Structural Design of Offshore Ships: Offshore Standard DNV-OS-C102
Structural Design of Offshore Ships: Offshore Standard DNV-OS-C102
Structural Design of Offshore Ships: Offshore Standard DNV-OS-C102
DNV-OS-C102
STRUCTURAL DESIGN OF
OFFSHORE SHIPS
APRIL 2004
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Offshore Standard DNV-OS-C102, April 2004
Changes – Page 3
CONTENTS
B. Design Loads for Minimum Structural Capacity ................. 20 D. Design Loads and Calculation of Stress Ranges ..................28
B 100 General principles ........................................................... 20 D 100 Load factors .................................................................... 28
D 200 Fatigue loads................................................................... 28
C. Design Loads for Global Hull Girder Capacity Assessment 20 D 300 Topside structures ........................................................... 29
C 100 Application...................................................................... 20 D 400 Turret structure, bow recess and moonpool area ............ 29
D 500 Calculation of global dynamic stress ranges .................. 29
D. Still Water Loads .................................................................. 20 D 600 Calculation of local dynamic stress ranges..................... 29
D 100 General............................................................................ 20 D 700 Combination of stress components................................. 29
E. Environmental Loads............................................................ 20 E. Calculation of Fatigue Damage ............................................29
E 100 General............................................................................ 20 E 100 Environmental loads ....................................................... 29
CHAPTER 0
CONTENTS PAGE
Sec. 1 Introduction ................................................................................................................................ 9
SECTION 1
INTRODUCTION
302 The hull of conventional ships intended for conversion course of action is preferred or particularly suitable. Alterna-
to an offshore unit comply with the requirements to structural tive courses of action are allowable under the standard where
strength, welds and material qualities provided the hull comply agreed between contracting parties but shall be justified and
with the ‘Main Class Requirements’ and satisfy the criteria for documented.
Benign Water according to 401. 103 May: Indicates a permission, or an option, which is per-
B 400 Decision criteria for world-wide and benign mitted as part of conformance with the standard.
waters C 200 C 200 Terms
401 Benign Waters is defined by: 201 Standard terms are given in DNV-OS-C101.
MWB γfi γnc ≤ 1.17 ΜWR + 0.17MS 202 Transit conditions: All unit movements from one geo-
and worldwide operation is defined by: graphical location to another.
MWB γfi γnc > 1.17 ΜWR + 0.17MS 203 Floating production and offloading unit: A unit used for
where the production of oil with arrangement for offloading to a shut-
tle tanker. The units normally consist of a hull, with turret or
MWB= linear wave bending moment at an annual probability spread mooring arrangement, and production facilities above
of exceedance 10-2 (100 years return period) the main deck. The unit can be relocated, but is generally lo-
γfi = partial load coefficient = 1.15 cated on the same location for a prolonged period of time.
γnc = non-linear correction factor 1.1 in sagging and 0.9 in 204 Floating storage and offloading unit: A unit used for
hogging condition unless otherwise documented 1) storage of oil with arrangement for offloading to a shuttle tank-
ΜWR= absolute value of wave bending moment as given in the er. The units normally consist of a hull, with turret or spread
Rules for Classification of Ships Pt.3 Ch.1 mooring system. The unit is equipped for crude oil storage.
MS = absolute value of maximum still water bending mo- The unit can be relocated, but is generally located on the same
ment. location for a prolonged period of time.
1) The default values given are for ships of conven- 205 Floating production, storage and offloading unit: A
tional hull form. The non-linear correction factors for unit used for the production and storage of oil with arrange-
unconventional hull forms shall be documented by di- ment for offloading to a shuttle tanker. The unit is equipped for
rect calculations. crude oil storage. The units normally consist of a hull, with tur-
ret or spread mooring arrangement, and production facilities
Guidance note: above the main deck. The unit can be relocated, but is general-
Applicable chapter of this standard for worldwide and benign ly located on the same location for a prolonged period of time.
waters is illustrated as guidelines in Fig.1.
206 Floating production, drilling, storage and offloading
unit: A unit used for drilling, storage and production of oil with
arrangement for offloading to a shuttle tanker. The unit is
13
equipped for crude oil storage.
C-102 Part I. 207 Drilling vessel: A unit used for drilling in connection
Significant wave height (m)
12
World wide with exploration and/or exploitation of oil and gas. The unit is
11 operation generally operating on the same location for a limited period of
(above curve) time and is normally equipped with dynamic positioning sys-
tem with several thrusters. The unit follows the normal class
(100 yrs)
10
survey program.
9
C-102 Part II.
208 Well stimulation vessel or well intervention vessel: A
8 Benign waters
unit equipped for performing wireline intervention (without
(below curve) riser) of subsea wells and or coiled tubing of subsea. The unit
7 is generally operating on the same location for a limited period
of time and is normally equipped with dynamic positioning
6 system with several thrusters. The unit follows the normal
100 150 200 250
Lpp (m)
300 350 class survey program.
209 LNG/LPG Floating Production and Storage units: A
Figure 1 unit with facilities for oil and gas producing and storage. The
Applicable chapter of DNV-OS-C102 unit is typically permanently moored. Due to the complexity of
the unit more comprehensive safety assessment are typically
carried out. The unit is normally equipped with solutions for
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- quick disconnection of mooring lines between the shuttle tank-
er and the oil and gas producing and storage unit.
402 If the significant wave height is less than 8.5 m for a 210 Turret: A device providing a connection point between
probability of exceedance of 10-2 (100 years return period), be- the unit and the combined riser- and mooring- systems, allow-
nign waters can be assumed without further calculation. ing the unit to freely rotate (weather vane) without twisting the
risers and mooring lines.
211 Main Class Requirements: Provisions and requirements
C. Definitions given in DNV Rules for Classification of Steel Ships Pt.3. Ch.1
or Ch.2.
C 100 Verbal forms 212 Load and Resistance Factor Design (LRFD): Method
101 Shall: Indicates a mandatory requirement to be fol- for design where uncertainties in loads are represented with a
lowed for fulfilment or compliance with the present standard. load factor and uncertainties in resistance are represented with
Deviations are not permitted unless formally and rigorously a material factor.
justified, and accepted by all relevant contracting parties. 213 Benign Waters: Environments at which the required to
102 Should: Indicates a recommendation that a certain hull girder capacity calculated according to the LRFD method
is less than the minimum section modulus according to the C 400 Abbreviations
‘Main Class Requirements’. 401 The abbreviations given in Table C3 are used in this
standard. Definitions are otherwise given in DNV-OS-C101
C 300 Symbols ‘Design of Offshore Steel Structures, General’ (LRFD meth-
301 The following Latin characters are used in this standard: od)
CHAPTER 1
WORLD-WIDE OPERATION
CONTENTS PAGE
Sec. 1 Introduction ............................................................................................................................. 15
Sec. 2 Structural Categorisation, Material Selection and Inspection Principles ................................. 16
Sec. 3 Design Basis and Principles ..................................................................................................... 18
Sec. 4 Design Loads............................................................................................................................ 20
Sec. 5 Structural Analyses for Capacity Checks................................................................................. 24
Sec. 6 Ultimate Limit States (ULS) .................................................................................................... 25
Sec. 7 Fatigue Limit States (FLS)....................................................................................................... 28
Sec. 8 Accidental Limit States (ALS)................................................................................................. 31
Sec. 9 Special Considerations ............................................................................................................. 32
Sec. 10 Welding and Weld Connections............................................................................................... 33
SECTION 1
INTRODUCTION
A. General
A 100 Assumptions and applications
104 The requirements given in this chapter are supplementa-
101 This chapter provides requirements and guidance to the ry to the “main class requirements” This implies that the unit
structural design of offshore ships constructed in steel for any
defined environmental condition. Reference is made to Ch.0 of shall comply with the “main class requirements” for the mid-
this standard for detailed description of the area of application. ship section modulus. These requirements are based on the
wave bending moments for the North Atlantic at an annual
102 The hull girder capacity is based on the principles of the probability of 10-1.3 (20 year return period).
Load and Resistance Factor Design method, referred to as
LRFD method, and is described in DNV-OS-C101. 105 The standard has been written for general world-wide
103 The design wave bending moments and shear forces at application. Costal State requirements may include require-
an annual probability of 10-2 (100 year return period) are de- ments in excess of the provisions of this standard depending on
termined by means of direct calculations based on a site specif- size, type, location and intended service of the offshore unit or
ic wave scatter diagram. installation.
SECTION 2
STRUCTURAL CATEGORISATION, MATERIAL SELECTION AND
INSPECTION PRINCIPLES
SECTION 3
DESIGN BASIS AND PRINCIPLES
A. Design Basis 106 Hull structural elements with less importance for overall
structural integrity such as deckhouses, elements within the
A 100 Operational modes fore and aft unit structure, may be assessed according to the
main class requirements unless otherwise noted. See also Sec.6
101 A unit shall be designed for all relevant modes of oper- B303.
ation. Typically, the assessment of the unit shall be based on
the following operational modes: Guidance note:
Fore and aft unit is normally areas outside 0.4 L amidships or the
— all operating conditions, intact and damaged, at the design cargo area whichever is the larger.
location(s)
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
— all transit conditions
— docking condition afloat
— dry-docking condition. B 200 Global strength
201 For the intact and damage operating conditions, the hull
A 200 Still water load conditions girder capacity shall be based on the LRFD method as defined
201 All relevant still water loading conditions shall be con- in DNV-OS-C101. The hull girder will typically comprise the
sidered to determine the limit curves for maximum permissible following stiffened panels in:
bending moments and shear forces.
— deck and bottom
A 300 Environmental loads — longitudinal bulkheads
301 Environmental loads shall in principle be based on site — sides and inner sides
specific data representative for the areas in which the unit is to — inner bottom
operate. For units, such as FPSOs, operating on a location for — longitudinal stringers and girders.
long period of time, the specified scatter diagram will govern
the areas of operation for the unit. 202 The structural capacity of the hull girder shall comply
with the ULS and ALS requirements given in the respective
302 For units, such as drilling vessels, operating on a given sections in this standard.
location for a limited period of time, the North Atlantic scatter
diagram as defined in DNV Classification Note 30.5 is consid- 203 The strength of the hull girder shall be assessed based on
ered sufficient for non-restricted operations, and shall be used the load conditions that result in maximum longitudinal ten-
as basis for assessment of the ULS and FLS. sion and compression stresses in deck and bottom plating. This
will normally be the extreme full load and ballast condition.
A 400 Prolonged survey periods The effect of topside facilities shall be included in the hull
girder assessment for these load conditions.
401 Units intended to stay on location for a prolonged survey
period, i.e. without dry-docking, shall also comply with the re- 204 The effect of large openings in the hull (e.g. moonpool)
quirements in Appendix A. which affect the distribution of global stresses shall be consid-
ered accounting for three dimensional effects.
B 300 Local strength assessment
B. Strength Assessment 301 Local strength assessment shall be carried out for typical
elements like:
B 100 Compliance with main class requirements
101 The requirements given in this standard are supplemen- — supporting structure for topside structure
tary to the requirements for main class. — supporting structure for thrusters
— turret
102 The main class requirements for plates and stiffeners ex- — crane pedestals
posed to local loads ensure sufficient safety level for local ca-
pacity, and need no further assessment unless otherwise noted. — bow recess area for submerged type of turrets.
103 Non-operating conditions like transit conditions, dock- 302 The supporting structure for topside structure (e.g. top-
ing condition afloat and dry-docking condition, are considered side modules, derrick, flare etc.) shall be assessed according to
to be covered by the main class requirements. the LRFD format. The extent of the supporting structure is lim-
ited to the structural members affected by the local loads from
104 In the transit condition, the design values of global ac- the topside structure. Structural capacity of the supporting
celerations for assessment of topside facilities and supporting structure shall be assessed according to DNV-OS-C101.
structure may be taken from the wave load analysis provided
the assessment is in accordance with 302. The wave load anal- 303 Supporting structure for thrusters are normally consid-
ysis shall in such cases be based on an annual probability of ex- ered for a specified thrust using the acceptance criteria given in
ceedance of 10-1.3 (20 year return period). the Rules for Classification of Ships Pt.3 Ch.1.
For units which are intended to operate on a specific location 304 The turret shall be analysed according to the LRFD for-
for the main part of the design life, wave loads can be based on mat based on the specific loads from the mooring system in the
the actual transit route and season at an annual probability of operational mode. Both global and local response of the turret
exceedance of 10-1.0 (10 year return period), or on the Rules shall be considered. In addition the local structure shall be con-
from a recognised Marine Warranty. sidered for the special load cases as defined in Sec.8 C101.
105 The main class requirements for plates and stiffeners on 305 The supporting structure for the turret shall be assessed
transverse bulkheads are considered to provide sufficient based on the LRFD format. Hull deformations shall be consid-
structural capacity. ered.
C. Fatigue Assessment 102 The fatigue capacity is calculated assuming that the lin-
ear accumulated damage (Palmgren – Miner rule). The differ-
C 100 General principles ent methods given in Classification Note 30.7 are used at
101 Fatigue capacity shall be carried out according to Clas- different stages in the design loop. Applicable method can also
sification Note 30.7 or DNV-RP-C203. be selected depending on the results from a screening process
to identify fatigue critical details.
SECTION 4
DESIGN LOADS
E. Environmental Loads
C. Design Loads for Global Hull Girder
Capacity Assessment E 100 General
C 100 Application 101 Environmental loads are loads caused by environmental
phenomena. The characteristic value of an environmental load
101 The design loads given in D and E are used to assess the is the maximum or minimum value (whichever is the most un-
hull girder capacity (global). Loads used in the global check favourable) corresponding to a load effect with a prescribed
shall be consistent. This implies that the longitudinal stresses probability of exceedance.
based on global load conditions in D and E shall be combined
with transverse stresses based on sea pressures and tank pres- 102 The long-term variation of environmental phenomena
sures as defined in DNV-OS-C101. such as wind, waves and current shall be described by recog-
nised statistical distributions relevant to the environmental pa-
rameter considered, see DNV-OS-C101. Information on the
joint probability of the various environmental loads may be
D. Still Water Loads taken into account if such information is available and can be
adequately documented.
D 100 General 103 Consideration shall be given to responses resulting from
101 With reference to DNV-OS-C101, the still water loads the following listed environmental loads:
consist of the permanent and variable functional loads.
— wave induced loads
102 Permanent functional loads relevant for offshore units — wind loads
are: — current loads
— snow and ice loads
— mass of the steel of the unit including permanently in- — sloshing in tanks
stalled modules and equipment, such as accommodation, — green water on deck
helicopter deck, cranes, drilling equipment, flare and pro- — slamming (e.g. on bow and bottom in fore and aft ship)
duction equipment. — vortex induced vibrations (e.g. resulting from wind loads
— mass of mooring lines and risers. on structural elements in a flare tower).
103 Variable functional loads are loads that may vary in E 200 Wave induced loads
magnitude, position and direction during the period under con-
sideration. 201 The wave loads shall be determined by methods giving
adequate description of the kinematics of the liquid, reflecting
104 Typical variable functional loads are: the site specific environment in which the unit is intended to
— hydrostatic pressures resulting from buoyancy operate, see DNV-OS-C101 and Classification Note 30.5.
— crude oil 202 Global linear wave induced loads such as bending mo-
ments and shear forces shall be calculated by using either strip These methods may include several different types of station-
theory or three dimensional sink source (diffraction) formula- keeping systems such as internal and submerged turret sys-
tion. tems, external turret, buoy, fixed spread mooring and dynamic
203 Linear wave induced loads are normally calculated by positioning. Each mooring system configuration will impose
3D sink-source theory. Strip theory may be used provided: loads on the hull structure. These loads shall be considered in
the structural design of the unit, and combined with other rele-
L pp vant load components.
- ≥ 3.0
--------
B E 400 Sloshing loads in tanks
Guidance note: 401 In partly filled tanks sloshing occurs when the natural
Three-dimensional effects in fore and aft ship will reduce the periods of the tank fluid is close to the periods of the motions
drag force compared to a strip theory approach. of the unit. Factors governing the occurrence of sloshing are:
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
— tank dimensions
204 When a 3-D diffraction program is used, due considera- — tank filling level
tion shall be given to the analytical model to determine the hull — structural arrangements inside the tank (wash bulkheads,
response with sufficient accuracy. web frames etc.)
205 The following wave induced linear responses shall be — transverse and longitudinal metacentric height (GM)
calculated: — draught
— natural periods of unit and cargo in roll (transverse) and
— motions in six degrees of freedom pitch (longitudinal) modes.
— vertical bending moment at a sufficient number of posi-
tions along the hull. The positions shall include the areas 402 The pressures generated by sloshing of the cargo or bal-
where the maximum vertical bending moment and shear last liquid shall be considered according to the requirements
force occur and at the turret position. The vertical wave in- given in the Rules for Classification of Ships Pt.3 Ch.1 Sec.4
duced bending moment shall be calculated with respect to C300.
the section’s neutral axis
— horizontal bending moment Guidance note:
— torsional moment if relevant The Rules for Classification of Ships differentiate between ordi-
— accelerations nary sloshing loads (non-impact) and sloshing impact loads.
— axial forces ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
— external sea pressure distribution.
206 The mass model shall be made sufficiently detailed to E 500 Green water
give centre of gravity, roll radius of inertia and mass distribu- 501 The green water is the overtopping by water in severe
tion as correct as practically possible. wave conditions. The forward part of the deck and areas aft of
207 Non-linear effects such as slamming, water on deck and midship will be particularly exposed to green water. Short
bow flare forces shall be considered with respect to local and wave periods are normally the most critical.
global consequences. 502 Appropriate measures shall be considered to avoid or
208 The midship bending moments and shear forces shall be minimise the green water effects on the hull structure, accom-
calculated considering the weather vaning characteristics of modation, deckhouses and topside equipment. These measures
the unit. E.g. for turret moored units, the calculations are nor- include bow shape design, bow flare, bulwarks and other pro-
mally carried out for head seas. tective structure. Adequate drainage arrangements shall be
209 Torsional moments may be of interest depending on the provided.
structural design. 503 Structural members exposed to green water shall be de-
210 The wave shear forces shall be determined at a sufficient signed to withstand the induced loads. Green sea loads is con-
number of sections along the hull to fully describe the limit sidered as local loads, but shall be combined with the effect
curve for the maximum value. from global response.
211 If roll resonance occurs within the range of wave periods 504 In lack of more exact information, for example from
likely to be encountered, the effect of non-linear viscous roll model testing, the design pressure acting on weather deck shall
damping shall be taken into account. be:
212 Viscous effects, such as eddies around the hull, act 2
mainly as a damping mechanism, especially at large roll an- p = ab ( p dp – ( 4 + 0.2k s )h 0 ) ( kN ⁄ m )
gles, and these effects shall be included.
213 The effects from roll damping devices, like bilge keels, Minimum pressure is 5.0 kN/m2.
shall be evaluated. The roll damping shall be evaluated for the The design pressure on topside support structure, unprotected
return period in question. bulkheads of deck houses and superstructures located forward
of 0.15 L from F.P. shall be calculated according to the pres-
E 300 Mooring loads sures given in Table E1, whichever is the greatest for the posi-
301 A unit may be kept on location by various methods. tion in question.
Guidance note:
Table E1 Design pressure for topside supports, deckhouses and Note that the speed V = 8 knots should also be used as minimum
superstructure for moored or dynamically positioned units to ensure sufficient
minimum pressure.
Structure Pressure kN/m2
p1 = 5.7 khs (2 +L/120)(kw Cw – h0) c ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
Unprotected p2 = 3.4 (2 + L/120)[(hs/8.5)0.25 1.07 kw Cw – h0] c 505 The required local scantlings shall be according to the
front bulkheads p = 12.5 + 0.05 L for first 4 m above the forecastle
3 Rules for Classification of Ships Pt.3 Ch.1 using the design
deck p3 = 6.25 + 0.025 L elsewhere pressure as given in 504.
Unprotected p4 = Pdp – (4 + 0.2ks) h0 minimum p3
sides and top- 506 Glass thickness of windows in unprotected front bulk-
side supports heads as well as the design of the fastening arrangement to the
Unprotected aft p5 = 0.85 p4 minimum p3
bulkheads shall be considered using the design pressures given
end bulkheads in Table E1.
507 Topside members located in the midship or aft area of
a = 1.0 for weather decks forward of 0.15 L from F.P., the unit shall be based on p4 in Table E1.
or forward of deckhouse front, whichever is the Guidance note:
foremost position
It is advised that provisions are made during model testing for
= 0.8 for weather decks elsewhere suitable measurements to determine design pressures for local
b = 1.5 at unit's side and 1.75 at the centre line. Linear structural design. This implies that model tests should be per-
interpolation shall be used for intermediate loca- formed at design draught, for sea states with a spectrum peak pe-
tions riod approximately 70 to 100% of the pitch resonance period of
y 2 the unit. The unit model should be equipped with load cells on
Pdp = P l + 135 ---------------- – 1.2 ( T – z ) (kN/m ) the weather deck at positions of critical structural members or
B + 75 critical topside equipment.
Pl = ks Cw + kf ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
V V
= ( k s C w + k f ) 0.8 + 0.15 ---------- if ---------- > 1.5 E 600 Strengthening against bottom slamming
L1 L1
601 When lacking more exact information, for example from
V = speed in knots, minimum 8 model testing, relevant requirements to strengthening against
CB = block coefficient bottom slamming in the bow region are given in the Rules for
2.5 Classification of Ships Pt.3 Ch.1 Sec.6 H200. The bow region
ks = 3 C B + ----------- at A.P. and aft
CB is normally to be taken as the region forward of a position 0.1 L
= 2.0 between 0.2 L and 0.7 L from aft aft of F.P. and above the summer load waterline.
602 The bottom aft of the unit may be strengthened against
4.0 bottom slamming according to the Rules for Classification of
= 3 C B + -------- at F.P. and forward
CB Ships Pt.3 Ch.1 Sec.6 H200 dependent on draught, hull shape,
environment, heading and relative velocity of the unit.
Between specified areas, ks shall be varied linearly.
Z = vertical distance from the baseline to the load E 700 Strengthening against bow impact
point, maximum T (m) 701 The design of the bow structure exposed to impact loads
Y = horizontal distance from the centre line to the load shall be carried out according to Rules for Classification of
point, minimum B/4 (m) Ships Pt.3 Ch.1 Sec.7 E300. The speed V in knots shall not be
kf = the smallest of T and f less than 8.0.
F = vertical distance from the waterline to the top of
the unit's side at transverse section considered,
maximum 0.8 CW (m)
F. Deformation Loads
L1 = unit length, need not be taken greater than 300 m
C = 0.3 + 0.7(b1/B1) For unprotected parts of machin- F 100 General
ery casings, C is not to be taken less than 1.0 101 Relevant deformation loads for units covered by this
b1 = breadth of deckhouse at position considered standard shall be considered according to the principles given
B1 = maximum breadth of unit on the weather deck at in DNV-OS-C101.
position considered(b1/B1) not to be taken less
than 0.25
khs = – 0.016 h 2 + 0.62 h – 3.15 maximum 1.8 G. Accidental Loads
s s
hs = significant wave height minimum 8.5 m G 100 General
x x 101 Accidental loads are loads related to abnormal operation
kw = 1.3 – 0.6 --- for --- ≤ 0.5
L L or technical failure.
x x
= 0.3 + 1.4 --- for --- > 0.5 102 Attention shall be given to layout and arrangements of
L L facilities and equipment in order to minimise the adverse ef-
x = longitudinal distance in m from A.P. to the load fects of accidental events.
point. G 200 Safety assessment
201 Accidental events that will be a basis for the design shall
be stated in the design brief. Such events are normally identi-
fied in a risk analysis. Typical events are:
SECTION 5
STRUCTURAL ANALYSES FOR CAPACITY CHECKS
SECTION 6
ULTIMATE LIMIT STATES (ULS)
A 100 General Table B1 Partial coefficients for the Ultimate Limit States
Load category
101 According to the LRFD format, see DNV-OS-C101, two Combination
sets of partial coefficient combinations shall be analysed. Still water loads Environmental loads
These combinations are referred to as the a) and b) combina- a) 1.2 0.7
tions. b) 1.0 1.15
102 The material factor to be used in the ULS assessment of 103 The environmental loads for hull girder global response
the hull girder is 1.15. are mainly wave induced loads. Other environmental loads can
103 The capacity assessment in the ULS condition shall in- normally be neglected.
clude buckling and yield checks. 104 The dimensioning condition for different Mw/Ms ratios
104 Buckling capacity checks shall be performed in accord- is shown in Figure 1. Offshore units also complying with the
ance with DNV-OS-C101 Sec.5. main class requirements will typically have Mw/Ms ratios of
1.4 to 1.6. In such cases the b) combination is dimensioning.
105 Gross scantlings may be utilised in the calculation of the
buckling capacity of the hull structural elements, provided a 105 Combination a) need not be assessed for the hull girder
corrosion protection system in accordance with DNV-OS- capacity if:
C101 is maintained. M W ≥ 0.44 M s
Guidance note:
Note that the Ms in the equations given above include the global
B. Hull Girder Longitudinal Strength effect of top side loads.
B 100 Hull girder bending and shear checks ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
101 The hull girder bending and shear capacity in the oper-
ating conditions shall be checked according to B200 and B300.
The capacity checks are based on the two equations below:
γ f, G , Q M s + γ f, E M w ≤ M g ⁄ γ m
γ f, G , Q Q s + γ f, E Q w ≤ Q g ⁄ γ m
which are critical for the structural integrity of the turret. The — longitudinal strain of upper deck due to global bending.
following list contains typical areas which should be consid-
ered: E 200 Partial load coefficients
— structure in vicinity of riser connection(s) 201 The partial load coefficients to be used are given in Ta-
— riser hang-off structure ble E1.
— structure in way of fairleads
— hang-off structure for mooring line Table E1 Partial coefficients for the Ultimate Limit States
— local structure transferring bearing reactions Combination Load category
— chain lockers G Q E
— pipe supports (single supports and complex structures) a) 1.2 1) 1.2 1) 0.7
— equipment supports
— foundation for transfer system (especially for swivel solu- b) 1.0 1.0 1.3 2)
tions) Load categories are:
— lifting appliances and pad-eyes including structure in way G = permanent load
of these. Q = variable functional load
E = environmental loads
1) To be 1.3 if the loads can not be determined with a high accuracy. Tank
E. Topside Facilities Structural Support loads and topside modules with documented weight better than 10% ac-
curacy can use a load factor of 1.2.
E 100 General 2) A factor of 1.15 can be used for inertia loads caused by hull accelera-
tions.
101 The supporting structure is defined as the area where the
stress pattern in the structural elements is significantly affected 202 The ULS assessment shall be carried out according to
by the topside loads. the requirements in DNV-OS-C101. Both a) and b) combina-
102 The strength of the supporting structure for the topside tions shall be considered.
facilities shall be evaluated considering all relevant operational
load conditions and combinations. For loads in transit condi-
tions, see Sec.3 B103.
103 The following loads shall be considered: F. Fore and Aft Ship
— permanent loads (weight of structures, process and drilling F 100 General
equipment, piping etc.) 101 The local requirements for the structural members in the
— variable loads (equipment functional loads related to liq- fore and aft ship including deck houses and accommodation
uid, machinery, piping reaction forces, helicopter, cranes shall comply with the technical requirements given in the
etc.) Rules for Classification of Ships Pt.3 Ch.1, see also Sec.3
— wave loads B100.
— wave accelerations (inertia loads)
— hull girder vertical deflections 102 The main longitudinal structural members needed for
— wind on topside facilities the global continuity and hull integrity shall be assessed ac-
— snow and ice cording to B200. For evaluation of slamming, sloshing and
— green water green water effects, see Sec.4.
SECTION 7
FATIGUE LIMIT STATES (FLS)
A. Introduction and repair. A design fatigue factor of 1.0 may therefore, be ap-
plied to all structural elements.
A 100 General
101 The general requirements and guidance concerning fa-
tigue criteria are given in DNV-OS-C101 Sec.7. The fatigue C. Structural Details and Stress Concentration
capacity shall be determined according to Classification Note
30.7 or DNV-RP–C203. Evaluation of the fatigue limit state Factors
shall include consideration of all significant loads contributing
to fatigue damage. C 100 General
102 The required fatigue life of new units shall be minimum 101 Fatigue sensitive details in the hull shall be documented
20 years. The effect of mean stresses should normally be ig- to have sufficient fatigue strength. Particular attention should
nored. be given to the following details:
103 The fatigue capacity of converted units will be consid- — main deck, including deck penetrations, bottom structure
ered on a case-by-case basis, and is a function of the following and side shell
parameters: — hull longitudinal stiffener connections to transverse
frames and bulkheads
— results and findings form surveys and assessment of criti- — hopper tank knuckles and other relevant discontinuities
cal details — attachments, foundations, supports etc. to main deck and
— service history of the unit and estimated remaining fatigue bottom structure
life. — topside and hull connections
Guidance note: — hull and turret connections
New structural steel in converted units older than 10 years, may — fairleads and supporting structure
normally be accepted with minimum 15 years documented fa- — openings and penetrations in longitudinal members
tigue life from the time of conversion. — transverse frames
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- — submerged turret and supporting structure
— flare tower
104 The fatigue life shall be calculated considering the com- — riser interfaces
bined effects of global and local structural response. The ex- — major process equipment foundations.
pected dynamic load history shall be the basis for the
calculations. 102 Calculations of stress concentration factors of local de-
tails may be undertaken in accordance with Classification Note
105 Local effects, due to: 30.7. For details not covered by Classification Note 30.7, or
documented in other recognised publications, detailed finite
— slamming element analysis should be carried out for determination of
— sloshing SCFs, according to the procedure given in Classification Note
— vortex shedding 30.7.
— dynamic pressures
— mooring and riser systems 103 Intersection between unit's side longitudinals and trans-
verse bulkheads shall be fitted with double sided brackets.
shall be considered in the fatigue damage assessment, if rele- Guidance note:
vant. In order to cover typical unit structural details, the design should
106 Calculations carried out in connection with the fatigue allow for the following stress concentration factors:
limit state may be based on gross thicknesses (i.e. without de- - the deck should at least be designed for KgKw ≥ 2.5. Openings
ducting the corrosion additions), provided a corrosion protec- may require higher values of SCF
tion system in accordance with DNV-OS-C101 is maintained. - brackets on side longitudinals have typical KgKw of 1.8 to 2.4.
107 In the assessment of fatigue life, consideration shall be ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
given to the stress concentration factors including those due to:
— large openings affecting the global stress distribution
— fabrication tolerances, including due regard to tolerances
— openings and penetrations D. Design Loads and Calculation of Stress
— local effects at connections or attachment of structural el- Ranges
ements, e.g. scallops, brackets etc.
D 100 Load factors
101 In the fatigue calculations, a load factor of 1.0 shall be
used on all dynamic loads.
B. Design Fatigue Factors
D 200 Fatigue loads
B 100 General 201 An overview of fatigue loads is given in Sec.4. Site spe-
101 Units covered by this standard have considerable redun- cific environmental data shall be used for calculation of long
dancy. All elements can therefore be classified as, “without term stress range distribution. Units intended for multi field
substantial consequences for total structural failure”. Offshore developments shall base the fatigue life on the expected dura-
units covered by this standard have regular dry-docking for in- tion on each location employing the scatter diagram for each
spection and repair, and the term “splash” zone has no signifi- field. The most severe environmental loads may be applied for
cance. It implies that all elements are accessible for inspection the complete lifetime of the unit, as a conservative approach.
202 A representative range of load conditions shall be con- topside and other environmental loads resulting in local stress-
sidered. It is generally acceptable to consider the ballast and es in parts of the structure.
the fully loaded condition with appropriate amount of time in 602 Dynamic pressures shall be calculated by means of the
each condition. hydrodynamic model. The transfer function for the dynamic
Guidance note: pressure can be used to calculate local stress transfer functions.
60% in full load and 40% in ballast for the two conditions may As a minimum, the following dynamic local stress components
be used unless otherwise documented, ref. DNV-RP-C102 Struc- shall be considered:
tural Design of Offshore Ships.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
— double hull stresses due to bending of double hull sections
between bulkheads
203 An appropriate range of wave directions and wave ener- — panel stresses due to bending of stiffened plate panels
gy spreading shall be considered. — plate bending stresses due to local plate bending
— deflection induced stresses.
Guidance note:
For weather waning units, and in lack of more detailed documen- 603 Further details regarding calculation of local stress com-
tation, head sea direction may be considered with cos2 wave en- ponents is given in Classification Note 30.7.
ergy spreading.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- D 700 Combination of stress components
701 Global and local stresses shall be combined to give the
The following dynamic loads shall be included in a FLS anal- total stress range for the detail in question. In general, the glo-
ysis as relevant: bal and the local stress components differ in amplitude and
phase. The method of combining these stresses for the fatigue
— global wave bending moments damage calculation will depend on the location of the structur-
— external dynamic pressure due to wave and unit motion al detail. A method for combination of loads is given in Clas-
— internal dynamic pressure due to unit motion sification Note 30.7.
— sloshing pressures due to fluid motion in tanks for units
with long or wide tanks
— loads from equipment and topside due to unit motion and
acceleration. E. Calculation of Fatigue Damage
D 300 Topside structures E 100 Environmental loads
301 The following loads shall be considered for the topside 101 Fatigue analyses shall be based on the site specific envi-
structure: ronmental data and, take appropriate consideration of both glo-
— vertical and horizontal hull deformations due to wave bal and local (e.g. pressure fluctuation) dynamic responses.
bending moment acting on the hull Guidance note:
— wave induced accelerations (inertia loads) These responses do not necessarily have to be evaluated in the
— vortex induced vibrations from wind same model but the cumulative damage from all relevant effects
— vibrations caused by operation of topside equipment should be considered when evaluating the total fatigue damage.
— external dynamic pressure due to wave and unit motion. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
302 Additionally, the following low cycle loads should be
considered where relevant for the topside structure: E 200 Methodology
201 The long term stress range distribution required for the
— hull deformations due to temperature differences fatigue analysis can either be established based on spectral
— hull deformations due to change in filling condition e.g. methods or on a postulated form of the long-term stress range
ballasting or deballasting. distribution defined by a maximum stress amplitude and a
D 400 Turret structure, bow recess and moonpool area Weibull shape parameter.
401 The following loads shall be considered for the fatigue 202 Principal stresses should be used as basis for fatigue life
design of turret structures: calculations. Local plate surface stresses due to plate bending
shall be used in the calculations.
— dynamic loads of mooring lines 203 Simplified fatigue life calculations based on a Weibull
— dynamic loads (tension and bending moment) from risers shape distribution of the long term response is acceptable for
— varying hydrodynamic pressure due to wave load well defined structural details, i.e. with known stress concen-
— varying hydrodynamic pressure due to unit accelerations, tration factors, primarily exposed to hull global bending stress-
including added mass effects es, provided ample fatigue life is documented.
— bearing reactions loads
— inertia loads due to accelerations of the unit 204 Fatigue life calculations based on spectral methods are
— fluctuating reactions in pipe supports due to thermal and either employing component stochastic or full stochastic anal-
pressure induced pipe deflections. ysis, see Classification Note 30.7. A component based stochas-
tic analysis implies that non linear effects can be applied to the
D 500 Calculation of global dynamic stress ranges relevant load component.
501 Global stress ranges shall be determined from the global 205 Local, detailed finite element analysis (e.g. unconven-
hull bending stresses, axial stresses and torsional stresses. Tor- tional details with insufficient knowledge about typical stress
sional stresses may be relevant for structures with extremely distribution) should be undertaken in order to identify local
large deck openings. If applicable, both vertical and horizontal stress distributions, appropriate SCFs, and/or extrapolated
bending moments shall be included. Shear lag effects and stresses to be utilised in the fatigue evaluation, see Classifica-
stress concentrations shall be considered. tion Note 30.7 for further details. Dynamic stress variations
through the plate thickness may have to be considered in such
D 600 Calculation of local dynamic stress ranges evaluations.
601 Local stress ranges are determined from dynamic pres- 206 Explicit account shall be taken for local details such as
sures acting on panels, accelerations acting on equipment and access openings, cut-outs and penetrations.
E 300 Applicable S-N-curves 302 Fatigue calculations for conversions of old tankers to
301 The S-N curves, as defined in Classification Note 30.7, production or storage units shall be based on the stresses from
to be used in the fatigue calculation are given in Table E1. the actual scantlings.
SECTION 8
ACCIDENTAL LIMIT STATES (ALS)
SECTION 9
SPECIAL CONSIDERATIONS
SECTION 10
WELDING AND WELD CONNECTIONS
CHAPTER 2
BENIGN WATERS
CONTENTS PAGE
Sec. 1 Introduction ............................................................................................................................. 37
Sec. 2 Selection of Material and Extent of Inspection........................................................................ 38
Sec. 3 Design Basis and Principles ..................................................................................................... 39
Sec. 4 Design Loads............................................................................................................................ 41
Sec. 5 Structural Analyses for Capacity Checks................................................................................. 42
Sec. 6 Structural Capacity................................................................................................................... 43
App. A Permanently Installed Units ..................................................................................................... 44
SECTION 1
INTRODUCTION
A. General B. Definitions
A 100 Assumptions and applications B 100 Benign waters
101 This Chapter provides requirements and guidance for 101 Benign waters is defined as the environments where re-
offshore ships constructed in steel restricted to operate in “be- quired hull girder capacity calculated according to the LRFD
nign waters”. method is less than the minimum section modulus calculated
102 Ch.2 is also applicable to conversions of old tankers to according to the “main class requirements”.
production or storage units. See Ch.0 of this standard for further explanations.
A 200 Assumptions
201 In addition to the requirements given in Ch.2 of this
standard Ch.1 Sec.9 and Ch.1 Sec.10 shall be complied with.
SECTION 2
SELECTION OF MATERIAL AND EXTENT OF INSPECTION
SECTION 3
DESIGN BASIS AND PRINCIPLES
SECTION 4
DESIGN LOADS
C. Environmental Loads
C 100 General E. Fatigue Loads
101 Definitions, general considerations and required meth- E 100 General
odology for determination of environmental loads are given in
Ch.1 Sec.4. 101 Fatigue loads shall be considered as in Ch.1 Sec.4.
SECTION 5
STRUCTURAL ANALYSES FOR CAPACITY CHECKS
SECTION 6
STRUCTURAL CAPACITY
APPENDIX A
PERMANENTLY INSTALLED UNITS
A. Introduction D. Fatigue
A 100 General D 100 Design fatigue factors
101 The requirements and guidance given in this Appendix 101 Design fatigue factors (DFF) are introduced as fatigue
are supplementary requirements for units that are intended to safety factors. DFF shall be applied to structural elements ac-
stay on location for prolonged periods. cording to the principles given in DNV-OS-C101.
102 The requirements apply in principle to all types of off- 102 The DFF applied to the structural detail depend on the
shore ships.| accessibility for inspection and repair.
103 The units can normally be ballasted to different
draughts, and the term “splash” zone has thus no significance.
Sufficient margin in respect to the lowest inspection waterline
B. Inspection and Maintenance should however be considered depending on the expected
wave heights during the inspection periods.
B 100 Facilities for inspection on location Guidance note:
101 Inspections may be carried out on location based on ap- Normally 1-2 m is considered sufficient margin on the lowest in-
proved procedures outlined in a maintenance system and in- spection waterline in world wide operation.
spection arrangement, without interrupting the function of the ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
unit. The following matters should be taken into consideration
to be able to carry out condition monitoring on location: The DFF applied to offshore ships will therefore be dependent
on the accessibility for inspection and repair and the position
— arrangement for underwater inspection of hull, propellers, of the lowest inspection waterline.
thrusters, rudder and openings affecting the units seawor-
thiness 104 Examples of DFF assigned to different structural ele-
— means of blanking of all openings including side thrusters ments according to the principles given above are given in
— use of corrosion resistant materials for shafts, and glands DNV-OS-C101.
for propeller and rudder 105 Substantial consequences other than pure strength con-
— accessibility of all tanks and spaces for inspection siderations may require higher design fatigue factors. Such
— corrosion protection of hull factors should be given in the design brief.
— maintenance and inspection of thrusters 106 When defining the appropriate design fatigue factor for
— ability to gas free and ventilate tanks a specific fatigue sensitive detail, consideration shall be given
— provisions to ensure that all tank inlets are secured during to the following:
inspection
— Evaluation of likely crack propagation paths (including di-
— measurement of wear in the propulsion shaft and rudder rection and growth rate related to the inspection interval),
bearings may indicate the use of a higher design fatigue factor, such
— testing facilities of all important machinery. that:
a) Where the likely crack propagation indicates that a fa-
tigue failure affect another detail with a higher design
C. Corrosion Protection fatigue factor.
C 100 Maintenance program b) Where the likely crack propagation is from a location
satisfying the requirement for a given "Access for in-
101 A maintenance program shall be made taking into con- spection and repair" category to a structural element
sideration that no dry-docking is planned for the unit. having another access categorisation.