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DNV-OS-C201 Structural Design of Offshore Units (WSD method) [Table of Contents] Sec.2: Design Principles

DNV-OS-C201 Structural Design of Offshore Units (WSD method)

[-] Sec.1: Introduction

SECTION 1
Introduction

Sec.1
A. General

Sec.1
A 100   Introduction

Sec.1 A
101   This offshore standard provides principles, technical requirements and guidance for the structural design of offshore structures, based on the Working Stress Design (WSD) method.

Sec.1 A
102   This standard has been written for general world-wide application. Statutory regulations may include requirements in excess of the provisions by this standard depending on size, type, location and intended service of the offshore unit or installation.

Sec.1 A
103   The standard is organised with general sections containing common requirements and sections containing specific requirement for different type of offshore units. In case of deviating requirements between general sections and the object specific sections, requirements of the object specific sections shall apply.

Sec.1
A 200   Objectives

Sec.1 A
201   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 referred standards, recommended practices, guidelines, etc.)
—serve as a contractual reference document between suppliers and purchasers
—serve as a guideline for designers, suppliers, purchasers and regulators
—specify procedures and requirements for offshore structures subject to DNV certification and classification.

Sec.1
A 300   Scope and application

Sec.1 A
301   This standard is applicable to the following types of offshore structures:
—column-stabilised units
—self-elevating units
—tension leg platforms
—deep draught floaters.


Sec.1 A
302   For utilisation of other materials, the general design principles given in this standard may be used together with relevant standards, codes or specifications covering the requirements to materials design and fabrication.

Sec.1 A
303   The standard is applicable to structural design of complete units including substructures, topside structures and vessel hulls.

Sec.1 A
304   This standard gives requirements for the following:
—design principles
—structural categorisation
—material selection and inspection principles
—loads and load effect analyses
—design of steel structures and connections
—special considerations for different types of units.

Requirements for foundation design are given in DNV-OS-C101.

Sec.1
A 400   Other than DNV codes

Sec.1 A
401   Other recognised codes or standards may be applied provided it is shown that the codes and standards, and their application, meet or exceed the level of safety of the actual DNV standard.

Sec.1 A
402   In case of conflict between requirements of this standard and a reference document other than DNV documents, the requirements of this standard shall prevail.

Sec.1 A
403   Where reference is made to codes other than DNV documents, the latest revision of the documents shall be applied, unless otherwise specified.

Sec.1 A
404   When code checks are performed according to other than DNV codes, the usage factors as given in the respective code shall be used.

Sec.1
A 500   Classification

Sec.1 A
501   Classification principles, procedures and applicable class notations related to classification services of offshore units are specified in the DNV Offshore Service Specifications given in Table A1.

Sec.1 A
Table A1 DNV Offshore Service Specifications 
Reference Title 
DNV-OSS-101 Rules for Classification of Offshore Drilling and Support Units  
DNV-OSS-102 Rules for Classification of Floating Production, Storage and Loading Units 
DNV-OSS-103 Rules for Classification of LNG/LPG Floating Production and Storage Units or Installations 
DNV-OSS-121 Classification Based on Performance Criteria Determined by Risk Assessment Methodology 
 Rules for Planning and Execution of Marine Operations 



Sec.1 A
502   Documentation for classification shall be in accordance with the NPS DocReq (DNV Nauticus Production System for documentation requirements) and DNV-RP-A201.

Sec.1
B. References

Sec.1
B 100   General

Sec.1 B
101   The DNV documents in Table B1 are referred to in the present standards and contain acceptable methods for fulfilling the requirements in this standard.

Sec.1 B
102   The latest valid revision of the DNV reference documents in Table B2 applies. See also current DNV List of Publications.

Sec.1 B
103   The documents listed in Table B2 are referred in the present standard. The documents include acceptable methods for fulfilling the requirements in the present standard and may be used as a source of supplementary information. Only the referenced parts of the documents apply for fulfilment of the present standard.

Sec.1 B
Table B1 DNV Reference Documents 
Reference Title 
DNV-OS-A101 Safety Principles and Arrangement 
DNV-OS-B101 Metallic Materials 
DNV-OS-C101 Design of Offshore Steel Structures, General (LRFD method) 
DNV-OS-C301 Stability and Watertight Integrity 
DNV-OS-C401 Fabrication and Testing of Offshore Structures 
DNV-OS-D101 Marine Machinery Systems and Equipment 
DNV-OS-E301 Position Mooring 
DNV-OS-F201 Dynamic Risers 
DNV-RP-A201 Plan Approval Documentation Types - Definitions 
DNV-RP-B401 Cathodic Protection Design 
DNV-RP-C103 Column Stabilised Units 
DNV-RP-C201 Buckling Strength of Plated Structures 
DNV-RP-C202 Buckling Strength of Shells 
DNV-RP-C204 Design against Accidental Loads 
DNV-RP-C203 Fatigue Strength Analysis of Offshore Steel Structures 
DNV-RP-C205 Environmental Conditions and Environmental Loads 
Classification Note 30.1 Buckling Strength Analysis of Bars and Frames, and Spherical Shells 
Classification Note 30.4 Foundations 
Classification Note 31.5 Strength Analysis of Main Structures of Self-elevating Units 

Sec.1 B
Table B2 Other references  
Reference Title 
AISC-ASD Manual of Steel Construction ASD 
API RP 2A - WSD with supplement 1 Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design 
API RP 2T Planning, Designing and Constructing Tension Leg Platforms 
BS 7910 Guide on methods for assessing the acceptability of flaws in fusion welded structures 
NACE TPC Publication No. 3. The role of bacteria in corrosion of oil field equipment 
SNAME 5-5A Site Specific Assessment of Mobile Jack-Up Units 

Sec.1
C. Definitions

Sec.1
C 100   Verbal forms

Sec.1 C
101   Shall: Indicates a mandatory requirement to be followed for fulfilment or compliance with the present standard. Deviations are not permitted unless formally and rigorously justified, and accepted by all relevant contracting parties.

Sec.1 C
102   Should: Indicates a recommendation that a certain course of action is preferred or particularly suitable. Alternative courses of action are allowable under the standard where agreed between contracting parties but shall be justified and documented.

Sec.1 C
103   May: Indicates a permission, or an option, which is permitted as part of conformance with the standard.

Sec.1
C 200   Terms

Sec.1 C
201   Accidental condition: When the unit is subjected to accidental loads such as collision, dropped objects, fire explosion, etc.

Sec.1 C
202   Accidental loads: Loads which may occur as a result of accident or exceptional events, e.g. collisions, explosions, dropped objects.

Sec.1 C
203   Atmospheric zone: The external surfaces of the unit above the splash zone.

Sec.1 C
204   Cathodic protection: A technique to prevent corrosion of a steel surface by making the surface to be the cathode of an electrochemical cell.

Sec.1 C
205   Characteristic load: The reference value of a load to be used in the determination of load effects. The characteristic load is normally based upon a defined fractile in the upper end of the distribution function for load.

Sec.1 C
206   Characteristic strength: The reference value of structural strength to be used in the determination of the design strength. The characteristic strength is normally based upon a 5% fractile in the lower end of the distribution function for resistance.

Sec.1 C
207   Characteristic value: The representative value associated with a prescribed probability of not being unfavourably exceeded during the applicable reference period.

Sec.1 C
208   Classic spar: Shell type hull structure.

Sec.1 C
209   Classification Note: The Classification Notes cover proven technology and solutions which is found to represent good practice by DNV, and which represent one alternative for satisfying the requirements given in the DNV Rules or other codes and standards cited by DNV. The Classification Notes will in the same manner be applicable for fulfilling the requirements in the DNV Offshore Standards.

Sec.1 C
210   Coating: Metallic, inorganic or organic material applied to steel surfaces for prevention of corrosion.

Sec.1 C
211   Column-stabilised unit: A floating unit that can be relocated. A column-stabilised unit normally consists of a deck structure with a number of widely spaced, large diameter, supporting columns that are attached to submerged pontoons.

Sec.1 C
212   Corrosion allowance: Extra wall thickness added during design to compensate for any anticipated reduction in thickness during the operation.

Sec.1 C
213   Damaged condition: The unit condition after accidental damage.

Sec.1 C
214   Deep draught floater (DDF): A floating unit categorised with a relative large draught. The large draught is mainly introduced to obtain reduced wave excitation in heave and sufficiently high eigenperiod in heave such that resonant responses in heave can be omitted or minimised.

Sec.1 C
215   Design brief: An agreed document presenting owner's technical basis, requirements and references for the unit design and fabrication.

Sec.1 C
216   Design temperature: The design temperature for a unit is the reference temperature for assessing areas where the unit can be transported, installed and operated. The design temperature shall be lower or equal to the lowest mean daily temperature in air for the relevant areas. For seasonal restricted operations the lowest mean daily temperature in air for the season may be applied.

Sec.1 C
217   Driving voltage: The difference between closed circuit anode potential and the protection potential.

Sec.1 C
218   Dry transit: A transit where the unit is transported on a heavy lift unit from one geographical location to another.

Sec.1 C
219   Dynamic upending: A process where seawater is filled or flooded into the bottom section of a horizontally floating DDF hull and creating a trim condition and subsequent water filling of hull or moonpool and dynamic upending to bring the hull in vertical position.

Sec.1 C
220   Environmental loads: Loads directly and indirectly due to environmental phenomena. Environmental loads are not a necessary consequence of the structures existence, use and treatments. All external loads which are responses to environmental phenomena shall be regarded as environmental loads, e.g. support reactions, mooring forces, and inertia forces.

Sec.1 C
221   Expected loads and response history: Expected load and response history for a specified time period, taking into account the number of load cycles and the resulting load levels and response for each cycle.

Sec.1 C
222   Expected value: The most probable value of a load during a specified time period.

Sec.1 C
223   Fail to safe: A failure shall not lead to new failure, which may lead to total loss of the structure.

Sec.1 C
224   Fatigue: Degradation of the material caused by cyclic loading.

Sec.1 C
225   Fatigue critical: Structure with calculated fatigue life less than three times the design fatigue life.

Sec.1 C
226   Functional loads: Loads which are a necessary consequence of the structure's existence, use and treatment under ideal circumstances, i.e. no environmental loads, for each design condition. All external loads which are responses to functional loads shall be regarded as functional loads, e.g. support reactions and still water buoyancy forces.

Sec.1 C
227   Gross scantlings: Scantlings, including thickness as shown on structural drawings, i.e. the actual (full) thickness provided at the newbuilding stage applies.

Sec.1 C
228   Guidance note: Information in the standard added in order to increase the understanding of the requirements.

Sec.1 C
229   Gust wind velocity: The average wind velocity during a time interval of 3 s. The "N years gust wind velocity" is the most probable highest gust velocity in a period of N years.

Sec.1 C
230   Hard tank area: Usually upper part of the hull providing sufficient buoyancy for a DDF unit.

Sec.1 C
231   High frequency (HF) responses: Defined as rigid body motions at, or near heave, roll and pitch eigenperiods due to non-linear wave effects.

Sec.1 C
232   Hindcasting: A method using registered meteorological data to reproduce environmental parameters. Mostly used for reproducing wave parameters.

Sec.1 C
233   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.

Sec.1 C
234   Installation condition: A temporary condition where the unit is under construction such as mating or in preparation for operational phase such as upending of DDFs, lowering the legs and elevating the self-elevating units or tether pretension for TLPs.

Sec.1 C
235   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.

Sec.1 C
236   Lowest mean daily temperature: The 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 temperature may be defined for specific seasons. In the above definition:
Mean daily average temperature: The statistical mean average temperature for a specific calendar day.
Mean: Statistical mean based on number of years of observations.
 Average: Average during one day and night.

Sec.1 C
237   Low frequency (LF) responses: Defined as TLP rigid body non-linear motions at, or near surge, sway and yaw eigenperiods.

Sec.1 C
238   Lowest waterline: Typical light ballast waterline for ships, wet transit waterline or inspection waterline for other types of units.

Sec.1 C
239   Material strength: The nominal value of material strength to be used in the determination of the design resistance. The material strength is normally based upon a 5% fractile in the lower end of the distribution function for material strength.

Sec.1 C
240   Mean: Statistical mean over observation period.

Sec.1 C
241   Moulded baseline: A horizontal line extending through the upper surface of hull bottom shell.

Sec.1 C
242   Non-destructive testing (NDT): Structural tests and inspection of welds with radiography, ultrasonic or magnetic powder methods.

Sec.1 C
243   Offshore Standard: The DNV Offshore Standards are documents which presents the principles and technical requirements for design of offshore structures. The standards are offered as DNV's interpretation of engineering practice for general use by the offshore industry for achieving safe structures.

Sec.1 C
244   One hour wind velocity: The average wind velocity during a time interval of one hour.

Sec.1 C
245   Operating condition: A conditions wherein a unit is on location for purposes of production, drilling or other similar operations, and combined environmental and operational loadings are within the appropriate design limits established for such operations (including normal operations, survival, accidental).

Sec.1 C
246   P-delta effect: Second order effect due to vertical forces in combination with second order displacements. For self-elevating units the P-delta effect describes non-linear amplification due to second order bending of the legs for the unit in the hull elevated mode. For DDF units the P-delta effect describes global bending or shear effects due to relatively high roll or pitch angles in harsh environment.

Sec.1 C
247   Potential: The voltage between a submerged metal surface and a reference electrode.

Sec.1 C
248   Recommended Practice (RP): The Recommended Practice publications cover proven technology and solutions which have been found by DNV to represent good practice, and which represent one alternative for satisfying the requirements given in the DNV Offshore Standards or other codes and standards cited by DNV.

Sec.1 C
249   Redundancy: The ability of a component or system to maintain or restore its function when a failure of a member or connection has occurred. Redundancy can be achieved for instance by strengthening or introducing alternative load paths.

Sec.1 C
250   Reference electrode: Electrode with stable open-circuit potential used as reference for potential measurements.

Sec.1 C
251   Reliability: The ability of a component or a system to perform its required function without failure during a specified time interval.

Sec.1 C
252   Representative value: The value assigned to each load for a design situation.

Sec.1 C
253   Resistance: The reference value of structural strength to be used in the determination of the design strength. The resistance is normally based upon a 5% fractile in the lower end of the distribution function for resistance.

Sec.1 C
254   Retrieval condition: A condition, normally applicable for self-elevating units only, and for which the unit is lowering the hull and elevating the legs.

Sec.1 C
255   Ringing: The non-linear high frequency resonant response induced by transient loads from high, steep waves.

Sec.1 C
256   Riser frame: Framed steel structures installed at different vertical elevations along the hull or moonpool in order to separate the different risers.

Sec.1 C
257   Risk: The qualitative or quantitative likelihood of an accidental or unplanned event occurring considered in conjunction with the potential consequences of such a failure. In quantitative terms, risk is the quantified probability of a defined failure mode times its quantified consequence.

Sec.1 C
258   Self-elevating unit or jack-up: A mobile unit having hull with sufficient buoyancy to transport the unit to the desired location, and that is bottom founded in its operating mode. The unit reaches its operating mode by lowering the legs to the seabed and then jacking the hull to the required elevation.

Sec.1 C
259   Service temperature: The service temperature is a reference temperature on various structural parts of the unit used as a criterion for the selection of steel grades.

Sec.1 C
260   Shakedown: A linear elastic structural behaviour is established after yielding of the material has occurred.

Sec.1 C
261   Slamming: Impact load on an approximately horizontal member from a rising water surface as a wave passes. The direction of the impact load is mainly vertical.

Sec.1 C
262   Specified minimum yield strength (SMYS): The minimum yield strength prescribed by the specification or standard under which the material is purchased.

Sec.1 C
263   Specified value: Minimum or maximum value during the period considered. This value may take into account operational requirements, limitations and measures taken such that the required safety level is obtained.

Sec.1 C
264   Splash zone: The external surfaces of the unit that are periodically in and out of the water. The determination of the splash zone includes evaluation of all relevant effects including influence of waves, tidal variations, settlements, subsidence and vertical motions.

Sec.1 C
265   Springing: The high frequency non-linear resonant response induced by cyclic (steady state) loads in low to moderate sea states.

Sec.1 C
266   Strake: Usually helical devices (strake) welded to outer hull with the purpose of reducing the vortex induced cross-flow motion of DDF hull due to current (mainly). Also the term suppression device may be used to describe the strake.

Sec.1 C
267   Submerged zone: The part of the installation, which is below the splash zone, including buried parts.

Sec.1 C
268   Survival condition: A condition during which a unit may be subjected to the most severe environmental loadings for which the unit is designed. Drilling or similar operations may have been discontinued due to the severity of the environmental loadings. The unit may be either afloat or supported on the seabed, as applicable.

Sec.1 C
269   Sustained wind velocity: The average wind velocity during a time interval (sampling time) of 1 minute. The most probable highest sustained wind velocity in a period of N years will be referred to as the "N years sustained wind". This is equivalent to a wind velocity with a recurrence period of N years.

Sec.1 C
270   Target safety level: A nominal acceptable probability of structural failure.

Sec.1 C
271   Temporary conditions: Design conditions not covered by operating conditions, e.g. conditions during fabrication, mating and installation phases, transit phases, accidental.

Sec.1 C
272   Tensile strength: Minimum stress level where strain hardening is at maximum or at rupture.

Sec.1 C
273   Tension leg platform (TLP): A buoyant unit connected to a fixed foundation by pre-tensioned tendons. The tendons are normally parallel, near vertical elements, acting in tension, which usually restrain the motions of the TLP in heave, roll and pitch. The platform is usually compliant in surge, sway and yaw.

Sec.1 C
274   Transit conditions: The unit conditions in wet transit from one geographical location to another.

Sec.1 C
275   Truss spar: A spar buoy with truss structure for the hull part below hard tank area.

Sec.1 C
276   Unit: A general term for an offshore installation such as ship shaped, column stabilised, self-elevating, tension leg or deep draught floater.

Sec.1 C
277   Usage factor: The ratio between permissible stress and the characteristic strength of the structural member.

Sec.1 C
278   Verification: Examination to confirm that an activity, a product or a service is in accordance with specified requirements.

Sec.1 C
279   Wave frequency (WF) responses: Linear rigid body motions at the dominating wave periods.

Sec.1 C
280   Wet transit: A transit where the unit is floating during the move from one geographical location to another.

Sec.1 C
281   Ultimate strength: Corresponding to the maximum load carrying resistance.

Sec.1
D. Abbreviations and Symbols

Sec.1
D 100   Abbreviations

Sec.1 D
101   The abbreviations given in Table D1 are used in this standard.

Sec.1 D
Table D1 Abbreviations 
Abbreviation In full 
AISC American Institute of Steel Construction 
API American Petroleum Institute  
ASD allowable stress design 
BS British Standard (issued by British Standard Institution) 
CTOD crack tip opening displacement 
DDF deep draught floaters 
DFF design fatigue factor 
DNV  Det Norske Veritas 
DP dynamic positioning 
EHS extra high strength 
FE finite elements 
HAT highest astronomical tide 
HF high frequency 
HISC hydrogen induced stress cracking 
HRTLP heave resisted TLP 
HS high strength  
IC inspection category 
IIP in service inspection program 
ISO International Organisation for Standardisation 
LAT lowest astronomic tide 
LF low frequency 
LRFD load and resistance factor design 
MPI magnetic particle inspection 
MSL mean sea level 
NACE National Association of Corrosion Engineers 
NDT non destructive testing  
NS normal strength 
QTF quadratic transfer function 
RAO response amplitude operator 
RP recommended practice 
SCF stress concentration factor 
SMYS specified minimum yield stress 
SNAME Society of Naval Architects and Marine Engineers 
TLP tension leg platform 
TLWP tension leg wellhead platform 
VIV vortex induced vibrations 
WF wave frequency 
WSD working stress design 

Sec.1
D 200   Symbols

Sec.1 D
201   The following units are used in this standard:
g gram 
k kilo 
m meter 
cm centimetre 
mm millimetre 
t tonne 
N Newton 
s second. 



Sec.1 D
202   The following Latin characters are used in this standard:
a sectional area of weld 
raster  the intercept of the design S-N curve with the log N axis 
a0 total connection area at supports of stiffeners 
ah horizontal acceleration 
av vertical acceleration 
b breadth of plate flange 
be effective flange width 
c flange breadth 
d web height 
dp diameter of pipe 
f distributed load factor for primary design 
fE elastic buckling stress 
fr strength ratio 
fu lowest ultimate tensile strength 
fw strength ratio 
fy yield stress 
g0 acceleration due to gravity 
h the shape parameter of the Weibull stress range distribution 
hop vertical distance from the load point to the position of maximum filling height 
k roughness height 
ka factor for aspect ratio of plate field 
km bending moment factor 
kpp factor dependent on support condition for plate 
kps factor dependent on support condition for stiffener 
kt shear force factor 
l stiffener span 
l0 distance between points of zero bending moments 
m the inverse slope of the S-N curve 
ni the number of stress variations in i years 
n0 total number of stress variations during the lifetime of the structure 
p lateral tank or sea pressure 
pd lateral pressure  
Pdyn pressure due to flow through pipes 
ps permanent sea pressure 
pe environmental sea pressure 
q distributed load 
qc contact pressure 
r root face 
s stiffener spacing 
t thickness  
t0 net thickness abutting plate 
tf thickness of flange 
tk corrosion addition 
tm factor used in formulas for minimum plate thickness 
tp thickness of pipe 
tw web thickness 
tW throat thickness of weld 
xD load effect with a return period of D-year 
zb vertical distance from moulded base line to load point 
A area 
AW web area 
C buckling coefficient 
Ce effective plate flange factor 
CD hydrodynamic coefficient, drag 
CM hydrodynamic coefficient, added mass 
CS shape coefficient for wind force 
CW reduction factor due to wave particle motion 
D number of years 
DD vertical distance from moulded base line to underside of deck structure  
Dm diameter of member 
DB depth of barge 
E modulus of elasticity, 2.1 105 N/mm2 
FV maximum axial force 
FVP maximum required preload  
Fx(x) long-term peak distribution 
Hs significant wave height 
KC Keulegan-Carpenter number 
L length 
Li variables used in determining splash zone 
M bending moment 
Mc mass of component 
Me eccentricity moment 
MO overturning moment 
Mp plastic moment resistance 
MS stabilising moment 
MU maximum moment restraint 
My elastic moment resistance 
N number of stress cycles to failure 
ND total number of load effect maxima during D years 
Np number of supported stiffeners on the girder span 
Ns number of stiffeners between considered section and nearest support 
P load 
PE Euler buckling load 
PH horizontal force 
Pp average point load 
PV vertical force 
R radius of curvature, or equivalent radius of spudcan contact area  
S stress range 
Sg girder span 
SZL lower limit of the splash zone 
SZU upper limit of the splash zone 
T wave period 
TE extreme operational draught 
TTH heavy transit draught 
TZ average zero-upcrossing period 
Ui variables used in determining splash zone 
Um maximum orbital particle velocity 
Z steel grade with proved through thickness properties 
Zs section modulus for stiffener section 
Zg section modulus for simple girder section. 



Sec.1 D
203   The following Greek characters are used in this standard:
alength ratio
bcoefficient depending on type of structure, failure mode and reduced slenderness
bwcorrelation factor
erelative strain
G( )the complete gamma function
gssafety coefficient
h0basic usage factor
hpmaximum permissible usage factor
jangle between the stiffener web plane and the plane perpendicular to the plating
lreduced slenderness parameter
qrotation
rdensity
sstress
sfwyield stress of weld deposits
sjequivalent stress for global in-plane membrane stress
Dsampl_n0extreme stress amplitude
Dsniextreme stress range
Dsn0extreme stress range
sppermissible stress
sp1permissible bending stress
sp2permissible bending stress
s^normal stress perpendicular to an axis
sxmembrane stress in x- direction
symembrane stress in y- direction
tshear stress
tppermissible shear stress
t^shear stress perpendicular to an axis
t||shear stress parallel to an axis
ystress ratio.
DNV-OS-C201 Structural Design of Offshore Units (WSD method) [Table of Contents] Sec.2: Design Principles