is above a given requirement. The gauge plate size
shall be according to DNVGL-ST-F101 Sec.10.
Guidance note:
Care must be taken during gauge pig loading and unloading so as not to cause any damage.
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3.3.4 Special considerations
4.1 Objectives
According to DNVGL-ST-F101 the pipeline system shall be subject to a pressure test after installation. The
system pressure test shall be considered as a test for:
— gross errors
— leakages.
4.1.1 Gross errors
For a new pipeline system the system pressure test will enable the confirmation that the pipeline system is
acceptable with reference to risk associated with gross errors, such as:
— Damages caused during pipe handling, transportation, storage, pipeline installation and tie-ins.
— Damages caused during ploughing, other trenching methods, by anchor, anchor chains, wires etc.
4.1.2 Leakages
The system pressure test will enable the confirmation that the pipeline system, including pipeline and
components, is free of leakages. Note, however that the system pressure test only will detect leakages above
a certain threshold depending on pipeline volume and holding period.
Procedure for localization of any leaks discovered during system pressure testing should be developed as part
of the system pressure test procedure.
4.1.3 Limitations
The system pressure test is not suitable to prove structural capacity of the pipeline system; hence the test
should not be considered a strength test.
Guidance note:
The structural capacity of each linepipe should be confirmed by the mill pressure test, see DNVGL-ST-F101. The mill pressure test
should constitute the highest utilisation (hoop stress) of the line pipe since the consequence of failure will be least.
In the same way as for linepipes, all pipeline components (e.g. connector, flanges, wyes, tees, valves etc.) must have been
previously tested at a pressure in line with the governing code for than component.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note:
For a typical system test pressure utilisation of the girth weld will not be sufficient to detect a structural weakness in the girth
weld apart from gross errors. The structural capacity of the girth weld must be ensured by proper welding procedures and quality
control, see DNVGL-ST-F101.
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4.1.4 Waiving of system pressure test
The pipeline system pressure test may be waived. See DNVGL-ST-F101 Sec.5.
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SECTION 4 SYSTEM PRESSURE TEST
4.2.1 When to perform the system pressure test
All intervention work on the submarine pipeline system, including crossings, trenching, gravel dumping,
artificial backfill, subsea assemblies, riser installation, as-built surveys etc. should be completed before the
final pressure test commences, see DNVGL-ST-F101 Sec.10.
Guidance note:
Project specific requirements for inspection and/or installation constraints may require that the system pressure test is performed
prior to completion of all intervention work. Trenching, post-lay rock dumping and as-built surveys may be performed after the
system pressure test if it is considered unlikely that the pipeline system will be damaged during these activities.
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4.2.2 System battery limits and isolation
The extent of the system pressure test should normally include all permanent components and connections
within the pipeline system.
The system may be tested as separate sections. This requires that the tie-in welds between sections are
‘Golden welds’.
Pipeline components that will be exposed to the test pressure shall previously have been tested to a pressure
not less than the local system test pressure.
Testing should not be performed against in-line valves, unless possible leakage and strength of the valve is
considered. If testing towards valves cannot be avoided, the most conservative approach would be to assume
that the valve is leak tight.
Tie-in of a new pipeline system into an existing system will require special considerations.
The pipeline system code breaks shall be defined during pipeline system design.
4.2.3 Specification of test pressure
The required system test pressure along the entire pipeline shall be established in accordance with DNVGLST-F101.
The required local system test pressure during the test shall meet the following requirement:
— safety class (in operation): medium and high
— test pressure (local) = 1.05 x incidental pressure (local)
— safety class (in operation): low
— test pressure (local) = 1.03 x incidental pressure (local).
Guidance note:
The test pressure is the internal pressure at a specified elevation. Normally the reference point for the specified test pressure is the
same as the point of pressure monitoring. If this is not the case the test pressure at the point (elevation) of pressure monitoring
shall be established based on difference (in elevation) between point of reference and point of monitoring.
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The checklist attached in App.C may be used as reference for specifying test pressure.
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For a pipeline system comprising of different parts (rigid pipeline with flexible riser or flexible tail or pipeline
with different wall thickness), the part with the lower allowable test pressure shall determine the overall
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4.2 Pressure test operation
4.2.4 Specification of hold period
The required test pressure hold period after stabilisation is minimum 24 hours, see DNVGL-ST-F101. For
3
pipeline volumes of less than 5000 m shorter holding periods may be agreed (between company and
contractor during the design phase).
Guidance note:
In case a shorter holding period is considered, other parameters than the volume also need to be considered such as the leak
potential, coating, complexity of the system et cetera. See also [4.2.5] and App.C.
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Longer hold periods should be applied to confirm leak tightness for longer pipelines, see [4.2.5].
4.2.5 Confirmation of pipeline leak thightness
One of the objectives of the system pressure test is to confirm that the pipeline is ‘free from leaks’. It is,
however, not possible to confirm absolute leak tightness from the test due to allowance for some pressure
variation and limitation in hold period.
For practical purposes it is of interest to correlate the results from the system pressure test with a
corresponding loss of product when the pipeline is put in operation. In other words, the system pressure test
may be used to confirm that the pipeline is free of leaks above a certain size. If the calculated leak size is
found unacceptable for longer pipelines, longer hold periods may be agreed to reduce maximum theoretical
leak rate.
Formulas that may be used to correlate a (pinhole) theoretical size of a leak path with the recorded pressure
decay rate are given in App.C.
4.2.6 Equipment and instrumentation
4.2.6.1 General
For the purpose of the system pressure test, the instrumentation and equipment used should be able to
measure:
— flow/mass rate of added test fluid during pressurisation
— pressure
— temperature.
4.2.6.2 Pressure recording
The test pressure shall be measured using a dead weight tester or a high accuracy pressure transducer. In
addition a high accuracy large diameter pressure gauge/subsea display shall be used. Dead weight testers
shall not be used before a stable condition is confirmed, and shall not be used offshore when positioned
onboard a vessel or subsea. Time history of the test pressure shall be recorded.
Guidance note:
Pressure recording frequency will depend on the used instrumentation. Typical values are:
—
every 15 min by an operator
—
every second by a data logger.
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4.2.6.3 Temperature recording
Instrumentation for monitoring and recording ambient and/or test fluid temperature is only mandatory if the
acceptance criteria for the test allows for temperature correction of the test results.
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test pressure while maintaining required system test pressure. For rigid pipelines 0.96 of SMYS shall not be
exceeded. Alternatively, the parts may have to be tested separately.
4.2.6.4 Equipment sensitivity and accuracy
Requirements on system pressure test equipment's sensitivity and accuracy in DNVGL-ST-F101 Sec.10 are
applied for instruments and test equipment.
4.2.6.5 Calibration
Instruments and test equipment have to be calibrated in accordance with requirement in DNVGL-ST-F101
Sec.10.
4.2.7 Pipeline condition prior to testing
4.2.7.1 Test medium
With reference to the main objective of the system pressure test, the test medium should be water or other
suitable liquids, e.g. glycol (MEG).
For arctic conditions sub-zero ambient temperature conditions need special consideration. It should be noted
that sub-zero temperatures may also occur in deep waters. Freezing temperature of water depends on the
salinity of the water and pressure. Different freeze depressants such as methanol or glycol may be used in
combination with water to lower the freezing temperature.
Testing with air or inert gases may be feasible for a pure leak test. The system shall then be inspected for
leaks during the test. Test pressure is normally equal to design pressure.
4.2.7.2 Requirement to residual air content
Air shall be removed from the pipeline system during flooding, cleaning and gauging operations in order to
minimise the amount of compressed air in the pipeline during pressure testing operations.
The residual air content during pressure testing should not exceed 0.2% of the total volume of the pipeline
and be confirmed at 35% of test pressure, or at maximum 3.5 MPa (35 bar), see Figure 4-1.
Figure 4-1 Determination of volume of air
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Independent of whether temperature corrections are allowed for in the specified acceptance criteria, the
temperature should be monitored at the test head.
Where the test head constitutes a high point in the system it should be ensured that any residual air is
vented off prior to pressure testing.
4.2.8 Volume/pressure calculations
The total volume of test medium required to pressurise/depressurise the pipeline to/from the target test
pressure shall be calculated prior to test execution to ensure that sufficient test medium and pump capacity
is available and that no test medium is lost during pressurisation.
A theoretical pressure response curve linking the pipeline pressure to the added volume of test medium
should be established before start of the test, accounting for pipeline expansion and compressibility of test
medium, see App.C (formula in step 9).
4.2.9 Pressurisation
Pressurisation of the pipeline shall be performed as a controlled operation with consideration for maximum
allowable velocities in the pipeline and inlet piping. The last 5% up to the test pressure shall be raised at a
reduced rate to ensure that the test pressure is not exceeded.
The mill test pressure shall never be exceeded.
Time shall be allowed for confirmation of temperature and pressure stabilisation before the test hold period
begins.
4.2.10 Stabilisation period
From the time the test pressure has been reached and the pipeline system is isolated from the active test
equipment (pump accumulator) a certain stabilisation period will be required. The stabilisation period is from
the time the test pressure has been reached and until the commencement of the hold period, i.e. the time of
acceptance of the pressure test minus the specified hold period.
Re-pressurisation and restart of stabilisation period may be required.
Guidance note:
The stabilisation period will vary depending on temperature gradient and any volume expansion. Typically well insulated pipe-inpipe systems and systems containing flexible pipe sections may require longer stabilisation periods.
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4.2.11 Hold period monitoring and inspection
The system test pressure and temperature (if relevant) shall be monitored and recorded during
pressurisation, stabilisation and test hold periods.
Data logging should be performed at a sufficient frequency in order to evaluate the test results and
acceptance of the test.
Guidance note:
If possible the pressure variations should be recorded electronically to allow for continuous evaluation (i.e. calculations) of system
test pressure variations.
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In general, visual inspection is not required during system pressure test.
For subsea connections ROV leak observations may be facilitated by use of local dye injection/dye-sticks.
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The pressure test can be performed even when the air content exceed 0.2% provided a linear trend between
added volume and pressure response can be established and the precision of the leak test will not be
impaired, and the larger safety risk is verified to be acceptable following a risk assessment.
The most likely leak points in a pipeline system are mechanical connectors due to ROV or diver tie-in operations, if relevant.
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For the purpose of a pure leak test, e.g. with inert gas, inspection of the entire pipeline route must be
performed in order to confirm leak tightness.
The test should be terminated if it is evident that the acceptance criteria, see [4.3], cannot be met.
Guidance note:
Abnormal system response such as unexpected pressure increase or decrease occurring during system pressure test execution
may be related to:
—
Pipeline expansion.
—
External heating or cooling of test fluid.
—
Moving (raising) air pockets. Note: This phenomenon may create a safety risk for equipment and personnel, particularly in case
of deep water pipelines.
—
Pipeline rupture or leak.
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4.2.12 De-pressurisation
De-pressurisation of the pipeline system shall be performed as a controlled operation with consideration for
maximum allowable velocities in the pipeline and the discharge piping.
De-pressurisation rates shall be recorded.
Guidance note:
Subsea de-pressurisation operation could be left 'unmanned' provided that during engineering phase a proper calculation note
demonstrate that use of suitable orifices will fulfil de-pressurization requirements.
In this case a minimum 30 min monitoring will be required during the de-pressurisation.
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4.3 Acceptance criteria
4.3.1 Pressure variations
Acceptance criteria for the system pressure test shall be established prior to execution of the test.
For a rigid pipeline designed according to DNVGL-ST-F101 the pressure test is acceptable if the submarine
pipeline system is free from leaks, and the pressure variation is within ± 0.2% of the test pressure. A
pressure variation up to an additional ± 0.2% of the test pressure is normally acceptable if the total variation
(i.e. ± 0.4%) can be documented that the additional pressure variations is caused by a combination of:
— temperature variations
— tidal water variations.
If pressure variations greater then ± 0.4% of the test pressure are observed, the holding period shall be
extended until a full hold period with acceptable pressure variations has occurred.
The pressure test is acceptable when a minimum hold period (see [4.2.4]) of acceptable pressure variations
have been recorded.
For a pipeline system comprising of rigid pipeline and flexible parts (e.g. flexible tails), an equivalent
acceptable pressure variation criterion should be determined on a project basis, e.g. by weighing the criteria
for the different parts with respect to their volumes.
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Guidance note:
For a leak-tight pipeline the rate of change will typically show an exponential trend with time until a level is reached where the test
pressure is influenced only by variations in ambient conditions. Compliance with ±0.2% pressure variation within a hold period
of 24 hrs corresponds to a rate of change of 0.00083 MPa/hr (0.0083 bar/hr) for a test pressure of 10 MPa (100 bar). The plot
intervals need to consider the sensitivity of the pressure recording system.
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4.3.1.1 Temperature corrections
Pressure variations caused by documented temperature variation can be accounted for, see DNVGL-ST-F101
Sec.10.
Guidance note:
Corrections based on temperature variations will normally require a number of measuring points along a pipeline. The set-up of
measuring points for temperature measurements and methodology for evaluation shall be decided prior to the pressure test.
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As a first principle the test should comply with the acceptance criteria of ±0.2% variation in test pressure
with no temperature correction.
In case the test results are to be corrected for variations in ambient temperature, the requirements to
temperature recordings and method for correction shall be established prior to test execution. The method
shall include description of ambient temperature measurements, see [4.2.6.3], and equations for corrections
are given in App.C.
If temperature correction is to be applied a correlation between the local temperatures of test fluid as
function of local variation in ambient temperature shall be demonstrated with reference to the pipeline design
and insulation properties.
4.3.1.2 Tidal corrections
Maximum variation in tides shall be established and effect on the system test pressure determined prior to
the test.
For a system pressure test monitored from either a fixed (non-floating) offshore facility or from land, no
compensation for variation in tidal water should be allowed for.
For a system pressure test monitored from a floating facility where the pressure at the vessel may vary due
to variation in hydrostatic column height, compensation for variation in tides may be allowed, see also App.C.
In cases where tidal water variations may have a significant effect, the duration of the system pressure test
should be sufficient to reflect both a reducing and increasing tide (i.e. hold period >12 hrs).
For a system pressure test monitored from a floating facility the effect of swell may also be filtered away
from the measured pressure in case the effect is well documented.
Guidance note:
Effect of tidal variation on the differential pressure across the pipe wall and subsequent variation in effective pipeline volume is
normally negligible. However, when the pressure is recorded from a floating facility (vessel) the pressure recorded at the vessel will
be influenced by the variation in hydrostatic column height from the seabed to the vessel. For moderate tidal variations the effects
are normally negligible.
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Guidance note:
5.1 Dewatering
5.1.1 Objective
The objective of dewatering is to remove the initial fill/pre-commissioning fluid to prepare for introducing
product to the pipeline system or to make the pipeline system ready for drying, see [5.2].
The objective may be achieved following two different ways; either to minimize the residual precommissioning fluid content or to condition the residual fluid. The approach taken will depend on the
intended pipeline service and with consideration for subsequent drying operation.
In addition to fluid removal, removing any construction devices which are normally removed from the system
prior to pre-commissioning, e.g. isolation spheres used for tie-ins should be accounted for.
5.1.2 Dewatering operation
5.1.2.1 Dewatering technique selection
The following dewatering techniques may be selected depending on the requirement for water removal:
— Dry gas pipelines: dewatering to be followed by a drying operation. Glycol train may be considered.
— All other pipelines: dewatering to include e.g. MEG/methanol swabbing, or just product filling.
Dewatering pig train may include batches with fresh water to remove/dilute salt water on the pipe wall along
with e.g. MEG to condition remaining water film on pipe wall.
Dewatering engineering and sequence will always consider the removal/drainage of the residual fluid in
branches, low points, valve cavities and dead legs of piping or at least minimizing/conditioning the remaining
fluid.
5.1.2.2 Dewatering medium
Dewatering medium is typically:
—
—
—
—
—
air
nitrogen
glycol (batches or small volume pipelines)
product (oil or gas) or stabilised hydrocarbon (diesel)
process water (water injection lines).
Dewatering medium shall be selected in accordance with the following criteria:
— need and type of the subsequent drying operation
— risk of flow assurance issues such as hydrate formation and emulsion formation during the dewatering
operation
— disposal of the dewatering fluid
— criticality in case of contingency including environmental impact
— corrosion (with prolonged exposure to high air pressure/oxygen content, relevant for deep water
pipelines)
— safety risk by use of product.
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SECTION 5 DEWATERING, DRYING, INERTING AND PRODUCT
FILLING
The following guidelines are highlighted in relation to the use of different propelling fluids:
1)
Air: preferred for the dry export pipelines that will be subjected to a subsequent air/vacuum drying. Glycol, instead of methanol,
may be used in case of swabbing in order to prevent explosive mixture risk. Evaluate risk of nitrogen hydrates when pressures
and temperatures during dewatering are greater than 30 MPa (300 bar) and lower than 8°C.
2)
Nitrogen: attractive in case of limited system volume considering that line dewatering will achieve also the inerting. Not
convenient when a subsequent drying is required.
3)
MEG or MEG gel: mainly considered for short subsea piping arrangements, manifolds piping, spools and jumpers. To verify
any impact for the required displacement pressure and any issue caused by the liquid slug generated during the start-up of
gas systems.
4)
Hydrocarbon gas: advantage of combining product filling and dewatering by use of glycol train, more stringent operating and
safety requirements, to verify the risk of hydrate formation at the expected pressure and temperature during dewatering. In
order to carry out the dewatering of gas pipelines with product, risk of discharge to environment, system details and expected
pig performance must be carefully assessed prior to choosing this alternative. Should subsea receival of pigs be considered,
then adequate safety precautions must be provided to control pig receival. This alternative combines product filling, dewatering,
and drying in one single operation.
5)
Export product: mainly used for liquid export lines. Special attention should be paid in case of using un-stabilized liquid stream.
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Air used as dewatering medium for a pipeline that will be followed by a drying operation, shall be dried to
such an extent to avoid re-introduction of water behind the dewatering train.
Guidance note:
Dew points of injected dewatering medium shall be limited to the practical limit of -40°C @ atm pressure.
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5.1.2.3 Dewatering direction
Dewatering direction shall be selected based on system configuration, considering the following issues:
—
—
—
—
—
—
space availability and logistics at the two ends
pipeline bathymetric profile and water depth
pipeline design features such as change in pipeline ID, Wyes, bends, valves, tees etc.
discharge location for the pre-commissioning and dewatering fluids
different required performance (in terms of pressure and flow rate) for the dewatering spread
dewatering control, acceptance criteria and monitoring requirement.
5.1.2.4 Dewatering speed
Dewatering speed shall be controlled to maintain pig integrity and operation performance. The following
issues shall be considered when selecting dewatering speed:
—
—
—
—
—
line diameter
roughness of the line internal surface
pipeline length
presence and types of irregularities along the pipeline
liquid vs. gas propellant of the pigs.
Guidance note:
Pig speed to be used for dewatering operation is normally limited to:
- Steel mandrel disc/cup pigs.
0.3 - 1.2 m/s
- All-urethane/hollow urethane pigs:
0.4 - 0,8 m/s
For deep-water pipeline dewatering, lower pigging velocity down to 0.1 – 0.2 m/s may be evaluated.
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Guidance note:
5.1.2.5 Pressure
The dewatering station delivery pressure shall be estimated considering the characteristics of the pipeline,
the configurations at the launching and receiving ends and all the different contributions to the system
pressure drop.
Estimation of the main operating parameters of the dewatering operation shall be made. These calculations
may be done by simulation models. Model accuracy and detail shall be consistent with the project stage and
the operation criticality.
Guidance note:
The contributions to be taken into account in the dewatering pressure estimate are:
1)
Friction pressure drop of the pipeline section filled with water. The pressure drop of the pipeline section filled with air may
be neglected.
2)
Static heads: the air head should not be neglected in the case of deep-water pipelines.
3)
Concentrated pressure drops such as dewatering pigs, connection hoses and intermediate joints, pig launching and receiving
heads, back-pressure control.
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Guidance note:
1)
In the case of water discharge on to platform or onshore, the input to the simulation model will typically be the discharge
water flow rate.
2)
In case of subsea discharge, the input to the simulation model will typically be the injected air flow rate considering that the
outlet pressure is determined by the environment.
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Shut-down of the compressors before completion of the dewatering operation (i.e. pig train displaced up to
the receiver location by gas expansion) may be performed. Appropriate time and procedure for dewatering
compressor shut-down shall be defined considering the operating scenario, the operation criticality and any
constraint/uncertainty.
5.1.2.6 Equipment
The dewatering station design and sizing shall be based on the calculated requirements (expected range of
flow rates and pressures for air, nitrogen or liquid displacement stream) and with a defined spare capacity
and spare philosophy.
5.1.2.7 Discharge of fluids
The pre-commissioning fluid discharge strategy shall be addressed for the dewatering operation, in particular
to assess the environmental impact, see also [6.1.2].
5.1.3 Special considerations
5.1.3.1 Dewatering of non-piggable systems
Dewatering operation could be required also for systems that are not piggable. The following alternatives
should be evaluated for these systems:
— Gravity displacement is applicable to pipelines with constant down-hill slope. If gas is used as
displacement medium, an interface with a glycol batch(s) is recommended to limit the water content and
to prevent the hydrate formation.
— Alternatively nitrogen and glycol displacement is applicable to systems having limited volume.
Connections with valves shall be arranged and methodology to minimise the risk of potential leaks
during the operating life should be implemented upon completion of the pre-commissioning phase. The
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In case of dewatering without the use of pigs by a liquid, topics such as contamination length, mixing
distance, over-pumping requirements, impact of the displacement liquid on the system during the start-up
phase should be evaluated.
5.1.4 Technical requirements
For requirements to pigs and pig trains, see [6.3].
For temporary equipment, see [6.5].
5.1.5 Acceptance criteria
Dewatering acceptance criteria shall be established prior to operation. The following parameters should be
considered when specifying dewatering acceptance criteria:
— received fluid volume and trend (in front of the dry sweeping pigs)
— purity of received fresh water (desalination water), alternatively glycol batches
— pig retrieval and pig status through visual inspection.
Guidance note 1:
Desalination slug fresh water purity: a target for maximum chlorides content of 200 mg per kg of water for carbon steel pipelines
is normally accepted. In the case of CRA pipelines, the above specified maximum chloride content must be reduced based on the
type of material.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For dry gas pipelines the acceptance criteria should be at least 97% glycol concentration in the last batch and could be relaxed
case by case.
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Details regarding sampling the received glycol/methanol batches as well as fresh water, detailing number,
volume and timing between subsequent sampling should be prepared as part of the operating procedure.
Evaluation of the glycol content in the received batch(es) can be performed using the following methods:
3
— measure of the density (kg/m ) of the sample
— measure of the refractive index of the sample.
5.2 Drying
5.2.1 Objective
The objective of drying is the removal of the water left in the pipeline after the dewatering operation in order
to:
— prevent corrosion development
— avoid hydrate formation
— prevent transport product contamination.
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residual water content should be typically lower than 20% vol. or such as to maintain a margin to hydrate
generation.
— Glycol displacement is applicable to systems of limited volume like non-piggable spools or jumpers
connecting to subsea structures that become free flooded during installation. Normally, volumes of flushed
glycols in the order of 2 or 3 times the volume to be treated, would allow for the conditioning of the
system. After the flushing operation the system may be left filled with glycol in readiness for start-up of
operations.
— Onshore pre-filling with MEG gel or liquid MEG. To be considered for spools and jumpers.
5.2.2.1 Drying method selection
The selection of method to be used for drying a pipeline is dependent on the pipeline characteristics, system
lay-out, feasibility, schedule, environmental condition, and dryness requirement.
The following drying methods are standard for pipelines:
— convection drying using dry gas, normally air
— vacuum drying.
Drying with air may be combined with running foam pigs (for shorter pipelines) to enhance the effectiveness
of the drying operation.
Evaluation of the optimum time for drying should be addressed.
Drying operation should be performed on the complete system i.e. after completion of all the intermediate
tie-ins with the target of minimising any re-contaminated area.
Vacuum drying is not considered efficient for longer, smaller bore pipelines.
Drying, as described above, may be avoided by use of glycol train during dewatering operations.
5.2.2.2 Conventional drying using a dry air
A typical drying process includes the following phases:
1)
2)
3)
Drying phase when the dry air flowing over the wet surface remove the water left by the dewatering
operation.
Soak test (if any), see [5.2.2.4] for details.
Final purging when the full pipeline volume is replaced and the documentation of the pipeline content
status taken during the soak test is analysed at the outlet.
Guidance note:
The factors affecting the convective drying efficiency are:
—
The operation pressure should be minimised. In particular, the outlet back pressure should be kept at a minimum in order to
maximise drying efficiency.
—
The drying fluid (normally air) flow rate. An optimum rate for drying can be established, however the consequences on the
increased operating pressure profile should be understood and evaluated.
—
The ambient temperature of the operation affects the water saturation content for a unit mass of drying gas at a certain pressure.
Normally this parameter is not controllable.
—
The pipeline 'geometrical effects' including 1) how the liquid film is distributed on the wall, 2) the adhesion and absorption of
the liquid film on the pipe wall, 3) the actual pressure profile along the pipeline during drying when considering also the air
gravitational head.
—
The dryness (water dew point) of the drying fluid, normally air. It should be noted that improvement of this parameter beyond a
certain value has marginal effect on the operation efficiency while significantly impacting mobilised spread performance/size.
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A water dew point of the inlet air (drying gas) of minus 40 °C at 0.1 MPa (1 atm) is normally a good
compromise between the operation efficiency and the required spread complexity and performance for most
of the operating scenarios. Lower air dew point could be required in cold climates and deep water pipelines.
Compressors shall provide oil-free air, ref [6.6.3], to prevent fouling/degradation of the downstream drying
column/sieves and to avoid oil deposits forming in the pipeline which will shield water moisture from the dry
air or depress its actual vapour pressure due to the oil-water interfacial tension force.
Launch of foam pigs during drying is an acceptable practice. However, it is to be noted that the pipeline is
not dried by using foam pigs. Dry air will dry-out the pipeline while the foam pigs are used to increase the
process efficiency.
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5.2.2 Drying operation
Maximum pipeline distance that can be run by foam pigs is mainly dependent on the pipeline internal wall conditions. For new
pipelines (better if internally coated) distance of up to 200-300 km may be considered feasible.
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The optimum range for dry gas flow rate shall be identified. For an efficient operation the drying gas flow rate
shall normally be sufficiently high to maintain turbulent flow throughout the pipeline length.
Guidance note:
Up to a certain level an increase of gas flow rate reduces the time required for drying, besides, by increasing the pressure profile
along the line (as effect of the larger flow rate), the water saturation decreases to the extent of not being compensated by the
increase of introduced gas mass.
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5.2.2.3 Vacuum drying
Vacuum drying operation is performed through the following phases:
1)
2)
Start air evacuation.
Leak test. The vacuum plant shall be turned off and isolated and pipework shall be checked for in-leaks.
In-leaks shall then be rectified or, if not possible, quantified. Any in-leakage rate still remaining from unrectifiable in-leaks as estimated from this test is used when analysing the final soak test results.
Guidance note 1:
Typically it is performed in the absolute pressure range of 0.01-0.02 MPa (100-200 mbara).
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3)
Water evaporation. During this phase the rate of vapour generation drastically increases resulting in a
reduction in the rate of pressure decline.
Guidance note 2:
Attempt to rapidly reduce the pressure could generate ice from the free water in the pipeline in case of insufficient heat
exchange coefficient with the ambient. Ice formation has to be avoided as it will reduce the potential of the water to
evaporate and as such to be evacuated from the pipeline.
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4)
Pressure draw-down. The draw-down pumping shall be maintained until a pressure lower than that
required for the specified dew point is achieved. Typical final pressure draw-down absolute pressure is
lower than 0.1 kPa (1 mbara).
Guidance note 3:
If a dew point of -20°C is required, equivalent to absolute pressure of about 0.1 kPa (1 mbara), pumping should be continued
until a pressure of approximately absolute pressure of 0.05 kPa (0.5 mbara) or less is achieved. This is carried out to ensure
that a stable balanced absolute pressure of 0.1 kPa (1 mbar) is obtained throughout the pipeline system.
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5)
6)
7)
Soak test. See [5.2.2.4] for details.
Purging phase. Purging shall be conducted by feeding nitrogen (or dry air) under vacuum conditions.
Line packing. Final packing pressure is generally defined on the basis of project requirements and
following the required conditions for the start up. If no specific requirements are given, the pipeline shall
be pressurized up to absolute pressure of 0.11 – 0.15 MPa (1.1 – 1.5 bara).
5.2.2.4 Soak test
Soak test may be used to prove that no significant water is present in the pipeline.
Soak test is performed by suspending drying operation (convectional drying or vacuum drying) and close in
the pipeline for a period dependent upon the length and volume of the pipeline (typically 12 – 48 hours). In
this way, any remaining water will be picked up by under-saturated air.
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Guidance note:
The effectiveness to obtain a representative documentation of the pipeline content status via a soak test is uncertain in particular
for longer length. The water picked-up during the soak period can then re-condense in a downstream section at lower temperature
or turbulence/mixing effects occurring when the flow is re-started could actually significantly disturb or even hide the taken
pipeline content status.
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5.2.3 Technical requirements
For air drying it shall be confirmed that all valves are drained and normally set in half open position (after
foam pigging to be completed, if any) prior to commencing drying operation.
For vacuum drying it shall be confirmed that:
— Valve seats can withstand foreseen vacuum.
— Vacuum does not generate ovalisation of the pipeline.
For temporary equipment and instrumentation, see [6.5].
5.2.4 Acceptance criteria
5.2.4.1 General
The drying acceptance criterion for pipelines shall be based on the amount of water left in the pipeline for a
unit of volume of gas at standard conditions. The final acceptance criteria for the dew point should consider
the gas specification requirements.
5.2.4.2 Acceptance criteria for convection drying
For the convection drying the following two alternative criteria can be applied:
1. Outlet dew point trend and a successful soak test.
Drying until the following conditions are verified:
— achievement of outlet dew point
— performance of a soak test for a given duration
— no significant increase of the outlet dew point when flowing a minimum of 1 pipeline volume at drying
operating pressure during the 'final purging' phase (i.e. following the soak test) still reaching a dew point
plateau below the minimum.
2. Difference between inlet and outlet dew point.
Drying until the difference of inlet and outlet dew points is less than a predetermined quantity as reported
here below. Typical requirements are:
o
- Inlet dew point:
- 40 C @ atm.
- Gap between inlet and outlet dew point:
lower thank 7 C.
o
Guidance note 1:
Field experience indicates that 10-15 pipeline volumes (after depressurisation) of dry air are required to reach the acceptable
results in an internally coated pipeline. Larger volumes shall be expected for non-internally coated pipeline.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Theoretically, foam pig launch should continue until the pigs are received nearly 100% dry. In order to confirm that the line is
sufficiently dry to perform a 'soak' test, 2 or 3 'dry' pigs should be received. From a practical point of view, if two successive pigs
are received carrying 10% or less of the estimated water capacity of the pig, then further foam pigging can be ceased.
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Guidance note:
Pipeline pressure trend during the performance of the soak test: the resulting pressure after an initial
stabilization period should remain below the dew point equivalent pressure correlating to the desired dew
point (for a dew point of -20°C, this is 0.1kPa (1mbar)) , notwithstanding any allowance for in-leakage
observed during step 2) of [5.2.2.3].
Pressure and corresponding dew point shall be continuously monitored and recorded during the final drawdown phase (following soak test) until reaching the vacuum level available before the start of the soak test.
5.3 Inerting
5.3.1 Objective
The objective of inerting is to reduce the oxygen content to an acceptable level before introducing
hydrocarbon gas.
5.3.2 Inerting operation
5.3.2.1 Nitrogen intering
Either the entire pipeline system can be purged with nitrogen, or a slug of nitrogen can be injected ahead
of the gas being used to commission the pipeline. This creates an inert environment that cannot support
combustion when the final product is introduced and also it may reduce or eliminate corrosion prior to
operation.
The nitrogen gas can be produced from vaporized liquid nitrogen, or be membrane generated.
Guidance note:
Where membrane generated nitrogen is used, the purity of nitrogen entering the pipeline will typically be 95%. In most cases a
final oxygen level of 5% will be acceptable.
Where vaporised liquid nitrogen (LN2) is used this has purity better than 99.99%, hence oxygen levels well below the accepted
requirement are achieved.
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5.3.2.2 Vacuum
By pulling a vacuum on the pipeline system the pressure is reduced and hence close to all air is removed
from the system. Relative to an initial absolute pressure of 0.1 MPa (1 bara), the mass of air after reducing
the absolute pressure in the line down to 0.1 kPa (1mbara) is reduced by 99.9%.
If required, it is possible to purge the pipeline with nitrogen at low pressures to displace the partial pressure
of air with a partial pressure of nitrogen. The dew point measurement during the purging phase must
be monitored and recorded, and confirmed to be below the specified acceptance level until reaching the
specified packing absolute pressure (typically 0.11 – 0.15 MPa (1.1 - 1.5 bara)).
5.3.3 Technical requirements
For temporary equipment, see [6.5].
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5.2.4.3 Acceptance criteria for vacuum drying
A commonly used acceptance criterion for vacuum drying is a dew point of -20°C. This corresponds to a
saturation absolute pressure of 0.103 kPa (1.03 mbara) and to residual water content in the pipeline of 0.9
3
g/m . More stringent requirements may be specified, but will normally require significantly longer drying
times.
The nitrogen purity achieved shall be as specified for the project.
If a slug of nitrogen in front of the natural gas is used, then the mixing zones must be calculated in order to
verify that the slug length is sufficient to prevent an explosive mixture.
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5.3.4 Acceptance criterion
6.1 Operational principles
6.1.1 Management
Management of pre-commissioning projects and activities should be in compliance with the requirements
of the individual contracts, governing regulations/codes/standards and in compliance with industry best
practice. Good communication in all phases of the project is essential and sufficient systems should be in
place to ensure proper and correct exchange of information, notifications and messages.
6.1.2 Quality and HSE
Project specific QHSE requirement should be defined.
A quality plan should be developed based on the principles of ISO 9001 or equivalent and should identify
required organisation, resources, activities and control points to ensure work is performed in compliance with
relevant regulations, standards and specifications.
An HSE manual should be developed, taking into consideration the occupational health, safety of personnel
and assets, and the environment during all phases of the work. Essential elements that should be covered
are:
—
—
—
—
—
—
—
—
—
personnel safety and training
operational safety, inclusive of PPE
chemical handling, inclusive safety data sheets
noise
pollution
risk management and evaluation
management of change
communication and emergency procedures
odour, if any.
In order to minimize the environmental impact of operation the following issues should be addressed:
— Environmental friendly fluid treatment should always be selected. The effectiveness of the treatment shall
be considered.
— Disposal of chemically treated pre-commissioning fluid by e.g. an environmental risk assessment. A test
programme should be carried out, if required.
— When discharging to the marine environment, consideration should be given to utilizing mechanical
discharging devices that will allow mixing and diluting of the test fluid in the marine environment.
— Potential for erosion from discharge/turbidity of pre-commissioning fluid.
— Location of equipment to minimize consequences of intentional/unintentional discharge of oil, diesel or
chemicals. Consider use of bunds.
— Noise from pre-commissioning equipment or gas venting from the pipeline.
— Operational duration: extended operations may cause disturbance to the local environment.
— Seasonal and tidal factors: these factors should be investigated before discharging operations.
Guidance note 1:
When discharging treated seawater overboard, the discharge depth, location, and sea currents shall take into consideration the
intake for vessel and/or platform water makers.
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SECTION 6 MISCELLANEOUS ISSUES
A guideline for environmental risk assessment is the comparison of the PEC (predicted environmental concentration) vs. PNEC
(predicted no-effect concentration). If the ratio PEC/PNEC is lower than 1, the product poses no risk. Besides, if this ratio is equal
to or higher than 1, management measures have to be taken to reduce the exposure to an acceptable level.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
A hazard and operability study (HAZOP) should be performed dedicatedly for each operation/procedure.
DNVGL-RP-N101 may be followed.
6.1.3 Planning and engineering
For planning and engineering the following list of required documentation may be used as guidance:
— philosophy or method statement
— project/execution schedule
— calculations and selection of treatment, equipment (based on required capacities/ performance) and
spares
— layout and arrangement drawings for permanent and temporary equipment
— execution manuals/procedures, providing all information needed for execution of the individual operations,
including operation manuals for temporary equipment/components
— interface and mobilisation manuals
— contingency plans
— spare philosophy
— pipeline systems piggability study.
Risk evaluation and HAZOP findings should be implemented in the operational manuals/ procedures prior to
start of any operations.
6.1.4 Execution
Execution of offshore/onshore operations shall be in accordance with the project philosophy/ method
statement and/or execution manual/procedures and take into account safety and risk elements brought
forward relative to the individual operations. Special focus should be put on operations involving work on/
with/in connection with operating pipelines or use of hydrocarbon fluid as propelling medium.
Pre-commissioning personnel should be sufficiently trained and briefed for the operations to be conducted.
Leading/supervisory personnel should have relevant and prior experience from the types of operations to be
performed.
6.2 Documentation
6.2.1 General
Pre-commissioning documentation shall at a minimum consist of:
—
—
—
—
—
—
specifications
design reports of equipment and systems
manufacturing and calibration certificates
test reports of temporary equipment and assembly
procedures
final documentation.
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Guidance note 2:
A pre-commissioning specification should be developed for the pipeline system, describing principal
methodology and requirements for execution of all operations relevant for the work.
Reference levels/heights for design and test pressures should be stated explicitly.
Functional requirements of spreads, equipment, personnel and acceptance criteria/results of the operations
should be clearly defined. Limitations and boundary conditions for the work, relative to the pipeline system or
any appurtenant systems, should be highlighted.
Detailed procedures shall be prepared for the pre-commissioning activities and be made available to all
parties prior to the operation. The following checklist is provided for guidance:
—
—
—
—
—
—
—
—
—
—
—
—
—
step by step execution plan including contingency operations established
operational limitations identified
communication plan established
mobilisation plan established
pre-commissioning spread mobilised and calibrated and function tested with appropriate certificates
inspection and test plan established
agreed reporting format and frequency
valve status check
treatment dosage established
environmental log established
toolbox meetings/JSA held and findings communicated to all involved parties
operational acceptance documents
management of change system established.
6.2.3 Final documentation
The final documentation shall include all relevant information, calculations and considerations made in order
to ensure the traceability of the pre-commissioning performed.
Final documentation produced in connection with pre-commissioning of the pipeline system shall as a
minimum include:
— approved procedures
— management of change
— flooding, cleaning and gauging operations:
—
—
—
—
—
—
volume of fluid introduced
log of additives
debris record
gauge results
pig condition report
acceptance documents
— pressure test:
— pressure log and charts including temperature log
— pressure test acceptance documents
— dewatering, drying and inerting operations:
—
—
—
—
calculated volume or removed fluid (during drying)
pig condition report
level of dryness achieved
for inerting, final oxygen concentration achieved
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6.2.2 Documentation prior to execution
— environmental accounting:
— list of chemicals used, discharges to sea an air
— report of noise pollution during operation, if any
— log of activities
— statement of compliance
— instrument and calibration certificates
— original records from all recorded information for pre-commissioning
— daily log of operations
— lessons learnt/recommendations for later pre-commissioning operations.
Guidance note 1:
A description of the activities and calculations to be performed as part of the pre-commissioning process should be considered in
early design phases to prevent misperception of purpose of the test during execution and the test results.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The checklist for system pressure testing given in App.B may be used as part of the final documentation.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
6.3 Pigs and pig trains
6.3.1 General
This section gives recommendations for pigs and pigging operations required for filling, cleaning, gauging and
dewatering.
The following shall be considered when selecting pig type/train design and operation:
— Objective of the pigging operation. Pre-commissioning pigs are normally divided into groups to serve the
following purposes:
—
—
—
—
—
—
batching or separation of fluids/products
displacement of fluids
cleaning
gauging
dewatering
or combinations of the above.
— Pipeline characteristics, see [2.2] and also App.A and App.D for details.
— Operational conditions, see [2.3] and [2.5], see App.A and App.D for details.
6.3.2 Pig train design
6.3.2.1 Flooding, cleaning and gauging
For larger diameter pipelines (≥0.152 m (6")) flooding, cleaning and gauging pig trains should be used.
This may range from one single pig, to a train of four or more when combined with cleaning and gauging
operations.
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— acceptance documents
Pig body and ancillary items must be compatible with pipeline material and components. Brush pigs are often
used for cleaning, but normally not applicable for pipe with internal coating, flexibles and CRA materials.
Foam pigs can be used as separation pigs, but are not recommended in the cleaning and gauging precommissioning stages as they are less effective compared to metal bodied disc pigs.
It is not recommended to perform fill operations on pipelines < 0.152 m (6") diameter by use of metal
bodied pigs. If a pig is to be used a solid cast or foam type should be considered.
All pigs used during flooding and cleaning operations should be bi-directional.
Pigs with bypass ports may be used to keep debris suspended in front of the pig if the pipeline is expected
to contain large amount of debris. Bypass ports shall be used with caution if multiple pigs are used in the
cleaning train as these will have to be sized appropriately throughout the train to ensure pig separation is
maintained.
The pig design shall ensure that pressure cannot be trapped inside any part of the pig.
Guidance note 1:
Bidirectional separation pigs are typically made up from a metal body with flexible sealing discs. The sealing discs are typically
made from types of polyurethane of varying hardness, depending on the requirements to durability, support and flexibility of a
particular pig.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For optimum separation pig design and choice, pig trials may be carried out on a custom built pipe test loop that represents the
unique pipeline system characteristics and any special features e.g. ID changes, valves, tees, wye, etc. In this way the pig sealing
length, number of sealing discs, sizes, disc/guide configuration and material can be optimised for a particular pipeline.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
For air filled pipelines it is common practice to inject a slug of pre-commissioning fluid ahead of the first pig
to provide lubrication for the pig discs and provide for a cleaning flow ahead of the leading pig. The precommissioning fluid volume to be injected ahead of the first pig should consider the pipeline characteristics
and expected debris in the pipeline.
Guidance note 3:
The volume pumped in front of first pig and between each pig should be based upon the following minimum sizes:
—
Pipelines ≤ 2 km – volume to fill up 50m pipeline length
—
Pipelines > 2 km to ≤ 5 km – volume to fill up 75 m pipeline length
—
Pipelines > 5km to ≤ 10 km – volume to fill up 100 m pipeline length
—
Pipelines > 10 km to ≤ 20km – volume to fill up 200 m pipeline length
—
Pipelines > 20 km to ≤ 50 km – volume to fill up 250 m pipeline length
—
Pipelines ≥ 150 km – volume to fill up 300-400 m pipeline length.
A sliding scale between separation distances can be used to fit the exact length of the pipeline with the spacing required for that
length of pipeline adjusted between two separation bands.
When gas is used as separation or propellant fluid, due consideration should be taken for the compressibility of the gas with
respect to pig spacing. Larger pig spacing will normally be required.
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Figure 6-1 Typical FCG pig train design
In some cases there may be a requirement to pump additional water in front of the filling pig train to reduce high pig velocities.
Water pumped in front of the pig train can have a ‘dampening’ effect on pig train velocity, see [3.1.3].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
6.3.2.2 Dewatering
The main variables that define a pig train composition can be summarised as follows:
—
—
—
—
pipeline service
design of pigs composing the dewatering train
number of pigs composing the dewatering train
type, size and number of batches included in the dewatering train.
The pipeline service is the main driver in selecting the dewatering train configuration as it defines the
efficiency required for the operation and requirement for chemical/rinsing treatment (if any).
Dewatering trains for pipelines required to be dried after dewatering should be designed to remove as
much water as possible and reduce any possible causes that could impact negatively the succeeding drying
operation (e.g. salt deposit) via rinsing.
Dewatering trains for other pipelines should be designed to remove water but also to chemically treat/dilute
(glycol or methanol swabbing) the film of water left after the operation to reduce or eliminate the possibility
of hydrate formation during the succeeding start-up phase.
Guidance note 1:
Dilution models should be used to assess the optimum combination relevant to number of batches and their sizing, considering
requirements (e.g. the required final glycol purity), operation criticalities (consequences associated to repeating the run) and
constraints (e.g. maximum number of pig in the dewatering/swabbing train). The model detail and expected precision should be
established considering the risks and related consequences of not achieving the intended operation results (i.e. mainly the dilution
of the residual water film).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The main functional requirement to a dewatering pig/pig train is the ability to displace as much water as possible during the
operation. Strict tolerances on disk sizes and hardness may improve the efficiency.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
The operational safety aspects related to dewatering of longer and deep-water pipelines using air as
propelling fluid should address both pig wear/pig damage and explosion risks.
6.3.3 Pig speed
Pig speed is normally in the order of 0.5 – 1.0 m/s. Sealing capabilities of modern pigs are generally very
good and pigging operations may very well be conducted at much lower velocities.
Lower pig speed for dewatering may be considered, see [5.1.2.4].
Optimum pigging speed should be established and defined based on the type of operation and pipeline
system to be pigged.
Extremely high pig speeds (+10 m/s) should be avoided, especially in connection with compressed air/
nitrogen pigging as friction forces between pig and pipe wall may generate high temperatures that may
damage (melt) the sealing discs.
When composing pig trains for combined operations the pig train speed shall be limited by the pig having the
most stringent requirement.
6.3.4 Pig tracking
The primary method for controlling and monitoring pig speed and separation and consequently pig location is
the rate of propelling fluid and/or discharge rate of pre-commissioning fluid in front of the pig train.
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Guidance note 4:
The following principles/systems are most common for tracking and location of pipeline pigs:
—
—
—
—
acoustic
magnetic
electromagnetic
radioactive.
Pig detection shall be qualified for the pipeline configuration considering pipeline content and characteristics
as e.g. coating, pipe-in pipe, burial or rock dumping. Each detection type/principle has its pros and cons;
which should be carefully evaluated in order to obtain the required reliability and functionality of the tracking
system.
The number of and location of tracking devices in pig trains should be considered for each operation.
Common practice is to have tracking devices on the first and last pig in a pig train.
Considerations should be given to ensure sufficient battery life time of tracking devices accounting for
possible delays in pigging operation and time for detection (i.e. time for positioning tracking vessel and ROV
along pipelines.
6.4 Pig traps (launchers and receivers) and test heads
Pig launchers and receivers should be designed and tested according to the applicable design standard and
be suitable to facilitate pre-commissioning operations.
Temporary launchers/receivers used during system pressure test should have a design pressure at least
equal to the pipeline design pressure, see DNVGl-ST-F101 Sec.10.
The design should prevent unintended opening while being pressurised.
The pig launcher and receiver facilities need to be sized and configured to accommodate all types of
pigs planned for during pre-commissioning, including any isolation spheres etc. used for tie-in. Special
considerations should be given to design of subsea pig launchers/receivers with respect to ROV interface and
weight and sizing for installation and removal.
Considerations shall be made concerning double valves in start-up and lay down heads when used for
pressure test as well as launchers/receivers.
Wet buckle contingency shall be considered.
6.5 Temporary pre-commissioning equipment
6.5.1 General requirements
The following requirements should be included for as relevant for the individual operations:
— Design and testing of all temporary equipment shall be according to recognised standards.
— Pressure producing equipment should have capacities of at least 10% in excess of the planned operational
level/point.
— Design pressure of equipment should be equal to or greater than the maximum expected pressure during
the pre-commissioning operations.
— Pre-commissioning equipment shall have sufficient flow capacity to accommodate the specified volumes
and rates of filling/treatment/discharge of pre-commissioning fluid.
— Contingency shall be provided for all critical spread components.
— Mechanical maintenance of rotating and hydraulic equipment for long durations of operations.
— Frost protection of equipment spread under arctic/cold conditions.
— Cooling water/medium temperature checks under tropic/warm conditions.
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As an additional control pig tracking can be used to verify pig position and flow meter readings and to
confirm arrival of pigs.
6.5.2 Function testing
All critical and essential parts of the pre-commissioning spread should be function tested and accepted by
operator prior to mobilisation.
Final function testing of the complete pre-commissioning spread should be performed after rig-up on site, in
due time prior to start of operations.
6.5.3 Instrumentation, logging and recording
Reading resolution, ranges and accuracies of pressure test instruments shall be in accordance with [4.2.6].
Reading resolution, ranges and accuracies of general pre-commissioning instruments should be in accordance
with Table 6-1 below.
Table 6-1 Reading resolution, ranges, and accuracies of general pre-commissioning instruments
Instrumentation
Resolution
Range
Accuracy
Dew point meter
1.0°C
+40°C to -50°C
± 1.0°C
Ambient temperature
recorder (24 hr.)
0.5°C
-20°C to 80°C
± 1°C
Temperature probes (digital)
0.1°C
-50°C to +50°C
± 0.2°C
2)
Pressure gauge and recorder 0.05 MPa (0.5barg)
0-120%
± 1%
Pressure gauge and recorder 2/0.1/0.05/0.01 kPa
3)
(vacuum operation)
(20/1/ 0.5/0.1mbar)
0 kPa - 100/10/5/1 kPa
(0mbar-1000/100/50/10mbar)
Flow meter and recorder
Dependent upon pipeline
volume
Dependent upon pipeline volume ± 2%
and/or spread capacity
Barometer
0.01 kPa (0.1mbar)
Relevant to site
1)
± 1%
1)
± 0.08 kPa (0.8mbar)
Notes:
1) Percentage of full range.
2) Percentage of maximum pressure expected during the relevant pre-commissioning operations.
3) Different pressure gauges required for different vacuum ranges.
Back-up/contingency units should be provided for all essential instrumentation.
In addition to electronic data recording, manual recording of operational data should be made regularly at
sufficient and adapted intervals to provide control of the status and progress of the on-going operation(s).
6.5.4 Equipment on board vessels
Pre-commissioning operations from vessels should be subject to the following additional evaluations:
— Establishment of environmental restrictions to the operations.
— Hose type and capacity, handling and configuration during operation, including hose reels position on
deck. Hose collapse to be addressed separately.
— Quick disconnect system.
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For temporary equipment that is used for more than one pressure test, the cyclic loading needs to be
considered to avoid failures due to e.g. fatigue, exposure to chemicals or other mechanical damage.
6.6.1 General
All fluids used for pre-commissioning shall be in compliance with local and statutory rules and regulations.
The fluids marked X in Table 6-2 below are typically used during pre-commissioning (less frequent use is
marked in brackets).
Table 6-2 Fluids used during pre-commissioning
Commissioning
fluid
Cleaning
Gauging
Pressure test
Dewatering
X
X
X
X
Water
Drying
X
X
Nitrogen
X
(X)
X
(X)
(X)
(X)
(X)
Preservation
(X)
Air
MEG
Inerting
(X)
X
X
(X)
Guidance note 1:
Water is the main type of pre-commissioning fluid used during pre-commissioning operations; filling, cleaning, gauging and
hydrostatic pressure testing. Water is used during filling and pigging due to its low cost and eases of access (subsea pipelines) and
for testing operations due to its incompressibility and inert properties. Low temperatures (i.e. winter conditions) may require use of
MEG as an additive to water.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
If gas is used as fluid for pressure test, it may cause increased harm to personnel in case of pipeline or equipment failure due to
its higher energy storage. In this case, strength of the pipelines/equipment need to be confirmed (e.g. by strength test) before
pressure test can be performed at least for pipelines/equipment located at location class 2.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
6.6.2 Water
Filtered and chemically treated seawater is commonly used as the pre-commissioning fluid when the pipeline
volume and location exclude the use of potable/fresh water.
The need for and use of chemical additives shall be documented, see [6.7]. All water entering the pipeline
system shall as a minimum be filtered to remove suspended particles larger than 50 micron and should have
3
an average content of suspended matters not exceeding 20g/m to prevent larger particles/volumes to enter
the pipeline, see DNVGL-ST-F101.
Guidance note:
The type of filtration required depends largely upon the water source conditions and the volumes required. For relatively small
3
volumes from a clean source (e.g. < 2m /min), basket filters may be suitable. Where the water supply is less clean, self-cleaning
filters may be required in order to meet the required cleanliness whilst maintaining the required pumping rates of larger diameter
3
pipelines (e.g. ≥ 2m /min).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
If potable/fresh water is used the requirements to maximum levels of chloride should be defined.
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6.6 Pre-commissioning fluid
Use of air as pre-commissioning fluid is normally limited to dewatering and drying, through use of
compressor spreads that injects air as the driving medium for pigs or for purging the pipeline systems.
For air drying operations, the air injected in to the pipeline should have an oil content not exceeding the
levels set out in ISO 8573-1 oil class 1. For other pre-commissioning operations involving compressed air, oil
levels set out in ISO 8573-1 oil class 2 and 3 should be acceptable. Such standards are commonly referred to
as 'oil-free'.
The condition of the air with respect to pressure, temperature and dew point, shall be established.
Special care should be paid to use of air in connection with hydrocarbon operating systems as the oxygen in
the air may cause spontaneous combustion/explosion.
6.6.4 Nitrogen
Use of nitrogen as pre-commissioning fluid is normally limited to dewatering, drying, inerting and
preservation. Nitrogen can be supplied in racks (as gas), in tanks (as liquid) and also through nitrogen
generation plants (membrane units) on site.
Nitrogen is used in pipeline pre-commissioning operations due to its inert nature/properties.
The concentration of the nitrogen shall be established, see [5.3].
6.7 Corrosion assessment and water treatment
6.7.1 Corrosion assessment
Routines and specifications for assessing both environmental risks related to disposal and corrosion risks
related to the pre-commissioning and operational phases shall be established.
Pre-commissioning fluid discharge permits shall be obtained, as required.
If seawater is used it shall be tested for concentrations of oxygen and organic matter. It is recommended
to acquire the water from a certain depth above the seabed in order to avoid contaminations by seabed
sediments.
6.7.2 Corrosion threats
In Table 6-3 are listed the potential effects of pre-commissioning fluid on corrosion for different types of
linepipe materials.
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6.6.3 Air
Potable water (treated fresh water)
Pipe material
Corrosion mechanism
and form
CMn steel
13Cr
O2-corrosion
Under deposit corrosion
MIC(aerobic)
Localised corrosion (O2)
Recommended
Treated seawater
Corrosion mechanism
O2-corrosion
MIC (aerobic/anaerobic)
Yes
Recommended
and form
Yes
Under deposit corrosion
Yes
3,4)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
316 (clad/lined)
No corrosion
Yes
4)
22Cr
No corrosion
Yes
4)
25Cr
No corrosion
Yes
4)
Alloy 625 (clad/
lined)
No corrosion
Yes
4)
Alloy 825 (clad/
lined)
No corrosion
Yes
4)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
MIC(aerobic/anaerobic)
Localised corrosion (O2)
No
Treated fresh water
1)
is preferred
Treated fresh water
1)
is preferred
Yes
2)
Yes
2)
Treated fresh water
1)
is preferred
Notes:
1) For Alloy 825, 316 and 22Cr seawater may be used if precautions are taken regarding treatment (filtered and added
chemicals to reduce the corrosivity) and maximum exposure time.
2) Untreated seawater may be used for stainless steel 25Cr and Alloy 625.
3) Treated fresh water shall be added oxygen scavenger to avoid corrosion; primarily by aerobic bacteria.
4) Treated fresh water shall have low chloride content typically 50 mg/l for 13 Cr and 200 mg/l for other stainless steel
types.
Microbiologically influenced corrosion (MIC) may cause corrosion damage to CMn steel and CRA linepipe
materials. Bacteria introduced during pre-commissioning may also cause MIC during subsequent operation.
Guidance note:
MIC is not critical for pipelines to be operated with dry gas or dehydrated condensate.
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6.7.3 Water treatment
It is normal practice to treat the water entering a pipeline system during filling and/or pigging operations in
order to prevent pipeline internal corrosion.
Selection of additives should be based on evaluation of objective, interaction with pipeline materials and
effects on personnel and environment during handling, injection and discharge and documented in the final
documentation.
As a minimum the following shall be addressed:
— national/authority regulations
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
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Table 6-3 Potential effects of pre-commissioning fluid on corrosion for different types of linepipe
materials
project requirements
pipeline geographical location (environmentally sensitive areas)
filling water temperature and quality
pipeline material
pipe wall surface (coated/uncoated)
additive retention period required
disposal of treated water.
An assessment of the compatibility of the different additives shall be carried out in order to eliminate
detrimental chemical interactions.
Guidance note:
Additives will generally not be used in the pig separation slugs during filling, cleaning and gauging operations since these slugs will
be discharged from the system during the pigging operation and will therefore have no effects relative to pipeline protection. In
cases where pig trains are inserted in the pipeline system at an early stage of the project – chemical inhibiting of the separation
slugs may be required.
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The following lists examples of different additives typically used:
Table 6-4 Examples of different additives
Type of additive
Corrosion mechanisms
Mitigating effect
Oxygen scavenger
Oxygen corrosion and MIC aerobic bacteria
Reduce amount of oxygen
Biocide
Microbiologically influenced corrosion
Reduce bacteria content
Sodium hydroxide
(NaOH)
Enhanced bacteria growth and oxygen
corrosion
Increase pH to mitigate bacteria growth and
decrease corrosion of carbon steel
Corrosion inhibitors
Oxygen corrosion
Reduce oxygen corrosion
Scale inhibitor
Scaling due to high pH in seawater
Reduce scaling
Removes/dissolves water from the pipe wall
during dewatering/swabbing operations.
Effective as frost protection and may be used
to prevent bacterial growth
MEG
Oxygen scavenger may be required to avoid formation of colloidal rust affecting discharge of water and to
avoid the biocide to react with oxygen.
Biocide addition to seawater is always recommended for CMn steel pipelines if the duration of exposure is
exceeding a certain period of time. This is typically set to 28 days in warm waters, necessary for bacteria to
form a biofilm causing MIC or if there is otherwise a risk of introducing anaerobic bacteria (primarily SRB)
into the production system causing corrosion damage at a later stage. The amount of biocide to be added is
dependent on the temperature, pH and the length of time the water will be left in the pipeline.
Biocides for SRB control are not necessary for fresh water if the sulphate content is low.
Adjusting pH > 10 is a method of suppressing the bacteria growth and rendering the water non-corrosive to
CMn steel and CRA linepipe materials. Increasing pH of seawater may result in precipitation of scale on the
pipe walls and in the pipeline, which may give detrimental effects on subsequent cleaning and drying. Scale
may further be treated with scale inhibitors, if required.
UV treatment may be considered as an alternative to biocide treatment, subject to specific project
evaluation. Acceptable exposure time, i.e. time period the pipe wall can be exposed to the pre-commissioning
fluid, shall be established ensuring efficiency of additives and limit any corrosion processes during precommissioning.
Contingency plans to avoid exceeding the acceptable exposure time shall be established.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
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—
—
—
—
—
—
—
Due to the environmental impact of fluid discharge, consideration shall always be given to the use of more environmentally friendly
additives or limit the amount used.
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6.7.4 Additive dosages and consumption
Systems shall be in place ensuring that the correct additive dosages are injected. Spare capacity for chemical
injection system shall be available, to take into account deviation in pre-commissioning fluid as compared to
planned flow rate.
Guidance note:
Chemical suppliers should be consulted to define a concentration range that will ensure flexibility related to deviations from
planned flow rate.
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6.8 Other considerations/special systems
6.8.1 Pipeline system components
All pipeline system components shall be designed according to the applicable design standard and tested
separately prior to installation. Test pressure shall be at least equal to the pipeline system test pressure.
Valves should not be used as end closures during system testing unless the valve design allows for full
differential pressure across the valve body. If testing is performed against closed valves, considerations
should be given to through-valve leakage rates and the possibility for measuring these. Conservatively no
leakage should be allowed unless the amount is measured and documented.
Inline valves and arrangements should be full bore and without internal obstructions that may restrict or
damage pigs during pigging. Considerations should be given to the necessity for including valve cavities
during pressure testing by having half open valves.
Bends should be designed for pigging and have a bend radius of minimum 5 x ID. Smaller bend radii may be
allowed subject to qualification of piggability.
Tee sections should be designed for pigging. Bars in barred tees should not protrude into the main line bore.
Sizing and spacing, also including distance to bends, should be specifically considered to prevent by-pass
during pigging.
Wye pieces should be designed to allow pigging of both the connecting lines. Due considerations should be
made to the branch line angle into the main bore with respect to pigging (sealing distance).
It shall be confirmed that expansion spools are suitable for all planned pre-commissioning activities, in
particular considering pigging operations. Expansion spool inner diameter and bend radius should be
checked.
Prior to installation each expansion spool should be cleaned, gauged and tested to minimum the same
requirements as for the complete pipeline system.
6.8.2 Combination of rigid and flexible pipelines
The behaviour of flexible flow lines during pressurisation shall be taken into account for system pressure test.
Due considerations shall be given to establishing acceptance criteria for combined flexible-rigid line systems.
The supplier of flexible pipe or riser shall be involved in the selection of pig design.
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Pre-commissioning of submarine pipelines
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Guidance note:
Special design considerations should be made to lines (new/old) connecting to lines in operation through
e.g. hot-tapping. Double isolation valves with bleed should be included in order to ensure adequate precommissioning/commissioning of the connecting line. Ventilation and drain points for testing and purging
(gas lines) should be included and placed at functional and appropriate locations.
6.8.4 Multi diameter pipelines
If a pipeline system consists of multi-diameter sections due considerations should be made with respect to:
—
—
—
—
—
required pigging direction (launch and receiving facilities)
inner diameter transition design
gauging technique (gauge plate versus calliper)
pigging speed and control
later pigging during operational phase.
In order to provide sufficient sealing effects throughout the entire pipeline the smaller ID section should
preferably be towards the end of the pig run(s).
Guidance note:
ID variations up to 8-10% are normally acceptable for modified standard disc pigs.
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6.8.5 Wet buckle contingency
Pigs may be inserted into start-up heads to allow for cleaning and dewatering in case of wet buckle during
pipeline installation.
Contingency plans should include pumps, dewatering compressors, pipeline retrieval equipment, etc.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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6.8.3 Connecting to pipelines in operation
7.1 References
/1/
rd
Pipeline design and construction – A practical approach; M. Mohitpour, H. Golshan, A. Murray, 3
Ed. 2008
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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SECTION 7 REFERENCES
A.1 Checklist for flooding, cleaning and gauging
Project:
Item
Pipeline system
configuration
Date:
Y
N
Question
Comments
Is the pipeline onshore or offshore?
Are both ends accessible?
Will there be need for vessel(s) to flood?
Is subsea flooding an option?
Is one flowing direction preferred?
Pipeline
characteristics
Is it a one diameter or multi-diameter pipeline?
(state minimum and maximum internal diameter)
What is the minimum bend radii (and bend
angles)?
Are there any pipeline features and types: valves,
Tees, wyes, branch connections, elevations etc.?
Is the pipeline internally coated or not?
Material quality, use of CRA?
What is the maximum seawater exposure time
acceptable for the submarine pipeline system?
Flexible tails or flexible risers?
Length of submarine pipeline system, giving
implications for pig design?
How is the permanent pig trap design?
How is the temporary pig trap design? (is it rated
for pressure test? Are pig traps adequate for
the required number of pigs/calliper and for the
accumulated debris in front of the cleaning train?)
Flooding
Will free flooding be used?
In case pump flooding is required, is the pump
spread specified?
Will water quality/water testing be required
Water treatment required?
Level of filtration required?
Water in front of first pig?
Pigs
Is the pig design accounting for the pipeline
characteristic stated above?
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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APPENDIX A CHECKLIST FOR FLOODING, CLEANING AND GAUGING
Item
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Project:
Date:
Y
N
Question
Comments
Is the pig material compatible with pipeline
materials (including internal coatings)?
Pigging directions (one way or bi-directional)
Use of magnets?
Number of pigs?
Pig spacing established?
Type of pigs to be used (brushes, bypass pigs,
jetting pigs)?
Gauge plate/location of damage/on each pig?
Use of instrumented gauge plate (smart gauge)?
Use of calliper gauging (uni-directional)?
Operational
conditions
What are the necessary driving pressures for the
pigging operations?
What will the pig velocity be (based on available
and/or necessary flow)
Will pig tracking be required? Will a pig tracking
vessel be required?
Onshore launch/receiving?
Offshore (topside) launch/receiving?
Subsea launch/receiving? Any weather criteria
limitations for the offshore operations?
Use of hot stabs, design of hot stabs required?
Hoses (type/handling)?
Calliper/electronic pigs, transponder/pingerbattery
life time? Remote start/sleep mode required?
Use of isotope as tracking device? (strength,
specialist personnel)
May access be partial restricted due to other
simultaneous operations, e.g. drilling rig?
Acceptance criteria
Acceptance criteria for flooding?
Acceptance criteria for cleaning?
Acceptance criteria for gauging?
Contingency
Has contingency operations been established in
case anything goes wrong?
Has a wet buckle contingency been established?
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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APPENDIX B CHECKLIST FOR SYSTEM PRESSURE TESTING
B.1 Checklist for system pressure testing
Project:
Item
Date:
Ref. sec.
Y
N
Comment
Comment
Definition of objective (to be performed prior to test execution)
Application and [1.3]
limitation
The recommended practice applies for the
current pipeline system
Test objective
The objective with the test is to check for gross
errors and leakages (and not to document
structural capacity)
Waiving
of system
pressure test
[4.1.4]
Definition and
documentation
The system pressure test should be waived.
(Special considerations should be made
according to DNVGL-ST-F101)
Definition and documentation of test objective
is included in design documentation
Planning and design (to be performed prior to test execution)
Specification of [4.2.2]
test
The test should be performed as a subsea
hydrostatic test after installation
Battery limits
and code
break
Battery limits and code break have been clearly
established
When to
perform
the system
pressure test
[4.2.1]
When to perform the system pressure test is
considered and planned
Test medium
[4.2.7.1]
Applicable test medium for working
environment is decided; the availability and
special chemical considerations are reflected.
Test pressure
[4.2.3]
The test pressure is calculated and meets the
requirement in DNVGL-ST-F101 along the entire
pipeline system. (Checklist in ?Appendix C may
be used)
Equipment and [4.2.3]
instrumentation
The equipment and instrumentation are fit
for purpose and certified as required. The
availability, number and location of equipment
and instrumentation is considered
HSE
HSE issues such as location of equipment, test
area, and personnel safety are considered and
documented
[6.1.2]
Equipment and [4.2.6.4]
instrumentation
sensitivity
Equipment sensitivity is according to
recommended specifications
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Project:
Date:
Ref. sec.
Y
N
Comment
Comment
Calibration of
[4.2.6.5]
equipment and
instrumentation
The equipment have been calibrated according
to the recommendations
Acceptance
criteria
[4.3]
The acceptance criteria are established prior to
the test commences.
Volume of test
medium
[4.2.8]
The total amount of test medium required to
pressurise the pipeline prior to testing to ensure
that sufficient pump capacity and test medium
is available
Air content
[4.2.7.2]
The allowable air content and its influence on
the safety and test results are considered
Stabilisation
period
[4.2.10]
The required stabilisation period is considered
Hold period
[4.2.4]
The minimum required hold period is
established.
Confirmation
[4.2.5]
of pipeline leak
tightness
Confirmation of pipeline leak tightness is
reflected
Temperature
corrections
[4.3.1.1]
Temperature corrections should be accounted
for during the test.
Temperature
corrections
[4.3.1.1]
If temperature corrections should be
considered: The requirements to temperature
recordings and method for correction is
established prior to test execution
Tidal
corrections
[4.3.1.2]
Tidal corrections should be considered
(only relevant if the system pressure test is
monitored from a floating vessel)
Tidal
corrections
[4.3.1.2]
If tidal corrections should be considered: The
method for correction is established prior to
test execution.
Documentation
Documentation of the planning activities
(including relevant calculations) is included in
official design documentation
Execution
Test initiation
[4.2.7]
All relevant aspects related to definition of
test objective and planning (given above) are
considered and documented. Testing may start.
Pressurisation
ratio
[4.2.9]
Pressurisation ratio is according to
recommendations
Monitoring
[4.2.11]
The test is continuously monitored during
pressurisation, stabilisation and test hold
periods
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
Page 58
Date:
Item
Ref. sec.
Data logging
frequency
[4.2.11]
The data logging frequency is sufficient
Visual
inspection
[4.2.11]
For the purpose of a pure leak test, visual
inspection is considered
Y
N
Abnormal
system
response
Depressurisation
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Project:
Comment
Comment
Relevant abnormal system response is
considered. The system pressure test shall be
terminated if any abnormal system response is
identified during testing
[4.2.12]
Applicable de-pressurization ratio is established
Acceptance and final documentation
Acceptance
criteria
[4.3]
Final
documentation
Acceptance criteria are met and the test is
considered successful
Final documentation is prepared and contains
all relevant information for traceability.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
Page 59
C.1 Checklist for system test pressure calculation
Project:
Date:
Step
Description
no.
1
Find the design pressure and its reference
elevation,
2
Guidance note
Value
From design brief.
,
Find the incidental to design pressure ratio,
From design brief.
Calculate the incidental pressure (at
reference level
4
5
),
Find the operational density of content,
Determine the local incidental pressure,
If the design pressure is defined at
a low elevation, a low density will be
conservative unless the test medium has a
lower density than during operation.
'Local' pressure means that it is adjusted
to the relevant elevation (at height 'h',
positive upwards) with the column weight
pressure.
Note that the
requirement,
(DNVGL-ST-F101 Eq. 5.1)
=..........
=..........
, only applies to the
reference level, i.e. the difference between
local design pressure and local incidental
pressure is a fixed pressure, not a relative
value.
Determine the system test pressure
= ..........
=..........
.
(DNVGL-ST-F101 Eq. 4.1)
6
=..........
..........
(DNVGL-ST-F101 Table 3-1)
3
Unit
=..........
The required local system test pressure,
, is
times the local incidental
pressure (typically 5% higher than the
local incidental pressure) along the entire
pipeline.
Note that this is not the actual test
pressure that you will get as you normally
will have different densities in the test
media and operational media. It is the
minimum required value.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
=..........
=..........
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APPENDIX C CHECKLIST FOR SYSTEM TEST PRESSURE
CALCULATION
Date:
Step
Description
no.
7
Guidance note
Determine governing elevation and system
test pressure,
,
(DNVGL-ST-
F101 Sec.5)
Value
Unit
To exceed the required test pressure
along the whole pipeline, the system test
pressure will be governed by the highest
point in the tested system if the density
ratio between system pressure test content
and operation content is larger than
.
=..........
(=1.05, ref. DNVGL-ST-F101 Table 5-9) As
the system pressure test content normally
is water which normally has a significantly
higher density than the operational content
(i.e. larger than 1.05 times the operational
content) this is often the case.
In the rare case that the test medium
has a density lower than 1.05 times the
operational content density, the reference
point for the system test pressure
calculation will be the lowest point in the
system. This will then typically apply to a
water injection pipeline.
8
Calculate the local system test pressure
based on the reference point, e.g. at
pressure gauge (DNVGL-ST-F101, Eq. 4.2)
From the local test pressure, calculated the
test pressure at the elevation where the
pressure is monitored.
=
..........
=..........
=..........
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Project:
Date:
Step
Description
no.
9
Guidance note
The pipeline pressure response as function
of added volume mass may be calculated
for an axially constrained pipeline according
to, ref. Sec.7 /1/:
Value
Unit
Establish the total volume of water required
to perform the pressure test.
For an non-axially-restrained
where:
3
= [m ] initial pipeline volume
= [m] pipeline (steel) outer diameter
= [m] pipeline wall thickness
= [-] Poisson ratio
= [Pa] Young's modulus of steel pipe material
= [Pa] test pressure
= [Pa] initial pressure
= [Pa] bulk modulus of test medium
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Project:
Date:
Step
Description
no.
10
Guidance note
The following expressions may be used to
correlate a (pinhole) theoretical size of a
leak path with the recorded pressure decay
rate.
Defining the decay rate from the system
pressure test in MPa as:
Value
Unit
Establish theoretical pinhole size leak.
Using the equations for a non-axially
restrained pipe, the loss rate of test
medium correlates with variation in test
pressure at constant temperature according
to, ref. Sec.7 /1/:
where:
= [Pa/hr] variation in pressure recorded
3
= [m /s] loss rate of test water
= [Pa] bulk modulus of test medium
= [m] pipeline outer diameter
= [m] pipeline wall thickness (average if variable wall thickness)
= [Pa] Young's Modulus
= [-] Poisson ratio
10
The equivalent pinhole diameter can then
be calculated according to:
= .......
Where:
= [Pa] Differential pressure across pipe wall during test taken as the average difference between
internal test pressure an external pressure
3
= [m /s] loss rate of test water
3
= [kg/m ] density of test medium
= [m] equivalent pinhole size
= [-] discharge coefficient (typical value 0.6)
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
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Project:
Date:
Step
Description
no.
11
Guidance note
For a pipeline that is free to expand both in
axial and lateral direction the temperature
correction of the system pressure test
results may be estimated according to the
following expression, ref /1/:
Value
Unit
Establish temperature correction terms.
= .......
Correspondingly, for a pipeline that
is restrained axially the temperature
correction of the system pressure test
results may be calculated according to the
following expression, ref /1/:
In lieu of specific data the coefficients of volumetric expansion and bulk modulus of water can be found in ref.
Sec.7 /1/.
[1/°C] Coefficient of volumetric thermal expansion factor of test medium
[1/°C] Coefficient of volumetric thermal expansion coefficient of pipe wall material
12
For a system pressure test monitored
from a floating facility where the pressure
at the vessel may vary due to variation
in hydrostatic column, compensation for
variation in tidal water may be allowed for
according to the following correction:
where:
Establish tidal correction terms.
= .......
= [Pa] variation in pressure recorded at vessel
= [m] tidal variation @ location of test vessel/rig (positive for increasing WD)
3
= [kg/m ] density of test medium
2
= [m/s ] gravity
13
Any corrections performed to the pressure
test data shall be limited to the medium
that can actually be accounted for (i.e.
volume of collected test medium,
).
Establish correction terms for loss of test
water.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
= .......
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APPENDIX D CHECKLIST FOR DEWATERING, DRYING AND
INERTING
D.1 Checklist for dewatering, drying and inerting
Project
Item
Pipeline system
configuration
Date
Y
N
Question
Comments
Is the pipeline onshore or offshore?
Are both ends accessible?
Will there be need for vessel(s) to dewater?
Is one flowing direction preferred?
Is system start-up philosophy established and will it
influence the pre-commissioning?
Overall project schedule relevance?
Is drying operation required after/in conjunction with
dewatering?
Pipeline
characteristics
Is it a one diameter or multi-diameter pipeline? (state
minimum and maximum internal diameter)
What is the minimum bend radii (and bend angles)?
Are there any pipeline features and types: valves,
Tees, wyes, branch connections, or dead legs that
need special considerations regarding dewatering?
Is the pipeline internally coated or not?
Material quality, use of CRA?
Flexible tails or flexible risers?
Length of pipeline system, giving implications for
drying technique selection?
Discharge
Is the discharge location chosen?
Is a diffuser required
Will permits to discharge be required?
How will discharging be monitored?
Handling of discoloured water/use of settling pond
required?
Analysis of discharged water required?
Quality of discharged MEG batches, if any used
during dewatering has to be monitored?
MEG batches, if any used during dewatering can to be
discharged to sea?
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
Page 65
Item
Pigs and pig receiver
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Project
Date
Y
N
Question
Comments
Pig and pig train designed according to the pipeline
characteristics above?
Pig tracking/pig tracking vessel required?
Pig tracking by outlet water velocity measurement?
Pig receiver design to accommodate use of
hydrocarbon as propelling medium?
Compressors
Compressor package designed for the required
dewatering pressured and flowrate to maintain the
required dewatering velocity?
Use of 'oil free' air?
Dryer requirements established?
Is any noise protection required, e.g. silencer for
depressurization, noise from compressor station?
Dew point measurements requirements established
(2 different designs/make at each end)?
Acceptance criteria
Acceptance criteria are established
Inerting
Inerting by nitrogen required?
Purity of nitrogen?
Use of nitrogen slug (typ. 10%) or full volume
(100%) to inert?
Inerting as part of vacuum drying?
Contingency
Has contingency operations been established in case
anything goes wrong?
Has the reliability of gas supply been addressed?
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
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Changes – historic
CHANGES – HISTORIC
There are currently no historical changes for this document.
Recommended practice — DNVGL-RP-F115. Edition September 2019
Pre-commissioning of submarine pipelines
DNV GL AS
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About DNV GL
DNV GL is a global quality assurance and risk management company. Driven by our purpose of
safeguarding life, property and the environment, we enable our customers to advance the safety
and sustainability of their business. We provide classification, technical assurance, software and
independent expert advisory services to the maritime, oil & gas, power and renewables industries.
We also provide certification, supply chain and data management services to customers across a
wide range of industries. Operating in more than 100 countries, our experts are dedicated to helping
customers make the world safer, smarter and greener.
SAFER, SMARTER, GREENER