OFFSHORE STANDARDS
DNV-OS-C101
Edition July 2023
Structural design of offshore units
The PDF electronic version of this document available at the DNV website dnv.com is the official version. If there
are any inconsistencies between the PDF version and any other available version, the PDF version shall prevail.
DNV AS
FOREWORD
DNV offshore standards contain technical requirements, principles and acceptance criteria related
to classification of offshore units.
©
DNV AS July 2023
Any comments may be sent by e-mail to rules@dnv.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 other parties than DNV is at the user's sole risk. DNV does not accept any liability or responsibility
for loss or damages resulting from any use of this document.
This document supersedes the July 2019 edition of DNVGL-OS-C101.
The numbering and/or title of items containing changes is highlighted in red.
Changes July 2023
Topic
Reference
Description
Renaming and restructure
All
Updated the standard extensively, including merging with DNVOS-C201.
Changed the title of the standard to Structural design of
offshore units.
Symbols
All
Aligned symbols with DNV rules for ships.
Included symbol list for each section.
Design principles
Loads and load effects
Previous Ch.2 Sec.1
Removed previous Ch.2 Sec.1 content related to design method
testing.
Previous Ch.2 Sec.1
Removed previous Ch.2 Sec.1 content related to probabilistic
design method.
Ch.2 Sec.1
Restructured section. Added working stress design (WSD)
method to scope of standard.
Ch.2 Sec.1 Table 1
Changed load factor γf from 1.3 to 1.2.
Ch.2 Sec.1 Table 4
Increased the basic usage factor
a) from 0.6 to 0.7.
Ch.2 Sec.1 [2]
Added design load conditions.
Ch.2 Sec.1 [3]
Added structural access requirements.
Ch.2 Sec.1 [4.2]
Deleted previous Sec.9 and moved the relevant requirements
on corrosion protection and corrosion control to Ch.2 Sec.1
[4.2], including reference to DNV-RP-B401.
Ch.2 Sec.2
Restructured section.
Ch.2 Sec.2 Table 1
Updated table for variable loads and included a new factor
accounting for several deck tiers.
Ch.2 Sec.2 Table 2
New table describing limit states versus design conditions and
design values.
Ch.2 Sec.2 [2.3.4]
Added wind load formula and correlation table for wind loads.
Added guidance note for correlation table versus elevation
above sea level and average wind time.
Removed existing minimum wind pressure requirements (2.5
2
kN/m ).
Ch.2 Sec.2 [2.5]
Aligned design liquid pressures and terms with DNV rules for
ships.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
η0 for static load combination
Page 3
DNV AS
Changes - current
CHANGES – CURRENT
Reference
Description
Material selection and
inspection principles
Ch.2 Sec.3
Updated and restructured section.
Ch.2 Sec.3 Table 3
Aligned table with DNV-OS-B101 Ch.2 Sec.2.
Strength assessment
Ch.2 Sec.4
New section where previous sections for ULS, FLS, ALS and SLS
have been moved.
Ch.2 Sec.4 [1.3.3]
Added formula for effective breadth based on the requirements
in DNV rules for ships.
Removed old figure for effective flange.
Ch.2 Sec.4 [2]
Aligned scantling control with DNV rules for ships.
Changed formulas for prescriptive scantling control and added
new formulas.
Ch.2 Sec.4 [3]
Added yield control acceptance criteria for both beam and
plate/shell FE-analyses.
Added acceptance criteria for LRFD and WSD design methods
for coarse mesh and fine mesh FE-analyses
Ch.2 Sec.4 [4.2]
Aligned new slenderness requirements with DNV rules for ships.
Ch.2 Sec.4 [4.3]
Added buckling requirement criteria for stiffened panels and
pillars, beams and girders, for both design methods, LRFD and
WSD.
Ch.2 Sec.4 [6]
Moved existing fatigue requirements from previous Ch.2 Sec.5.
Ch.2 Sec.4 [6.3]
Added simplified fatigue design method.
Ch.2 Sec.4 [6.4]
Added stochastic fatigue design method.
Ch.2 Sec.4 [7]
Added descriptions and applications for global, partial, and local
FE-analyses.
Ch.2 Sec.4 [8]
Added descriptions of non-linear FE-analyses.
Weld and bolt connections
Ch.2 Sec.5
Combined weld and bolt connections into one section.
Weld connections
Ch.2 Sec.5 [3]
Removed formula for partial penetration weld. Direct weld
calculations alternatively given in Ch.2 Sec.5 [3.8].
Ch.2 Sec.5 [3.6],
Ch.2 Sec.5 [3.7]
Introduced weld gap tgap to align with DNV rules for ships.
Ch.2 Sec.5 Table 1
Updated and aligned weld yield stress and strength coefficient
factor with DNV rules for ships.
Ch.2 Sec.5 [3.6.4]
Aligned fillet weld formula with DNV rules for ships.
Added leg length requirements for fillet welds.
Ch.2 Sec.5 Table 2
Added minimum leg length requirements.
Ch.2 Sec.5 Table 3
Added weld factors for different structural member type
connections.
Ch.2 Sec.5 [3.7.2],
Ch.2 Sec.5 [3.7.3]
Updated weld connection details to stiffeners and girders to
align with DNV rules for ships.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 4
DNV AS
Changes - current
Topic
Bolt connections
Reference
Description
Ch.2 Sec.5 [3.8.2]
Included 0.9 factor for design resistance for direct calculation of
weld connection.
Ch.2 Sec.5 [4.2.5]
Added requirement for critical single bolt connections.
Ch.2 Sec.5 Table 11
Adjusted partial safety factors for support of life saving
appliances (LSA).
Ch.2 Sec.5 [4.5],
Ch.2 Sec.5 [4.6],
Added requirements for bolted connections in oval bolt holes.
Ch.2 Sec.5 [4.7]
Hull equipment and support
structure
Ch.2 Sec.5 [4.8]
Added requirements for bolted connections in oval bolt holes.
Added requirement for thread length engagement for bolts
fastened directly into base material.
Ch.2 Sec.6
New section providing requirements for hull equipment
including:
— moved temporary mooring, quayside mooring and towing
requirements from DNV-OS-E301 and aligned with DNV-RUSHIP Pt.3 Ch.11
— moved anchor bolster requirements from DNV-OS-E301
— moved quayside mooring requirements and aligned with
DNV-RU-SHIP Pt.3 Ch.11
— moved winches and other pulling accessories requirements
and aligned with DNV-RU-SHIP Pt.3 Ch.11
— moved requirements for foundations and supporting
structures for lifting appliances and aligned with DNV-RUSHIP Pt.3 Ch.11 and DNV-ST-0378.
Special provisions for unit
types
Ch.2 Sec.7
New section for other unit types.
Classification and certification
requirements
Ch.3 Sec.1 [1.4],
Ch.3 Sec.1 [1.5]
New subsection for documentation and required compliance
documentation.
Ch.3 Sec.1 [2]
Added subsection for additional classification requirements
related to Ch.2.
App.B
Moved technical requirements for soil foundation from previous
Ch.2 Sec.10.
Soil foundation design
Editorial corrections
In addition to the above stated changes, editorial corrections may have been made.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 5
DNV AS
Changes - current
Topic
Changes – current............................................................ 3
Chapter 1 Introduction..................................................... 8
Section 1 General................................................................................................................ 8
1 Introduction......................................................................................................... 8
2 Objectives............................................................................................................ 8
3 Scope................................................................................................................... 8
4 Application........................................................................................................... 9
5 References........................................................................................................... 9
6 Definitions and abbreviations............................................................................ 11
7 Procedural requirements....................................................................................15
Chapter 2 Technical content........................................... 16
Section 1 Design principles............................................................................................... 16
1 Overall principles............................................................................................... 16
2 Design load conditions....................................................................................... 19
3 Structural arrangement......................................................................................21
4 Scantlings, tolerances, corrosion protection and corrosion control.................... 22
Section 2 Loads and load effects....................................................................................... 25
1 Introduction....................................................................................................... 25
2 Characteristic loads............................................................................................25
3 Accidental loads................................................................................................. 34
Section 3 Material selection and inspection principles...................................................... 35
1 General...............................................................................................................35
2 Temperatures for selection of material..............................................................35
3 Structural category............................................................................................ 35
4 Structural steel.................................................................................................. 37
Section 4 Strength assessment......................................................................................... 42
1 General...............................................................................................................42
2 Scantling control................................................................................................ 47
3 Yield control.......................................................................................................51
4 Buckling control................................................................................................. 53
5 Deflection........................................................................................................... 60
6 Fatigue............................................................................................................... 61
7 Finite element analyses..................................................................................... 64
8 Non-linear analyses........................................................................................... 66
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 6
DNV AS
Contents
CONTENTS
1 Symbols..............................................................................................................67
2 Butt joints.......................................................................................................... 67
3 Tee and cross joints.......................................................................................... 68
4 Bolt connections.................................................................................................82
Section 6 Hull equipment and supporting structure.......................................................... 93
1 Temporary mooring, quayside mooring and towing equipment..........................93
2 Support structure for permanent mooring/position mooring equipment.......... 104
3 Heavy equipment, deck machinery, marine equipment and topside module
foundations and supports............................................................................... 106
4 Support for lifting appliances and crane pedestals.......................................... 107
Section 7 Special provisions for DNV unit types.............................................................. 113
1 Basis for all DNV unit specific types................................................................ 113
2 Ship-shaped and cylindrical units.................................................................... 113
3 Column stabilised units....................................................................................113
4 Self-elevating units (including self-elevated wind turbine installation units)... 113
5 Tension leg platforms...................................................................................... 113
6 Deep draught units.......................................................................................... 113
Chapter 3 Classification and certification......................114
Section 1 Procedural requirements..................................................................................114
1 Introduction..................................................................................................... 114
2 Classification requirements for technical requirements in Ch.2........................ 122
Appendix A Cross sectional types................................. 127
1 Cross sectional types.......................................................................... 127
Appendix B Soil foundation design............................... 131
1 General................................................................................................131
2 Stability of seabed.............................................................................. 134
3 Design of pile foundations.................................................................. 135
4 Design of gravity foundations............................................................. 138
5 Design of anchor foundations............................................................. 140
Changes – historic........................................................145
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 7
DNV AS
Contents
Section 5 Weld and bolt connections.................................................................................67
SECTION 1 GENERAL
1 Introduction
This offshore standard provides principles, technical requirements and guidance for structural design of
floating offshore units made of steel.
DNV unit specific standards may have specific requirements that replace or clarify requirements specified in
this standard. For new or novel designs where no DNV unit specific exists, this standard may be used as a
stand-alone document.
2 Objectives
The objectives of this standard are to:
— provide an internationally acceptable level of safety by defining minimum requirements for structures and
structural components, in combination with referenced standards, recommended practices, guidelines,
etc.
— serve as a contractual reference document between suppliers and purchasers
— serve as reference document for designers, suppliers, purchasers and regulators.
3 Scope
The scope of this standard is to provide principles, technical requirements and guidance for structural design
of floating offshore units made of steel.
Load and resistence factor design (LRFD) and the working stress design (WSD) are covered as applicable
design methods. The following topics are covered:
—
—
—
—
—
—
design principles
design loads and load effects
material selection and inspection principles
strength assessment
weld and bolt connections
hull equipment and supporting structure.
This standard has structural requirements for the following:
—
—
—
—
—
scantling control of plates, stiffeners and girders
yield
buckling
fatigue
deflection
covered by the limit states:
—
—
—
—
ultimate limit states (ULS)
accidental limit states (ALS)
fatigue limit states (FLS)
serviceability limit states (SLS).
This standard uses a deterministic design method. Design by testing and probabilistic design methods are not
covered.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 8
DNV AS
Chapter 1 Section 1
CHAPTER 1 INTRODUCTION
Flag and shelf state requirements are not covered by this standard.
Guidance note:
Flag and shelf state regulations may include requirements in excess of the provisions of this standard depending on the type,
location and intended service of the offshore unit.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4 Application
This standard is applicable for structures made of steel, with the primary application being floating offshore
units.
The following specific application requirements apply:
— For application of this standard as a technical basis for DNV classification, see Ch.3 Sec.1.
— Where a DNV unit specific standard exists, the unit specific standard shall be applied as the unit specific
standard gives references to this standard when appropriate. In case of deviating requirements between
this standard and the DNV unit specific standard, the requirements of the DNV unit specific standard
prevail.
— The latest revision of the referenced document shall be applied, unless contractual documents specify
otherwise.
5 References
Table 1 lists DNV references used in this document.
Table 1 DNV references
Document code
Title
DNV-CG-0550
Maritime services – terms and systematics
DNV-CG-0127
Finite element analysis
DNV-CG-0128
Buckling
DNV-CG-0129
Fatigue assessment of ship structures
DNV-CG-0156
Conversion of ships
DNV-OS-A101
Safety principles and arrangement
DNV-OS-B101
Metallic materials
DNV-OS-C102
Structural design of offshore ships-shaped units
DNV-OS-C103
Structural design of column-stabilised units - LRFD method
DNV-OS-C104
Structural design of self-elevating units - LRFD method
DNV-OS-C105
Structural design of TLP - LRFD method
DNV-OS-C106
Structural design of deep draught floating units
DNV-OS-C401
Fabrication and testing of offshore structures
DNV-OS-D101
Marine and machinery systems and equipment
DNV-OS-E301
Position mooring
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 9
DNV AS
Chapter 1 Section 1
The acceptance criteria in this standard are based on a linear elastic strength approach, i.e. material yield
stress limit. However, for special structural items it may be acceptable to use the plastic material capacity,
i.e. ultimate material limit.
Title
DNV-OS-E302
Offshore mooring chain
DNV-OS-E303
Offshore fibre ropes
DNV-OS-E304
Offshore mooring steel wire ropes
DNV-RU-SHIP Pt.2 Ch.1
General requirements for materials and fabrication
DNV-RU-SHIP Pt.3 Ch.3
Structural design principles
DNV-RU-SHIP Pt.3 Ch.10
Special requirements
DNV-RU-SHIP Pt.3 Ch.11
Hull equipment, supporting structure and appendages
DNV-RU-SHIP Pt.6 Ch.6
Cold climate
DNV-RP-B401
Cathodic protection design
DNV-RP-B101
Corrosion protection of floating production and storage units
DNV-RP-C201
Buckling Strength of Plated Structures
DNV-RP-C202
Buckling strength of shells
DNV-RP-C203
Fatigue design of offshore steel structures
DNV-RP-C204
Design against accidental loads
DNV-RP-C205
Environmental conditions and environmental loads
DNV-RP-C208
Determination of structural capacity by non-linear finite element analysis methods
DNV-ST-0377
Shipboard lifting appliances
DNV-ST-0378
Standard for offshore and platform lifting appliances
Chapter 1 Section 1
Document code
Table 2 lists other references used in this document.
Table 2 Other references
Document code
Title
AISC
Steel construction manual
API RP 2A-LRFD
Planning, Designing, and Constructing Fixed Offshore Platforms - Load and Resistance Factor
Design
API RP 2A-WSD
Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design
CB/T 3143-2013
Bolt type cable releaser
EN 1993-1 series
Design of steel structures
EN 1999-1 series
Design of aluminium structures
EN 10204
Metallic products - Types of inspection documents
DIN 81860-1
End fastenings for anchor chain cables on board ships - Part 1: Assembly
DIN 81860-2
End fastenings for anchor chain cables on ships - Part 2: Components
MODU code
Code for the construction and equipment of mobile offshore drilling units
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 10
DNV AS
Title
NORSOK N-003
Actions and action effects
NORSOK N-004
Design of offshore structures
IMO Res. MSC.215(82)
Performance standards for protective coatings for dedicated seawater ballast tanks in all
types of ships and double-side skin spaces of bulk carriers
ISO 4014
Fasteners - Hexagon head bolts - Product grades A and B
ISO 898 Part 1
Mechanical properties of fasteners made of carbon steel and alloy steel - Part 1: Bolts,
screws and studs with specified property classes - Coarse thread and fine pitch thread
ISO 10474
Steel and steel products — Inspection documents
ISO 19901-2
Petroleum and natural gas industries — Specific requirements for offshore structures — Part
2: Seismic design procedures and criteria
ISO 19902
Petroleum and natural gas industries - Fixed steel offshore structures
ISO 19905-1
Petroleum and natural gas industries — Site-specific assessment of mobile offshore units —
Part 1: Jack-ups
JIS F2025
Shipbuilding - Cable clenchies
6 Definitions and abbreviations
6.1 Definition of verbal forms
The verbal forms defined in Table 3 are used in this document.
Table 3 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
may
verbal form used to indicate a course of action permissible within the limits of the document
6.2 Definition of terms
The terms defined in Table 4 are used in this document.
Table 4 Definition of terms
Term
Definition
accidental condition
events caused by abnormal operation or technical failures
accidental limit states
damage of the structure due to an accidental event or operational failure
characteristic value
representative value associated with a prescribed probability of not being
unfavourably exceeded during the applicable reference period
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 11
DNV AS
Chapter 1 Section 1
Document code
Definition
design life
defined period the unit is expected to operate
design fatigue life
design life times the design fatigue factor
design resistance
reference value of structural strength to be used in the determination of the design
strength
DNV unit specific standard
DNV offshore standard for a unit type, e.g.
— DNV-OS-C102 structural design of offshore ship-shaped and cylindrical units
— DNV-OS-C103 structural design of column-stabilised units
— DNV-OS-C104 structural design of self-elevating units
— DNV-OS-C105 structural design of TLP's
— DNV-OS-C106 structural design of deep draught floating units
expected value
most probable value of a load during a specified time period
external structure
outside structural parts of the unit exposed to weather or submerged in sea, e.g.
shell plates, deck and bottom plates
fatigue limit states
possibility of structural failure due to cyclic loading, e.g. cumulative damage due to
repetitive loads
floating offshore unit
unit where the full weight is supported by buoyancy
For the purpose of this standard self elevating units/ jack-ups are considered floating
offshore units.
hawse pipe
steel pipe fitted in the unit to ensure smooth running of the anchor chain, and
stowage of the anchor
inspection
activities such as measuring, examination, testing, gauging one or more
characteristics of an object or service and comparing the results with specified
requirements to determine conformity
lifting appliance
crane, A-frame, derrick or lifting mast
limit state
state beyond which the structure no longer satisfies the requirements
load effect
effect of a single design load or combination of loads on the equipment or system,
such as stress, strain, deformation, displacement, motion, etc.
lowest mean daily average
temperature
lowest value on the annual mean daily average temperature curve for the area in
question
For temporary phases or restricted operations, the lowest mean daily average
temperature may be defined for specific seasons.
— Mean daily average temperature: the statistical mean average temperature for
a specific calendar day based on a number of years of observations (normally at
least 20 years).
— Mean: statistical mean based on number of years of observations.
— Average: average during one day and night.
lowest waterline
typical light ballast waterline for ships, wet transit waterline or inspection waterline
for other types of units
main towing
towing assisted by tug boat(s), e.g. from shipyard to the location of operation for the
unit, or from one location of operation to another
marine equipment
equipment related to the marine system and equipment onboard the unit
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 12
DNV AS
Chapter 1 Section 1
Term
Definition
non-destructive testing
structural tests and inspection of welds with radiography, ultrasonic or magnetic
powder methods
offshore crane
lifting appliances on board units, intended for load handling outside the deck area at
open sea, e.g. loading and discharging offshore support vessels, barges or from the
seabed
For wind turbine installation units, see also platform crane.
offshore installation
construction engaged in offshore operations, and which is designed and intended for
use at one particular location for an extended period
operating condition
when the unit is operating in the location and for the purpose which it was built for
(e.g. drilling)
operation draught
interval between normal ballast and full load draught for the unit
platform crane
lifting appliances on board units intended for load handling within and outside the unit
while in harbours and within the cargo deck area whilst at sea
Guidance note:
Cranes on self-elevating units in an elevated position, and other bottom fixed units,
where lifting is carried out to other fixed installations, are categorized as platform
cranes.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
primary supporting members
members of the beam, girder or stringer type which provide the overall structural
integrity of the unit
quayside mooring
mooring used when the unit is moored at a quay
redundancy
ability of a component or system to maintain or restore its function when a failure of
a member or connection has occurred
Redundancy may be achieved for instance by strengthening or introducing alternative
load paths.
serviceability limit states
criteria applicable to normal use or durability, e.g. deflections, vibration and motions
related to operation
specified minimum yield
strength
minimum yield strength prescribed by the specification or standard under which the
material is purchased
spurling pipe
pipe between the chain locker and the weather deck
safe towing load
maximum load the equipment is designed for with respect to towing purposes
safe working load
for mooring purpose: the maximum load the equipment is designed for with respect
to mooring purposes
For crane lift: the maximum load which the lifting appliance is certified to lift.
submerged zone
part of the unit which is below the splash zone, including buried parts
supporting structure
strengthening of the vessel structure, e.g. a deck, in order to accommodate loads and
moments from a heavy or loaded object
survival condition
most severe environmental loads the unit will be exposed to at its location
temporary mooring
mooring used when the unit is temporarily moored at a location with anchor(s) in
moderate sea conditions (when Hs is below 2 m, and the 1 minute wind speed is
below 11 m/s)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 13
DNV AS
Chapter 1 Section 1
Term
Definition
tensile strength
minimum stress level where strain hardening is at maximum or at rupture
topside module
modules related to the drilling or processing of hydrocarbons onboard the unit
transit condition
when the unit moves, either using it's own propulsion or via towing, between different
locations
ultimate limit states
ultimate resistance of the structure for carrying further loads, e.g. excessive yielding
and buckling
unit
general term for a mobile offshore unit or offshore installation such as e.g. ship
shaped and cylindrical, column stabilised, self-elevating, tension leg or deep draught
unit
6.3 Symbols
Global symbols are given below. Local symbols used in subsections are specially defined when used.
= characteristic load
Hs
= significant wave height
= design resistance
= specified minimum yield stress
= thickness of plate
η0
γf
γm
ρ
= basic usage factor
= load factor
= partial safety factor
= density.
6.4 Abbreviations
The abbreviations described in Table 5 are used in this document.
Table 5 Abbreviations
Abbreviation
Description
AISC
American Institute of Steel Construction
ALS
accidental limit states
API
American Petroleum Institute
DFF
design fatigue factor
FLS
fatigue limit state
ISO
international organisation of standardisation
LAT
astronomical tide level in agreement
LMDAT
lowest mean daily average temperature
LRFD
load and resistance factor design
MBL
minimum breaking load
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 14
DNV AS
Chapter 1 Section 1
Term
Description
NDT
non-destructive testing
OI
offshore installation
PMA
permanent means of access
PSM
primary supporting members
SCF
stress concentration factor
SLS
serviceability limit state
SWL
safe working load
TOW
towing load
ULS
ultimate limit states
WSD
working stress design
Chapter 1 Section 1
Abbreviation
7 Procedural requirements
7.1 Documentation requirements
Documentation shall be submitted as required by Ch.3 Sec.1 [1.4].
7.2 Required compliance documentation
Compliance documentation shall be submitted as required by Ch.3 Sec.1 [1.5].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 15
DNV AS
SECTION 1 DESIGN PRINCIPLES
1 Overall principles
1.1 General
1.1.1 This section defines the underlying design principles of this standard in terms of loads, structural
capacity models and assessment criteria, and in-service aspects of importance for the structural integrity of
the unit.
1.1.2 For novel designs, or unproven applications where limited experience is available, increased scope such
as model testing or more extensive use of FE-analyses may be required to demonstrate sufficient strength.
1.1.3 This standard is based on the following overall principles:
— the safety of the structure can be assessed by addressing the potential structural failure mode(s) when
the unit is subjected to operational loads and environmental loads/conditions
— the design complies with the unit's specified design basis
— the structural requirements shall be based on consistent design load sets which cover the appropriate
operating modes.
1.1.4 The unit’s structure shall be designed such that it:
— has a redundant structure. The unit’s structure shall work in a hierarchical manner and, in principle,
failure of structural elements lower down in the hierarchy do not result in immediate consequential failure
of elements higher up in the hierarchy
— sustains loads liable to occur during all design conditions of its design life
— has adequate structural redundancy to survive in case of structural failure of a vital element or component
— has low probability of fatigue cracks in-service
— has good access conditions during construction to ensure satisfactory quality of welding, coating and safe
survey/inspection
— has good access conditions during the operational phase to allow adequate maintenance and safe
periodical survey by crew, and other authorities.
1.2 Limit state design principle
For the limit state design, each structural member shall be designed for the most unfavourable limit state.
The level of safety of a structural element is acceptable if the design load effect Fd does not exceed the
design resistance
Rd, i.e:
where:
Fd
= Rd defines a limit state.
The limit states are defined in Ch.1 Sec.1 Table 4.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 16
DNV AS
Chapter 2 Section 1
CHAPTER 2 TECHNICAL CONTENT
Chapter 2 Section 1
1.3 Design methods
1.3.1 General
The following two design principle methods are used in this standard:
a)
b)
the load and resistance factor design (LRFD) method
the working stress design (WSD) method.
The design methods accounts for:
— possible unfavourable deviations of the loads
— uncertainties in the model and analysis used for determination of load effects
— possible unfavourable deviations in the resistance of materials.
Each structural member shall be designed for the most unfavourable design load condition.
1.3.2 Consistency in design method
One design method shall be used, either LRFD or WSD, in the global design of the unit.
Guidance note:
DNV unit specific standards may specify design methods to be used.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.3.3 LRFD method
1.3.3.1 General
The LRFD method is a design method by which the target safety level is obtained by applying partial load and
partial safety factors.
1.3.3.2 Design loads
The design load is obtained by multiplying the characteristic load with a load factor:
where:
Fd
γf
Fk
= design load
= load factor
= characteristic load.
The load factors in Table 1 shall be applied, unless otherwise specified in an applicable DNV unit specific
standard.
Table 1 Load factors
Combination
of design loads
a)
b)
γf
for the limit states
ULS limit state
G
1.2
1) 2)
1.0
Q
1.2
1) 2)
1.0
FLS, ALS and SLS limit state
E
D
0.7
1.2
3)
1.0
1.0
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
G
Q
E
D
1.0
1.0
1.0
1.0
Page 17
DNV AS
G
Q
FLS, ALS and SLS limit state
E
D
G
Q
E
Chapter 2 Section 1
ULS limit state
Combination
of design loads
D
Load categories are:
= permanent load
G
Q
E
D
1)
= variable load
= environmental load
= deformation load.
The load factor shall be increased to 1.3 if the loads are not well defined.
A load factor γf = 1.0 for permanent loads (G) and variable loads (Q) in combination a) shall be used, if
this results in a higher design load effect.
2)
The load factor γf for the environmental loads may be reduced to 1.15 in combination b), if the unit is
unmanned or evacuated in predefined extreme environmental conditions.
3)
1.3.3.3 Design resistance
The design resistance
Rd is defined by a partial safety factor γm given in Table 2.
Table 2 Partial safety factor γm
Limit state
γm
ULS
1.15
ALS
1.0
SLS
1.0
An additional β-factor shall be applied for buckling of cylindrical and unstiffened conical shell structure. i.e:
Guidance note:
See DNV-RP-C202 for description of cylindrical and unstiffened conical shell structure.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
The β-factor is given in Table 3 and depends on a reduced slenderness parameter
λ
=
ReH
σE
= specified minimum yield stress [N/mm ]
Rk
λ defined by:
reduced slenderness parameter
2
= elastic buckling stress for the buckling mode [N/mm2], given in DNV-RP-C202 (denoted by fE)
= characteristic resistance.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 18
DNV AS
β-factor for cylindrical and unstiffened conical shell structure
Structural
member type
Chapter 2 Section 1
Table 3
β
λ ≤ 0.5
Cylindrical and
unstiffened conical
shell structure
0.5 <
1.0
λ < 1.0
0.74 + 0.52
λ ≥ 1.0
λ
1.26
1.3.4 WSD method
1.3.4.1 General
For the WSD method the target safety level is achieved by use of an allowable usage factor
ηall.
where:
.
1.3.4.2 Design loads
The design load
Fd = characteristic load Fk.
1.3.4.3 Design resistance
The design resistance Rd is defined by an allowable usage factor ηall, which depends on a basic usage factor
η0 given in Table 4, and a β-factor for cylindrical and unstiffened conical shell structure given in Table 3.
Table 4 Basic usage factors
Combination
of design loads
η0
Description
ULS limit state
a) Static
Permanent and variable loads
b) Static + dynamic
Maximum combination of environmental
loads and associated permanent/variable
loads
FLS, ALS and SLS limit state
0.70
0.80
1)
1.0
The basic usage factor η0 may be increased to 0.85 in combination b), provided the unit is unmanned or evacuated in
predefined extreme environmental conditions.
1)
2 Design load conditions
2.1 General
The basic design conditions shall use the most unfavourable combination of variable loads and environmental
loads for the limit states given in [1.2].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 19
DNV AS
2.2.1 Transit condition
The transit conditions are:
a)
b)
c)
unrestricted transit based on the North Atlantic environmental condition defined in DNV-RP-C205 App.C
restricted transit based on site specific environmental conditions (scatter diagram) for the intended
transit route(s), see also DNV-RP-C205 App.B
restricted transit defined by a maximum allowable wave height (Hs).
The wave response calculation shall be calculated by either:
i)
ii)
L
-8
long term response analysis using scatter diagram for a probability level of 10 , which corresponds to a
20-years response level, or
iii) short term response analysis using a design wave height (Hs), or
iv) use of rule based prescriptive loads as applicable, e.g. DNV rules for ships.
2.2.2 Operating condition
The applicable requirements for the operating condition are:
1)
2)
3)
The operation is limited to a maximum sea state (Hs) at which the unit must suspend operation.
The maximum environmental loads for the operation shall be used, i.e. waves and wind shall be
combined with operational loads, e.g. loads from drilling operations.
Limitations shall be specified in the design basis documentation, and shall be included in the unit's
operation manual.
2.2.3 Survival condition
The applicable requirements for the survival condition are:
1)
2)
3)
The design loads for the survival condition shall be based on site specific loads. For units intended to
operate wordwide, the North Atlantic environmental conditions shall be applied.
The 100-year environmental response level shall be applied.
Units designed to leave the location in case of an extreme weather conditions (e.g. forewarned
hurricanes, icebergs), shall have a procedure for disconnection and re-connection. The disconnection
criteria shall be described in the unit's operation manual, and the 100-years response level may be
based on the disconnected weather conditions (Hs).
2.2.4 Accidental condition
The accidental conditions shall be assessed based on the unit's function, arrangements, operational
procedures, safety systems, etc. according to the principles in DNV-OS-A101.
The accidental events shall be checked in the following steps:
1)
2)
The structure shall withstand the design accidental loads caused by the design accidental events, as
found applicable for the unit in operational condition.
In case of damage in step 1), the unit shall be able to resist a 1-year environmental condition, without
loss of floatability, stability or global structural integrity.
Guidance note:
The DNV unit specific standards may give further guidance and requirements to accidental events.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 20
DNV AS
Chapter 2 Section 1
2.2 Basic design conditions
2.3.1 Single transit to location
Single transit to location is a one single transit to a location, typically from a shipyard to the location where
the unit shall operate. The unit may either use its own propulsion or be towed, within a defined towing/transit
route.
The response analysis for a single transit may be based on by either:
a)
b)
site specific environmental condition (scatter diagram) for the intended single transit route using an
annual probability level of 1-year
specified restricted maximum allowable wave height (Hs) for the intended single transit route.
Guidance note:
Dry tow, e.g transit on a heavy lift vessel, is considered as a marine operation.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.3.2 Installation and retrieval condition
Installation and retrieval condition for units where this is applicable, e.g. self-elevated unit when lowering
and jacking the hull and legs. The allowable environmental conditions shall be defined.
2.3.3 Inspection and maintenance condition
Inspection and maintenance conditions are conditions carried out in sheltered waters, or in restricted weather
conditions. Actual tank filling configurations and draft shall be defined. Environmental loads based on a wave
height (Hs) of 2.0 m should be used for this condition, unless otherwise specified.
3 Structural arrangement
3.1 Access arrangement
3.1.1 All units shall be provided with means of access which give safe and practical access to all structural
parts. For spaces with limited access, or where areas are not possible to enter, special measures for
inspection and maintenance shall be applied.
3.1.2 The manholes shall be arranged to provide easy access to all parts of the structure. The edges of
manholes shall be smooth.
3.1.3 Relevant parts of the PMA requirements should be embedded in the early design phase, considering:
— access arrangement and structural arrangement of stairs, ladders, platforms and handrails
— number of openings, accessibility, types, sizes and positions related to fatigue sensitive areas
— location of horizontal stiffeners, girders and stringers such that they may be used as access platforms.
3.1.4 The means of access shall be described in an access manual made for the unit.
Guidance note:
The PMA requirements are specified in SOLAS Ch.II-1 Reg.3-6 for oil tankers and bulk carriers, and in the MODU code Ch.2 [2.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 21
DNV AS
Chapter 2 Section 1
2.3 Special conditions
3.2.1 The splash zone is the part of the unit intermittently exposed to air and immersed in the sea. Splash
zone is mainly relevant for units operating with a constant draught where any change of the draught during
access for inspection and repair in the splash zone not is possible, e.g. typical for TLPs and self elevating
units. In the splash zone stricter design fatigue factors (DFF) are required, see Sec.4 [6].
The splash zone shall extend 5 m above highest operation draught and 4 m below lowest operation draught.
Guidance note:
Additional corrosion margins may be relevant to use in the splash zone based on expected efficiency and durability of the unit's
corrosion protection system.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.2.2 For bottom fixed structures, e.g. self-elevating units and TLPs, the Hs-splash shall be used to determine
the upper and lower limits of the splash zone.
Hs-splash is defined as 67% of the site specific 100-years significant wave height Hs at the location(s) the unit
shall operate.
The upper limit of the splash zone
SZu is defined as:
The lower limit of the splash zone
SZL is defined as:
where:
U1
U2
U3
U4
L1
L2
L3
= 60% of Hs-splash
= highest astronomical tide level (HAT)
= foundation settlement, if applicable
= motion of the unit, as applicable
= 40% of Hs-splash
= lowest astronomical tide level (LAT)
= motion of the unit, as applicable.
4 Scantlings, tolerances, corrosion protection and corrosion control
4.1 Scantlings
4.1.1 The required scantling thickness is obtained by rounding required calculated thickness to the nearest
half millimetre.
4.1.2 Scantlings may be designed without corrosion additions, provided those parts of the structure are
protected by a corrosion protection system, e.g. coating, anodes, impressed current during the entire lifetime
of the unit.
Guidance note:
DNV unit specific standards, e.g. DNV-OS-C102 for ship shaped offshore units, may require use of corrosion addition.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 22
DNV AS
Chapter 2 Section 1
3.2 Splash zone
Guidance note:
E.g. API-5L allows minus tolerances up to 12.5% of nominal thickness in seamless pipes and 10% in welded pipes.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.2 Corrosion protection and corrosion control
4.2.1 Corrosion protection/corrosion control of structural steel comprises:
—
—
—
—
—
coatings
cathodic protection
use of a corrosion allowance
inspection and monitoring of corrosion
control of humidity for internal zones (compartments).
4.2.2 All structures shall be protected from corrosion. Combination of different corrosion protection systems
may be applied.
4.2.3 Requirements to surface preparation, coating application, fabrication and installation of cathodic
protection systems are given in DNV-OS-C401 Ch.2 Sec.9.
4.2.4 Coating
All external surfaces shall be protected by coating. For coating requirements, see DNV-RP-B401.
A coating technical file (CTF) considering coating work specification, inspection and repair, shall be prepared.
Guidance note:
See also MODU code Ch.2 [2.12], SOLAS Ch.II-1 Reg.3-2 and IMO Res. MSC.215(82).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.2.5 Corrosion addition
If corrosion allowance is applied, the necessary corrosion addition for the structural elements shall be based
on:
—
—
—
—
the design life of the unit
the maintenance philosophy
temperature
single or double side exposure.
If no corrosion protection system is installed for the individual structural elements, the following corrosion
addition [mm] for one side of a structural member shall be used:
— ballast and cargo tanks: the highest value of 1.0 mm, or 0.05 mm per year times the unit's design life in
years
— other tanks: the highest value of 0.5 mm, or 0.025 mm per year times the unit's design life in years
— void and dry spaces: 0.0 mm.
where:
= total corrosion addition [mm]
tc
tc1, tc2 = corrosion addition, in mm, on each of the two sides of the considered structural member
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 23
DNV AS
Chapter 2 Section 1
4.1.3 If applicable standard for dimensional tolerances, e.g. for tubular products, specifics minus tolerance
on nominal thicknesses of more than 0.3 mm, then this shall be accounted for in the design.
4.3.1 The cathodic protection system shall be designed equal to the design life of the unit.
4.3.2 Cathodic protection may be achieved by use of galvanic anodes (also referred to as sacrificial anodes)
or impressed current from a rectifier, and shall be designed according to a recognized standard, see e.g.
DNV-RP-B401 and DNV-RP-B101.
4.3.3 The effect of cathodic protection or exposure to anaerobic environments on excessively strained areas
of extra high strength steel shall be evaluated for the risk of hydrogen induced stress cracking (HISC).
Special requirements for welding are given in DNV-OS-C401 Ch.2 Sec.5 [3.3.5.2].
4.3.4 Anodes shall not be fitted at intersection between structural elements or at stress concentration areas
(hot-spots).
4.3.5 For cathodic protection of ballast tanks that may become affected by hazardous gas from adjacent
tanks for storage of oil or other liquids with flash point less than 60°C, anodes on zinc base are preferred.
Due to the risk of thermal ignition, aluminium base anodes shall be installed so that a detached anode can
generate maximum 275 J of energy. The energy calculation shall be based on anode weight and falling
height.
4.3.6 Galvanic anodes
For determination of numbers, positions, sizes of galvanic anodes, see DNV-RP-B401.
4.3.7 Impressed current systems
Impressed current anodes shall be protected, e.g. mounted flush, with thick non-conducting coating or sheet
(dielectric shield) in order to avoid any disbondment of paint coatings or hydrogen induced damage of the
steel.
For calculation of impressed current cathodic protection systems, see e.g. DNV-RP-B401 and DNV-RP-B101.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 24
DNV AS
Chapter 2 Section 1
4.3 Cathodic protection
1 Introduction
1.1 General
The requirements in this section define and specify load components and load combinations to be applied for
the global and local design analyses.
For symbols not defined in this section, see Ch.1 Sec.1 [6.3].
1.2 Scope
1.2.1 Local impact pressure caused by the sea (slamming, hull impact from wave run-up) or by liquid
cargoes in partly filled tanks (sloshing) are not covered by this section. Such loads are described in e.g. DNVRU-SHIP Pt.3 Ch.10, object specific DNV offshore standards and DNV-RP-C205.
1.2.2 For design loads related to mooring and towing, see Sec.6.
2 Characteristic loads
2.1 Permanent loads
Permanent loads are loads caused by dead weight of the construction and from fixed installed equipment
which will not vary in magnitude, position or direction during the unit's design life, e.g.:
—
—
—
—
self weight of structure
topside structure and permanently installed equipment
pre-tension of permanently installed mooring lines
permanent ballast.
2.2 Variable loads
2.2.1 General
Variable loads are loads which will vary in magnitude, position, and direction during operation of the unit.
Examples of variable loads are:
—
—
—
—
personnel
cargo and movable equipment
operational loads from topside, drilling and lifting
content of independent tanks/pipes typically applicable for topsides.
The following apply:
a)
b)
c)
The variable load shall be taken as either the maximum or minimum specified value, using the value
which gives the most unfavourable resulting load effects for the structure elements considered.
The distributed deck loads q shall be defined by the designer, but should not be taken lower than the
values in Table 1.
When considering the overall variable design load on deck areas of topside modules, this may be limited
to loads of storage and lay down areas.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 25
DNV AS
Chapter 2 Section 2
SECTION 2 LOADS AND LOAD EFFECTS
For accommodation areas extending over several tiers, the reaction loads for pillars and bulkhead
stiffeners supporting multiple decks may be reduced by a factor kp-f.
Chapter 2 Section 2
d)
where:
n is the number of decks above the pillar or bulkhead stiffener considered.
Guidance note:
Example: For a pillar supporting number of deck as shown in Figure 1, the total force
FTot to be applied for the pillar will be:
.
Figure 1 Pillar force across multiple decks example
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Table 1 Minimum variable loads on deck areas
Area
Description
2
Distributed load q [t/m ]
1)
Point load Pu [tonnes]
a)
Storage and lay down areas
1.2
1.8
b)
Lifeboat platforms
0.7
0.7
c)
Area between equipment
0.4
0.4
d)
Walkways, staircases and platforms,
wheelhouse deck
0.35
0.35
e)
Accommodation areas
0.35
0.35
f)
Walkways and staircases for inspection only
0.25
0.25
g)
Platforms in machinery spaces
0.8
0.8
2)
1)
Wheel loads shall be applied where relevant (wheel loads may be assumed to act on an area of 300 mm x 300 mm).
Further guidance may be found in DNV-RU-SHIP Pt.3 Ch.10 Sec.5.
2)
Point loads shall be applied on an area 100 mm x 100 mm at the most severe location.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 26
DNV AS
The minimum distributed design deck loads pdl shall be taken as the distributed load
vertical acceleration for the location considered, but shall not be taken less than:
q times resulting
where:
q is the distributed load given in Table 1.
2.2.3 Concentrated force due to point loads
The deck structure (plate and stiffeners) shall be checked for concentrated force
FU based on defined deck
PU for the actual deck area.
FU shall be taken as the point load PU times resulting vertical acceleration for the location considered, but
point mass
shall not be taken less than:
where:
PU
= point load given in Table 1.
2.3 Environmental loads
2.3.1 General
The following applies:
— Dynamic environmental loads are loads which may vary in magnitude, position and direction during the
period under consideration.
— For units that operate at different locations, the most severe environmental loads that the structure may
experience during its design life shall be applied.
— For units moored at one location, site specific loads for the actual location shall be applied. The site
specific environmental data shall be based on observations from, or in the vicinity, of the actual location
the units shall operate.
— The characteristic environmental dynamic loads for ULS and ALS are given in Table 2.
— Scatter diagrams for North Atlantic and worldwide are defined in DNV-RP-C205 App.C.
— Nautical zones for estimation of long term wave distribution given in DNV-RP-C205 App.B.
— For serviceability criteria (SLS), see Sec.4 [5.1.1].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 27
DNV AS
Chapter 2 Section 2
2.2.2 Pressure due to distributed loads
Design condition
Dynamic
load
category
Waves
Limit
state
ULS
Accidental
Transit
Operation
- North Atlantic
scatter diagram for
a probability level
–8
of 10 (20-years
return period).
- Specific scatter
diagram for defined/
limited transit
area(s).
- Site specific
scatter.
- Site specific scatter
or North Atlantic
- A defined sea state scatter diagram for
an annual probability
(Hs) limitation.
–2
level of 10 (100years return period).
Damaged
structure
-
-
-
-
- Site specific
condition, i.e. max
wave height(s) with
associated wave
period(s), Hmax/Tass
- Rule based
prescriptive loads,
e.g. DNV rules for
ships.
Wind
velocity
Intact
structure
Design wave
represented by a
100-years design
value.
- A defined sea state
(Hs) limitation.
FLS
Survival
1)
World wide trade
scatter diagram.
Site specific scatter
or North Atlantic
scatter diagram.
ALS
-
ULS
Site specific, or
U1min,10m = 36 m/s.
-
-
Site specific, or
U1min,10m =36 m/s
-
Site specific, or
U1min,10m =51.5 m/s
Specified
1 year level
value, see
return period.
Sec.1 [2.2.4].
-
-
To be
specially
considered
based on area
of operation.
-
The given wind velocities are in accordance with the MODU code
current
ULS
To be applied as applicable.
Ice
ULS
ALS
To be applied as applicable.
Earthquake
ALS
-
1)
Short term design wave response analyses for the 100-years survival condition is normally represented by use of 90%
fractal value of the extreme response distribution developed from contour lines
Guidance note:
Flag and shelf states may have requirements above the design loads listed in Table 2.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 28
DNV AS
Chapter 2 Section 2
Table 2 Characteristic environmental dynamic loads for the basic design conditions
Static sea pressure
2
shall be applied for all relevant design conditions [kN/m ].
where:
3
= density of sea water 1.025 [t/m ]
2
= 9.81 [m/s ]
= draught of the unit for the load condition considered [m]
z
= distance from bottom (baseline) to load point [m].
Dynamic sea pressure shall be calculated from a wave load analysis, see [2.3.3], or use prescriptive rule
loads e.g. DNV rules for ships, as found applicable.
2.3.3 Waves
Hydrodynamic wave load analyses shall be used to determine the dynamic responses, e.g. excitation
forces, pressures and accelerations. When theoretical predictions are subjected to significant uncertainties,
theoretical calculations shall be supported by model test measurements, see DNV-RP-C205 Sec.10.
Wave theory or kinematics shall be selected according to recognized methods with due consideration of
actual water depth and description of wave kinematics at the surface and the water column below. Linearized
wave theories, e.g. airy, described in DNV-RP-C205 is accepted.
For large volume structures where the wave kinematics is disturbed by the presence of the structure,
radiation and diffraction effects shall be taken into consideration.
For slender structures (typically chords and bracings, tendons, risers) drag effects shall be taken into
consideration.
Non-linear effects shall be applied when relevant, e.g. in air gap analysis, for slamming and impact loads,
and evaluation of second order wave effects.
2.3.4 Wind
The basic wind pressure is expressed by the following formula:
where:
p
ρa
UT,z
Cp
= wind pressure or suction [Pa]
= mass density of air, to be taken as 1.226 kg/m
3
= wind velocity averaged over a time interval T at a height z meter above the mean water level [m/s]
= pressure coefficient, 1.0 in for plane surfaces.
Shielding effects may be taken into account if documented, see DNV-RP-C205 [5.3.3].
The mean wind velocity UT,z for a one (1) minute period at the actual position above the sea level
shall be used unless otherwise specified. See Table 2 for required wind velocities.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
U1min,z
Page 29
DNV AS
Chapter 2 Section 2
2.3.2 Sea pressure
Chapter 2 Section 2
Guidance note:
Correlation between different average times for the wind speed and elevation above sea level are as follows:
Elevation above
Average time [sec]
sea level (z)
3 sec
5 sec
15 sec
1 min.
10 min.
60 min.
1m
0.843
0.821
0.774
0.716
0.618
0.542
5m
1.042
1.020
0.973
0.914
0.817
0.741
10 m
1.127
1.106
1.059
1.0
0.903
0.827
20 m
1.213
1.191
1.144
1.086
0.988
0.912
30 m
1.264
1.242
1.195
1.136
1.039
0.962
40 m
1.299
1.277
1.230
1.171
1.074
0.998
50 m
1.327
1.305
1.258
1.199
1.101
1.025
60 m
1.348
1.327
1.281
1.222
1.124
1.048
100 m
1.412
1.390
1.344
1.285
1.187
1.111
Wind spectra as defined in DNV-RP-C205 may alternatively be used.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Slender members, e.g. helideck substructure and members in a flare tower structure, shall be checked for
Vortex shedding according to the principles in DNV-RP-C205 [9.5].
2.3.5 Current
The current shall be taken into account for:
— mooring and riser systems related to steady excursions and slow drift motions
— units standing on the seabed, e.g. self-elevating units
— motion behaviour and structural response for units with deep draught, e.g. deep draught units.
2.3.6 Ice impact
When determining the magnitude and direction of the loads, the following factors shall be considered:
—
—
—
—
—
shape, size and mechanical properties of the ice
velocity and direction of the ice
contact area between ice and structure
ice failure mode as a function of the structure geometry
inertia effects for both ice and structure.
Where impact with sea ice or icebergs may occur, the contact loads shall be determined according to relevant
recognized theoretical models.
Guidance note:
Theoretical models for ice loads may need to be calibrated by use of model tests.
DNV rules for ships operating in cold climate, see DNV-RU-SHIP Pt.6 Ch.6, may be used for guidance where applicable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.3.7 Tidal
The tidal effect shall be taken into account for:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 30
DNV AS
b)
Tension moored units, e.g. TLPs, where change in the tide may change the unit's buoyancy and increase
the mooring tension.
Units that are standing on the seabed, e.g. self-elevating units.
2.3.8 Marine growth and snow/ice accumulation
The additional weight due to sea spray, snow, ice and marine growth shall be taken into account where
relevant.
In addition to the direct increase of additional weight of the unit, marine growth, snow and ice accumulation
may also cause an increase in drag and added mass due to the effective increase in members dimensions,
and may also alter the roughness characteristics of the surface. These effects shall be accounted for where
relevant.
Guidance note:
Guidance on values for marine growth may be found in DNV-OS-E301.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.3.9 Earthquake
Relevant earthquake effects shall be considered for units standing on the seabed based on site specific
assessments.
Earthquake excitation design loads and load histories may be described either in terms of response spectra
or in terms of time histories. For the response spectrum method all modes of vibration which contribute
significantly to the response shall be included.
Guidance note:
Guidance for risk and actions for earthquakes can be found in e.g. ISO 19905-1, ISO 19901-2 and NORSOK N-003.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.4 Deformation loads
2.4.1 Temperature
Temperature differences between structural elements shall be taken into account, as found relevant.
The most extreme temperature differences shall be used, unless otherwise documented.
Guidance note:
For temperature difference lower than 100°C between the structural elements, the thermal effect is considered to be negligible.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.4.2 Settlements and subsidence of seabed
Settlement of the foundations shall be taken into account for units permanently located on the seabed.
The possibility and consequences of subsidence of the seabed due to changes in the subsoil and in the
production reservoir during operation, shall be taken into account.
2.4.3 Fabrication and mating sequence
The effect from deformations (built-in stresses) related to fabrication, i.e. assembly of blocks, crop-in new
structure, shall be taken into account where relevant.
2.5 Tank pressures
2.5.1 Symbols
Symbols used in this subsection are listed below. For symbols not defined in this subsection, see Ch.1 Sec.1
[6.3].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 31
DNV AS
Chapter 2 Section 2
a)
2
= dynamic liquid tank pressure [kN/m ]
2
= hydrostatic liquid tank pressure [kN/m ]
= additional dynamic tank pressure due to flow through pipes or overfilling
2
To be minimum 25 kN/m , unless otherwise specified and documented
3
= density of liquid [t/m ]
2
= 9.81 [m/s ]
2
= maximum vertical acceleration in the centre of the actual tank [m/s ]
= Z coordinate of the highest filling point of the tank, excluding small hatchways [m].
For tanks adjacent to the sea that are situated below the highest operational draught of the unit,
the maximum filling point of the tank shall not be taken lower than the highest operational draught
= height of air pipe or overflow pipe above the top of the tank [m]
z
= distance from bottom (baseline) to load point [m].
2.5.2 General
The tank structure shall be designed to resist the maximum hydrostatic pressure for all relevant loading
conditions given in Sec.1 [2] for the actual tank filling arrangement.
3
The density of seawater of 1.025 t/m shall be used for all tanks, except for tanks that are intended for
fluids with higher density. When tanks are intended for higher densities, e.g. drilling mud, the tanks shall be
designed for the highest density fluid they are intended to be exposed to. The maximum allowable density
of all tanks shall be specified on the unit's tank plan drawing, and stated in the unit's operating manual, as
applicable.
2.5.3 Hydrostatic liquid tank pressure
The maximum hydrostatic pressure
Pls shall be taken as:
The pressure height hair in the formula above may be replaced with a "cut-off pressure head " if an
automatic stop of the pumping system, that prevent overfilling of the tanks, is installed and documented. The
requirements to automatic stop pumping systems are given in DNV-OS-D101 Ch.2 Sec.3.
2.5.4 Dynamic liquid tank pressure
The dynamic liquid pressure
Pld due to the unit's motions shall be taken as:
2.5.5 Flow through pipes or overfilling
An additional dynamic pressure of minimum, Pdrop= 25 kN/m , due to flow through pipes caused by liquid
exchange or overflow of ballast water or through air pipe shall be applied.
2
2.5.6 Tank testing
All tanks shall be designed to satisfy the tank testing conditions at the building berth. The maximum tank
2
testing heights, including a dynamic over pressure of 25 kN/m , shall be applied, see also DNV-OS-C401
Ch.2 Sec.8 [3].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 32
DNV AS
Chapter 2 Section 2
2
= design liquid tank pressure [kN/m ]
If an automatic pump cut-off system is installed, the tank testing shall take into account the actual design
pressure height hair.
2.5.7 Design liquid pressure
The design tank pressure shall take into account the actual design method applicable for the project as given
in Table 3.
Table 3 Design liquid pressure
2
Design method
Design tank pressure PlD [kN/m ]
LRDF
and
are defined in Sec.1 [1.3.3]
WSD
2.6 Combination of loads
2.6.1 All load effects shall be combined, e.g. static loads, dynamic loads and deformation loads, as
applicable. Unless stochastic analyses are performed, or correlation between the loads are documented, all
loads shall be assumed to appear with their maximum simultaneously.
2.6.2 The environmental loads for the ULS survival condition may be combined with respect to the annual
probabilities of exceedance given in Table 4, unless otherwise documented.
Table 4 Combinations of different environmental loads for ULS survival condition
Limit state
ULS
Wind
Waves
-2
10
-1
10
-1
10
10
10
10
Current
-2
10
-1
10
-1
10
Ice
Sea level
-1
10
-2
10
-1
-2
-2
-2
10
Mean water level
Guidance note:
For most constructions, wind and waves are the governing loads for the unit's global and local strength and combination of these
loads are given in the DNV unit specific standard. However, special unit types, environmental operation areas and construction
elements may require combination of many load types as given in Table 4. For loads application of permanent mooring system, see
DNV-OS-E301.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.6.3 High and low frequency load response bands shall be considered, as applicable. The following three
frequency bands defined in Table 5 shall be checked.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 33
DNV AS
Chapter 2 Section 2
When the tank is designed for a density
greater than sea water, and when the testing is carried out with
sea or fresh water, the testing pressure height shall take into account the actual tank density.
Frequency type
Definition
High frequency (HF)
Rigid body natural periods below dominating wave periods (e.g. ringing and springing
responses).
Wave frequency (WF)
Area with wave periods in the range of 4 to 25 s typically.
Low frequency (LF)
Slowly varying responses with natural periods above the dominating wave periods, e.g. slow
varying surge and sway motions for column-stabilised and ship-shaped units, and slow varying
roll and pitch motions for deep draught floaters.
3 Accidental loads
Accidental loads are possible loads caused by abnormal operation or technical failure, with a frequency of
-4
occurrence in the order of 10 per year, see also Sec.1 [2.2.4]. The requirements in DNV-OS-A101 related to
accidental loads shall be complied with.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 34
DNV AS
Chapter 2 Section 2
Table 5 Frequency types
1 General
This section defines structural categorisation for the selection of steel materials and the fabrication inspection
requirements.
Guidance note:
Structural categorisation for the object unit types are given in DNV unit specific standards.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2 Temperatures for selection of material
2.1 Definitions
2.1.1 The design temperature for a unit is the reference temperature for assessing area(s) where the unit
will operate.
The design temperature shall be lower or equal to the lowest mean daily average temperature in air (LMDAT)
for the relevant area(s). For seasonal restricted operations, the LMDAT for the season may be applied.
2.1.2 The service temperature is the reference temperature for actual structural parts of the unit used as the
criteria for selecting material steel grades.
2.1.3 External structure: Structural parts of the unit exposed to weather or submerged in sea e, e.g. shell
plates, deck and bottom plates.
2.2 General
2.2.1 External structures, including structural members connected to the shell plate within 600 mm from
the shell plate, above the lowest waterline shall be designed for service temperature equal to the design
temperature for the area(s) where the unit shall operate.
2.2.2 External structures below the lowest waterline, including structural members connected to the shell
plate within 600 mm from the shell plate, shall be designed for service temperatures of 0°C.
2.2.3 Internal structures in way of permanently heated rooms, cargo and ballast tanks, shall use the design
temperature for the actual area the unit shall operate, but need not to be taken lower than 0°C.
2.2.4 In areas where the temperature is locally reduced, i.e. in cryogenic storage or other cooling conditions,
the actual local temperature shall be used.
2.2.5 For units standing on the seabead, e.g. jack-ups, the actual draft in this condition shall use the lowest
astronomical tide (LAT). Below the LAT a service temperature of 0°C shall be applied.
3 Structural category
3.1 General
The purpose of structural categorisation is to ensure adequate:
a)
material qualities
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 35
DNV AS
Chapter 2 Section 3
SECTION 3 MATERIAL SELECTION AND INSPECTION PRINCIPLES
requirements to inspection.
3.2 Selection of structural category
3.2.1 Structural categorisation is based on the following criteria:
— consequence of failure of the strength element considered
— expected stress level in the strength element considered.
3.2.2 The structural categories are based on the principles given in Table 1.
Table 1 Structural categories
Structural category
1)
Principles for determination of structural category
Special
Structural parts where failure will have substantial consequences and which are subject to a
2)
stress condition that may increase the probability of a brittle fracture .
Primary
Structural parts where failure will have substantial consequences.
Secondary
Structural parts where failure will be without significant consequence.
1)
Determination of structural categories are given in the various DNV unit specific standards.
2)
Complex joints, triaxial or biaxial stress patterns will increase the possibility for brittle fracture.
3.3 Inspection of welds
3.3.1 Inspection category of welds is based on the following criteria:
— consideration of fatigue damage (fatigue critical)
— assessment of fabrication quality.
Whether a detail is considered fatigue critical shall be determined on the following factors:
a)
b)
c)
d)
consequence of failure
calculated fatigue damage (D) including design fatigue factor (DFF)
possibility of detecting a crack at an early stage
accessibility for repair of possible crack(s).
3.3.2 Requirements for type and extent of inspection are given in DNV-OS-C401 depending on the assigned
inspection category for the welds.
3.3.3 The inspection category is related to the structural category given in Table 2.
Table 2 Inspection categories
Inspection category
Structural category
I
Special
II
Primary
III
Secondary
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 36
DNV AS
Chapter 2 Section 3
b)
3.3.5 Fatigue critical details, see [3.3.1], within structural category primary and secondary shall be
fabricated according to tolerances for structural category special, and inspected according to requirements in
category I.
3.3.6 Fatigue critical details/welds, not accessible for inspection and repair during operation, shall be
inspected according to requirements in category I.
3.3.7 The extent of NDT for welds in block joints and erection joints transverse to main stress direction shall
not be less than for inspection category II.
Guidance note:
The extent of inspection and the locations for NDT should take into account relevant fabrication parameters such as:
—
location of block (section) joints
—
welding assembly and complexity of the welding job
—
manual versus automatic welding
—
start and stop of weld.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.3.8 Weld connections with limited or difficult fabrication access shall be evaluated with respect to
applicable non-destructive testing (NDT) methods. Such details shall be inspected according to category I
and identified on the inspection category plan.
4 Structural steel
4.1 General
4.1.1 The requirements in this subsection specify structural steel grades in compliance with DNV-OS-B101.
Guidance note:
For more details and requirements, e.g. related to chemical composition, elongation, extra high tensile steels, see DNV-OS-B101.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.1.2 Steel grades for structural components are named based on strength and material toughness
properties. The toughness properties are based on Charpy V-notch tests carried out at a given temperature
and meet a minimum specified toughness value.
Selection of material steel grade for a structural member shall be based on the structural member category,
the relevant service temperature, and the actual plate thickness according to Table 3.
4.1.3 The material toughness may alternatively be documented by fracture mechanics testing, but
acceptance of such testing shall be agreed with the verifier, see [4.4] and DNV-OS-C401.
4.1.4 In structural cross-joints, e.g. tee and cruciform connections employ full penetration welds, where
high tensile stresses are acting perpendicular to the plane of the plate, the plate material shall be tested to
demonstrate the ability to resist lamellar tearing, Z-quality, see DNV-OS-B101.
For secondary structure or redundant connections where the tensile stress in the connection is lower than
50% of the base material yield stress, the Z-quality requirement may be waived.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 37
DNV AS
Chapter 2 Section 3
3.3.4 Between two components, the weld connection shall be assigned the inspection category according to
the highest of the joined components.
Guidance note:
Requirements to sulphur content and NDT of plate before welding are given in DNV-OS-B101 Ch.2 Sec.2 [5]. Requirements for
NDT of the final weld are given inDNV-OS-C401.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.1.5 Requirements for steel castings are given in DNV-OS-B101.
4.2 Material designations and steel grade
The alphanumeric designation of the steel grade is:
—
—
—
NV xyd for steels of normal weldability
NV xWyd for steels of improved weldability
NV xOyd for specific offshore grade extra high strength steels (with designation O). These are steels
where the requirement for yield and tensile strength are independent of the product thickness.
Where:
NV
x
W
O
= designation of a steel grade delivered with a DNV certificate
y
= a figure designating the strength group according to the specified minimum yield stress. The figure
y is omitted for NS steels
d
= capital letters to give the delivery condition, e.g. N, NR, TM, QT. Optional for steels of normal and
high strength, mandatory for steels of extra high strength.
Z
= steel grade of improved through-thickness properties. This symbol is omitted for steels of improved
weldability although improved through-thickness properties are required.
= a capital letter corresponding to a specified impact toughness test temperature
= letter included to designate a steel grade of improved weldability
= letter included to designate a specific offshore grade steel where the requirements for yield and
tensile strength are independent of the product thickness
Table 3 Thickness limitations [mm] of structural steels for different structural categories and
service temperatures [ºC]
Structural
Category
Strength group
Secondary
NS
HS
Grade
≥ 10
0
-10
-20
-25
-30
A
35
30
25
20
15
10
B/BW
70
60
50
40
30
20
D/DW
150
150
100
80
70
60
E/EW
150
150
150
150
120
100
A/AW
60
50
40
30
20
15
D/DW
120
100
80
60
50
40
E/EW
150
150
150
150
120
100
F
150
150
150
150
1)
1)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 38
DNV AS
Chapter 2 Section 3
Material with non-proven through thickness properties may be accepted based on a case-by-case approval,
with special attention to the sulphur content. An ultrasonic test of the plate, before and after welding, shall
be carried out at the tension exposed areas.
Strength group
EHS
Primary
NS
HS
EHS
Special
NS
HS
EHS
Grade
≥ 10
0
-10
-20
-25
-30
A/AO
70
60
50
40
30
20
D/DW/DO
150
150
100
80
70
60
E/EW/EO
150
150
150
150
120
100
F/FO
150
150
150
150
1)
1)
A
30
20
10
N.A.
N.A.
N.A.
B/BW
40
30
25
20
15
10
D/DW
70
60
50
40
35
30
E/EW
150
150
100
80
70
60
A/AW
30
25
20
15
12.5
10
D/DW
60
50
40
30
25
20
E/EW
120
100
80
60
50
40
1)
F
150
150
150
150
1)
A/AO
35
30
25
20
17.5
15
D/DW/DO
70
60
50
40
35
30
E/EW/EO
150
150
100
80
70
60
1)
F/FO
150
150
150
150
1)
D/DW
35
30
25
20
17.5
15
E/EW
70
60
50
40
35
30
A/AW
15
10
N.A.
N.A.
N.A.
N.A.
D/DW
30
25
20
15
12.5
10
E/EW
60
50
40
30
25
20
F
120
100
80
60
50
40
A/AO
20
15
10
N.A.
N.A.
N.A.
D/DW/DO
35
30
25
20
17.5
15
E/EW/EO
70
60
50
40
35
30
F/FO
150
150
100
80
70
60
1) For service temperatures below -20°C, the thickness limits shall be specially evaluated.
For intermediate service temperatures, linear interpolation may be used.
4.3 Selection of structural steel
Definition of steel grades are given in Table 4. See DNV-OS-B101 Ch.2 Sec.2 for details.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 39
DNV AS
Chapter 2 Section 3
Structural
Category
Impact testing
Strength
group
NS
HS
EHS
Symbol x
Normal
weldability
Improved
weldability
Offshore
grades
-
Tensile properties
Test
temperature
[°C]
Symbol
y
Minimum yield
1), 3)
stress (ReH)
[MPa]
Tensile strength
2)
(Rm)
[MPa]
-
Omitted
235
400-530
A
5)
-
B
4)
BW
0
D
DW
–20
E
EW
–40
A
AW
0
27
265
-
D
DW
-
–20
32
315
440-570
E
EW
–40
36
355
490-630
F
-
–60
40
390
510-660
A
-
AO
0
420
420
520-680
D
DW
DO
–20
460
460
540-720
E
EW
EO
–40
500
500
590-770
F
-
FO
–60
550
550
640-820
620
620
700-800
690
690
770-940
1)
For extra high strength steels of normal and improved weldability the indicated minimum yield stress is reduced
for increasing product thickness. The indicated minimum yield stress is applicable for thicknesses up to 50 mm. For
thickness above 50 mm, see DNV-OS-B101 Ch.2 Sec.2 Table 18.
2)
For extra high strength steels of normal and improved weldability the indicated minimum tensile stress is reduced for
increasing product thickness. The indicated minimum tensile stress is applicable for thicknesses up to 100 mm. For
thickness above 100 mm, see DNV-OS-B101 Ch.2 Sec.2 Table 18.
3)
For offshore grade steels, the indicated minimum yield stress is independent of the product thickness.
4)
Testing of the impact toughness is not required for grade NV B steel with t ≤ 25 mm. Manufacturer shall ensure (e.g.
through regular in-house tests) that material of grade NV B with thickness less than 25 mm meets minimum average
impact toughness energy of 27 J at 0°C.
5)
Testing of the impact toughness is not required for grade NV A over 50 mm thickness when the material is produced
using fine grain practice and supplied in N condition. Manufacturer shall ensure (e.g. through regular in-house tests)
that material of grade NV A with thickness more than 50 mm meets minimum average impact toughness energy of 27
J at +20°C.
4.3.1 Welded steel plates and sections with thickness exceeding the upper limits for the actual steel grade
given in Table 3 shall be evaluated in each individual case, with respect to the fitness for purpose of the
weldments. The evaluation should be based on fracture mechanics testing and analysis, see [4.4].
2
4.3.2 The use of steels with a specified minimum yield stress greater than 550 N/mm (NV 550) shall be
subject to special consideration for applications where anaerobic environmental conditions, such as stagnant
water, organically active mud (bacteria) and hydrogen sulphide, may predominate. Relevant for e.g. jack-up
legs and spud cans, see also Sec.1 [4.3.3].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 40
DNV AS
Chapter 2 Section 3
Table 4 Definitions of steel grades
Predominantly anaerobic conditions can, for this purpose, be characterized by a concentration of sulphate reducing bacteria (SRB)
3
in the order of magnitude >10 SRB/ml.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.3 All materials shall be delivered with a material certificate.
4.4 Fracture mechanics testing
Fracture mechanics testing shall be applied in the qualification of welding procedures for joints when all of
the following are applicable:
—
—
—
—
the unit shall operate continuously at the same location for more than five (5) years
the design temperature is lower than +10ºC
the welded joint is in a structural category area defined as special
at least one of the adjoining members is fabricated from steel with ReH ≥ 420 MPa.
For details on fracture mechanic testing methods, see DNV-OS-C401 Ch.2 Sec.5 [3.3.7].
4.5 Post weld heat treatment
Post weld heat treatment (PWHT) shall be applied for joints in C-Mn steels when all of the following are
applicable:
— the unit shall operate continuously at the same location for more than five (5) years
— the welded joint is in a structural category area defined as special
— the material thickness at the weld exceeds 50 mm.
For details, see DNV-OS-C401 Ch.2 Sec.6 [9.6].
PWHT may be omitted if satisfactory performance in the as-welded condition are documented by a fitnessfor-purpose assessment applying fracture mechanics testing, fracture mechanics and fatigue crack growth
analyses in agreement with the verifier.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 41
DNV AS
Chapter 2 Section 3
Guidance note:
1 General
Symbols used in this section are listed below. For symbols not defined in this section, see Ch.1 Sec.1 [6.3].
a
= length of plate panel [mm]
b
= breadth of plate panel [mm]
= correction factor for aspect ratio of plate
Ca
= design bending stress coefficient for plates
Cs
= design bending stress coefficient for stiffeners
fbdg
= bending moment factor
fshr
= shear force factor
fu
= factor for unsymmetrical profiles
lbdg
= effective stiffener/girder bending span [m]
lshr
= effective stiffener/girder shear span [m]
Pd
= design lateral pressure given in Sec.2, according to the applicable design method (LRFD, WSD) in
2
Sec.1 [1.3] [kN/m ]
ReH
= minimum material yield stress given in Sec.3 Table 3 [N/mm ]
ReHd
= design allowable yield strength [N/mm ]
2
2
For the WSD method:
For the LRFD method:
where:
η0 is the basic usage factor given in Sec.1 Table 4
γm is the partial safety factor given in Sec.1 Table 2
s
= stiffener spacing [m], measured along the plating
S
= girder spacing [m]
σjd
= equivalent design in-plane membrane stress [N/mm ]
τeH
= specified shear yield stress [N/mm ]
τeHd
= design shear stress [N/mm ].
2
2
2
1.1 Basis
1.1.1 The basis for a strength assessment is to cover the ULS and ALS limit states with respect to yield and
buckling capacity of structural elements.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 42
DNV AS
Chapter 2 Section 4
SECTION 4 STRENGTH ASSESSMENT
Structural elements where deflection may be important for operation should also be controlled, but is not part of the Society's
scope, see Ch.3 Sec.1 [2.1].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.1.2 A basic overall requirement is that all failure modes are sufficiently ductile such that the structural
behaviour will be in accordance with the anticipated model used during the determination of the responses.
1.2 Use of other codes and standards
Capacity checks of members may alternatively be carried out according to other recognized standards,
e.g. EN 1993-1, NORSOK N-004, ISO 19902, API RP 2A-LRFD, API RP 2A-WSD and AISC Manual of Steel
Construction, using the allowable criteria for yield and buckling given in Table 2 and Table 7, as applicable.
1.3 Structural idealisation
1.3.1 Effective bending span of girders and stiffeners
The effective bending span of stiffeners lbdg shall be taken as given in Figure 1.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 43
DNV AS
Chapter 2 Section 4
Guidance note:
Chapter 2 Section 4
Figure 1 Effective bending span of stiffeners
1.3.2 Effective shear span of stiffeners and girders
The effective shear span of stiffeners lshr may be taken as given in Figure 2.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 44
DNV AS
Chapter 2 Section 4
Figure 2 Effective shear span of stiffeners and girders
1.3.3 Effective breadth
The effective breadth [mm] of the attached plate
is given as follows:
beff, for stiffeners and girders, exposed to lateral pressure
for
for
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 45
DNV AS
The bending moment factor fbdg and shear force factor fshr for stiffeners and girders exposed to lateral
pressure are given in Table 1.
Table 1 Definition of bending moment and shear force factors, fbdg and fshr
Bending moment and shear force
distribution factors (based on load
at mid span, where load varies)
Load and boundary condition
Position
1
2
3
fbdg1
fshr1
fbdg2
-
fbdg3
fshr3
A
12.0
0.50
24.0
-
12.0
0.50
B
0.38
14.2
-
8.0
0.63
C
0.50
8.0
-
0.50
D
15.0
0.30
23.3
-
10.0
0.70
E
0.20
16.8
-
7.5
0.80
Load model
1
Support
2
Field
3
Support
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 46
DNV AS
Chapter 2 Section 4
1.3.4 Bending moment and shear force factors
Position
1
Support
Load model
2
Field
3
Support
F
1
2
3
fbdg1
fshr1
fbdg2
-
fbdg3
fshr3
-
-
2.0
1.0
Note 1
The bending moment distribution factor fbdg for the support positions is applicable for a distance of 0.2S
from the end of the effective bending span of the primary supporting member.
Note 2
The shear force distribution factor fshr for the support positions is applicable for a distance of 0.2S from the
end of the effective shear span of the primary supporting member.
Note 3
Application of fbdg and fshr:
The section modulus requirement within 0.2S from the end of the effective span shall be determined using
the applicable fbdg1 and fbdg3, however the fbdg shall not be taken as greater than 12.
The section modulus of mid-span area shall be determined using fbdg = 24, or fbdg2 from the table.
The shear area requirement for end connections within 0.2S from the end of the effective span shall be
determined using fshr = 0.5 or the applicable fshr1 or fshr3, whichever is greater.
For models A through F, the value of fshr may be gradually reduced outside of 0.2S towards 0.5fshr at midspan, where fshr is the greater value of fshr1 and fshr3.
2 Scantling control
2.1 General
The scantling requirements stated in this subsection are based on simple plate and beam theory. The
scantling requirements may be replaced with equivalent strength control by use of yield and buckling criteria
given in [3] and [4], but the requirement in [2.2.1] shall be met.
2.2 Plates
2.2.1 Minimum plate thickness
The thickness of plates [mm] shall not be less than:
where:
t0 = 7 mm for primary structural elements
t0 = 5 mm for secondary structural elements.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 47
DNV AS
Chapter 2 Section 4
Bending moment and shear force
distribution factors (based on load
at mid span, where load varies)
Load and boundary condition
where:
Pd
2
= design lateral pressure [kN/m ] given in Sec.2, according to the applicable design method (LRFD,
WSD) in Sec.1 [1.3].
= correction factor for aspect ratio of plate field,
maximum 1.0 when
minimum 0.72 when
a
= length of panel [mm]
b
= breadth of plate panel [mm]
Ca
= design bending stress coefficient for plates, when accounting for reduced capacity due to in-plane
membrane stress:
2
= equivalent design in-plane membrane stress [N/mm ].
fpp
= fixation factor for the plate:
= 1.0 when edges are clamped (normally for plates used in stiffened panel structures)
= 0.5 when the edges are simply supported.
2.3 Stiffeners
2.3.1 General
Stiffeners covered here are part of a stiffened plate field.
2.3.2 Section modulus
The section modulus
Zs [cm3] for stiffeners subjected to lateral pressure shall not be less than:
where:
fu
= factor for unsymmetrical profiles, to be taken as:
— 1.00 for flat bars and symmetrical profiles (T-profiles)
— 1.03 for bulb profiles
— 1.15 for unsymmetrical profiles (L-profiles)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 48
DNV AS
Chapter 2 Section 4
2.2.2 Lateral pressure on plates
The thickness of plates [mm] which are subjected to lateral pressure shall not be less than:
= stiffener spacing [mm]
lbdg
= effective stiffener span with respect to bending moment [mm]
fbdg
= bending moment factor given in Table 1
Cs
= design bending stress coefficient for stiffeners, accounting for in-plane membrane stress:
Guidance note:
The design bending stress
for the plate side of the stiffener is the in-plane membrane stress considering all stress
components (longitudinal, transverse and shear stresses).
The design bending stress
for stiffener flange side is the longitudinal membrane stress only.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.3.3 Tilted stiffeners
For stiffeners connected with an oblique angle to the plate, the required section modulus in [2.3.2] shall be
multiplied with the following factor:
where
is the angle between the stiffener web plane and the plane perpendicular to the plating.
2.3.4 Sniped stiffeners
Stiffeners with sniped ends are accepted when the dynamic stresses are small, i.e. nominal stress range
lower than 30 MPa at the plate. The sniping angle shall be maximum 30º.
The plate thickness
t [mm] connected to the stiffener shall be minimum:
l is the stiffener span [m] of the sniped stiffener.
The section modulus requirements Zs in [2.3.2] shall be checked using a bending moment factor fbdg2= 8.
where
Sniped stiffeners shall not be used:
— on structures in the vicinity of engines, generators or propeller impulse zones
— at boundaries of sea chests
— at deckhouses and the first tier of accommodation structures which may be exposed to green sea.
2.3.5 Shear area
The shear area
As for stiffeners subjected to lateral pressure shall not be less than:
where:
fshr
= shear force factor given in Table 1
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 49
DNV AS
Chapter 2 Section 4
s
lshr
Chapter 2 Section 4
= effective stiffener span wrt shear force [mm]
2
= design shear stress [N/mm ]
σxd = In-plane membrane axial stress in the web plate.
Guidance note:
The design shear stress τeHd representing the remaining capacity when reduced for in-plane membrane axial stress
.
For stiffeners subject to high shear loads the combined effect of stiffener bending and shear loads should be considered by
corresponding reduction in the web thickness when checking
Zs in [2.3.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.4 Girders
2.4.1 General
The requirements in this subsection are based on the following assumptions::
— the minimum thickness requirement for plates in [2.2.1] is satisfied for the girder web plate and the
girder flange
— girders are exposed to linearly distributed lateral pressure
— girders are approximately evenly spaced and have similar support conditions at both ends, else direct
strength control according to [2.5] shall be carried out
— effective breadth given in [1.3.3] is applied in the calculation of Z
— all openings in the web are deducted from the shear capacity control.
2.4.2 Section modulus
Girders subjected to lateral pressure shall comply with the following minimum requirement for the section
3
modulus [cm ]:
where:
S
= girder spacing [mm]
CS
= design bending stress coefficient for girders, accounting for in-plane membrane stress:
Guidance note:
The design bending stress
for the plate side of the girder is the in-plane membrane stress considered all stress
components (longitudinal, transverse and shear stresses).
The design bending stress
for girder flange side is the longitudinal membrane stress only.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 50
DNV AS
where:
2
= design shear stress [N/mm ]
σxd = In-plane membrane axial stress in the web plate.
2.5 Complex grillage systems
For girders which are part of a complex grillage structural system, a direct strength FE-analysis shall be
carried out considering:
—
—
—
—
—
—
global and local loads as applicable
boundary conditions
shear area variation, e.g. cut-outs
moment of inertia variations
effective flanges
lateral buckling of girder flanges.
The acceptance criteria for yield and buckling are given in Table 2 and in Table 7.
3 Yield control
3.1 General
Structural members shall be checked for yielding. The acceptable yield criteria for the individual design stress
components, and the von Mises equivalent design stress, are given in Table 2 and Table 3, based on the
design resistance principles in Sec.1 [1.3].
3.2 Beam FE-analyses
2
When FE-beam analyses are applied, the calculated stresses [N/mm ] shall comply with the following
criteria:
where:
τd
2
= design average shear stress in the member [N/mm ] at the considered position
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 51
DNV AS
Chapter 2 Section 4
2.4.3 Shear area
Girders subjected to shear forces shall comply with the following minimum requirement for the web shear
2
area [cm ] :
2
= design bending stress [N/mm ] at the considered position
2
= design axial stress [N/mm ] at the considered position
2
= design von Mises equivalent stress [N/mm ] at the considered position
= allowable normal stress coefficient given in Table 2
= allowable shear stress coefficient given in Table 2
= allowable von Mises stress coefficient given in Table 2.
Table 2 Allowable yield criteria
Acceptance criteria
LRFD
Cs
Ct
WSD
Cvm
ULS
0.87
0.95
ALS
1.0
1.1
Cs
Ct
Cvm
a)
0.70
0.78
b)
0.80
0.87
1.0
1.1
Guidance note:
The member check for normal stress and shear stress may be omitted if the von Mises stress control is performed using the
minimum acceptance factor for normal stress or shear stress, i.e.
Cvm = Min(Cs , Ct).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.3 Plate/shell FE-analyses
3.3.1 General
For FE-analyses with plate/shell elements, or combination of plate/shell and rod/beams, the element
membrane nominal and shear stress components and the von Mises stresses shall be checked. The stresses
shall be calculated at the element centroid of the mid-plane (layer).
where:
σxd , σyd = design element normal membrane stresses [N/mm2]
2
= design element shear stress [N/mm ].
τxyd
3.3.2 Coarse mesh criteria
Coarse mesh is approximately the distance between stiffener spacing.
The structural elements shall comply with the following criteria:
where:
λy
= yield utilisation factor
λy =
λy =
for membrane normal stress of shell or plate elements
for membrane shear stress of shell or plate elements
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 52
DNV AS
Chapter 2 Section 4
σbd
σxd
σvmd
Cs
Ct
Cvm
for von Mises stress of shell or plate elements
λy =
σvmd
σaxiald
λyperm
for rod or beam elements
2
= design von Mises stress [N/mm ]
2
= design axial stress in rod or beam element [N/mm ]
= permissible yield utilisation factors for coarse mesh of plate/shell elements, and for integrated
rod/beam elements.
The permissible yield utilisation factors for coarse mesh of plate/shell elements, and for integrated rod/beam
elements are given in Table 3.
Table 3 Permissible yield utilisation factor
λyperm
LRFD
Acceptance criteria
WSD
Stress component control
λym , λyb
λyτ
λyvm
ULS
0.87
0.95
ALS
1.0
1.1
Design
combination
Stress component control
λym , λyb
λyτ
λyvm
a)
0.70
0.78
b)
0.80
0.87
1.0
1.1
3.3.3 Local yield, fine mesh criteria
Local peak stresses from linear elastic analyses in areas with pronounced geometrical changes, may exceed
the yield stress provided the adjacent structural parts are able to redistribute the stresses.
Guidance note:
a)
Areas exceeding yield determined by a linear finite element method analysis may give an indication of the actual area of
plastification. Otherwise, a non-linear finite element method analysis may be carried out in order to trace the full extent of
the plastic zone.
b)
Provided a fine mesh analysis is carried out with a maximum mesh size of 50 mm x 50 mm, and fatigue criteria is satisfied, a
0.8
local peak stress criteria of 1.7(235/ReH)
may be accepted.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4 Buckling control
4.1 General
Structural members shall be checked for buckling based on the characteristic buckling resistance for the most
unfavourable buckling mode.
Conformity between the initial imperfections in the buckling resistance formulas and the tolerances for
fabrication shall be ensured.
4.2 Slenderness requirements
4.2.1 General
Slenderness requirements to plate, stiffeners, girders and pillars are given below. Alternatively, the
slenderness requirements for members given in App.A, using cross section type III, may be applied.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 53
DNV AS
Chapter 2 Section 4
λy =
4.2.2 Plates
The thickness of plate panel tp [mm], shall comply with:
where:
b
C
= breadth between the stiffeners, measured at the middle length of the plate field [mm]
= slenderness coefficient = 125.
4.2.3 Thickness of stiffeners
The thickness of stiffeners [mm] shall comply with the following:
a)
Stiffener web plate:
b)
Stiffener flange:
where:
Cw, Cf
= slenderness coefficients given in Table 4 for the stiffener types shown in Figure 3.
Figure 3 Stiffener scantling parameters
Table 4 Slenderness coefficients for stiffeners
Type of profile
Cw
Cf
Angle, L2 and L3 bars
75
12
1)
T-bars
75
12
1)
Bulb bars
45
-
Flat bars
22
2)
-
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 54
DNV AS
Chapter 2 Section 4
In case the slenderness requirements below fail, the member may alternatively be checked for local buckling
using a buckling capacity analysis according to DNV-CG-0128 or DNV-RP-C201.
Cw
Cf
1)
Cf= 22, non-continuous straight flanges with end bracket.
2)
Cw= 26, for laterally loaded flat bar stiffeners which are not part of the primary
strength capacity.
4.2.4 Flange breadth of angle and T-bars
The total flange breadth for angle and T-bars shall comply with:
Provided that [4.2.3] a) is satisfied for the web plate using a slenderness coefficient CW = 45 (representing a
bulb bar), the above requirement for total flange breath of angle and T-bars may be disregarded.
4.2.5 Girder web plates and flanges
The thickness [mm] of web plates and flanges for girders and primary supporting members shall comply with
the following criteria:
a)
Web plate:
b)
Flange:
where:
sw
Cw
Cf
= girder plate breadth [mm]
= slenderness coefficient = 100
= slenderness coefficient = 12.
4
The moment of inertia [cm ] of web stiffeners Ist, with effective plate breadth, fitted on a girder web plate
shall not be less than the minimum moment of inertia given in Table 5.
Table 5 Stiffness criteria for web stiffeners
4
Stiffener arrangement
Minimum web stiffener moment of inertia [cm ]
Web stiffeners fitted along the girder span
A
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 55
DNV AS
Chapter 2 Section 4
Type of profile
4
Minimum web stiffener moment of inertia [cm ]
Chapter 2 Section 4
Stiffener arrangement
Web stiffeners fitted normal to the girder span
B
Where:
= slenderness coefficient = 0.72
C
ℓ
Aeff
tw
ReH
= length of web stiffener [m]
2
= web stiffener section area including effective attached plate flange [cm ]
= web thickness of the primary supporting member [mm]
2
= specified minimum yield stress of the girder web plate [N/mm ].
The thickness of the web plate and flange of the edge stiffener shall comply with the requirements specified
in [4.2.3] and [4.2.4].
4.2.6 Brackets
4.2.6.1 Tripping brackets
The distance [m] between tripping brackets for girders shall be:
, but minimum 3.0 m.
where:
C
= 0.020 for symmetrical flange
= 0.025 for unsymmetrical flange
bf
= flange breadth [mm].
The tripping brackets shall be stiffened by a flange or edge stiffener if the effective length of the edge ℓb
defined in Table 6 is greater than [mm]:
The height of the edge stiffener web shall not be less than [mm]:
where:
tb
C
ReH
= bracket net web thickness [mm]
= slenderness coefficient = 50
2
= specified minimum yield stress of the tripping bracket [N/mm ].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 56
DNV AS
Chapter 2 Section 4
4.2.6.2 End brackets
The web thickness of end brackets subject to compressive stresses shall not be less than [mm]:
where:
db
C
ReH
= depth of bracket [mm] defined in Table 6
= slenderness coefficient defined in Table 6
2
= specified minimum yield stress for the end bracket material [N/mm ].
Table 6 Buckling coefficient C of brackets
Mode
C
Brackets without edge stiffener
where:
Brackets with edge stiffener
C = 70
The height of the edge stiffener web shall not be less than [mm]:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 57
DNV AS
Chapter 2 Section 4
where:
C
ReH
= slenderness coefficient = 75
2
= specified minimum yield stress of the edge stiffener [N/mm ].
4.2.7 Pillars
The thickness of circular section pillars shall comply with the following criterion [mm]:
where:
r = mid thickness radius of the circular section [mm].
4.2.8 Edge reinforcements in way of openings
The web height of edge stiffeners in way of openings, see Figure 4, shall not be less than [mm]:
or 50 mm, whichever is greater.
where:
C
ReH
= slenderness coefficient = 50
2
= specified minimum yield stress of the edge stiffener [N/mm ].
Figure 4 Typical edge reinforcements
4.3 Allowable buckling criteria
4.3.1 General
A structural member has an acceptable buckling strength when the following criteria is satisfied:
where:
ηact = buckling utilisation factor
ηallb = allowable buckling safety factor defined in Table 7.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 58
DNV AS
where:
Wactd = actual design membrane stress
Wu = buckling capacity of the structural member
= stress multiplier factor at failure.
γc
The allowable buckling safety factors are given in Table 7 for LRDF and WSD design methods, based on the
design resistance principle in Sec.1 [1.3].
Table 7 Allowable buckling safety factor ηallb
Structural member
Acceptance criteria
LRFD
ULS
0.87
ALS
1.0
ULS
0.80 (0.87)
ALS
0.90 (1.0)
ULS
0.87
ALS
1.0
Plate and stiffened panels
Pillar, struts and cross-ties
2)
Frames, beams and girders
— Shell structures
1)
WSD
Design combination
a)
0.70
b)
0.80
1.0
a)
0.65 (0.70)
b)
0.75 (0.80
0.90 (1.0)
a)
0.70
b)
0.80
1.0
a)
ULS
b)
ALS
1)
β factor is defined in Sec.1 Table 3.
2)
— The acceptance criteria values given without brackets shall be used when ideal elastic buckling capacity given in DNVCG-0128 Sec.3 [4] is applied.
— The acceptance criteria values given in brackets shall be used when the type specific cross section buckling capacity
given in DNV-CG-0128 Sec.3 [5] is applied.
4.3.2 Plates and stiffened panels
The buckling capacity Wu of plates and stiffened panels shall be calculated according to one of the listed
codes, as applicable:
— DNV-CG-0128 Sec.3 [2]
— DNV-CG-0128 Sec.4
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 59
DNV AS
Chapter 2 Section 4
The buckling utilisation factor ηact is the ratio between the calculated stress, based on applicable loads in
Sec.2, and the corresponding buckling capacity:
The allowable buckling resistance factors
ηallb for plates and stiffened panels are given in Table 7.
Guidance note:
When using the buckling code DNV-CG-0128, the plated and stiffened panels are assumed to be properly supported along
their boundaries, i.e. by bulkheads, frames or stocky girders. In cases the supporting girders are heavy utilized, the girders
should additionally be checked to see if they are sufficiently stocky to support the stiffened plate field, e.g. by prescriptive rule
requirements or by the method described in DNV-RP-C201 Sec.7.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.3 Cylindrical shell structures
The buckling capacity
RP-C202.
Wu of cylindrical and un-stiffened conical shell structures shall be according to DNV-
The allowable buckling resistance factor
ηallb for shell structures is given in Table 7.
4.3.4 Pillars or columns, struts and cross-ties
Pillars or columns, struts, cross-ties are here defined as self-standing members exposed to axial compressive
loads and end moments.
The buckling capacity Wu are given in DNV-CG-0128 Sec.3 [4], using the allowable buckling resistance factor
ηallb given in Table 7.
4.3.5 Frames, beams and girders
Frames, beams and girders are here defined as integrated members in plated structures exposed to all types
of loads, including lateral pressure.
The buckling capacity Wu of frames, beams and girders shall be calculated according to one of the listed
codes, as found applicable:
— DNV-CG-0128 Sec.3 [5]
— DNV-RP-C201.
The allowable buckling resistance factor ηallb for frames, beams and girders is given in Table 7.
5 Deflection
5.1 General
5.1.1 Deflection criteria is related to the serviceability criteria (SLS) which may prevent the intended
operation.
5.1.2 The maximum deflection
max
of a beam should be limited to:
[mm]
where:
= span of the beam [mm].
S
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 60
DNV AS
Chapter 2 Section 4
— DNV-RP-C201.
6.1 General
6.1.1 Assumptions
The following assumptions and parameters are the basis for the fatigue strength calculations:
a)
b)
c)
d)
e)
f)
g)
h)
the calculated fatigue life shall form the basis for establishing efficient inspection programmes during
fabrication and the operational life of the structure
fatigue assessments shall be carried out for each individual element or component which is subjected to
fatigue loading
the analyses shall be performed utilising relevant site specific environmental data for the area(s) in
which the unit will be operated
design S-N curves with 97.5% probability of survival, corresponding to the mean-minus-two-standarddeviation curves of relevant experimental data
corrosion protection system and time in a corrosive environment
fraction of time in each loading condition
design fatigue factors (DFF) shall be accounted for in order to reduce the probability of fatigue failure
during the unit's design life
fatigue life improvements by methods such as grinding, hammer peening or TIG dressing of the weld
should not be accounted for at the design stage.
6.1.2 Fatigue detail design
The following principles shall be applied for detail design:
1)
2)
3)
4)
Special consideration shall be made to stress concentration when considering cut-outs for door
openings, hatch coamings, windows, etc. and groups of such cut-outs. The corner radius of openings
in areas categorized as special and primary structure shall not be less than 100 mm, unless otherwise
documented. Larger radii may be required based on actual stress levels. Multi cable transits (MCTs)
installations shall be specially evaluated with respect to the actual dynamic stress level at the location in
the structure, and the corner radius of the MCTs.
In areas categorized as secondary structure where openings are found to be not fatigue critical, see
Sec.3 [3.3.1], a corner radius of minimum 25 mm is accepted.
Brackets exposed to high dynamic stresses shall have soft toes, and preferably be made stocky to avoid
hot spots from the termination of buckling stiffeners.
Outfitting details connected to hull structures shall be such that stress concentrations are minimized.
In highly stressed areas such connections shall be avoided. Outfitting connections shall be placed a
minimum of 200 mm from bracket toes, unless detailed fatigue life is documented. Connections to top
flanges of girders or stiffeners shall be avoided.
Temporary attachments used for e.g. fabrication purposes, shall be removed and base material grinded.
The ground area shall be visually examined to be free of cracks by magnetic particle testing (MT) or by
penetrant testing (PT), as found applicable.
6.2 Design fatigue factors
6.2.1 Design fatigue factors (DFF) given in Table 8 are based on the unit being structurally redundant, see
Sec.1 [1.1.4]. The DFFs depend on the significance of the structural components with respect to structural
integrity and availability for inspection and repair. DNV unit specific standards may define specific DFFs to be
applied for structural details in areas. In case of conflict, the DNV unit specific standards shall be followed.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 61
DNV AS
Chapter 2 Section 4
6 Fatigue
Chapter 2 Section 4
Table 8 Design fatigue factors (DFF)
DFF related to survey cycle
Structural element
5-year inspection interval,
carried out in dry dock
5-year inspection
interval, carried out afloat
External structure, accessible for regular
inspection and repair in dry and clean
3)
conditions
1
1
External structure, accessible for inspection
but not accessible for repair in dry and clean
3)
conditions
1
Internal structure, accessible and not welded
directly to the submerged shell plate
1
1
Internal structure, accessible and welded
directly to the submerged shell plate
1
2
Non-accessible areas, not planned to be
accessible for inspection and repairs during
operation
3
3
1)
2
1) 2)
For units that are planned to be inspected afloat at a sheltered location:
— DFF of 1 from 1 m above lowest inspection waterline and upwards.
— DFF of 2 from 1 m above the lowest inspection waterline and downwards.
2)
For units intended to be inspected afloat at location:
— DFF of 1 above the splash zone.
— DFF of 2 below the splash zone.
— DFF of 3 in the splash zone.
3)
The DFF for external structure includes buttwelds/seams in the plate.
6.2.2 Where it is likely that a crack may propagate from an area with access into an area with no access, the
location shall be categorized as not accessible. E.g. a weld detail located on the inside of a submerged shell
plate, that is accessible, shall have the same DFF as an identical non-accessible weld detail located outside of
the plate.
6.3 Simplified fatigue analyses
6.3.1 Simplified fatigue analysis may be used:
— when the stress range is dominated by one governing stress component, i.e. where the principal stress
range is well defined
— where the hot spot stress concentrations are defined from tabulated values
— as a screening process in order to identify the most critical areas/details.
6.3.2 The dynamic stress ranges shall be calculated according to the principles given in Sec.2 [2.3], using
appropriate conservative design parameters.
6.3.3 The basis for a simplified fatigue analyse is that the long term stress range distribution of the loads can
be expressed by a two-parameter Weibull distribution, i.e:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 62
DNV AS
Q
= probability for exceedance of the stress range
h
= Weibull shape parameter
q
=
Δσ
Weibull scale parameter is defined from the stress range level,
where
Δσ0, as:
Δσ0 is the largest stress range out of n0 cycles.
When the long-term stress range distribution is defined applying Weibull distributions for the different load
conditions, and a one-slope S-N curve is used, the fatigue damage is given by:
where:
Td
h
q
ν0
= design life in seconds
= Weibull stress range shape distribution parameter
= Weibull stress range scale distribution parameter
= long-term average zero up-crossing frequency
= gamma function
= usage factor.
For details, see e.g. DNV-RP-C203 Sec.5.
6.4 Stochastic fatigue analyses
6.4.1 Stochastic fatigue analysis should be used:
— when the stress range is complex and includes several dominating stress components
— where the hot spot stress concentrations need to be based on fine mesh FE-analyses.
Guidance note:
Stochastic fatigue analysis may be required in project specifications or by DNV unit specific standards.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
6.4.2 Stochastic fatigue analysis shall:
— be based upon recognized procedures and principles utilising relevant site specific data
— be based on a screening process from simplified fatigue analysis, or a screening process using a global
stochastic fatigue analysis to identify locations for which a detailed stochastic fatigue fine-mesh analysis
should be undertaken
— be based on dynamic stresses from a wave load analysis
— consider the directional probability of the environmental data based on a scatter diagram
— use relevant wave spectra and energy spreading functions
— use an adequate number of wave directions.
For details, see DNV-CG-0129 Sec.5 [3] and DNV-CG-0129 Sec.5 [4].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 63
DNV AS
Chapter 2 Section 4
where:
7.1 General
7.1.1 Introduction
This subsection describes modelling techniques, loads, acceptance criteria and required documentation for
finite element (FE) analyses.
Three levels of FE-analyses are described:
a)
b)
c)
Global strength analysis, to assess the unit's global strength.
Partial strength analysis, to assess part of the unit's global strength.
Local structure analysis, to assess local stresses in the case that neither a) nor b) are satisfied, or to
calculate the stress concentrations of details for a fatigue evaluation.
7.1.2 Basis for FE-analyses
— Gross scantlings (as-built scantlings) shall be applied, unless otherwise agreed with the verifier.
— Linear elastic FE-analyses should be used. Use of simplified rigid-plastic or elastic-plastic analyses may be
applied, but the principles shall be agreed with the verifier, see also [8].
7.1.3 Loads
The analyses shall be based on the most severe loads for the structure.
The following loaded conditions shall be evaluated, as applicable:
— fully loaded
— ballasted
— during operation.
7.1.4 Element types
Two (2) node line elements and four (4) node isotropic shell elements are normally sufficient for the
representation of the hull structure. The mesh requirements given in this chapter are based on the
assumption that these elements are used in the finite element models. However, higher order elements may
also be used. Use of different elements types that shall be used are described in DNV-CG-0127 Sec.1.
Isotropic material properties should be used.
Anisotropic elements may be used to obtain a more accurate stress distribution in the element model, e.g.
for slender plate panels with high utilization in the transverse direction. The anisotropic material model shall
represent correct geometrical properties.
For structures subjected to high temperature gradients i.e. exceeding 100°C, thermal effects shall be taken
into account.
7.1.5 Reporting
Analysis reports shall include information specified in DNV-CG-0127 Sec.1 [2.1].
7.1.6 Software
All computational programmes shall be recognized and made for structural assessment.
A computer programme which has demonstrated the production of reliable results to the satisfaction of the
verifier is regarded as a recognized programme. Where the computer programme employed is not supplied
or recognized, full particulars of the computer programme, including example calculation outputs, shall
be submitted. It is recommended that the designers consult the verifier on the suitability of the computer
programme intended to be used prior to the commencement of any analysis work.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 64
DNV AS
Chapter 2 Section 4
7 Finite element analyses
7.2.1 General
The objective of a global FE analysis is to obtain a reliable description of the unit's overall global stiffness and
to calculate the global stresses and deformations of the main load bearing members.
A global FE-model shall cover the main parts of the unit's main structure, unless the global strength can be
sufficiently documented using a partial strength analysis described in [7.3].
Specific requirements for global FE analyses are given for each unit type in the specific DNV unit specific
standards, as applicable.
7.2.2 Structural model
The global FE-model shall extend over the entire unit and represent the actual geometry of the unit with
acceptable accuracy. All main structure contributing, or partly contributing, to the global strength shall be
included in the model.
7.2.3 Modelling
The mesh size and model idealisation shall be done in accordance with DNV-CG-0127 Sec.1 [1.7].
7.2.4 Loading conditions
The selection of loading conditions and the application of loads depend on the scope of the analysis. For
different unit types, specific load applications are described in DNV unit specific standards.
7.2.5 Analysis criteria
Element component stresses and von Mises stresses shall be checked according to the acceptance criteria
given in Table 3.
All structural elements in the FE-analysis shall be assessed individually against the buckling acceptance
criteria given in Table 7.
7.3 Partial strength analyses
7.3.1 General
Partial structural FE-analyses may, in agreement with the verifier, be applied as an alternative to a global
FE-analysis, or as a supplement to the global FE-analysis. In order to use a partial strength FE-analysis, the
following shall be evaluated:
— is the unit's global stiffness adequately taken into account?
— is it possible to apply correct boundary conditions in order to achieve correct global stresses for the
area(s) of interest?
— can all relevant loads be applied to the model?
For ship-shaped units, partial FE-analyses shall be used to document the unit's hull girder strength, see DNVOS-C102 Ch.2 Sec.3 [4.3].
7.3.2 Analysis criteria
Element component stresses and von Mises stresses shall be checked according to the acceptance criteria
given in Table 3.
All structural elements in the FE-analysis shall be assessed individually against the buckling acceptance
criteria given in Table 7.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 65
DNV AS
Chapter 2 Section 4
7.2 Global strength analyses
7.4.1 General
The purpose of a local structure analysis is:
— to perform local strength checks of details where the stress results cannot be adequately represented
using a normal coarse mesh size, e.g. from a global or partial strength analysis
— to find local hot spots relevant for fatigue assessments, when tabulated hot spots are not available
Where the local peak stress is dominated by dynamic loads, fatigue is governing and shall be checked.
7.4.2 Modelling
Structural modelling shall follow the principles stated in DNV-CG-0127 Sec.4.
7.4.3 Analysis criteria
Stress control shall be according to the acceptance criteria given in [3.3.3].
Alternatively, a non-linear analysis, using a recognized FE-program, may be applied if a plastic strain criteria
is applied.
Guidance note:
A local strain of maximum 5% may be accepted, provided that redistribution of loads is possible.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
8 Non-linear analyses
Non-linear static and dynamic FE-analyses may be utilized for the strength control for the accidental
condition, see Sec.1 [2.2.4]. All relevant failure modes (e.g. strain rate, local buckling, joint overloading)
shall be checked. Local overloading of the structural capacity is acceptable provided that redistribution of
loads is possible, see DNV-RP-C208 and DNV-RP-C204.
Guidance note:
The capability of the structure to redistribute loads during and after accidents depends on:
—
having materials with sufficient toughness for the actual service temperature and thickness of structural members
—
making joint connections of primary members stronger than the members themselves
—
having redundancy in the structure, so that alternate load redistribution paths may be utilized
—
avoiding dependency on energy absorption in slender members with a non-ductile post buckling behaviour
—
avoiding pronounced weak sections and abrupt changes in strength or stiffness.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 66
DNV AS
Chapter 2 Section 4
7.4 Local structure analyses
1 Symbols
Symbols used in this section are listed below. For symbols not defined in this section, see Ch.1 Sec.1 [6.3].
2
ReH
= specified minimum yield stress for the base material [N/mm ]
ReH_weld
= specified minium yield stress for the weld deposit [N/mm ]
Rm
= specified minimum tensile stress for the abutting plate [N/mm ]
f3
= correction factor for the weld type
fweld
= weld factor
fyd
= coefficient depending on the material strength of the weld deposit
k
= base material factor
t
= plate thickness of the actual member considered [mm]
tgap
= allowance for the fillet weld gap [mm]
tleg
= leg length of the fillet welds [mm]
tthroat
= throat thickness of the fillet welds [mm]
βw
= weld correlation factor
γMw
= partial safety factor for the weld connections
2
2
= basic usage factor for the weld connections.
2 Butt joints
2.1 General
2.1.1 Plate components of stiffened panel structures shall be joined by butt welds, see Figure 1.
2.1.2 Butt welding from one side against permanent steel backing should be avoided, but may be used at
locations where the dynamic stress level is low. One side butt welds shall not be used in tanks.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 67
DNV AS
Chapter 2 Section 5
SECTION 5 WELD AND BOLT CONNECTIONS
Chapter 2 Section 5
Figure 1 Typical butt welds
2.1.3 Tapering
When welding plates with difference in thicknesses equal to or greater than 4 mm, the thicker plate shall
be tapered. The taper shall have a length at least three (3) times the difference in the as-built thickness. If
the width of the groove is greater than three (3) times the difference in the as-built thickness, the transition
taper may be omitted.
3 Tee and cross joints
3.1 General
3.1.1 The connection of a plate abutting on another plate shall be made by fillet or penetration welds, as
shown in Figure 2.
Figure 2 Tee and cross joints
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 68
DNV AS
Figure 3 Leak stopper towards tank boundary
3.2 Fillet welds
3.2.1 Continuous fillet welds
Double continuous fillet weld shall be used for the following connections:
—
—
—
—
connection of the web plate to the face plate for all members
cargo tanks and watertight compartments
connections at supports and ends of girders, stiffeners and pillars
connections at foundations and supporting structures.
3.2.2 Intermittent fillet welds
With reference to Figure 4 the various types of intermittent welds are:
— chain weld
— staggered weld
— scallop weld (closed).
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 69
DNV AS
Chapter 2 Section 5
3.1.2 Leak stoppers
Where structural members pass through a tight boundary such as e.g. tank boundaries, leak stoppers shall
be arranged e.g. by small notches filled with weld material, scallop, or full penetration weld see Figure 3.
Chapter 2 Section 5
Figure 4 Recommended arrangement of intermittent welds
The following is applicable with respect to intermittent welds:
a)
b)
c)
d)
Chain or staggered welds may be used in dry spaces and tanks arranged for fuel oil only, and where the
weld connection is moderately stressed.
When chain and staggered welds are used on continuous members penetrating oil- and watertight
boundaries, the weld termination towards the tight boundary shall be closed by a scallop or full
penetration weld, see Figure 3.
Where beams, stiffeners, frames, etc., are intermittently welded and pass through slotted girders,
shelves or stringers, there shall be a pair of matched intermittent welds on each side of every
intersection. In addition, the beams, stiffeners and frames shall be efficiently attached to the girders,
shelves or stringers.
Where intermittent welding or one side continuous welding is permitted, double continuous welds shall
be applied in accordance with [3.6.4], as following:
— 50% of total length for connections in tanks
— 35% of total length for connections elsewhere.
e)
Double continuous welds shall be used at the stiffener end connections, see Figure 4.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 70
DNV AS
The size for a one side continuous weld is given in [3.6.4], where a f3 factor of 2.0 shall be used.
One sided fillet welds may only be used in dry spaces.
3.3 Full and partial penetration welds
3.3.1 General
The following apply for full and partial penetration welds:
a)
b)
c)
d)
For partial penetration welds, the root face f shall be minimum 3 mm or t0 /3, as applicable.
For full penetration welds, the root face shall be removed, e.g. by gouging, before welding the back side.
The groove angle , made to ensure that the welding bead penetrates in to the root of the groove,
should be between 40° to 60° and in accordance with a recognized fabrication standard, see Figure 5.
The welding bead of the full/partial penetration weld shall cover the root of the groove.
Figure 5 Partial penetration welds
3.3.2 Full penetration welds
Full penetration weld shall be used in the following locations:
a)
b)
c)
d)
Cruciform cross connections in areas categorized as special and primary, exposed to high stresses.
Partial penetration weld may be accepted on a case by case evaluating.
Weld connections defined to be fatigue critical, see Sec.3 [3.3.1].
Welds where abutting plate panels form a boundary to open sea.
Welds used for openings to sea e.g. pipes penetrations and sea chests.
3.4 Slot welds
3.4.1 Slot welds may be used for connections of plating to internal webs, where access for welding is not
practicable.
Slots welded against a backing plate, including butt welds against permanent backing as shown in Figure 6,
shall not be used when the pressure acts on the side of the abutting member.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 71
DNV AS
Chapter 2 Section 5
3.2.3 One sided continuous weld
Chapter 2 Section 5
Figure 6 Filled up continuous slot welds
3.4.2 Slot welds shall not be used in areas with high in-plane stresses acting transversely to the slots.
3.4.3 Slots shall be well-rounded and have a minimum slot length, ℓslot of 75 mm and width, wslot of twice
the as-built plate thickness. Slots on closing plates shall be spaced a distance sslot of maximum 140 mm, see
Figure 7.
The size of the fillet welds shall be determined from the second formula given in [3.6.4] with a weld factor
tweld = 0.60.
Figure 7 Slot welds
3.4.4 If a backing plate is used the minimum backing plate gross thickness shall be 0.7 times the gross
thickness of the adjacent web. Adequate overlapping between the backing plate and the slot opening shall be
provided.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 72
DNV AS
3.5.1 General
Lap joint welds, see Figure 8, may be used for connections dominated by shear- or in plane stresses acting
parallel to the weld. Lap joint welds shall not be used for connections of primary members, or where in-plane
stresses transverse to the weld are present.
3.5.2 Overlap width
Where overlaps are used, the width Wlap of the overlap shall not be less than 50 mm, but need not to be
greater than 100 mm, see Figure 8.
Figure 8 Fillet weld in lapped joint
3.5.3 Overlaps for lugs
The overlaps for lugs and collars used in cut-outs for the passage of stiffeners through webs and bulkhead
platings shall not be less than three (3) times the thickness of the lug, but need not be greater than 50 mm.
3.5.4 Lapped end connections
Lapped end connections shall have continuous welds on each edge, with leg lengths tleg, in mm, as shown
in Figure 8. The sum of the two leg lengths shall not be less than 1.5 times the average thickness of both
plates.
3.5.5 Overlapped seams
Overlapped seams shall have continuous welds on both edges, according to the sizes required by [3.6.4] for
the boundaries of tanks/holds or watertight bulkheads. Seams for plates with thicknesses of 12.5 mm or less,
which are clear of tanks/holds, may have one edge with intermittent welds in accordance with [3.6.4] for
watertight bulkhead boundaries.
3.6 Weld size criteria
3.6.1 The weld deposit yield stress
deposit are given in Table 1.
ReH_weld and the strength coefficient fyd between base material and weld
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 73
DNV AS
Chapter 2 Section 5
3.5 Lap joints
Base metal
Strength group
Designation
Weld deposit
Yield stress,
Yield stress,
ReH [N/mm2]
ReH_weld [N/mm2]
Strength coefficients
Base metal/weld
deposit factor
Base material
factor
k 1)
Normal strength
steels
NV NS
235
305
1.0
0.82
High strength
steels
NV 27S
375
0.91
High strength steels
NV 32
265 315
355
375
0.78
NV 36
390
375
0.72
400
0.68
NV 40
1)
For steels with other defined minimum yield stress
ReH than given above, the following shall be used:
.
Guidance note:
Requirements for weld consumables are given in DNV-OS-C401.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.6.2 If welding consumables with deposits of lower yield stress than what is specified in Table 1 are used,
the actual yield strength of the weld deposit shall be applied.
3.6.3 When deep penetrating welding processes are applied, the required leg length may be reduced by 15%
provided that sufficient weld penetration is documented.
3.6.4 For fillet welds dominated by shear and in-plane stresses acting parallel to the weld, the leg length tleg
of double continuous fillet welds shall be:
,
but shall not be taken less than the values given in Table 2.
Where:
fweld
= weld factor given in Table 3
fyd
= strength ratio for the base material, given in Table 1
f3
= correction factor for weld type:
t
= plate thickness of abutting plate [mm]
f3 = 1.0 for double continuous weld
f3 = Sctr / lweld = for intermittent or chain welding, see Figure 9
f3 = 2.0 for one side continuous fillet weld
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 74
DNV AS
Chapter 2 Section 5
Table 1 Strength coefficient between base material and weld deposit fyd
= allowance for fillet weld gap is not to be taken less than 2.0 mm when t ≥ 12.0 mm and 1.0 mm
when t0 ≤ 6.0 mm. For 6.0 mm < t < 12.0 mm linear interpolation applies.
Figure 9 Weld scantlings definitions
3.6.5 The minimum leg length of a fillet weld shall not be taken as less than the values given in Table 2.
Table 2 Minimum leg length
Minimum leg length [mm]
Effective plate or web thickness t [mm]
In liquid cargo tanks
and water ballast tanks
Elsewhere
t ≤ 5.5
4.0
3.5
5.5 < t ≤ 8.0
4.5
3.5
8.0 < t ≤ 12.5
5.0
4.0
t > 12.5
5.5
4.5
3.6.6 The weld factors for different structural members are given in Table 3.
Table 3 Weld factors fweld for different structural members
No
Structural members
fweld
Stiffeners
1
Buckling and sniped stiffeners
2
Stiffeners attached to primary plate members
0.20
0.35
1)
Girders and primary supporting structures (PSM)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 75
DNV AS
Chapter 2 Section 5
tgap
3
Structural members
Girders and PSM’s
2)
fweld
0.45
1)
Non-tight decks/bulkheads
4
Non-tight bulkheads
0.45
Perforated decks
Watertight boundaries
5
Boundary connection of watertight compartments and tanks
6
Boundary connection of liquid cargo tanks
0.6
Boundary connection of water ballast tanks
Boundary connection of weather deck
Collision bulkhead
Other structure
7
All other welds not specified in the table
1)
Inclusive end brackets.
2)
See also [3.7.3].
0.6
3.6.7 In order to account for the convexity of a fillet weld, the throat size [mm] shall not be less than:
3.7 Weld connection details
3.7.1 General
Stiffeners shall be connected to the web plate of girders, frames or bulkheads as follows:
a)
b)
c)
d)
welded directly to
welded directly to
welded directly to
welded directly to
stiffener flange.
the
the
the
the
connection
connection
connection
connection
plate
plate with an additional lug/collar plate
plate with a closed lug/collar plate
plate with an additional stiffener or bracket welded on top of the
In locations where high shear forces need to be transferred from the stiffener to the connection area, lug
plates may be required to achieve sufficient transition area. See also Figure 10.
3.7.2 Stiffener connection to primary supporting members
The required welding leg length tleg-s [mm] for a stiffener connection to primary members shall not be less
than:
and not less than the values given in Table 2, but need not be taken larger than:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 76
DNV AS
Chapter 2 Section 5
No
Chapter 2 Section 5
Where:
Wd
= the design load transmitted through the shear connection [kN]
τperm
= permissible nominal shear stress [N/mm ]:
2
WSD method:
LRFD method:
and
are given in Sec.1 [1.3].
ls
= total length of shear connection (ℓd +
ℓc) [mm], see Figure 10
t
= thickness of the plate at the shear connection [mm]. Lug plate thickness tc or web plate thickness tw,
as applicable, see Figure 10.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 77
DNV AS
Chapter 2 Section 5
Figure 10 Symmetric and asymmetric cut-outs of stiffener connections
3.7.3 End connection of primary supporting members
The weld dimensions of girder end connections and adjoining brackets shall be based on the calculated
stresses at the welded connection.
Double continuous fillet welds, or alternatively partial or full penetration welds, shall be used.
For high tensile stresses at the connection, the requirements given in [3.8] apply.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 78
DNV AS
Chapter 2 Section 5
The required weld leg length tleg-w for primary supporting members shall not be less than:
but not be taken larger than:
and shall not be taken less than the values given in Table 2.
Where:
hw
= web height of primary supporting members [mm]
tw
= web thickness of primary supporting members [mm]
lweld
= length of the welded connection [mm], as shown in Figure 11
= as given in [3.7.2].
Figure 11 Weld shear area of primary supporting members at end connections
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 79
DNV AS
3.8.1 Application
Direct calculation of welds shall be carried out if the requirements in [3.6] are replaced by direct stresses for
the connection. The minium leg length values given in Table 2 shall, independent of the direct calculation, be
complied with.
Direct calculation of the welds are based on that the following are considered:
a)
b)
c)
d)
e)
Residual stresses in the welded connection are accounted for.
Welded connections are designed to have adequate deformation capacity.
In joints where plastic hinges may form, the welds are designed to provide the same design resistance
as the weakest of the connected parts.
The design resistance of welds are adequate if, at every point in its length, the resultant of all the forces
per unit length transmitted by the weld does not exceed its design resistance.
Fatigue life for the welded connection is properly accounted for.
3.8.2 The design resistance of welds is acceptable provided that both a) and b) are satisfied:
a)
b)
where:
2
= normal design stress perpendicular to the throat, [N/mm ], see Figure 12
2
=
shear design stress (in plane of the throat) perpendicular to the axis of the weld, [N/mm ], see
Figure 12
=
shear design stress (in plane of the throat) parallel to the axis of the weld, [N/mm ], see Figure
12
2
= partial safety factor for weld, LRFD method, given in Table 5
= basic usage factor for weld, WSD method, given in Table 5
βw
= weld correlation factor given in Table 4
Rm
= specified minimum tensile stress for the abutting plate [N/mm ] given in Table 4.
2
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 80
DNV AS
Chapter 2 Section 5
3.8 Direct calculation of weld connections
Chapter 2 Section 5
Figure 12 Stresses in fillet weld
The correlation factor βw accounts for higher ultimate tensile stresses in the weld deposit. Values are given in
Table 4 for different basic steel grades.
Table 4 The correlation factor
βw
Lowest ultimate tensile strength
Correlation factor
Rm
βw
NV NS
400
0.83
NV 27S
400
0.85
NV 32
440
0.87
NV 36
490
0.90
NV 40
510
0.95
NV 420
530
1.0
NV 460
570
1.0
Steel grade
The partial safety factors
given in Table 5.
for the LRDF method, and the basic safety factors
for the WSD method, are
Table 5 Partial safety factor and basic safety factor for direct weld calculation
Design method
Limit state
LRFD
ULS
1.3
ALS
1.1
WSD
a)
0.62
b)
0.67
0.90
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 81
DNV AS
4.1 Symbols
Symbols used for bolted and bolt connections in this subsection are listed below. For symbols not defined in
this subsection, see Ch.1 Sec.1 [6.3].
2
A
= bolt nominal shank area [mm ]
As
= bolt tension area [mm ]
Bp, Rd
= punching shear resistance [N]
ds
= nominal bolt (shank) diameter [mm]
ds,min
= minimum shank diameter [mm]
d0
= bolt hole diameter [mm]
e1, e2, e3, e4
= end distances [mm]
ReHb
= material yield strength for bolt [N/mm ]
Rmb
= material ultimate tensile strength for bolt [N/mm ]
fM2m, fM2b
= material correction factor for base material and bolt
Ft,Ed
= design tensile load [N]
Fv,Ed
= design shear load [N]
Fb,Rd
= bolt bearing resistance [N]
Fs,Rd
= combined shear and tension resistance at ULS [N]
Fs,Rd,Ser
= combined shear and tension resistance at SLS [N]
Ft,Rd
= bolt tensile resistance [N]
Fv,Rd
= bolt shear resistance [N]
k2
= tension resistance factor
kb
= bearing capacity factor
ks
= hole clearance factor
Nnet,Rd
= net cross section capacity for base material [N]
p1, p1,0, p1,i,
p2
= spacing values [mm]
μ
= friction factor
ϒM0, ϒM1,
ϒM2b, ϒM2m,
ϒM3
= utilization factors
2
2
2
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 82
DNV AS
Chapter 2 Section 5
4 Bolt connections
= dynamic stress range [N/mm ].
4.2 General
4.2.1 This subsection contains strength requirements for different bolted connection types and purposes. The
principles are based on EN 1993-1 and EN 1999-1, adjusted for use on floating offshore units.
4.2.2 The following shall be taken into account for bolted connections:
a)
b)
c)
d)
e)
f)
g)
nature of external loading
load in bolt(s)
design/capacity of bolted connection
material properties in bolt and base material
environmental conditions (e.g. min./max. temperature, corrosive environment)
consequence of failure
fit-up tolerance.
4.2.3 The bolted connection may be of fitted type or slip resistance type with or without pre-loading. The
connection shall be controlled for all relevant design conditions, as applicable.
If the ULS or ALS conditions are governing the serviceability limit condition (SLS) does not need to be
evaluated
4.2.4 When a bolt connection is a critical structural connection, i.e. when a failure of one bolt in the
connection may cause a progressive collapse of the main structural elements, the bolt group capacity shall be
checked for the loss of one bolt.
Guidance note:
This may be the case for framework support interfaces, framework diagonals and other main bearing trusses.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.2.5 Critical single bolt connections shall be designed with an additional safety factor of 1.5 and be
mechanically secured to prevent accidental loss of bolt or nut.
4.2.6 Bolt type, connection type and material specification shall be stated on relevant drawings
(arrangement, assembly or structure drawings) for the purpose used.
4.3 Bolt scantlings and connection types
4.3.1 Typical bolt scantlings for relevant bolt dimensions are given in Table 6.
Table 6 Bolt scantlings
Symbol
Description
Steel bolt dimension
M10
M12
M16
M20
M24
M30
M36
M42
M48
ds [mm]
Nominal shank
diameter
10
12
16
20
24
30
36
42
48
As [mm2]
Tension area
58
84.3
157
245
353
561
817
1120
1470
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 83
DNV AS
Chapter 2 Section 5
2
Δσ
Minimum
shank
diameter
9.78
11.73
15.73
19.48
23.67
-
-
-
-
ds,min
Minimum
shank
diameter
9.64
11.57
15.57
19.48
23.48
29.48
35.38
41.38
47.38
Ads,min [mm2]
Minium shank
area
75.1
108.1
194.3
303.9
440
682.6
983.1
1344.8
1763.1
A [mm2]
Nominal shank
area
78.5
113.1
201.1
314.2
452.4
706.9
1017.9
1385.4
1809.6
1)
1)
[mm], class B
Class A and class B refer to production grade, see ISO 4014.
4.3.2 Bolted connections are grouped into the following two main categories:
— fitted connections
— slip resistance connections.
Slip resistance bolts are pre-loaded, while fitted bolt connections are used without any specific pre-loading.
Table 7 specifies bolt type connections and loading applications.
Table 7 Bolt connection types and applicable acceptance criteria
Bolt category
A
2)
Connection type
Shear connection
Pre-loaded
Acceptance criteria
Allowable
bolt classes
No
Fv,Ed < Fv,Rd
8.8 1)
10.9
Fv,Ed < Fb,Rd
B
(SLS)
2)
Shear connection
Yes
Fv,Ed < Fs,Rd,Ser
A4-80
1)
D
2)
Pre-loaded for slipresistance and dimensioned
for serviceability load levels.
Not accepted for reversal
shear connections.
8.8 - 10.9
Pre-loaded for slipresistance and dimensioned
for ultimate load levels.
Fv,Ed < Fv,Rd
Shear connection
Yes
Fv,Ed < Fs,Rd
Fv,Ed < Fb,Rd
Tension connection
No
Ft,Ed < Ft,Rd
Ft,Ed < Bp,Rd
E
Tension connection
Yes
Ft,Ed < Ft,Rd
To be arranged with
fitted bolts for structure
exposed to dynamic loads
or vibrations.
8.8 - 10.9
Fv,Ed < Fb,Rd
C
(ULS)
Comment
8.8 1)
10.9
Not accepted for structure
exposed to dynamic loads
or vibrations.
8.8 - 10.9
Ft,Ed < Bp,Rd
1)
Lower bolt grade may be accepted for secondary structure connections.
2)
Block tearing in the base material shall additional be checked using the net cross section between the bolt holes, as
applicable.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 84
DNV AS
Chapter 2 Section 5
ds,min [mm], class A1)
4.4.1 Bolts shall be delivered with a 2.2 certificate as a minimum. For DNV classed units, see certification
requirements given in Ch.3 Sec.1 Table 6.
4.4.2 Where the structural connection is categorized to be primary or special, the following bolt classes shall
be used:
— steel bolt class 8.8 or 10.9
— stainless steel class A4-80.
Bolts of a higher class than 10.9 or A4-80, given in Table 8, shall not be used, unless specially agreed with
the verifier.
4.4.3 Bolts for use in cold areas where the LMDAT is equal to or lower than - 20°C, or otherwise subject to
low temperatures, shall be subject to special consideration. The bolt material shall be of austenitic stainless
steel, or equivalent quality with documented impact test properties.
4.4.4 Relevant values for the material yield strength ReHb and ultimate tensile strength
shall be applied for the bolt classes, unless otherwise documented.
Table 8 Bolt class with corresponding yield
ReHb and ultimate tensile strength Rmb capacity
Steel bolt class
Symbol
Rmb given in Table 8
Stainless steel bolts
4.6
4.8
5.6
5.8
6.8
8.8
10.9
A4-70
A4-80
]
240
320
300
400
480
640
900
450
600
Rmb [N/mm2]
400
400
500
500
600
800
1000
700
800
ReHb [N/mm
2
4.4.5 A material correlation factor accounting for the relationship (fM2m, fM2b) between the base or the
bolt's material yield strength (ReH or Reb), and the base or the bolt's material ultimate tensile strength (Rm
Rmb), are given in Table 9 and Table 10. The fM2m and fM2b factors are used for the safety factors ϒM2m
ϒM2b given in Table 11.
or
Table 9 Material correction factor fM2b for bolts
Symbol
fM2b
Steel bolt class
Stainless steel bolts
4.6
4.8
5.6
5.8
6.8
8.8
10.9
A4-70
A4-80
1.33
1.0
1.33
1.0
1.0
1.0
1.0
1.24
1.07
Table 10 Material correction factor fM2m for base material
Symbol
fM2m
Base material designations
NV
NV 27
NV 32
NV 36
NV 40
NV 420 or higher
1.36
1.21
1.12
1.10
1.05
1.0
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 85
DNV AS
Chapter 2 Section 5
4.4 Material and certification requirements
for bolts, and
for base materials.
4.4.7 The safety factors for LRFD and WSD design methods are given in Table 11.
Table 11 Safety factors for steel connections
Design method
Type of calculation
Acceptance
criteria
Resistance of plates in
members
ULS
Resistance of members and
cross sections
ULS
Resistance of bolts in bearing
ULS
2)
Safety factor
LRFD
γM0
1.15
ALS
γM1
Slip resistance
ULS
2)
γM2b 1)
1.3fM2b
2)
3)
1.25
1.43
2)
γM2m 1)
1.3fM2m
1.25
1.68fM2b
2)
γM3
1.5
1.4fM2b
2.25
1.1fM2b
1.68fM2m
1.1fM2m
ALS
1.5
1.0
1.1fM2b
1.3
ALS
1)
1.43
1.0
ALS
ULS
b)
1.0
1.15
ALS
Resistance of plates in
bearing
a)
1.0
2)
Life saving
3)
applications
WSD
1.1
1.4fM2m
2.25
1.1fM2m
1.68
1.4
2.25
1.1
fM2b is given in Table 9 and fM2m is given in Table 10.
For aluminium bolted connection the safety factors for ULS shall be multiplied with 1.05.
The safety factors assume that a load factor of 2.2 is used for the static load.
4.5 Bolt capacity
4.5.1 The bearing capacity factors kb and the hole clearance factors ks are given in Table 12.
Table 12 Bearing capacity factor
kb and hole clearance factor ks
Description
Fitted bolts and bolts in normal holes.
Bolts in short slotted holes where the axis of the slot is perpendicular to the direction of the
load transfer.
Bolts in long slotted holes where the axis of the slot is perpendicular to the direction of the
load transfer.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
kb
ks
1.0
1.0
0.85
1)
0.6
0.7
1)
Page 86
DNV AS
Chapter 2 Section 5
4.4.6 For bolts or base materials not specified above, the following formula shall be used to define fM2:
kb
ks
Bolts in short slotted holes where the axis of the slot is parallel to the direction of the load
transfer
0.76
1)
Bolts in long slotted holes where the axis of the slot is parallel to the direction of the load
transfer.
0.63
1)
Chapter 2 Section 5
Description
1)
Aluminium bolted connections shall be arranged with a hole clearance that is the same as for fitted bolt
connections given in [4.7.1].
4.5.2 Friction coefficients
connections.
μ are given in Table 13 for steel connections, and in Table 14 for aluminium
Table 13 Surface categories and friction coefficient factors for steel connections
Surface category
A
μ
Surface treatment
Surfaces blasted with shot or grit:
- with any loose rust removed, no pitting
- spray metallised with aluminium coating certified to provide a slip
factor of no less than 0.5
0.5
- spray metallised with a zinc based coating certified to provide a
slip factor of no less than 0.5.
B
Surfaces blasted with shot or grit:
- spray metallised with aluminium based coating
0.4
- spray metallised with a zinc based coating.
Surfaces blasted with shot or grit, and painted with an alkali-zinc
silicate paint to produce a coating thickness of 50 um to 80 um.
C
Surfaces cleaned by wire brushing or flame cleaning, with any loose
rust removed.
0.3
D
Surfaces not treated.
0.2
Table 14 Friction coefficient factors for aluminium connections
Joint thickness [mm]
μ
12 - 18
0.27
18 -24
0.33
24 - 30
0.37
30 and above
0.40
4.5.3 The acceptance criteria for different bolt connections are given in Table 7, and are based on the bolt
capacity formulas given in Table 15.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 87
DNV AS
Failure mode
Capacity/requirement
Bolt shear
resistance
Factors/comments
av = 0.5, in general.
av = 0.6 for 8.8 bolts or where the shear plane is at the unthreaded part of the bolt.
A = bolt area at actual shear plane.
Bolt bearing
resistance
Edge bolts
Inner bolts
See Figure 13
Tension
resistance
k2 = 0.9
Punching shear
resistance
dm= mean diameter of the flat surface of bolt head or the nut,
k2 = 0.63 for countersunk bolts.
whichever is smaller [mm].
For hexagonal bolt head/nut,
across flats may be used.
Combined
shear and
tension
dm equal 1.077 times the width
General
Applicable for category B connections.
n = number of friction interfaces.
Applicable for category C connections.
n = number of friction interfaces.
Slip resistant
connections
For slip resistance connections class 8.8 or 10.9 bolts shall be
used, with pre-loading to 70% of
Rmb. I.e:
Fp,C= 0.7RmbAs
Other specific pre-loading may be accepted, provided it is
documented.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 88
DNV AS
Chapter 2 Section 5
Table 15 Bolt capacity/requirement
Chapter 2 Section 5
4.6 Bolt hole arrangement
Bolt holes shall be arranged with the spacing as given in Table 16. The capacity may alternatively be
documented separately. See also Figure 13.
Table 16 Values for maximum spacing, end and edge distance
Minimum
Distance and spacing
Maximum
1)
General
Dynamic exposed
connections
General
End distance, e1
1.2d0
1.5d0
4t+40 mm
End distance, e2
1.2d0
1.5d0
4t+40 mm
Edge distance in slotted holes, e3
1.5d0
End distance in slotted holes, e4
1.5d0
2.5d0
smallest of 14t and 200 mm
Spacing,
p1
2.2d0
Spacing,
p1,0
smallest of 14t and 200 mm
Spacing,
p1,i
smallest of 28t and 400 mm
Spacing,
p2 2)
2.4d0
2.5d0
smallest of 14t and 200 mm
1)
Maximum values for spacing and edge distance are limited by the buckling capacity of base material in the
compression and prevention of corrosion. The buckling capacity shall comply with DNV-CG-0128 or alternatively EN
1993-1.
2)
For staggered rows of fasteners, a minimum line spacing of
distance between any fasteners is greater than 2.4d0.
p2=1.2d0 may used provided that the minimum
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 89
DNV AS
Chapter 2 Section 5
Figure 13 End and edge distances
4.7 Bolt hole diameter
4.7.1 Fitted bolt connections
Bolt holes in fitted connections shall not have a diameter larger than the measured bolt diameter + 0.3 mm.
4.7.2 Non-fitted bolt connections
Bolt hole diameter for non-fitted bolts shall not exceed the values given in Table 17 for standard bolt holes,
and in Table 18 for slotted holes.
Table 17 Clearances for standard bolt holes
Bolt diameter ds (maximum) [mm]
1)
Bolt hole diameter d0 [mm]
12 and 14
ds +1
16 to 24
ds +2
27 and larger bolts
ds +3
1)
For aluminium connections, the maximum clearances is 1.0 mm independent of the bolt diameter
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
ds.
Page 90
DNV AS
Bolt diameter ds (maximun)
Short slotted
Long slotted
[mm]
Max length of slot [mm]
Max length of slot [mm]
12 -14
ds +4
2.5ds
16 - 22
ds +6
2.5ds
24
ds +8
2.5ds
27 and larger
ds +10
2.5ds
For bolts in slotted holes the nominal clearance across the width shall be the same as is specified for fitted or
standard bolt holes.
4.8 Bolt arrangement
4.8.1 General
The following is applicable for bolted connections:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
Bolted connections subject to a dynamic stress range should be pre-loaded to a minimum at 70% of the
bolt yield stress ReHb in order to reduce possible fatigue damage.
Bolted connections that are subject to reversal shear loading shall be arranged with fitted-bolts, or
alternatively category C or E bolt with shear stoppers.
For slip resistant connections, pretension shall be specified on the drawing.
Pretension bolts shall have a bolt shaft length (loaded part) at minimum 4.5 times the bolt shaft
diameter.
Bolt pre-loading procedures shall be established by the manufacturer, and all factors effecting correct
torque application shall be properly addressed.
For fitted bolts connections the threaded part of the bolt shank, included in the bearing length, shall not
exceed 1/3 of the base plate thickness, see Figure 14 a).
Bolts used in primary and special structures shall not be less than M16 mm.
All necessary information needed for the capacity control of the bolted connection shall be clearly
specified on relevant drawings.
Applications for slotted bolt holes shall be limited to the fastening of equipment where slide bearing are
required.
Slotted bolt holes shall be arranged with shear stoppers unless the connection shall serve as a sliding
bearing.
The threaded bolt area shall not be in the shear area for bolts subject to dynamic shear loads, see Figure
14 b).
The engaged threaded bolt length lthr_b that is fastened into the base material, see Figure 14 c), shall
not be taken less than either the maximum of:
or
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 91
DNV AS
Chapter 2 Section 5
Table 18 Clearances for slotted holes
Chapter 2 Section 5
Figure 14 Engaged threaded bolt length
4.8.2 Connection to other materials
For bolts connected to other materials than steel, the following applies:
a)
b)
The elasticity modulus of bolt versus base material shall be evaluated for bolted connections where other
materials than steel are used, e.g. bolted aluminium structures with steel bolts.
Galvanic protection shall be utilized between different materials.
4.9 Bolt securing
For bolts securing, the following apply:
a)
b)
c)
All bolt connections shall be secured by means of lock-nuts, mechanical locking or gluing (locktite).
Tack welding shall not be used.
Critical single bolt connections shall be mechanical secured/locked.
4.10 Fatigue
Fatigue shall be documented when the dynamic stress range Δσ exceeds the limit values given in Table 19,
and may be assessed based on recognized international standards, e.g. DNV-RP-C203.
Table 19 Reference values for critical axial stress ranges of bolts
2
Axial loaded bolts [N/mm ]
Dynamic stress range
Δσ
Δσ caused by the unit's interia
load at ultimate condition (ULS)
Constant Δσ caused by equipment
dynamics, such as vibrations
Shear connections
2
[N/mm ]
Bolts with cut threads
Bolts with cold
rolled threads
50
100
150
10
20
35
Guidance note:
The
Δσ listed represents approximately 50% of the allowable Δσ for a 20-years design life, see DNV-RP-C203.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 92
DNV AS
1 Temporary mooring, quayside mooring and towing equipment
1.1 Introduction
1.1.1 The requirements given in this section apply to equipment and its installation used for temporary
mooring, quayside mooring and towing.
For symbols not defined in this section, see Ch.1 Sec.1 [6.3].
1.1.2 The towing types that are covered by this standard are:
a)
b)
towing between different locations
emergency towing.
Other types of towing are not covered, e.g. towing assistance when manoeuvring into ports or towing
support for commissioning.
1.2 Temporary mooring
1.2.1 General
All offshore floating units shall have a temporary mooring arrangement, unless it is accepted by the verifier
that the unit is delivered without a temporary mooring arrangement, e.g. based on the type of unit or from
systems that may replace the temporary mooring arrangement (typically dynamic positioning systems with
high level of redundancy).
Guidance note:
Mooring requirements for permanently moored units, or units moored for longer periods are specified in DNV-OS-E301.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.2.2 Assumptions
The following assumptions are applicable for temporary mooring:
a)
b)
c)
The required anchoring equipment is the minimum required for temporary mooring of a unit in moderate
sea conditions when the unit is awaiting berth, tide, etc. The equipment is therefore not designed to hold
a vessel off fully exposed coasts in rough weather or for frequent anchoring operations in open sea. In
such conditions the loads on the anchoring equipment will increase to such a degree that its components
may be damaged or lost due to the high energy forces generated.
The required anchoring equipment is designed to hold a unit in good holding ground, avoiding situations
such as dragging of the anchor. In poor holding ground the holding power of the anchors is significantly
reduced.
It is assumed that the unit uses one anchor and chain cable at a time.
1.2.3 Structural arrangement for anchors
The following apply for the anchor arrangement:
a)
b)
c)
The anchors shall be housed in hawse pipes of a suitable size and form to prevent movement of the
anchor and chain caused by wave action.
The anchor arrangement shall provide an easy lead of the chain cable from the windlass to the anchors.
Upon release of the brake, the anchor shall immediately start falling due to its own weight.
At the upper and lower ends of hawse pipes, well rounded chafing lips shall be provided.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 93
DNV AS
Chapter 2 Section 6
SECTION 6 HULL EQUIPMENT AND SUPPORTING STRUCTURE
Anchor pockets or bell mouths shall be provided where the chain cable is supported during hoisting, and
when the unit is temporarily moored. Alternatively, roller fairleads of suitable design may be applied.
Guidance note:
The diameter ratio (Dc / dc/ > 6) between the curvature
pipe or bell mouth and the anchor chain cable diameter
Dc of the rounded parts at the lower end of anchor pocket, hawse
dc will normally provide sufficient contact length for the anchor chain.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
e)
f)
g)
The shell plating connected to the hawse pipes shall be increased in thickness, and the framing shall be
reinforced as necessary to ensure a rigid fastening of the hawse pipes to the hull.
For ship-shaped units with a bulbous bow, where it is not possible to obtain ample clearance between
shell plating and anchors during anchor handling, local reinforcements of the bulbous bow shall be made.
Provisions shall be made for securing the inboard ends of chain to the structure. This attachment shall
be able to withstand a force of greater than 15% but less than 30% of the minimum breaking strength
of the chain cable. The fastening of the chain to the unit shall be made in such a way that in case of an
emergency, the anchor and chain can be sacrificed. The chain shall be able to be readily released from
an accessible position outside the chain locker.
Guidance note:
Recognized standards for end fastening and release of anchors are:
i)
DIN 81860-1 and DIN-81860-2
ii)
CB/T 3143-2013
iii)
JIS F 2025.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.2.4 Structural arrangement for chain lockers
The following apply for the chain locker arrangement:
a)
b)
c)
d)
The chain lockers shall have adequate capacity and a suitable form to provide a proper stowage of the
chain cable, and an easy direct lead for the cable into the spurling pipes, when the cable is fully stowed.
Cables shall have separate spaces. Spurling pipes and chain lockers shall be watertight up to the weather
deck. The bulkheads separating adjacent chain lockers are not required to be watertight.
The chain lockers shall be closed by a substantial cover and secured by closely spaced bolts.
Where means of access to spurling pipes or cable lockers is located below the weather deck, the access
cover and its securing arrangements shall be equivalent to watertight manhole covers. Butterfly nuts
and/or hinged bolts are prohibited for securing the access cover.
Spurling pipes, through which anchor cables are lead, shall be provided with permanently attached
closing appliances to minimize water ingress. Drainage facilities of the chain lockers shall be included.
1.2.5 Selection of anchor and chain cables
The equipment number (EN) calculation for guidance on selection of anchors and chain cables shall be based
on the following formula:
where:
Δ
= moulded displacement [tonnes] in salt waters (
draft, unless otherwise specified
Α
= projected area [m ] of all the wind exposed surfaces, including deck cargo, above the unit's
maximum design draught. The most unfavourable orientation relative to the wind shall be used.
Shielding and solidification effects shall not be taken into account, unless documented.
3
= 1.025 t/m ) for the unit's maximum design
2
For units able to weathervane, i.e. units equipped with only one anchor, a heading angle against the incoming
weather of +/- 30 degree may be assumed, and the equipment number calculation can be adjusted (EN*) as
follows:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 94
DNV AS
Chapter 2 Section 6
d)
ΑF
ΑS
2
= projected exposed front area of the unit [m ] above the unit's maximum design draught
2
= projected exposed side area of the unit [m ] above the unit's maximum design draught.
From the calculated equipment number,
EN or EN*, selection of anchor and chain cable is given in Table 1.
Stockless
anchors
Stud-link chain cables
Total
1)
length
Number
Equipment number
Equipment letter
Table 1 Equipment for temporary mooring
Mass per
anchor
[kg]
Diameter [mm] per steel grade
NV K3
[m]
NV K1
NV K2
or
NV R3
720 to 779
NV
R3S
NV R4
NV
R4S
NV R5
s
2
2280
467.5
48
42
36
780 to 839
t
2
2460
467.5
50
44
38
840 to 909
u
2
2640
467.5
52
46
40
910 to 979
v
2
2850
495
54
48
42
980 to 1059
w
2
3060
495
56
50
44
1060 to 1139
x
2
3300
495
58
50
46
1140 to 1219
y
2
3540
522.5
60
52
46
1220 to 1299
z
2
3780
522.5
62
54
48
1300 to 1389
A
2
4050
522.5
64
56
50
1390 to 1479
B
2
4320
550
66
58
50
1480 to 1569
C
2
4590
550
68
60
52
1570 to 1669
D
2
4890
550
70
62
54
1670 to 1789
E
2
5250
577.5
73
64
56
54
50
1790 to 1929
F
2
5610
577.5
76
66
58
54
52
1930 to 2079
G
2
6000
577.5
78
68
60
56
54
2080 to 2229
H
2
6450
605
81
70
62
58
54
2230 to 2379
I
2
6900
605
84
73
64
60
56
2380 to 2529
J
2
7350
605
87
76
66
62
58
2530 to 2699
K
2
7800
632.5
90
78
68
64
60
2700 to 2869
L
2
8300
632.5
92
81
70
66
62
2870 to 3039
M
2
8700
632.5
95
84
73
68
64
3040 to 3209
N
2
9300
660
97
84
76
70
66
63
61
3210 to 3399
O
2
9900
660
100
87
78
73
68
65
63
3400 to 3599
P
2
10500
660
102
90
78
73
68
65
63
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 95
DNV AS
Chapter 2 Section 6
where:
Mass per
anchor
[kg]
Diameter [mm] per steel grade
NV K3
[m]
NV K1
NV K2
or
NV R3
NV
R3S
NV R4
NV
R4S
NV R5
3600 to 3799
Q
2
11100
687.5
105
92
81
76
70
67
65
3800 to 3999
R
2
11700
687.5
107
95
84
78
73
69
67
4000 to 4199
S
2
12300
687.5
111
97
87
81
76
72
70
4200 to 4399
T
2
12900
715
114
100
87
81
76
72
70
4400 to 4599
U
2
13500
715
117
102
90
84
78
74
72
4600 to 4799
V
2
14100
715
120
105
92
87
81
77
75
4800 to 4999
W
2
14700
742.5
122
107
95
90
84
80
78
5000 to 5199
X
2
15400
742.5
124
111
97
90
84
80
78
5200 to 5499
Y
2
16100
742.5
127
111
97
90
84
80
78
5500 to 5799
Z
2
16900
742.5
130
114
100
92
87
82
80
5800 to 6099
A*
2
17800
742.5
132
117
102
95
90
85
83
6100 to 6499
B*
2
18800
742.5
137
120
107
100
95
90
88
6500 to 6899
C*
2
20000
770
124
111
105
97
92
89
6900 to 7399
D*
2
21500
770
127
114
107
100
95
92
7400 to 7899
E*
2
23000
770
132
117
111
102
97
94
7900 to 8399
F*
2
24500
770
137
122
114
105
99
96
8400 to 8899
G*
2
26000
770
142
127
120
111
105
102
8900 to 9399
H*
2
27500
770
147
132
124
114
109
105
9400 to 9999
I*
2
29000
770
152
132
124
114
109
105
10000 to 10699
J*
2
31000
770
137
130
120
114
110
10700 to 11499
K*
2
33000
770
142
132
124
117
114
11500 to 12399
L*
2
35500
770
147
137
127
120
117
12400 to 13399
M*
2
38500
770
152
142
130
123
119
13400 to 14599
N*
2
42000
770
157
147
137
129
125
14600 to 16000
O*
2
46000
770
162
152
142
134
130
1)
The total length of chain cable required shall be equally divided between the two anchors. For units where one
anchor is accepted, the required anchor chain length may be similarly reduced, i.e. 50% of the required total length.
If steel wire rope is used the length shall be at least be 50% above the values given.
1.2.6 Requirements to anchor, anchor chain cables, windlass and chain stoppers
— The requirements for anchors are given in DNV-RU-SHIP Pt.3 Ch.11 Sec.1 [4].
— The requirements for anchor chain cables are given in DNV-RU-SHIP Pt.3 Ch.11 Sec.1 [5].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 96
DNV AS
Chapter 2 Section 6
Stud-link chain cables
Total
1)
length
Number
Equipment letter
Equipment number
Stockless
anchors
1.2.7 Anchor bolsters
For anchor bolsters the following apply:
a)
b)
The geometry of bolsters shall accommodate the anchor in a way which prevents sliding of the anchor
on the bolster. The bolster shall not solely rely on friction between the anchor and the bolster to prevent
sliding.
Bolsters and hull supporting structures shall be designed for a load equal to two times the winch load
applied to keep the anchors secured in the bolsters, with a maximum allowed equivalent stress σe:
where:
ReH = material yield stress of the bolster or hull support structure.
c)
d)
e)
Slamming loads acting on the anchor shall be included in the structural assessment of the bolster
structure, taking into account the relative velocity between the water particles and the unit's motion.
DNV-RP-C205 [8.6] may be used to calculate the slamming load. Unless particle velocity is found by
direct calculations, 7 m/s shall be applied.
Anchor bolsters shall be designed with a failure sequence where ultimately the bolster is torn from the
hull at connections which are defined as weak links.
If doubler plates are used as weak links, the weld between doubler and bolster structural members shall
be designed with inferior breaking strength compared with the weld to the hull. The welding between the
doubler plate and the side shell shall be aligned with internal structure to prevent hard points which may
act as initiation points for hull plate tear out.
Loss of single members of the bolsters shall not lead to a scenario where the anchor is impacting the
hull. A second barrier shall be present in case of member failure.
1.3 Quayside mooring
1.3.1 General
Offshore floating units that are designed to be moored along side shall have a quayside mooring arrangement
in compliance with this sub-section.
1.3.2 Mooring line arrangement
The following definitions are applicable for mooring lines, see Figure 1:
— breast line is a mooring line that is deployed perpendicular to the unit restraining the unit in the off-berth
direction
— spring line is a mooring line that is deployed almost parallel to the unit, restraining the unit in fore or aft
direction
— head and stern lines are mooring lines that is oriented between longitudinal and transverse direction,
restraining the unit in the off-berth and in fore or aft directions. The amount of restraint in the fore or aft
and off-berth directions depend on the line angle relative to these directions.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 97
DNV AS
Chapter 2 Section 6
— The requirements for windlass and chain stoppers are given in DNV-RU-SHIP Pt.3 Ch.11 Sec.1 [6].
1.3.3 Design minimum breaking load
The design minimum breaking load
MBLSC [kN] for the quayside mooring line (per line) shall be taken as:
where A1 is the side-projected area [m ] for the unit's at minimum draft. For units with small variation in
draft, the side-projected area may be based on actual operational draft.
2
— The wind shielding of the pier may be included in the calculation of A1, unless the unit is intended to be
regularly moored to jetty type piers. A pier surface height of 3 m over the waterline may be assumed.
— Topside and deck equipment for normal operation shall be included in the calculation of A1.
MBLSD is based on a design wind velocities of vW = 25 m/s. In case the mooring lines are intended for use of
other wind velocity vW*, the MBLSD* shall be adjusted as follows:
The wind velocity vW* shall not be taken as lower than vW* = 21 m/s.
1.3.4 Head, stern and breast lines
The total number of head, stern and breast mooring lines, see Figure 1, shall be taken as:
The total number of mooring lines shall be rounded to the nearest whole number.
The required number of mooring lines may be adjusted, i.e. increased or decreased, when the strength of the
individual lines are considered.
The adjusted strength,
MBL**, is given as:
, for increased number of lines
, for reduced number of lines
where:
or
, given in [1.3.3], as appropriate
where:
n** is the adjusted (increased or decreased) total number of lines.
n is the number of lines calculated without rounding.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 98
DNV AS
Chapter 2 Section 6
Figure 1 Mooring lines definition
— two lines where EN < 5000
— four lines where EN ≥ 5000.
The unit's design breaking load for spring lines shall be the sum equal to the individual mooring line strength
for head, stern and breast lines, see Figure 1. When the design breaking strength for the individual mooring
line is increased during the adjustment in [1.3.4], the number of spring lines shall be:
, rounded up to the nearest even number.
Where:
or
, as found applicable in [1.3.3].
ns is the number of spring lines
ns* is the increased number of spring lines.
1.4 Towing
1.4.1 General
1.4.1.1 All units shall have both a main towing arrangement and an emergency towing arrangement. Main
towing arrangements may be omitted for self-propelled units (i.e. units fit for independent transits).
1.4.1.2 The emergency towing arrangement shall have the same capacity as the main towing arrangement
given [1.4.1.1]. If the emergency towing arrangement differ from the main towing arrangement, the
emergency towing arrangement shall be additionally documented.
1.4.2 Towing arrangement
The following is required:
a)
b)
c)
d)
e)
f)
The emergency towing arrangement shall be possible to deploy during a blackout of the unit.
Bridle pennants shall consist of chain or steel wire ropes, or a combination of these, and have a clear
way from the fastening devices to the fairlead.
Chains shall be used in connection with chafing areas such as e.g. fairleads.
Fairleads shall be designed to accommodate the chafing chain and shall be shaped to prevent excessive
bending stress in the chain links.
There shall be arrangements for hang-off and retrieval of the unit's towing bridle(s) and towing
pennant(s).
Application of the unit's mooring arrangement may be used for emergency towing, provided the
arrangement is feasible and has equivalent strength.
A typical towing arrangement (main and emergency) is shown in Figure 2.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 99
DNV AS
Chapter 2 Section 6
1.3.5 Spring lines
Total number of spring lines, see Figure 1, shall not be taken less than:
Chapter 2 Section 6
Figure 2 Typical towing arrangement
1.4.3 Towing design load
1.4.3.1 The design load FD for the towing arrangement shall be based on the required towing force
for the unit when floating in its normal transit condition, including any deck cargo.
FT [kN]
The unit under tow shall be able to maintain position against a specified sea state, wind and current velocity
acting simultaneously, without the static force in the towing arrangement exceeding its towing design load.
1.4.3.2 As a minimum the following weather conditions shall be used:
U1 min, 10 = 20 m/s
a)
sustained wind velocity:
b)
current velocity: VC = 1 m/s
significant wave height: Hs = 5 m
c)
d)
zero up-crossing wave period: 6 s ≤
Tz ≤ 9 s.
Guidance note:
Environmental forces may be calculated according to DNV-RP-C205.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
FD to be used in the strength analyses for the towing bridles and pennants is
a function of the required towing force FT and the number of tugs to be used in the design given by:
1.4.3.3 The towing design load
where:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 100
DNV AS
= design load factor [kN]
ftow
=
NTug
= number of tugs to be used in the towing arrangement design.
NTug = 1
1.5/NTug, if NTug > 1
= 1.0, if
Guidance note:
It is advised that:
—
the towing design load for each towing bridle or pennant is not taken less than 100 tonnes
—
the towing arrangement is designed for use of a single tug.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.4.4 Towing equipment and fittings
The following requirements apply to the components of the towing arrangement and the unit's fittings
(fairleads and bridle/pennant attachments), and the supporting structures of the fittings:
a)
b)
c)
d)
e)
f)
The minimum breaking strength Smbs of the unit's towing bridle(s) and/or towing pennant(s), including
end connections (hard eyes and shackles), shall be three (3) times the towing design load FD.
The strength analysis shall be made using the most unfavourable directions for the bridle/pennant.
The nominal equivalent stress
σe in the triangular plate ≤ 0.9 times material's yield stress, when
subjected to Smbs.
Towline fittings (typically stong-points and fairleads) shall be designed for a load equal to the minimum
breaking strength Smbs.
The allowable nominal equivalent stress σe in the fittings shall be ≤ 0.9 times the material's yield stress.
Chain cables and shackles used in the towing arrangement shall be of R-quality, or ship chain quality K3,
see Table 1.
1.5 Safe towing load (TOW) and safe working load (SWL) for mooring and towing
equipment
1.5.1 General
The safe working load (SWL) and safe towing load (TOW) is the safe load limit to be applied for quayside
mooring and towing equipment. The intended use shall be stated on the towing and mooring arrangement
plan, as applicable.
1.5.2 Safe working load (SWL)
The safe working load (SWL) is the safe load limit of shipboard fittings used for the mooring purpose. The
following requirements for SWL apply for the use of one mooring line per fitting:
a)
b)
The SWL shall not exceed the minimum breaking strength of the mooring line (chain, wire, rope) used
for the quayside mooring.
The SWL (in tonnes) shall be marked on each shipboard fitting by weld bead or equivalent. When fittings
are used for both towing and mooring, the SWL [tonnes] shall be marked on the fittings, in addition to
TOW.
1.5.3 Safe towing load (TOW)
The safe towing load (TOW) is the safe load limit of the fittings used for the towing purpose. The following
requirements for TOW apply, assuming single tug boat assistance:
a)
b)
TOW shall not exceed 80% of the breaking strength, Smbs, given in [1.4.4].
The greater of the TOW for main towing and emergency towing shall be used.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 101
DNV AS
Chapter 2 Section 6
ftow
ftow
TOW [tonnes] shall be marked on each shipboard fitting by weld bead or equivalent. When fittings are
used for both towing and mooring, the SWL [tonnes] shall be marked on the fittings, in addition to TOW.
1.6 Supporting structure for mooring equipment and towing fittings
1.6.1 Structural arrangement
1.6.1.1 Equipment and fittings for towing and mooring, including winches and capstans, shall be located on
stiffeners and/or girders which are part of the deck structure, to efficiently distribute the load.
1.6.1.2 The deck strengthening shall be effectively arranged for any directional variation (horizontally and
vertically) of the design loads acting on the equipment and fittings, see Figure 3 for a typical arrangement.
The structural arrangement shall provide continuity of strength with proper alignment to the supporting hull
structure.
Figure 3 Typical arrangement for support of mooring equipment and towing fittings
1.6.1.3 The acting point of the towing/mooring forces on the deck fittings shall be taken at the attachment
point of the towing/mooring line or at the direction change location. For bollards the attachment point of
the line shall be taken no less than 4/5 of the tube height above the base, see Figure 4 and Figure 5 a) for
towing and mooring respectively.
If fins are fitted to the bollard tubes in order to keep the mooring line as low as possible, the attachment
point of the mooring lines may be taken at the location of the fins, see Figure 5 b).
Figure 4 Attachment point of the towing line
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 102
DNV AS
Chapter 2 Section 6
c)
Chapter 2 Section 6
Figure 5 Attachment point of the mooring line
1.6.2 Design loads
1.6.2.1 General
For equipment and fittings used for both temporary mooring and towing purpose, the support structure shall
be checked based on the greater of load from either mooring (SWL) or towing (TOW), take into account all
relevant angles for the mooring or towing purpose.
1.6.2.2 Temporary mooring, anchoring windlass and chain stopper
a)
The following design load cases shall be checked, as appropriate:
Windlass:
— Windlass where a chain stopper is provided: 45% of BS.
— Windlass where a chain stopper is not provided: 80% of BS.
Chain stopper:
— 80% of BS.
b)
c)
Where BS is the minimum breaking strength (BS) of the anchor chain cable.
Where a separate foundation is provided for the windlass brake, the distribution of the resultant forces
shall be calculated based on the assumption that the brake is applied for the load cases given in a).
Green sea loads shall be included, as applicable.
Guidance note:
Guidance for green sea design loads for ship-shaped units are given in DNV-RU-SHIP Pt.3 Ch.11 Sec.2 [2.5].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.6.2.3 Quayside mooring, winches and fittings
The design load shall be the greater of the following, as applicable:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 103
DNV AS
—
F = 1.25 times the intended maximum brake holding load, and not less than 0.8 times the MBLSD
calculated in [1.3].
— Capstans:
—
F = 1.25 times maximum hauling in force.
— Mooring fittings:
—
—
F = 1.15 times MBLSD calculated in [1.3].
F = SWL specified for the mooring fitting.
The most unfavourable operational direction of the mooring lines with interaction to the mooring fittings shall
be applied.
1.6.2.4 Towing fittings
The design load shall be the greater of the following, as applicable:
—
—
F = 1.25 times the minimum breaking load (Smbs) of the towing line connected to the towing fitting
F = TOW specified on the towing fitting.
The most unfavourable operational direction of the towing lines with relation to the towing fittings shall be
applied.
1.6.3 Structural categorization
The supporting structures for mooring and towing equipment/fittings are categorized as primary, see
Sec.3 [3]. The support structures shall extend minimum 0.5 m in vertical and horizontal direction from the
intersection line between the equipment and hull the structure.
1.6.4 Acceptance criteria
Strength assessments of support structures for mooring and towing fittings shall be checked using a FEanalysis with the following acceptance criteria:
— Yield control: 1.0 ReH (von Mises)
— Fine mesh yield control: local peak stress requirements are given in Sec.4 [3.3.3]
— Buckling control: ηall = 1.0
— Welds according to Sec.5 [3]. For bolted connections, the requirements in Sec.5 [4] apply.
If the mooring equipment or towing fitting is prone to fatigue damage, e.g. from frequent use, it shall be
documented to be suitable for the intended use.
2 Support structure for permanent mooring/position mooring
equipment
2.1 General
2.1.1 Units permanent moored or positioned moored at a location for a shorter or longer period, may have
one of the following mooring systems:
— A turret system inside the hull of the unit that enables the unit to weathervane. Typically used for shipshaped FPSOs.
— A turret system outside the unit, e.g. a yoke structure, that enables the unit to weathervane. Typically
used for ship-shaped FPSOs.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 104
DNV AS
Chapter 2 Section 6
— Mooring winches:
Guidance note:
Permanent mooring/position mooring shall comply with the requirements given in DNV-OS-E301.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.1.2 The supporting structures for permanent mooring systems are categorized as special, see Sec.3 [3],
and shall be inspected according to requirements in category IC-I. The support structures shall extend
minimum 0.5 m in vertical and horizontal direction from the intersection line between the mooring equipment
and hull structure.
2.2 Design loads
2.2.1 General
The design loads for the mooring line system shall be according to a recognized standard.
2.2.2 Spread mooring
For spread mooring systems, the minimum breaking load (MBL) of the mooring lines shall be used, assuming
the most unfavourable relative angle between the unit and the anchor line, see Figure 6.
Figure 6 Fairlead with vertical inlet angle (γ) and horizontal working angle (φ)
The loads from the mooring lines shall be combined with global and local hull loads as found relevant.
When several mooring lines are connected to a cluster, the minimum breaking load (MBL) for the mooring
line shall be used for one individual mooring line at a time. The remaining part of the cluster force (cluster
force minus MBL of one line) shall be distributed on the remaining lines in the cluster according to their
direction relative to the total cluster force for the ULS condition. The most unfavourable directions for the
cluster forces shall be checked.
In addition, the accidental case (ALS) with one mooring line broken shall be checked. The cluster force shall
then be distributed on the remaining lines in the cluster. The most unfavourable directions for the cluster
forces shall be checked.
2.2.3 Turret mooring
For a turret mooring system, the unit will weathervane against the incoming waves and the turret loads shall
be determined based on a mooring analysis where a weather influx sector is considered.
The extreme 100-years restoring loads from the turret, provided by the turret designer, shall be used and
combined with the unit's global and local loads.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 105
DNV AS
Chapter 2 Section 6
— A spread mooring system, where the unit is moored by clusters of mooring lines, normally at the unit's
sides fore and aft. Used for e.g. column-stabilized units, cylindrical units and ship-shaped units FPSOs
(conversions).
Chapter 2 Section 6
2.3 Acceptance criteria
2.3.1 Design conditions
The design conditions specified in [2.3.2] to [2.3.5] shall be analysed.
2.3.2 Survival condition (ULS)
Survival condition (ULS) based on 100-years restoring loads:
— yield control: 0.8ReH (von Mises)
— fine mesh yield control local peak stress requirements are given in Sec.4 [3.3.3]
— buckling control: ηall = 0.8.
2.3.3 Accidental condition (ALS)
Accidental condition (ALS) with one or more mooring line broken line scenario:
— yield control: 1.0ReH (von Mises)
— fine mesh yield control: Local peak stress requirements are given in Sec.4 [3.3.3]
— buckling control: ηall = 1.0.
2.3.4 Welds and bolts
Welds shall be checked according to Sec.5 [3]. Bolt connections requirements are given in Sec.5 [4].
2.3.5 Fatigue
Hull strength supporting details for mooring shall be designed with soft brackets to avoid stress
concentrations.
Time simulations of the combined stress process from the hull motions and the mooring loads shall be
applied. Alternatively, the fatigue damage from hull motions and mooring may be calculated separately and
combined using the following formula:
where:
D1
D2
ν1
ν2
m
= calculated fatigue damage for the high cycle fatigue (hull motions)
= calculated fatigue damage for the low cycle fatigue (mooring loads)
= mean zero up crossing frequency for the high cycle fatigue (hull motions)
= mean zero up crossing frequency for the low cycle fatigue (mooring loads)
= negative inverse slope of the S-N curve.
3 Heavy equipment, deck machinery, marine equipment and topside
module foundations and supports
3.1 Application
3.1.1 Supporting structures and foundations for heavy equipment
This subsection applies to supporting structures and foundations for heavy equipment, deck machinery,
independent tanks and topside modules.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 106
DNV AS
— Proper alignment of foundations and supporting hull structure.
— Support positions shall be arranged so that the attachment to the unit's structure is clear of stress
concentrations caused by e.g. openings and bracket toes.
— Design of supports shall be such that the attachment to the hull minimises the creation of hard points.
3.2 Structural categorisation
3.2.1 The supporting structures of heavy equipment and topside modules are categorized as primary, see
Sec.3 [3]. The support structures shall extend minimum 0.5 m in vertical and horizontal direction from the
intersection line between the deck level and the equipment support foundation.
3.2.2 Fatigue critical details, see Sec.3 [3.3.1], shall be inspected according to IC-I, see Sec.3 [3.3.5] and
Sec.3 [3.3.6].
3.3 Design loads
3.3.1 Design loads shall take into account all relevant design conditions given in Sec.1 [2].
3.3.2 The reaction loads from the equipment or topside modules shall be combined with the unit's global hull
responses.
3.3.3 Winches and other pulling accessories
The strength of the supporting structures shall fulfil the strictest of the following design loads as found
relevant:
a)
b)
c)
d)
Design load to be given by the respective SWL times dynamic coefficient, ψ, as specified by designer.
shall however not be taken less than 1.3.
Design load to be given by the force in the rope causing the brake to render.
For winches with constant tension control, design load to be taken as 1.1 times the maximum pulling
force.
For winches, e.g. trawl winches, where the rope/equipment can get stuck on the sea bottom or other
structures, the design load shall be equal to the breaking load of the rope.
ψ
3.4 Acceptance criteria
Acceptance criteria for:
a)
b)
c)
d)
FE-analyses according to the structural requirements for yield and buckling, see Sec.4.
Welds, see Sec.5 [3].
Bolt connections, see Sec.5 [4].
Fatigue requirements, see Sec.4 [6].
4 Support for lifting appliances and crane pedestals
4.1 Application and definition
4.1.1 The requirements given in this subsection applies to support of:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 107
DNV AS
Chapter 2 Section 6
3.1.2 Structural arrangement
For structural alignment the following shall be ensured:
The overturning moment is the maximum bending moment, calculated at the connection of the lifting
appliance to the unit considering:
— the lifting appliance safe working load (SWL)
— lifting gear, outreach, self-weight, dynamic amplification factors, list and heel, and environmental loads
— offlead/sidelead and safety factors.
4.1.2 The strength of supporting structures for lifting appliances, including those with lower safe working
loads/overturning moments, shall be confirmed with a load test according to applicable standards.
4.1.3 The requirements given in this subsection do not cover the following:
a)
b)
c)
Supports of lifting appliances for personnel or passengers, except supporting structure for life saving
appliances.
Holding down bolts and their arrangement, which are part of the lifting appliance.
The crane pedestal flange and bolts or the lifting gear.
4.2 Structural categorisation
4.2.1 For shipboard/platform and offshore cranes material and inspection categorisation for the crane
pedestal and its supporting structure are defined in Table 2. The extension area for the crane pedestal
support structure includes:
— the deck plate(s)
— support brackets to deck plate, if used
— minimum 0.5 m in vertical and horizontal direction from the intersection line between the pedestal
intersection points at the deck level.
Table 2 Material and inspection category
Crane pedestal
Crane pedestal support structure
Material
categorisation
Inspection
categorisation
Material
categorisation
Inspection
categorisation
Shipboard/platform crane
Primary
IC-II
Primary
IC-II
Offshore crane
Primary
IC-II
Special
IC-I
Crane type
Pedestal transitions (e.g. cylindrical shape to conical shape) shall be fabricated according to tolerances for structural
category special, and inspected according to IC-I.
4.2.2 When a pedestal is not continuous through the deck plate, materials with lamellar tearing properties
(Z-quality) shall be used for the deck plate, see Sec.3 [4.1.4].
4.3 Structural arrangement
4.3.1 Heavy loaded crane pedestals should be supported by a minimum of two decks or stringer levels.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 108
DNV AS
Chapter 2 Section 6
— Cranes and other lifting appliances where the safe working load is greater than 30 kN, and/or where the
maximum overturning moment is greater than 100 kNm.
— Appliances for launching and recovery of lifesaving equipment and work boats regardless of safe working
load and overturning moment.
4.4 Design loads
4.4.1 The structural strength of the supporting structure (including pedestal) shall be based on a design load
consisting of the working load W (SWL plus the weight of the lifting gear) multiplied by the dynamic factor ψ
(specified by the crane designer) plus the self weight, including the weight of any lifting gear. However, the
dynamic factor shall not be taken less than the following:
For cranes when operating in harbour:
–
ψ = 1.3.
For cranes when operating offshore:
ψ = 1.3 for 10 kN < W ≤ 2500 kN
– ψ = 1.1 for W > 5000 kN.
–
Linear interpolation shall be used for values of
W between 2500 kN and 5000 kN.
For offshore cranes, where the operator cabin is attached above the slewing ring, the design loads for the
supporting structure shall be multiplied with an additional offshore safety factor SF1 as follows:
— When the crane lifting capacity
— When the crane lifting capacity
W ≤ 2500 kN, SF1=1.3 shall be applied
W > 2500 kN, SF1=1.1 shall be applied
The crane supplier shall provide information regarding the crane lifting loads and reaction loads at the
connection to the pedestal to enable suitable design of the supporting structure.
4.4.2 For life saving appliances, e.g. davit and winch supporting structures for life boats, life rafts and manoverboard boats, design loads shall be taken as 2.2 times SWL.
Guidance note:
Using a dynamic factor
ψ of 2.2 with the acceptance criteria of 0.67 given in [4.4.6] is found to comply with the requirement in the
LSA code Sec.6.1.1.6 (a safety factor of 4.5 towards the material ultimate tensile strength).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.4.3 For man-overboard boat davits, the supporting structure shall also be designed to withstand a
horizontal towing force.
4.4.4 Inertia loads
Inertia loads from the unit's motions shall be included, both when the crane is in operation and when the
crane is stowed. For operation, the maximum operation criteria (e.g max Hs) shall be used. In stowed
condition, the 100-year environmental loads given in Sec.2 [2] shall be applied.
4.4.5 Wind and ice
Wind and ice loads shall be included, as applicable.
Standard ice loads for North Sea winter conditions may be taken as 5 cm ice deposits for wind and weather
exposed surfaces.
Wind loads shall be based on 1 minute wind velocity.
4.4.6 Acceptance criteria
Strength assessment of the supporting structure shall be checked for the following acceptance criteria:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 109
DNV AS
Chapter 2 Section 6
4.3.2 Full penetration welds shall be used between the aligned structure above and below the deck plate.
Structural continuity shall be ensured.
Welds shall be checked according to Sec.5 [3]. Bolting connections are given in Sec.5 [4].
The acceptance criteria offshore crane supports is aligned with the acceptance criteria for load case I given in
DNV-ST-0378 [4.3]. If other load cases, i.e. load case II or load case III, present in DNV-ST-0378 [4.2] are
found to be governing, the acceptance criteria given in DNV-ST-0378 [4.3.2] and DNV-ST-0378 [4.3.3] for
yield and buckling shall be used.
For cranes mounted on high pedestals, and where the crane operator is positioned above the slewing
ring, the SLS condition due to the deflection from crane lifts should be evaluated in accordance with the
requirements given in Sec.4 [5].
4.4.7 Fatigue
4.4.7.1 Application
Fatigue control of crane support structures and crane pedestals shall be carried out for:
— offshore cranes
— other lifting appliances if the design is deemed to be fatigue prone.
4.4.7.2 Fatigue damage from crane operation
Cranes subject to different design cycles (class of utilization Ν) and load spectra kp, depending on crane type
and the intended operation, are given in Table 3. Other values may be used if specified by the crane designer.
For further details reference is made to DNV-ST-0378.
Table 3 Design cycles and load spectra for different crane types
Crane type
Number of cycles [Ν]
Spectrum [kp]
Jib crane for container service
200 000
0.66
Shipboard crane
200 000
0.66
Offshore crane whip hoist
200 000
0.66
Offshore crane main hoist
63 000
0.33
Provision crane
63 000
0.33
Based on
N and kp the fatigue damage from crane operation is given by:
where:
= intercept of the applicable SN curve given in DNV-RP-C203
a1
σmax = maximum stress range at the hotspot.
Guidance note:
The dominating operating angle for the crane operation may be used as the basis for the stress range
σmax.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 110
DNV AS
Chapter 2 Section 6
— yield control: 0.67ReH (von Mises)
— fine mesh yield control: local peak stress of 1.0ReH shall be applied
— buckling control: ηall = 0.67.
where:
Td
a1
T0
Δσ0
n0
m
Γ
= design life in seconds
= intercept of SN curve, see e.g. DNV-RP-C203
= zero-crossing period
= maximum stress range (MPa) from n0 cycles
= number of cycles for actual stress
= slope of SN-curve, for most curves m = 3
= gamma function, Γ(4) = 6.0.
4.4.7.4 Total fatigue damage
The total fatigue damage DF shall include contributions from both operation DO and stowed conditions DS.
The fatigue damage from the crane operation and when stowed shall be assumed to be uncorrelated, unless
otherwise documented.
DF = DO + DS
4.4.8 Calculation of hot-spots stress
The relevant hot spots may be based on tabulated values when applicable given in e.g. DNV-RP-C203, or
calculated values from a local FE-analysis, see Figure 7.
Figure 7 Crane pedestal fine mesh for hot spot calculation
4.4.9 Crane boom rest
The structural design of the support structure for crane boom rests shall comply with the acceptance criteria
given in Sec.4, applying loads specified by the crane designer.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 111
DNV AS
Chapter 2 Section 6
4.4.7.3 Fatigue damage in stowed condition
Fatigue damage caused by inertia loads of the unit may be calculated using a long term Weibull shape
parameter for the stresses found from a wave load analysis. Alternatively a Weibull shape parameter of 1.0
may be applied. If linear cumulative fatigue damage is assumed, the expression for the fatigue damage is
given as:
If reaction forces are not specified by the crane designer, inertia loads based on 0.35 g in all directions is a conservative approach,
when applied simultaneously.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 112
DNV AS
Chapter 2 Section 6
Guidance note:
1 Basis for all DNV unit specific types
1.1 General
For new designs or unproven designs where limited or no direct experience exists, additional analyses as
relevant and model testing shall be performed in order to demonstrate that an acceptable level of safety is
obtained.
Guidance note:
Flag and shelf states may specify requirements for a unit type depending on the size, type, location and intended operation.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2 Ship-shaped and cylindrical units
Specific strength requirements of ship-shaped and cylindrical units, i.e:
—
—
—
—
—
—
floating production and storage unit (FPSO)
oil storage unit (FSU)
floating liquefied natural gas unit (FLNG)
drill ship
well intervention ship
cylindrical units
are given in DNV-OS-C102.
3 Column stabilised units
Specific strength requirements for column stabilized units are given in DNV-OS-C103.
4 Self-elevating units (including self-elevated wind turbine
installation units)
Specific strength requirements for self-elevating objects are given in DNV-OS-C104.
5 Tension leg platforms
Specific strength requirements for tension leg platforms are given in DNV-OS-C105.
6 Deep draught units
Specific strength requirements for deep draught objects are given in DNV-OS-C106.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 113
DNV AS
Chapter 2 Section 7
SECTION 7 SPECIAL PROVISIONS FOR DNV UNIT TYPES
SECTION 1 PROCEDURAL REQUIREMENTS
1 Introduction
1.1 Introduction
As well as representing the Society's recommendations on safe engineering practice for use by the offshore
industry, the offshore standards also provide the technical basis for the Society's classification, certification
and verification services.
This section specifies the design documentation, certification, fabrication and survey requirements to be
applied when using this standard for certification and classification purposes.
A complete description of the principles, procedures, applicable class notations and technical basis for
offshore classification is given by the applicable rules for offshore classification listed in Table 1.
Table 1 DNV rules for classification - offshore units
Document code
Title
DNV-RU-OU-0101
Offshore drilling and support units
DNV-RU-OU-0102
Floating production, storage and loading units
DNV-RU-OU-0103
Floating LNG/LPG production, storage and loading units
DNV-RU-OU-0104
Self-elevating units
1.2 Application
1.2.1 The Society may accept alternative solutions found to represent an overall safety level equivalent to
the requirements given in Ch.2 of this standard.
1.2.2 Any deviations and exceptions to the design codes and standards that are recognized shall be
approved by the Society.
1.3 Basic hull classification scope
The basic classification requirements for hull strength are given in Ch.2 of this standard.
Guidance note:
For full classification hull scope, see DNV rules for classification listed in Table 1.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.4 Documentation requirements
The basic structural documentation that shall be submitted to the Society for the main class notation 1A and
OI are given in Table 2. A documentation list shall be created and agreed for each unit type and each specific
project , i.e. a DNV unit specific document required list shall be created based on the items in Table 2.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 114
DNV AS
Chapter 3 Section 1
CHAPTER 3 CLASSIFICATION AND CERTIFICATION
Object
Documentation type
Additional description
Info
General
arrangement
Z010 - General arrangement plan
FI
Design basis
H010 - Structural design brief
FI
H020 - Design load plan
FI
Design loads
H084 - Wave load analysis
For details, see Ch.2 Sec.2.
FI
Tank plan
H030 - Tank and capacity plan
FI
Material and
inspection
H040 - Structural categorisation plan
AP
H041 - Structural inspection plan
AP
Structural
drawings
H050 - Structural drawing
Drawings showing main hull dimensions
and scantlings for the unit, including LQ,
superstructure, etc, as found applicable.
AP
Foundations
H053 - Foundation and support structure
Foundations of heavy equipment and heavy
deck equipment, see Ch.2 Sec.6 [3].
AP
Structural
drawings
H070 - Standard details
AP
Strength,
fatigue and
wave load
analyses
H080 - Strength analysis
FI
H081 - Global strength analysis
FI
H084 - Wave load analysis
For details, see Ch.2 Sec.2.
H085 - Fatigue analysis
Main
structure
Pedestal
and support
for offshore
cranes
FI
FI
H090 - Model test documentation
Mainly required for new and novel designs.
FI
H120 - Docking arrangement plan
As found applicable.
AP
H134 - Hole and penetration plan
A drawing or document showing position, type
and size and of all holes and cut-outs in the
hull structure, including edge reinforcements
where relevant.
AP
H140 - Welding tables
If not specified on the structural drawings.
AP
H200 - Access manual
Plan or drawings showing access arrangement
and access openings with requirement to safe
access to tanks, cofferdams and other spaces
within the whole unit.
Z030 - Arrangement plan
Position/locations, crane reaction forces at
sleewing ring and top of boom rest.
FI
H053 - Structural drawing
Crane pedestal and connection to hull support
structure, incl. material type/grade, scantling
and weld type.
AP
H080 - Structural analysis
FI
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 115
DNV AS
Chapter 3 Section 1
Table 2 Basic structural documentation (H) to be submitted for offshore floating units
Documentation type
Additional description
Applicable for OI units or 1A units without
propulsion.
Support
structure for
permanent
mooring
Z030 - Arrangement plan
Sacrificial
nodes
M050 - Cathodic protection specification,
calculation and drawings
H050 - Structural drawing
H080 - Structural analysis
FI
AP
FI
In ballast tanks.
Z030 - Arrangement plan
Quayside
mooring
Info
FI
AP
C030 - Detailed drawing
Fastening of anodes in ballast tanks.
AP
Z030 - Arrangement plan
Applicable for SOLAS compliant units, and
shall cover a plan providing information for
each item regarding:
FI
— location on the unit
— fitting types
— relevant industry standards for fittings
— dimensions
— safe working load (SWL)
— maximum breaking strength of the
mooring lines
— manner of applying the lines including
limiting fleet angles.
H100 - Equipment number calculation
Equipment number for quayside mooring.
AP
Supporting
structure of
shipboard
fittings used
for quayside
mooring
H050 – Structural drawing
Including foundation above deck and fixation
to deck and supporting structure below deck.
Mooring fitting devices (if provided) shall be
included.
AP
Temporary
mooring
(anchoring)
Z030 - Arrangement plan
Including main dimensions and design loads
(SWL, equipment weight, brake rendering load
and chain breaking load) and foot print loads.
FI
Z090 - Equipment list
Covering windlasses, anchors, grade of anchor
chain, type and breaking load of chain, wire
and fibre ropes.
AP
H100 - Equipment number calculation
Equipment number for temporary mooring
(anchoring), see Ch.2 Sec.6 [1.2].
FI
C010 – Design criteria
Applicable if different from standard or
previously approved design.
Anchor
H080 - Structural analysis
FI
C030 – Detailed drawing
Anchor chain
stopper
FI, TA
AP, TA
C040 – Design analysis
FI, TA
C010 – Design criteria
FI, TA
C020 – Assembly or arrangement drawing
FI, TA
C030 – Detailed drawing
AP, TA
C040 – Design analysis
FI, TA
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 116
DNV AS
Chapter 3 Section 1
Object
Anchor
windlass
Anchor
windlass
supporting
structure
Documentation type
Additional description
C010 – Design criteria
FI, TA
C020 – Assembly or arrangement drawing
FI, TA
C040 – Design analysis
FI, TA
C050 – Non-destructive testing (NDT) plan
AP, TA
C030 – Detailed drawing
AP, TA
H050 – Structural drawing
Including foundation above deck and fixation
(bolts and shear stoppers), supporting
structure below deck, SWL, equipment weight,
brake rendering load, chain/wire breaking load
and foot print loads.
H080 - Structural analysis
Anchor chain
stopper
supporting
structure
AP
FI
H050 – Structural drawing
Including foundation above deck and fixation
(bolts and shear stoppers), supporting
structure below deck, and equipment weight,
chain breaking load and foot print loads.
H080 - Structural analysis
Towing
arrangements
Info
AP
FI
Z030 – Arrangement plan
Plan providing the following:
FI
— location of towing equipment and fittings
on the unit
— fitting types
— relevant industry standards for fittings
— dimensions of towing equipment and
fittings
— safe towing load (TOW) = FT
— maximum breaking strength of the
equipment
— manner of applying the lines including
limiting fleet angles.
— weather conditions used as basis.
Emergency
towing
procedure
Z250 – Procedure
Covering DNV and applicable statutory
requirements.
FI
Supporting
structure for
shipboard
fittings
associated
with towing
operations,
including
emergency
towing
H050 – Structural drawing
Including foundation above deck and fixation
to deck including the supporting structure
below deck. Fittings and towing devices (if
provided) shall be included.
AP
H080 - Structural analysis
FI
AP = for approval, FI = for information, TA = type approval
For requirements to documentation, including definitions for the info codes, see DNV-CG-0550 Sec.6.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 117
DNV AS
Chapter 3 Section 1
Object
1.5 Required compliance documentation
1.5.1 Required compliance documentation for anchors, anchor windlass, chain stoppers and
anchor equipment
Compliance documentation shall be submitted as required by Table 3 through Table 6.
Table 3 Compliance documentation for anchors, anchor windlasses, chain stoppers and anchor
equipment
Object
Hull and hull
appendages
Compliance
document type
MC
Issued by
Society
Compliance standard
1)
Additional description
Including:
— rolled steel and
aluminium for hull
structures
— forgings, castings
and other materials
for special parts and
equipment, including
sunken bits fitted in side
shells.
See also DNV-RU-SHIP Pt.3
Ch.3 Sec.1 [1].
Mooring fittings
MD
Manufacturer
Anchors
PC
Society
MC
Society
PC
Society
MC
Society
Anchor chains
Quayside mooring fittings,
as found applicable.
Including accessories, e.g.
swivels.
Content shall include the
following:
— grade of chain, method
of manufacture,
condition of supply and
reference to material
certificate
— results of proof load
test, breaking load test
and, where applicable,
mechanical tests
— identification marking.
MC
Society
Bars for K2 and K3 anchor
chain cable.
Anchor steel wire
ropes
MD
Manufacturer
Anchor fibre ropes
MD
Manufacturer
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 118
DNV AS
Chapter 3 Section 1
For definitions of the documentation types, see DNV-CG-0550 Sec.5.
Anchor chain joining
shackles
Anchor windlasses
Compliance
document type
Issued by
PC
Society
MC
Society
PC
Society
MC
Society
Compliance standard
1)
Additional description
Cable lifter
Drum
Shaft
Clutch
Brake
Gear
Anchor chain stoppers
MD
Manufacturer
Frame
MC
Society
Material declaration
issued by manufacturer is
acceptable if type approved.
Associated electrical equipment (e.g. motors, power transformers, semi-conductor assemblies, electrical assemblies)
serving the anchor windlasses shall be delivered with compliance documents in accordance with DNV-RU-SHIP Pt.4 Ch.8
Sec.1 [1.7].
1)
Unless otherwise stated the compliance standard is the DNV rules.
PC = product certificate, PD = product declaration, MC = material certificate, MD = material declaration
For definitions of the compliance document types, see DNV-CG-0550 Sec.3 and DNV-RU-SHIP Pt.2 Ch.1
Sec.2 [4].
1.5.2 Required compliance documentation for towing equipment
Compliance documentation shall be submitted as required by Table 4. See also Ch.2 Sec.6 Figure 2 for
definitions.
Table 4 Compliance documentation for towing and towing equipment
Object
Smit bracket
Fairlead
Compliance
document
type
Issued by
PC
Society
MC
Manufacturer
PC
Society
MC
Manufacturer
Compliance standard
1)
Additional description
Ch.2 Sec.6 [1.4.4] and
Ch.2 Sec.6 [1.5.3]
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 119
DNV AS
Chapter 3 Section 1
Object
Studlink chain
Compliance
document
type
PC
MC
Issued by
Society
Compliance standard
1)
DNV-OS-E302, DNVOS-E303 or DNV-OSE304, as applicable.
Additional description
Content shall include the following:
— grade of chain, method of
manufacture, condition of supply
and reference to material certificate
Manufacturer Ch.2 Sec.6 [1.4.4],
Ch.2 Sec.6 [1.5.3]
— results of proof load test, breaking
load test and, where applicable,
mechanical tests
— identification markings.
Bridle leg
PC
Society
MC
Manufacturer
Ch.2 Sec.6 [1.4.4],
Ch.2 Sec.6 [1.5.3]
Including shackles
Content shall include the following:
DNV-OS-E304
— grade of steel wire, method of
manufacture, condition of supply
and reference to material certificate
— results of proof load test, breaking
load test and, where applicable,
mechanical tests
— identification markings.
Triangular plate
Pennants
PC
Society
MC
Manufacturer
PC
Society
MC
Manufacturer
Ch.2 Sec.6 [1.4.4],
Ch.2 Sec.6 [1.5.3]
Ch.2 Sec.6 [1.4.4],
Ch.2 Sec.6 [1.5.3]
Including shackles
Content shall include the following:
DNV-OS-E304
— grade of steel wire/chain, method
of manufacture, condition of supply
and reference to material certificate
— results of proof load test, breaking
load test and, where applicable,
mechanical tests
— identification markings.
1)
Unless otherwise stated the compliance standard is the DNV rules.
PC = product certificate, PD = product declaration, MC = material certificate, MD = material declaration
For definitions of the compliance document types, see DNV-CG-0550 Sec.3 and DNV-RU-SHIP Pt.2 Ch.1
Sec.2 [4].
1.5.3 Required compliance documentation for internal watertight doors and shell doors
Compliance documentation shall be submitted as required by Table 5.
Table 5 Compliance documentation for internal watertight doors and shell doors
Object
Compliance
document
type
Internal watertight doors
PC
Issued by
Compliance standard
1)
Additional description
Society
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 120
DNV AS
Chapter 3 Section 1
Object
Compliance
document
type
Issued by
Internal watertight doors
control and monitoring
system
PC
Society
Shell doors control and
monitoring system
PC
Society
1)
Compliance standard
1)
Additional description
Unless otherwise stated the compliance standard is the DNV rules.
PC = product certificate, PD = product declaration, MC = material certificate, MD = material declaration
For definitions of the compliance document types, see DNV-CG-0550 Sec.3 and DNV-RU-SHIP Pt.2 Ch.1
Sec.2 [4].
1.5.4 Material
1.5.4.1 All materials shall be delivered with a DNV certificate. However, for doublers and pipe sleeves the
criteria in [1.5.4.2] and [1.5.4.3] may be applied as an alternative.
1.5.4.2 For doublers used for the mounting of outfitting steel to the hull structure:
— The material of the doubler when welded to a material category secondary, non DNV certified material
may be accepted by the Society upon special consideration. The material shall then be equivalent to a
material within the same strength group as it is welded to. The material shall be delivered with a 2.2
certificate.
— The material of the doubler when welded to a material category primary, non DNV certified material may
be accepted upon special consideration. The material shall then be equivalent to a material within the
same strength group as it is welded to. The material shall be delivered with a 3.1 material certificate, from
a DNV approved steel mill.
— The material of the doubler when welded to a material category special, shall be certified by the Society.
— Use of stainless steel materials for doublers is accepted by DNV provided:
— proper coatings are used for the base material to avoid corrosion
— welding of the doubler to the base material is carried out with a suitable electrode, e.g. 309L.
Guidance note:
For definitions of material categories, see Ch.2 Sec.3 [3.2].
For definition of material certificates, see e.g. ISO 10474 or EN 10204.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.5.4.3 For pipe sleeves used in connection with penetrations in secondary structures, with the exception of
bulkheads/decks/shell plating:
— For the material of the pipe sleeves, the Society may accept materials not certified by the Society upon
special consideration. The sleeve material shall be equivalent to the material it is welded to and shall be
delivered with a 2.2 material certificate.
For pipe sleeves used in connection with penetrations pipe transitions through primary structures and
bulkheads/decks/shell plating:
— For the material of the pipe sleeves, the Society may accept materials not certified by the Society upon
special consideration.. The sleeve material shall be equivalent to the material it is welded to, and shall be
delivered with a 3.1 material certificate from a DNV approved steel mill.
Pipe penetrations shall not be used in special areas.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 121
DNV AS
Chapter 3 Section 1
Object
Table 6 Compliance documentation for bolt and bolt connections
Object
Compliance
document
type
Bolt category A
Issued by
Compliance standard
1)
Additional description
MC
Manufacturer Ch.2 Sec.5 [4]
2.2 certificate according to a recognized
standard e.g. ISO 898 Part 1.
MC
Manufacturer Ch.2 Sec.5 [4]
3.1 certificate according to a recognized
standard e.g. ISO 898 Part 1.
— Bolts not exposed
to tension stresses,
or when tension
stresses are less than
25% of the bolts
material yield stress.
— Connection is
redundant, i.e.
no single point
(connection) of
failure may cause
failure of the
structure.
Bolt category B
— Bolts used in
non-redundant
applications.
— Bolts that cannot
be categorized as
category A.
1)
Unless otherwise stated the compliance standard is the DNV rules.
PC = product certificate, PD = product declaration, MC = material certificate, MD = material declaration
For definitions of the compliance document types, see DNV-CG-0550 Sec.3 and DNV-RU-SHIP Pt.2 Ch.1
Sec.2 [4].
2 Classification requirements for technical requirements in Ch.2
2.1 Design principles
Additional classification requirements for design principles specified in Ch.2 Sec.1 are given in Table 7.
Table 7 Design principles
Subsection
Ch.2 Sec.1 [1.2]
Title
Limit state design principle
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Classification requirement
Class scope does not cover the SLS
condition.
Page 122
DNV AS
Chapter 3 Section 1
1.5.5 Bolt and bolt connections
Compliance documentation related to bolts shall be submitted as required by Table 6.
Title
Classification requirement
Ch.2 Sec.1 [1.3]
Design method
The Society accepts both LFRD and
WSD methods for the structural
calculations of all unit types, provided
the selected design method is
consistently applied. Relevant DNV unit
specific standard may require use of
a specific design method with specific
loads and acceptance criteria.
Ch.2 Sec.1 [2.1]
Design load conditions
Design conditions related to temporary
phases. e.g. fabrication, lifting,
commissioning and installation at
location are not part of the Society's
class scope.
Ch.2 Sec.1 [2.3.1]
Single transit to location
For units intended for single tow, the
towing execution is not part of the
Society's class scope.
2.2 Load and load effects
Additional classification requirements for load and load effects specified in Ch.2 Sec.2 are given in Table 8.
Table 8 Load and load effects
Subsection
Title
Classification requirement
Ch.2 Sec.2 [2.2.1]
Deck load plan
A deck load plan shall be submitted as
listed in Table 2.
Ch.2 Sec.2 [2.3.9]
Earthquake
Earthquake loads and effects are not
covered in the Society's basic class
scope.
2.3 Material selection and inspection principles
Additional classification requirements for Material selection and inspection principles specified in Ch.2 Sec.3
are given in Table 9.
Table 9 Material selection and inspection principles
Subsection
Ch.2 Sec.3 [4.3.3]
Title
Material certificate
Classification requirement
For units classed by the Society, the
material shall be delivered with a
material certificate, see [1.5.4].
2.4 Strength assessment
Additional classification requirements for Strength assessment specified in Ch.2 Sec.4 are given in Table 10.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 123
DNV AS
Chapter 3 Section 1
Subsection
Subsection
Ch.2 Sec.4 [6.4.1]
Title
Stochastic fatigue analysis
Classification requirement
The class notation FMS(Year)
requires stochastic fatigue analysis, see
DNV-OS-C102 Ch.3 Sec.1 [2.1].
2.5 Weld and bolt connections
There are no additional classification requirements for weld and bolt connections specified in Ch.2 Sec.5.
2.6 Hull equipment and supporting structure
Additional classification requirements for hull equipment and supporting structure specified in Ch.2 Sec.6 are
given in Table 11.
Table 11 Hull equipment and supporting structure
Subsection
Ch.2 Sec.6 [1.1.1]
Title
General
Classification requirement
— Mooring lines are not subject to
the Society's classification scope.
Lengths, breaking strength and
number of mooring lines are
given in the equipment tables as
guidance.
— Quayside mooring arrangement is
only required for SOLAS compliant
units.
— For units to be position moored at
a location for a shorter or a longer
period, the requirements in DNVOS-E301 apply.
— Documentation requirements are
given in [1.4] and [1.5].
Ch.2 Sec.6 [1.2.1]
Temporary mooring
The temporary mooring arrangement
shall be submitted to the Society for
approval, see [1.4].
Ch.2 Sec.6 [1.2.6]
Anchor, anchor chain cables, windlass
and chain stoppers
Requirements to documentation
for units classed by the Society are
specified in [1.5.1].
Ch.2 Sec.6 [1.3.1]
Quayside mooring
For SOLAS compliant units, a quayside
mooring arrangement is required,
and the arrangement plan shall be
submitted to the Society, see [1.4]
For non SOLAS units, a quayside
mooring arrangement is not required
by the Society.
Ch.2 Sec.6 [1.3.3]
Design minimum breaking load
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Limitations of wind velocity will be
listed in the Society's appendix to class
certificate.
Page 124
DNV AS
Chapter 3 Section 1
Table 10 Strength assessment
Ch.2 Sec.6 [1.4.1.2]
Ch.2 Sec.6 [1.4.2]
Title
Emergency towing
Towing arrangement
Classification requirement
For emergency towing:
1)
Exemptions may be given for
permanently installed units where
the single transit to location is
supported by tug boat(s).
2)
Flag states may have additional
requirements.
For units classed by the Society:
— Towing arrangement plans shall be
submitted, see [1.4].
— Towing equipment shall be certified,
see [1.5.2].
Ch.2 Sec.6 [1.4.4]
Towing equipment and fittings
Requirements to documentation
for units classed by the Society are
specified in [1.5.2].
Shackles tested at -20 °C may be
accepted provided they are certified by
the Society.
Ch.2 Sec.6 [2.2.1]
Design loads for mooring line system
For DNV class related projects the
design loads shall be based on DNVOS-E301.
Ch.2 Sec.6 [3.1.1]
Supporting structures and foundations
for heavy equipment
Topside modules are not part of the
Society' main class scope, unless one
of the class notations PROD or DRILL
is selected.
For the Society's scope related to
marine equipment, see DNV-OS-D101.
Ch.2 Sec.6 [3.1.2]
Temporary equipment
For temporary equipment:
The Society's class scope covers
support for permanently installed
equipment. For supporting structures
of temporarily mounted equipment, see
e.g. DNV-CG-0156.
The Society's class scope covers
strength documentation that shall be
submitted when one of the items below
is exceeded:
— the module/equipment weight is
above 10 tonnes
— the static overturning moment
exceeds 100 KNm.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 125
DNV AS
Chapter 3 Section 1
Subsection
Ch.2 Sec.6 [4.1.3]
Title
Classification requirement
Support for lifting appliances and crane The crane, including pedestal flange
pedestals
and bolts or the lifting gear itself, is
not subject to approval, unless class
notation Crane, DSV or Crane vessel
is requested.
If ILO certification of lifting appliances
is requested and the Society shall
issue the certificate, approval of
documentation will be required. See
DNV-ST-0377 and DNV-ST-0378.
2.7 Special provisions for unit types
The specific classification requirements for unit types specified in Ch.2 Sec.7 are given in the relevant DNV
unit type standard.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 126
DNV AS
Chapter 3 Section 1
Subsection
1 Cross sectional types
1.1 General
1.1.1 Cross sections of beams are divided into different types dependent of their ability to develop plastic
hinges as given in Table 1.
Table 1 Cross sectional types
I
Cross sections that can form a plastic hinge with the rotation at capacity required for plastic analysis
II
Cross sections that can develop their plastic moment resistance, but have limited rotation at capacity
III
Cross sections where the calculated stress in the extreme compression fibre of the steel member can reach its
yield strength, but local buckling is liable to prevent development of the plastic moment resistance
IV
Cross sections where it is necessary to make explicit allowances for the effects of local buckling when
determining their moment resistance or compression resistance
Figure 1 Relation between moment M and plastic moment resistance
sectional types.
My is the elastic moment resistance
Mp, and rotation θ for cross
1.1.2 The categorisation of cross sections for different compression elements are given in Table 3.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 127
DNV AS
Appendix A
APPENDIX A CROSS SECTIONAL TYPES
1.2 Cross section requirements for plastic analysis
1.2.1 At plastic hinge locations, the cross section of the member which contains the plastic hinge shall have
an axis of symmetry in the plane of loading.
1.2.2 At plastic hinge locations, the cross section of the member which contains the plastic hinge shall have
a rotation capacity of at least the required rotation at that plastic hinge location.
1.3 Cross section requirements when elastic global analysis is used
1.3.1 When elastic global analysis is used, the role of cross section classification shall identify the extent to
which the resistance of a cross section is limited by its local buckling resistance.
1.3.2 When all the compression elements of a cross section are type III, its resistance may be based on an
elastic distribution of stresses across the cross section, limited to the yield strength at the extreme fibres.
Table 2 Strain coefficients for different steels
NV Steel grade
1)
1)
ε 2)
NV NS
1
NV 27S
0.94
NV 32
0.86
NV 36
0.81
NV 40
0.78
NV 420
0.75
NV 460
0.72
NV 500
0.69
NV 550
0.65
NV 620
0.62
NV 690
0.58
The table is not valid for steel with improved weldability. See Ch.2 Sec.3 Table 3.
2)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 128
DNV AS
Appendix A
1.1.3 An element in a cross section may be in partly or total in compression, due to axial forces or
bending moments, and selection of cross sectional types shall be based on the least favourable type of it's
compression elements.
Cross section part
3)
d = h - 3 tw
Type I
d / tw ≤ 33
ε
d / tw ≤ 72
ε
2)
Type II
ε
d / tw≤ 42
d / tw ≤ 83
ε
d / tw ≤ 124
when
α > 0.5:
α ≤ 0.5:
when
α ≤ 0.5:
when
Type III
d / tw ≤ 38
when α > 0.5:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Appendix A
Table 3 Maximum width to thickness ratios for compression elements
ε
ε
when
ψ > -1:
when
ψ ≤ -1:
Page 129
DNV AS
Type I
Type II
Tip in compression
Tip in compression
Tip in tension
Tip in tension
d / tp ≤ 50
Type III
ε2
d / tp ≤ 70
Tip in compression
Tip in tension
ε2
d / tp ≤ 90
1)
Compression negative.
2)
ε is defined in Table 2.
3)
Valid for rectangular hollow sections (RHS) where h is the height of the profile.
4)
C is the buckling coefficient. See e.g. DNV-RP-C201 or EN 1993-1 (1993-1-5 Table 4.2 denoted kσ).
Valid for axial and bending, not external pressure.
5)
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Appendix A
Cross section part
ε2
Page 130
DNV AS
1 General
1.1 Introduction
1.1.1 The requirements in this section apply to pile foundations, gravity type foundations, anchor
foundations and stability of sea bottom. Foundation types not covered in this appendix shall be specially
considered.
1.1.2 Design of foundations shall be based on site specific information.
1.1.3 The LRFD method described in Ch.2 Sec.1 [1.3.3] should be used for the foundation design. The WSD
method described in Ch.2 Sec.1 [1.3.4] may be applied, provided the safety factors are agreed upon in each
case.
1.1.4 The design of foundations shall consider both the strength and deformations of the foundation
structure and of the foundation soils. This section states requirements for:
— foundation soils
— soil reactions upon the foundation structure
— soil-structure interaction.
The anchor, including the anchor pad eye, shall be designed for the loads and acceptance criteria specified in
DNV-OS-E301 Ch.2 Sec.4.
1.1.5 A foundation failure mode is defined as the mode in which the foundation reaches any of its limit
states. Examples of such failure modes are:
—
—
—
—
—
bearing failure
sliding
overturning
anchor pull-out
large settlements or displacements.
1.1.6 The definition of limit state design principle is given in Ch.2 Sec.1 [1.2]
1.1.7 The load factors γf for the different design conditions and limit states are given in Ch.2 Sec.1 [1.3.3].
For anchor foundations the load factors are specified in [5] for each anchor type.
1.1.8 The partial safety factor γm are specified in the relevant subsections [2] - [5] and shall be used as
follows:
For soil shear strength:
— For effective stress analysis, the tangent to the characteristic friction angle shall be divided by γm.
— For total shear stress analysis, the characteristic undrained shear strength shall be divided by γm.
For soil resistance to axial pile load, γm shall be applied to Ch.2 Sec.1 [1.3.3].
For anchor foundations, γm shall be applied to [5].
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 131
DNV AS
Appendix B
APPENDIX B SOIL FOUNDATION DESIGN
Guidance note:
Further elaborations on design principles and examples of design solutions for foundation design are given in DNV-RP-C212.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.2 Site investigations
1.2.1 The extent of site investigations and the choice of investigation methods shall take into account the
type, size and importance of the structure, uniformity of soil and seabed conditions and the actual type
of soil deposits. The area to be covered by site investigations shall account for positioning and installation
tolerances.
1.2.2 For anchor foundations the soil stratigraphy and range of soil strength properties shall be assessed
within each anchor group or per anchor location, as relevant.
1.2.3 Site investigations shall provide relevant information about the soil to a depth below which possible
existence of weak formations will not influence the safety or performance of the structure.
1.2.4 Site investigations are normally to comprize of the following type of investigations:
—
—
—
—
—
site geology survey
topography survey of the seabed
geophysical investigations for correlation with borings and in-situ testing
soil sampling with subsequent laboratory testing
in-situ tests, e.g. cone penetrations tests.
1.2.5 The site investigations shall provide the following type of geotechnical data for the soil deposits as
found relevant for the design:
— data for soil classification and description
— shear strength parameters including parameters to describe the development of excess pore-water
pressures
— deformation properties, including consolidation parameters
— permeability
— stiffness and damping parameters for calculating the dynamic behaviour of the structure.
Variations in the vertical, as well as, the horizontal directions shall be documented.
1.2.6 Tests to determine the necessary geotechnical properties shall be carried out in a way that accounts for
the actual stress conditions in the soil. The effects of cyclic loading caused by waves, wind and earthquake
shall be included, as applicable.
1.2.7 Testing equipment and procedures shall be adequately documented. Uncertainties in test results shall
be described. Where possible, mean and standard deviation of test results shall be present.
1.3 Characteristic properties of soil
1.3.1 The characteristic strength and deformation properties of soil shall be determined for all deposits of
importance.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 132
DNV AS
Appendix B
1.1.9 Settlements caused by increased stresses in the soil due to structural weight shall be considered for
structures with gravity type foundation. In addition, subsidence, e.g. due to reservoir compaction, shall be
considered for all types of structures.
1.3.3 The results of both laboratory tests and in-situ tests shall be evaluated and corrected as relevant on
the basis of recognized practice and experience. Such evaluations and corrections shall be documented. In
this process account shall be given to possible differences between properties measured in the tests and the
soil properties governing the behaviour of the in-situ soil for the limit state in question. Such differences may
be due to:
—
—
—
—
—
soil disturbance due to sampling and samples not reconstituted to in-situ stress history
presence of fissures
different loading rate between test and limit state in question
simplified representation in laboratory tests of certain complex load histories
soil anisotropy effects giving results which are dependent on the type of test.
1.3.4 Possible effects of installation activities on the soil properties should be considered.
1.3.5 The characteristic value of a soil property shall be a cautious estimate of the value affecting the
occurrence of the limit state, selected such that the probability of a worse value is low.
1.3.6 A limit state may involve a large volume of soil and it is then governed by the average of the soil
property within that volume. The choice of the characteristic value shall take due account for the number and
quality of tests within the soil volume involved. Specific care should be made when the limit state is governed
by a narrow zone of soil.
1.3.7 The characteristic value shall be selected as a lower value, being less than the most probable value, or
an upper value being greater, depending on which is worse for the limit state in question.
Guidance note:
When relevant statistical methods are used, the characteristic value should be derived such that the calculated probability of a
worse value, governing the occurrence of the limit state, is not greater than 5%.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
1.4 Effects of cyclic loading
1.4.1 The effects of cyclic loading on the soil properties shall be considered in foundation design, where
relevant.
1.4.2 Cyclic shear stresses may lead to a gradual increase in pore pressure. Such pore pressure build-up and
the accompanying increase in cyclic and permanent shear strains may reduce the shear strength of the soil.
These effects shall be accounted for in the assessment of the characteristic shear strength for use in design
within the applicable limit state categories.
1.4.3 The effects of cyclic loading on the soil's shear modulus shall be corrected for as relevant when
dynamic motions, settlements and permanent (long-term) horizontal displacements shall be calculated. See
also [1.5].
1.4.4 The effects of wave induced forces on the soil properties shall be investigated for single storms and for
several succeeding storms, where relevant.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 133
DNV AS
Appendix B
1.3.2 The characteristic value of a soil property shall account for the variability in that property based on an
assessment of the soil volume governing for the limit state being considered.
1.5 Soil-structure interaction
1.5.1 Evaluation of structural load effects shall be based on an integrated analysis of the soil and structure
system. The analysis shall be based on realistic assumptions regarding stiffness and damping of both the soil
and structural members.
1.5.2 Due consideration shall be given to the effects of adjacent structures, where relevant.
1.5.3 For analysis of the structural response to earthquake vibrations, ground motion characteristics valid
at the base of the structure shall be determined. This determination shall be based on ground motion
characteristics in free field and on local soil conditions using recognized methods for soil-structure interaction
analysis.
2 Stability of seabed
2.1 Slope stability
2.1.1 Risk of slope failure shall be evaluated. Such calculations shall cover:
—
—
—
—
—
natural slopes
slopes developed during and after installation of the structure
future anticipated changes of existing slopes
effect of continuous mudflows
wave induced soil movements.
The effect of wave loads on the sea bottom shall be included in the evaluation when such loads are
unfavourable.
2.1.2 When the structure is located in a seismically active region, the effects of earthquakes on the slope
stability shall be included in the analyses.
2.1.3 The safety against slope failure for the survival condition (ULS) shall use γm as follows:
= 1.2 for effective stress analysis
γm
γm
= 1.3 for total stress analysis.
2.1.4 For the accidental condition (ALS), γm = 1.0.
2.2 Hydraulic stability
2.2.1 The possibility of failure due to hydrodynamic instability shall be considered where soils susceptible to
erosion or softening are present.
2.2.2 An investigation of hydraulic stability shall assess the risk for:
— softening of the soil and consequent reduction of bearing capacity due to hydraulic gradients and seepage
forces
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 134
DNV AS
Appendix B
1.4.5 In seismically active areas, where the structure foundation system may be subjected to earthquake
forces, the deteriorating effects of cyclic loading on the soil properties shall be evaluated for the site specific
conditions and considered in the design where relevant. See also [4.2].
2.2.3 If erosion is likely to reduce the effective foundation area, measures shall be taken to prevent, control
and/or monitor such erosion, as relevant, see [2.3].
2.3 Scour and scour protection
2.3.1 The risk for scour around the foundation of a structure shall be taken into account unless it can be
demonstrated that the foundation soils will not be subject to scour for the expected range of water particle
velocities.
2.3.2 The effect of scour, where relevant, shall be accounted for according to at least one of the following
methods:
a)
b)
c)
Adequate means for scour protection is placed around the structure as early as possible after installation.
The foundation is designed for a condition where all materials, which are not scour resistant are assumed
removed.
The seabed around the platform is kept under close surveillance and remedial works to prevent further
scour are carried out shortly after detection of significant scour.
2.3.3 Scour protection material shall be designed to provide both external and internal stability, i.e.
protection against excessive surface erosion of the scour protection material and protection against
transportation of soil particles from the underlying natural soil.
3 Design of pile foundations
3.1 General
3.1.1 The load carrying capacity of piles shall be based on strength and deformation properties of the pile
material as well as on the ability of the soil to resist pile loads.
3.1.2 In evaluation of soil resistance against pile loads, the following factors shall be amongst those to be
considered:
—
—
—
—
—
shear strength characteristics
deformation properties and in-situ stress conditions of the foundation soil
method of installation
geometry and dimensions of pile
type of loads.
3.1.3 The data bases of existing methods for calculation of soil resistance to axial and lateral pile loads
are often not covering all conditions of relevance for offshore piles. This especially relates to size of piles,
soil shear strength and type of load. When determining soil resistance to axial and lateral pile loads,
extrapolations beyond the data base of a chosen method shall be made with thorough evaluation of all
relevant parameters involved.
3.1.4 It shall be demonstrated by a driveability study or equivalent that the selected solution for the pile
foundation is feasible with respect to installation of the piles.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 135
DNV AS
Appendix B
— formation of piping channels with accompanying internal erosion in the soil
— surface erosion in local areas under the foundation due to hydraulic pressure variations resulting from
environmental loads.
3.1.6 For determination of design soil resistance against axial pile loads in survival condition (ULS),
shall be applied to all characteristic values of soil resistance, e.g. to skin friction and tip resistance.
γm = 1.3
Guidance note:
This material coefficient may be applied to pile foundation of multilegged jacket or template structures. In this application, the
design pile load shall be determined from structural analyses where the pile foundation is modelled with elastic stiffness, or nonlinear models based on characteristic soil strength.
If the ultimate plastic resistance of the foundation system is analysed by modelling the soil with its design strength and allowing
full plastic redistribution until a global foundation failure is reached, higher material coefficients should be used.
For individual piles in a group lower material coefficients may be accepted, as long as the pile group as a whole is designed with
the required material coefficient. A pile group in this context shall not include more piles that those supporting one specific leg.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.1.7 For pile foundations of structures where there are no or small possibilities for redistribution of loads
from one pile to another, or from one group of piles to another group of piles, larger material coefficients
than those given in [3.1.6] shall be used. This may for example apply to pile foundations for TLPs or to deep
draught floaters. In such cases, γm = 1.7 shall be used for the survival condition (ULS).
3.1.8 For calculation of design lateral resistance according to [3.3], the following material coefficients shall
be applied to characteristic soil shear strength parameters for the survival condition (ULS):
γm
γm
= 1.2 for effective stress analysis
= 1.3 for total stress analysis.
3.1.9 For accidental condition (ALS) and for normal operation (SLS), γm = 1.0.
3.1.10 For conditions where large uncertainties are attached to the determination of characteristic shear
strength or characteristic soil resistance, e.g. pile skin friction or tip resistance, larger material factors
shall normally be used. Choice of material coefficients shall, in such cases, be in accordance with the
determination of characteristic values of shear strength or soil resistance.
3.2 Soil resistance against axial pile loads
3.2.1 Soil resistance against axial pile loads shall be determined by one, or a combination of, the following
methods:
— load testing of piles
— semi-empirical pile capacity formulae based on pile load test data.
3.2.2 The soil resistance in compression shall be taken as the sum of accumulated skin friction on the outer
pile surface and resistance against pile tip. In case of open-ended pipe piles, the resistance of an internal soil
plug shall be taken into account in the calculation of resistance against pile tip. The equivalent tip resistance
shall be taken as the lower value of the plugged (gross) tip resistance or the sum of the skin resistance of the
internal soil plug and the resistance against the pile tip area. The soil plug may be replaced by a grout plug
or equivalent in order to achieve fully plugged tip resistance.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 136
DNV AS
Appendix B
3.1.5 Structures with piled foundations shall be assessed with respect to stability for both operation and
temporary design conditions, e.g. prior to and during installation of the piles. See Ch.2 Sec.2 for selection of
representative loads.
3.2.4 Effects of cyclic loading shall be accounted for as far as possible. In evaluation of the degradation of
resistance, the influence of flexibility of the piles and the anticipated loading history shall be accounted for.
3.2.5 For piles in mainly cohesive soils, the skin friction shall be taken equal to or smaller than the undrained
shear strength of undisturbed clay within the actual layer. The degree of reduction depends on the nature and
strength of clay, method of installation, time effects, geometry and dimensions of pile, load history and other
factors.
3.2.6 The unit tip resistance of piles in mainly cohesive soils may be taken as 9 times the undrained shear
strength of the soil near the pile tip.
3.2.7 For piles in mainly cohesionless soils the skin friction may be related to the effective normal stresses
against the pile surface by an effective coefficient of friction between the soil and the pile element. It shall be
noticed that a limiting value of skin friction may be approached for long piles.
3.2.8 The unit tip resistance of piles in mainly cohesionless soils may be calculated by means of conventional
bearing capacity theory, taken into account a limiting value, which may be approached, for long piles.
3.3 Soil resistance against lateral pile loads
3.3.1 When pile penetrations are governed by lateral soil resistance, the design resistance shall be checked
for the survival (ULS) and accidental (ALS) conditions, using γm specified in [3.1.8].
3.3.2 For analysis of pile stresses and lateral pile head displacement, the lateral soil reaction shall be
modelled using characteristic soil strength parameters, with the soil material coefficient γm = 1.0. Non-linear
response of soil shall be accounted for, including the effects of cyclic loading.
3.4 Group effects
3.4.1 When piles are closely spaced in a group, the effect of overlapping stress zones on the total resistance
of the soil shall be considered for axial, as well as, lateral loads on the piles. The increased displacements of
the soil volume surrounding the piles due to pile-soil-pile interaction and the effects of these displacements
on interaction between structure and pile foundation shall be considered.
3.4.2 In evaluation of pile group effects, due consideration shall be given to factors such as:
—
—
—
—
—
pile spacing
pile type
soil strength
soil density
pile installation method.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 137
DNV AS
Appendix B
3.2.3 For piles in tension, no resistance from the soil below pile tip shall be accounted for, if the pile tip is in
sandy soils.
4.1 Stability of foundations
4.1.1 The risk of shear failure below the base of the structure shall be investigated for all gravity
type foundations. Such investigations shall cover failure along any potential shear surface with special
consideration given to the effect of soft layers and the effect of cyclic loading. The geometry of the
foundation base shall be accounted for.
4.1.2 The analyses shall be carried out for fully drained, partially drained or undrained conditions, whatever
represents most accurately the actual conditions.
4.1.3 For the survival (ULS) and accidental (ALS) conditions, the foundation stability shall be evaluated by
one of the following methods:
— effective stress stability analysis
— total stress stability analysis.
4.1.4 An effective stress stability analysis shall be based on effective strength parameters of the soil and
realistic estimates of the pore water pressures in the soil.
4.1.5 A total stress stability analysis shall be based on total shear strength values determined from tests
on representative soil samples subjected to similar stress conditions as the corresponding element in the
foundation soil.
4.1.6 Both effective stress and total stress methods shall be based on laboratory shear strength with pore
pressure measurements included. The test results should preferably be interpreted by means of stress paths.
4.1.7 Stability analyses by conventional bearing capacity formulae are only acceptable for uniform soil
conditions.
4.1.8 For structures where skirts, dowels or similar foundation members transfer loads to the foundation soil,
the contributions of these members to the bearing capacity and lateral resistance may be accounted for as
relevant. The feasibility of penetrating the skirts shall be adequately documented.
4.1.9 Foundation stability shall be analysed in the survival condition (ULS) by applying γm to the
characteristic soil shear strength parameters as following:
γm
γm
= 1.2 for effective stress analysis
= 1.3 for total stress analysis.
For accidental condition (ALS), γm = 1.0.
4.1.10 Effects of cyclic loads shall consider load factors in accordance with [1.1.7].
4.1.11 In an effective stress analysis, evaluation of pore pressures shall include:
—
—
—
—
initial pore pressure
build-up of pore pressures due to cyclic load history
the transient pore pressures through each load cycle
the effect of dissipation.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 138
DNV AS
Appendix B
4 Design of gravity foundations
4.2 Settlements and displacements
4.2.1 For SLS design conditions, analyses of settlements and displacements are, in general, to include
calculations of:
—
—
—
—
initial consolidation and secondary settlements
differential settlements
permanent (long term) horizontal displacements
dynamic motions.
4.2.2 Displacements of the structure, as well as of its foundation soil, shall be evaluated to provide basis for
the design of conductors and other members connected to the structure which are penetrating or resting on
the seabed.
4.2.3 Analysis of differential settlements shall account for lateral variations in soil conditions within
the foundation area, non-symmetrical weight distributions and possible predominating directions of
environmental loads. Differential settlements or tilt due to soil liquefaction shall be considered in seismically
active areas.
4.3 Soil reaction on foundation structure
4.3.1 The reactions from the foundation soil shall be accounted for in the design of the supported structure
for all design conditions.
4.3.2 The distribution of soil reactions against structural members seated on, or penetrating into the sea
bottom, shall be estimated from conservatively assessed distributions of strength and deformation properties
of the foundation soil. Possible spatial variation in soil conditions, including uneven seabed topography, shall
be considered. The stiffness of the structural members shall be taken into account.
4.3.3 The penetration resistance of dowels and skirts shall be calculated based on a realistic range of soil
strength parameters. The structure shall be provided with sufficient capacity to overcome maximum expected
penetration resistance in order to reach the required penetration depth.
4.3.4 As the penetration resistance may vary across the foundation site, eccentric penetration forces may be
necessary to keep the platform inclination within specified limits.
4.4 Soil modelling for dynamic analysis
4.4.1 Dynamic analysis of a gravity structure shall consider the effects of soil and structure interaction. For
homogeneous soil conditions, modelling of the foundation soil using the continuum approach may be used.
For more non-homogeneous conditions, modelling by finite element techniques or other recognized methods
accounting for non-homogenous conditions shall be performed.
4.4.2 Due account shall be taken of the strain dependency of shear modulus and internal soil damping.
Uncertainties in the choice of soil properties shall be reflected in parametric studies to find the influence on
response. The parametric studies should include upper and lower boundaries on shear moduli and damping
ratios of the soil. Both internal soil damping and radiation damping shall be considered.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 139
DNV AS
Appendix B
4.1.12 The safety against overturning shall be investigated for the survival (ULS) and the accidental (ALS)
conditions.
4.5.1 In order to assure sufficient stability of the structure or to provide a uniform vertical reaction, filling of
the voids between the structure and the seabed, e.g. by underbase grouting, may be necessary.
4.5.2 The foundation skirt system and the void filling system shall be designed so that filling pressures
do not cause channelling from one compartment to another, or to the seabed outside the periphery of the
structure.
4.5.3 The filling material used shall be capable of retaining sufficient strength during the lifetime of the
structure considering all relevant forms of deterioration such as:
— chemical
— mechanical
— placement problems such as incomplete mixing and dilution.
5 Design of anchor foundations
5.1 General
5.1.1 This subsection applies to the following types of anchor foundations:
—
—
—
—
—
pile anchors
gravity anchors
suction anchors
fluke anchors
plate anchors.
5.1.2 The analysis of anchor resistance shall be carried out for the survival (ULS) and the accidental ALS)
conditions, in accordance with the safety requirements given in [5.2]. Due consideration shall be given to the
specific aspects of the different anchor types and the current state of knowledge and development.
5.1.3 Determination of anchor resistance may be based on empirical relationships and relevant test data.
Due consideration shall be given to the conditions under which these relationships and data are established
and the relevance of these conditions with respect to the actual soil conditions, shape and size of anchors
and loading conditions.
5.1.4 When clump weight anchors are designed to be lifted off the seabed during extreme loads, due
consideration shall be paid to the suction effects that may develop at the clump weight and soil interface
during a rapid lift-off. The effect of possible burial during the subsequent set-down shall be considered.
5.2 Safety requirements for anchor foundations
5.2.1 In the survival (ULS) condition, the anchor shall withstand the loads arising in an intact mooring
system under extreme environmental conditions.
In the accidental (ALS) condition, the mooring system shall retain adequate capacity if one mooring line or
anchor should fail.
5.2.2 Two consequence classes are considered for the survival (ULS) and for the accidental (ALS) as follows:
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 140
DNV AS
Appendix B
4.5 Filling of voids
Appendix B
5.2.2.1 Consequence class 1 (CC1)
Failure is unlikely to lead to unacceptable consequences such as loss of life, collision with an adjacent
platform, uncontrolled outflow of oil or gas, capsize or sinking.
5.2.2.2 Consequence class 2 (CC2)
Failure may well lead to unacceptable consequences of these types.
5.2.3 The load factors for time domain and frequency domain analyses are given in Table 1 for survival
(ULS) condition, and in Table 2 for accidental (ALS) condition. For mooring in deep water (i.e. water depth
exceeding 200 m, see DNV-OS-E301 Ch.2 Sec.2 [2.1], a dynamic analysis is required.
Table 1 Load factors for survival (ULS)
Time domain analysis
Consequence class Type of unit
Frequency domain analysis
1)
Load factor on
Load factor on
Load factor on
Load factor on
pretension
env. tension
pretension
env. tension
γpret
γenv
γpret
1
Permanent
1.20
1.45
1.20
1.80
1
Mobile
1.20
1.35
1.20
1.50
2
Permanent &
mobile
1.20
1.90
1.20
2.30
γenv
1) The safety factors should be increased by 10% if Rayleigh distributed maxima is assumed for LF motion and WF
tension, unless it can be documented that the Rayleigh distribution provide good estimates of the response.
Table 2 Load factors for accidental (ALS)
Time domain analysis
Consequence class Type of unit
Frequency domain analysis
1)
Load factor on
Load factor on
Load factor on
Load factor on
pretension
env. tension
pretension
env. tension
γpret
γenv
γpret
1
Permanent
1.00
1.10
1.00
1.25
1
Mobile
1.00
1.05
1.00
1.10
2
Permanent &
mobile
1.00
1.45
1.00
1.70
γenv
1) The safety factors should be increased by 10% if Rayleigh distributed maxima is assumed for LF motion and WF
tension, unless it can be documented that the Rayleigh distribution provide good estimates of the response.
5.2.4 The design line tension Td at the touch-down point is the sum of the two line tension components Tpret
and TC-env at that point multiplied by their respective load coefficients γpret and γenv, i.e.:
where:
Tpret
TC‐env
= mooring line pretension
= characteristic environmental line tension induced by mean, low-frequency and wave-frequency
loads in the environmental state.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 141
DNV AS
5.3 Pile anchors
5.3.1 Pile anchors shall be designed in accordance with the relevant requirements given in [3].
5.3.2 The γm for the resistance of pile anchors shall not be taken less than:
= 1.3 for survival (ULS ) condition for consequence class 1 (CC1) and 2 (CC2)
γm
γm
= 1.0 for accidental (ALS) condition for consequence class 1 (CC1) and 2 (CC2).
See also requirements to tension piles in [3.1.7].
5.4 Gravity anchors
5.4.1 Gravity anchors shall be designed in accordance with the relevant requirements given in [4]. The
capacity against uplift of a gravity anchor shall not be taken higher that the submerged mass. However,
for anchors supplied with skirts, the contribution from friction along the skirts may be included. In certain
cases such anchors may be able to resist cyclic uplift loads by the development of temporary suction within
their skirt compartments. In relying on such suction one shall make sure, that there are no possibilities
for leakage, e.g. through pipes or leaking valves or channels developed in the soil, that could prevent the
development of suction.
5.4.2 The γm shall not be taken less than:
= 1.3 for survival (ULS ) condition for consequence class 1 (CC1) and 2 (CC2)
γm
γm
= 1.0 for accidental (ALS) condition for consequence class 1 (CC1) and 2 (CC2).
5.5 Suction anchors
5.5.1 Suction anchors are vertical cylindrical anchors with open or (normally) closed top, which are
installed initially by self-weight penetration followed by application of underpressure (suction) in the closed
compartment.
The failure mechanism in the clay around an anchor will depend on various factors, like the load inclination,
the anchor depth to diameter ratio, the depth of the load attachment point, the shear strength profile, and
whether the anchor has an open or a closed top.
5.5.2 If the load inclination is close to vertical, the anchor will tend to move out of the ground, mainly
mobilising the shear strength along the outside skirt wall and the inverse bearing capacity of the soil at skirt
tip level. If the anchor has an open top, the inverse bearing capacity will not be mobilized if the inside skirt
friction is lower than the inverse bearing capacity at skirt tip level.
5.5.3 If the load inclination is more towards the horizontal, the resistance at the upper part of the anchor
will consist of passive and active resistance against the front and back of the anchor, and side shear along
the anchor sides. Deeper down, the soil may flow around the anchor in the horizontal plane, or underneath
the anchor.
5.5.4 The coupling between vertical and horizontal resistance occurs when the failure mechanism is a
combination between vertical and horizontal translation modes. The coupling may reduce the vertical and
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 142
DNV AS
Appendix B
5.2.5 The γm to be used in combination with the load coefficients in Table 1 and Table 2 are given specifically
for the respective types of anchors in [5.3] to [5.7].
Figure 1 Schematic resistance diagram for suction anchor.
5.5.5 DNV recommendations for geotechnical design and installation of suction anchors in clay are provided
in DNV-RP-E303. The design method outlined in the code makes use of a relatively detailed resistance
analysis, and it is concluded that many existing analytical methods will meet the analysis requirements in this
code. For details, see DNV-RP-E303.
5.5.6 If a less detailed resistance analysis is applied, the designer should be aware of the limitations of the
method and make sure that the effects of any simplifications are conservative in comparison with the results
from the more advanced methods.
5.5.7 The γm for suction anchors shall be:
= 1.20 for survival (ULS ) condition for consequence class 1 (CC1) and 2 (CC2)
γM
γM
γM
= 1.20 for accidental (ALS) condition for consequence class CC2
= 1.00 for accidental (ALS) condition for consequence class 1 (CC1).
In the calculation of the anchor resistance, strength anisotropy and the effects of cyclic loading on the
undrained shear strength shall be accounted for. The characteristic undrained shear strength shall be taken
as the mean value with due account of the quality and complexity of the soil conditions.
5.5.8 Seabed impact landing and subsequent penetration by self-weight shall be addressed in terms of
required water evacuation areas to avoid excessive channelling and/or global instability during installation.
5.5.9 Load factors for impact landing, suction to target penetration depth and possible retrieval by means of
overpressure shall be based on the survival (ULS) condition according to Ch.2 Sec.1 [1.3.3]. For loads related
to permanent removal after design life, the load factor for accidental (ALS) condition according to Ch.2 Sec.1
[1.3.3] shall be used.
5.5.10 The γm for the soil material to be applied for a potential soil plug failure during suction assisted
penetration shall not be taken less than 1.5.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 143
DNV AS
Appendix B
horizontal resistance components at failure, and the resulting resistance will be smaller than the vector sum
of the uncoupled maximum vertical and horizontal resistance. This is illustrated in Figure 1.
5.6.1 Design of fluke anchors shall be based on recognized principles in geotechnical engineering
supplemented by data from tests performed under relevant site and loading conditions.
5.6.2 The penetration resistance of the anchor line shall be taken into considerations where deep penetration
is required to mobilize reactions forces.
5.6.3 Fluke anchors shall normally be used only for horizontal and unidirectional load application. However,
some uplift may be allowed under certain conditions both during anchor installation and during operating
design conditions. The recommended design procedure for fluke anchors is given in the DNV-RP-E301.
5.6.4 The required installation load of the fluke anchor shall be determined from the required design
resistance of the anchor, allowing for the inclusion of the possible contribution from post installation effects
due to soil consolidation and storm induced cyclic loading. For details, see DNV-RP-E301. For fluke anchors
in sand the load factors given in Table 1 and in Table 2 shall be applied. The target installation load should
normally not be taken less than the design load.
5.6.5 Provided that the uncertainty in the load measurements is accounted for and that the target
installation tension Ti is reached and verified by reliable measurements the main uncertainty in the anchor
resistance lies then in the predicted post-installation effects mentioned above. The soil material coefficient γM
on this predicted component of the anchor resistance shall then be:
γm
γm
γm
= 1.3 for survival (ULS) condition for consequence class 1 (CC1) and 2 (CC2)
= 1.0 for accidental (ALS) condition for consequence class 1 (CC1)
= 1.3 for accidental (ALS) condition for consequence class 2 (CC2).
5.7 Plate anchors
5.7.1 Design methodologies for plate anchors like drag-in plate anchors, push-in plate anchors, drive-in
plate anchors, suction embedment plate anchors, etc. should be established with due consideration of the
characteristics of the respective anchor type, how the anchor installation affects the in-place conditions, etc.
5.7.2 Recipes for calculation of characteristic line tension and characteristic anchor resistance are given in
DNV-RP-E302, together with their partial safety factors for each combination of limit state and consequence
class. Requirements for measurements during installation are also provided.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 144
DNV AS
Appendix B
5.6 Fluke anchors
July 2019 edition
Amendments August 2021
Topic
Rebranding to DNV
Reference
All
Description
This document has been revised due to the rebranding of DNV
GL to DNV. The following have been updated: the company
name, material and certificate designations, and references to
other documents in the DNV portfolio. Some of the documents
referred to may not yet have been rebranded. If so, please see
the relevant DNV GL document. No technical content has been
changed.
Changes July 2019
Topic
Reference
Description
Bolted connections
requirements update
Ch.2 Sec.11 [2]
Bolt connections modified and capacity requirements included.
Documentation and
certification
Ch.3 Sec.1 [1.3.2]
New sub section and guidance note for certification
requirements of material.
Ch.3 Sec.1 [1.3.2]
New sub section for certification requirements of bolts.
Ch.2 Sec.3 [4.1.5]
Guidance note for high tensile limit included.
Ch.2 Sec.3 [4.1.6]
Requirements to forgings and casting with respect to impact
test temperature included.
Ch.2 Sec.3 [4.3.8]
New sub section and guidance note for material certificates.
Ch.2 Sec.5 [1.1.9]
Requirements for detail design of cut-outs, brackets and
outfitting included.
Ch.2 Sec.9 [3.1.4]
Guidance note removed. Reference given to DNVGL-OS-C401.
previous Ch.2 Sec.11
[1.1.2]
Sub section [1.1.2] deleted, other sub-sections re-numbered
accordingly.
Miscellaneous changes
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 145
DNV AS
Changes – historic
CHANGES – HISTORIC
Changes July 2018
Topic
Minor update
Reference
Description
Ch.1 Sec.1 Table 1
Include DNVGL-RP-C212 and DNVGL-CG-0129 as reference
document.
Ch.1 Sec.1 Table 5
and Ch.1 Sec.1 Table
6
Update definition and abbreviation of lowest mean daily
average temperature (LMDAT).
Ch.2 Sec.3 [3.3.5]
Updated clause by including requirements on fabrication and
tolerances for fatigue critical details.
FLS
Ch.2 Sec.5 [1.2]
Updated subsection on design fatigue factors (DFF) including
restructured table.
ALS
Ch.2 Sec.6
Section on accidental limit states completely updated and
restructured, brought in line with the general principles in
DNVGL-OS-A101. Accidental events removed, referring to
DNVGL-OS-A101 instead.
Update based on output from
NORMOOR JIP
Ch.2 Sec.10 Table 1
and Ch.2 Sec.10 Table
2
Updated tables with safety factors to be applied in soil design
reflecting update of mooring system safety factors in DNVGLOS-E301.
July 2017 edition
Main changes July 2017
• Ch.2 Sec.3 Structural categorisation, material selection and inspection principles
— Ch.2 Sec.3 [4.2.2]: Extra high strength steel (EHS) included.
— Ch.2 Sec.3 Table 3, Ch.2 Sec.3 Table 4 and Ch.2 Sec.3 Table 5: Tables updated with new material
designations according to DNVGL-OS-B101.
April 2016 edition
Main changes April 2016
• Ch.1 Sec.1 Introduction
— [2.1.4]: References in tables updated.
— [3.2] Table 5: Service life removed and replaced with design life and design fatigue life.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 146
DNV AS
Changes – historic
July 2018 edition
Changes – historic
• Ch.2 Sec.2 Loads and loads effects
— Table 3: Notes in table updated for variable functional loads.
• Ch.2 Sec.3 Material selection
— Table 3: Material designation updated according to new DNV GL terms.
— Table 5: Strength group column added and material grade naming changed.
— [4.3.8]: Added.
• Ch.2 Sec.4 Ultimate limit states
— [1.4.1]: Added guidance.
— [6.4.4]: Shear area control of stiffeners included.
— Old subsection [8] Slip resistant bolt connections moved to new section Sec.11.
• Ch.2 Sec.6 Accidental limit states
— ALS is general updating in-line with exiting class practice and harmonized against other class object
standards.
• Ch.2 Sec.8 Weld connections
— Table 2 and Table 4: Material designation updated according to new DNV GL terms.
• Ch.2 Sec. 9 Corrosion control
— [2.4.7]: Added clarification.
• Ch.2 Sec.11 Miscellaneous
— New section.
— Crane pedestal and foundations for lifting appliance.
— Bolt and nuts moved from DNVGL-OS-C401 and slip resistant bolt connection moved from Sec.4.
July 2015 edition
Main changes July 2015
• General
The revision of this document is part of the DNV GL merger, updating the previous DNV standard into a DNV
GL format including updated nomenclature and document reference numbering, e.g.:
— Main class identification 1A1 becomes 1A.
— DNV replaced by DNV GL.
— DNV-RP-A201 to DNVGL-CG-0168. A complete listing with updated reference numbers can be found on
DNV GL's homepage on internet.
To complete your understanding, observe that the entire DNV GL update process will be implemented
sequentially. Hence, for some of the references, still the legacy DNV documents apply and are explicitly
indicated as such, e.g.: Rules for Ships has become DNV Rules for Ships.
Offshore standards — DNV-OS-C101. Edition July 2023
Structural design of offshore units
Page 147
DNV AS
About DNV
DNV is the independent expert in risk management and assurance, operating in more than 100
countries. Through its broad experience and deep expertise DNV advances safety and sustainable
performance, sets industry benchmarks, and inspires and invents solutions.
Whether assessing a new ship design, optimizing the performance of a wind farm, analyzing sensor
data from a gas pipeline or certifying a food company’s supply chain, DNV enables its customers and
their stakeholders to make critical decisions with confidence.
Driven by its purpose, to safeguard life, property, and the environment, DNV helps tackle the
challenges and global transformations facing its customers and the world today and is a trusted
voice for many of the world’s most successful and forward-thinking companies.
WHEN TRUST MATTERS