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Sec.4: Ships Pt.3 [Table of Contents] Sec.6: Strength of Topside Structures

DNV-OS-C102 Structural Design of Offshore Ships

[-] Sec.5: Ch.1 Strength

SECTION 5
Ch.1 Strength

Sec.5
A. Introduction

Sec.5
A 100   General

Sec.5 A
101   This section gives the requirements and guidance to the design of hull structure.

Sec.5 A
102   Operating mode for offshore ship shaped units such as well service/drilling units and FPSOs are normally different and depends on units characteristics both for transit and operating conditions. Detailed design basis and environmental loads are specified in unit specific provisions Sec.11 and Sec.12.

Sec.5 A
103   Hull structures affected by topside facilities are to be checked with additional requirements given in Sec.7.

Sec.5 A
104   The design criteria for other hull structures where not addressed in this standard are to conform to recognized practices acceptable to DNV.

Sec.5 A
105   Overview of design principles for hull is given in Table A1, A2 and A3 respectively.

Sec.5 A
Table A1 General design basis  
 

Design conditions 

General Transit Operation and Survival 
 Ship Rules Direct calculation Ship Rules Direct calculation 
Design basis and environmental load level Unit specific provisions Sec.11 and Sec.12 
Local scantlings of hull (plates, stiffeners, girders, beams in general) Ship Rules Pt.3 Ch.1 and unit specific provisions Sec.11 and Sec.12 
Direct strength calculation Ship Rules Pt.3 Ch.1 Sec.12 Part C, D & E + Sec.11 and Sec.12 Ship Rules Pt.3 Ch.1 Sec.12 Part C, D & E + Sec.11 and Sec.12 

Sec.5 A
Table A2 Design loads  
 Design conditions 
Design loads Transit Operation and Survival 
 Ship Rules Direct calculation Ship Rules Direct calculation 
Still water bending moments and shear forces Sec.4, limit curves Sec.4, limit curves 
Green sea Sec.4, Sec.11 and Sec.12 
Design accelerations Ship Rules Pt.3 Ch.1 Sec.4 
Sec.3, wave load analysis
(North Atlantic) 		Ship Rules Pt.3 Ch.1 Sec.4 
Sec.3, wave load analysis with specified seastate  
External sea pressure Ship Rules Pt.3 Ch.1 Sec.4 Ship Rules Pt.3 Ch.1 Sec.4 
Wave bending moments and shear forces  Ship Rules Pt.3 Ch.1 Sec.5 Ship Rules Pt.3 Ch.1 Sec.5 

Sec.5 A
Table A3 Longitudinal strength requirements 
 Design conditions 
Main longitudinal members of hull Transit Operation and Survival 
 Ship Rules Direct calculation Ship Rules Direct calculation 
Yield capacity Ship Rules Pt.3 Ch.1 Sec.5&12  Part C Ship Rules Pt.3 Ch.1 Sec.5&12  Part C 
Buckling capacity Ship Rules Pt.3 Ch.1 Sec.5&13  Part C Ship Rules Pt.3 Ch.1 Sec.5&13  Part C 
Shear strength 1) Ship Rules Pt.3 Ch.1 Sec.5 Part C Ship Rules Pt.3 Ch.1 Sec.5 Part C 
  1. Main shear members as side shell and longitudinal bulkheads
 



Sec.5 A
106   Design principles for foundations and supporting structures of hull equipment and machinery are given in Table A4.

Sec.5 A
Table A4 Design principles for foundations and supporting structures of hull equipment
and machinery 
Supporting structures of thruster  Ship Rules Pt.3 Ch.3 Sec.6 
Rudders, nozzle and steering gears Ship Rules Pt.3 Ch.3 Sec.2 
Supporting structures of helicopter deck and substructure DNV-OS-E401 
Foundations and supporting structures of temporary mooring equipment (e.g. chain stoppers, windlasses or winches, bollards, chocks, etc) Ship Rules Pt.3 Ch.3 Sec.5 
Supporting structures of position mooring equipment (e.g. turret, etc) Sec.12 
Crane pedestal and supporting structures DNV-RP-C102 
Davits and supporting structures of launching appliances
(e.g. life boat, raft, etc) 		Ship Rules Pt.3 Ch.3 Sec.5 

Sec.5
A 200   Corrosion addition

Sec.5 A
201   Corrosion addition tk for tanks containing liquids used for offshore service is given in Table A6. These additions shall be considered in scantlings calculations.

Sec.5 A
Table A5 Corrosion addition tk in mm 
Internal members and plate boundary between spaces of the given category. Tank/hold region 
Within 1.5 m below weather deck or hold top Elsewhere 
Mud tanks /Brine tanks/ Produced water tanks 3.0 1.5 
Methanol tanks /MEG tanks/ Condensate tanks 2.0 1.0 (0.0) 2) 
Plate boundary between given space category Tank/hold region 
Within 1.5 m below weather deck or hold top Elsewhere 
Ballast/Mud/Brine/Produced water tanks towards cargo oil/condensate/MEG/methanol tanks 2.5 1.5 (1.0) 2) 
Ballast/Mud/Brine/Produced water tanks towards other category space 1) 2.0 1.0 
Cargo oil/condensate/MEG/methanol tanks towards other category space 1) 1.0 0.5 (0.0) 2) 
  1. Other category space denotes the hull exterior and all spaces other than water ballast, cargo oil tanks and tanks mentioned above.
  2. The figure in brackets refers to non-horizontal surfaces.
 

Sec.5
B. Compliance with Main Class Requirement, 1A1

Sec.5
B 100   Local scantling requirement

Sec.5 B
101   DNV Rules for Classification of Ships Pt.3 Ch.1 shall be basis for the general hull structures such as:
—plates, stiffeners and simple girders in general
—bulkheads in general
—deck houses and superstructure
—fore- and aft ship structure.

Sec.5
C. Hull Girder Longitudinal Strength

Sec.5
C 100   Application

Sec.5 C
101   This section describes the requirements to the hull girder longitudinal strength for both survival and transit conditions in case it is based on direct calculations (hydrodynamic wave load analysis) according to the principles defined in unit specific provisions Sec.11 and Sec.12.

Sec.5 C
102   The hull girder longitudinal strength calculation is intended to verify ultimate strength of main longitudinal members of the hull such as:
—continuous decks, bottom and inner bottom
—side, inner side and longitudinal bulkheads
—longitudinal stringers and longitudinal girders.


Sec.5 C
103   If using the direct calculation method to check the main hull girder longitudinal members according to the requirement given in C500 and C600, the DNV Rule checks for longitudinal strength (Pt.3 Ch.1 Sec.5, 12 and 13) may be omitted.

Sec.5
C 200   General

Sec.5 C
201   Hull girder longitudinal strength shall be evaluated by FE analysis or using the DNV NAUTICUS Hull program for ultimate strength calculation. The probability of exceedance shall be consistent when global and local loads are combined.

Sec.5 C
202   Horizontal wave bending moments are generally disregarded in the assessment of the hull girder longitudinal strength.

Sec.5 C
203   The wave axial force determined by the hydrodynamic wave load analysis shall be considered.

Sec.5 C
204   The shear correction factor for the longitudinal bulkheads is to be calculated. The factor is defined as the ratio between the corrected still water shear force and actual still water shear force at the relevant section. Ref. DNV Classification Note No.31.3 Sec.4

Sec.5 C
205   Any phase angles between accelerations and bending moments are normally disregarded and it is assumed that maximum acceleration and design bending moments occurs at the same time.

Sec.5 C
206   Gross scantlings (scantlings with corrosion addition included) may be used for the calculation of the hull girder longitudinal strength.

Sec.5
C 300   Analysis model

Sec.5 C
301   The part ship model(s) or the whole ship model shall be used for the FE analysis depending on the structural arrangements of the unit. The purpose of the FE analysis is either to fully satisfy the structural requirements given in C500 and C600, or only to calculate the transverse stress and double bottom/side stress that shall be used as input to DNV NAUTICUS Hull program for ultimate strength check.

Sec.5 C
302   Longitudinal extent of the part ship model should be large enough that structural response is not significantly affected by the boundary conditions.

Sec.5 C
303   Main structural members of the hull are to be represented in FE model. These include inner and outer shell, floor and girder system in double bottom, transverse and vertical web frames, stringers, transverse and longitudinal bulkhead structures.

Sec.5 C
304   The topside loads shall be represented in FE model to account for the effect of topside interface with the hull structure.

Sec.5 C
305   Mesh boundaries in FE model are to simulate the stiffening systems on the actual structures as far as practical and are to represent the correct geometry of the panels between stiffeners. Element mesh size in the part ship model is normally based on the standard mesh size.

Sec.5 C
306   Finer mesh less than the standard mesh size may be necessary for correct representation of structural details in local areas, e.g. moonpool corner, other large deck/bottom openings, etc. For element mesh size in fine mesh areas, see E. Local Detailed Stress Analysis.

Sec.5
C 400   Design loading conditions

Sec.5 C
401   For still water hull girder loads, see Sec.4 B200.

Sec.5 C
402   Local static and dynamic loads from topside, tank pressure and sea pressure shall be considered.

Sec.5 C
403   Dynamic loads shall be determined from the wave load analysis according to the principles given in Sec.3 B500.

Sec.5 C
404   For the purpose of structural analysis, the loading conditions in the loading manual may need to be modified to satisfy the principles given in 401 and 402.

Sec.5 C
405   Detailed design loading conditions for the hull girder longitudinal strength are specified in unit specific provisions Sec.11 and Sec.12.

Sec.5
C 500   Hull girder yield check

Sec.5 C
501   The von Mises criteria should be used for the yield stress control according to the criteria given in Sec.3 C200

Sec.5 C
502   Where elements are smaller than the standard mesh size, the von Mises equivalent membrane stress may be obtained from the averaged stress over the elements within the standard mesh size.

Sec.5 C
503   Where the effect of openings is not considered in the FE model, the von Mises equivalent membrane stress in way of the opening is to be properly modified with adjusting shear stresses in proportion to the ratio of web height and opening height, ref. DNV Classification Note No.31.3 Ch.2.3.2

Sec.5 C
504   Local peak stresses by refined mesh density in local areas described in C306 shall comply with the requirement given in E. Local Detailed Stress Analysis

Sec.5
C 600   Hull girder buckling capacity check

Sec.5 C
601   The hull girder ultimate buckling capacity check of the main longitudinal members is performed by assessment of local stiffened panels subject to:
—longitudinal nominal stress (in direction of primary stiffener for stiffened panel)
—transverse nominal stress (in direction perpendicular to primary stiffener for stiffened panel)
—nominal in-plane shear stress
—local lateral pressure from sea or cargo.


Sec.5 C
602   For units with large opening such as moonpool, the distribution of the global longitudinal bending stress shall be considered with respect to global stress concentration.

Sec.5 C
603   For the hull girder ultimate buckling capacity check using DNV NAUTICUS Hull only, the transverse stresses determined from FE analysis due to bending of main primary members subject to local loads need to be considered. The local stress effects from bending of stiffeners and plates may be omitted. The average membrane stress is to be calculated from a group of elements representing one plate field between stiffeners.

Sec.5 C
604   The permissible still water bending moment and still water shear force curves (limit curves) defined based on the loading manual shall be used together with the characteristic wave bending moments and wave shear forces (limit curves) for the considered transverse section. Phase information between the wave bending moment and the wave shear force may be considered if available.

Sec.5 C
605   The ultimate buckling capacity control of local stiffened panels should be performed according to DNV-RP-C201 and shall comply with the permissible usage factor given in Sec.3 C200. The ultimate buckling capacity estimate of stiffened panels accepts local elastic buckling of plates between stiffeners.

Sec.5 C
606   The buckling usage factor is defined as the ratio between the applied loads and the corresponding ultimate capacity.

raster

Fig. 1   Stress components acting on the hull

Sec.5
D. Transverse Strength

Sec.5
D 100   Application

Sec.5 D
101   Transverse strength refers the strength of the main transverse girder system of the hull such as floors, transverse stringers, transverse bulkheads and transverse web frames which are not directly affected by the hull girder longitudinal loads.

Sec.5 D
102   The transverse strength should be evaluated by use of FE analysis and shall meet the requirement with respect to material yield and buckling capacity given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.12 and 13 respectively.

Sec.5 D
103   Main longitudinal members of the hull shall also be checked in the transverse strength analysis for the cases when direct calculations for the hull girder longitudinal strength are not required in accordance with unit specific provisions Sec.11 and Sec.12.

Sec.5
D 200   General

Sec.5 D
201   Hull girder normal stresses and hull girder shear stresses should not be considered directly from the FE part ship analysis unless special boundary conditions and loads are applied to represent the hull girder bending moments and shear forces correctly.

Sec.5 D
202   The effects of topside facilities and drilling equipment deck loads shall be included, where relevant.

Sec.5 D
203   Net scantlings are to be utilised in the calculation of the transverse strength according to Rules for Classification of Ships Pt.3 Ch.1 Sec.12.

Sec.5
D 300   Analysis model

Sec.5 D
301   Part ship model(s) used for the hull girder longitudinal strength as described in C300 may be used. Detailed considerations are given in unit specific provisions Sec.11 and Sec.12.

Sec.5
D 400   Design loading condition

Sec.5 D
401   All operating conditions specified in the loading manual should be checked for the selection of design loading condition and most onerous loading conditions with the unit resulting in maximum stress response of the main transverse girder system shall be selected.

Sec.5 D
402   Detailed considerations are given in unit specific provisions Sec.11 and Sec.12.

Sec.5
D 500   Material yield check

Sec.5 D
501   Nominal stress derived from FE analysis shall be checked according to the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.12 B400.

Sec.5 D
502   The final thickness of the considered structure is not to be less than the minimum thickness given in DNV Rules for Ships Pt.3 Ch.1 Sec.6 to Sec.10.

Sec.5 D
503   Local structural areas with local stress concentrations e.g. toe of girder bracket shall be evaluated by local detailed stress analysis.

Sec.5
D 600   Buckling capacity check

Sec.5 D
601   Buckling capacity of each individual plate panel between stiffeners shall comply with the requirements given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.13. 602 The buckling capacity check of unstiffened panels in the main transverse girder system of the hull may be performed according to DNV-RP-C201. Ideal elastic buckling strength without accepting any local redistribution of the loads shall be used as basis together with the acceptance criteria given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.13.

Sec.5
E. Local Detailed Stress Analysis

Sec.5
E 100   Application

Sec.5 E
101   Local detailed stress analyses are applicable to local areas of the hull where part ship FE model(s) used for the strength of the main girder system does not represent the local response sufficiently, e.g. toe of girder bracket, etc.

Sec.5 E
102   Areas to be checked are given in unit specific provisions Sec.11 and Sec.12.

Sec.5
E 200   General

Sec.5 E
201   Local structural details shall be evaluated by fine mesh FE analysis or equivalent methods to determine local stress distribution in the local areas which is difficult to achieve with coarse mesh.

Sec.5 E
202   The stress distribution in areas with global stress concentrations and discontinuities, e.g. moonpool openings, turret openings, etc. shall be derived from fine mesh FE analysis.

Sec.5 E
203   Net scantlings are to be utilised in the local strength analysis according to Rules for Classification of Ships Pt.3 Ch.1 Sec.12. Alternatively, gross scantlings may be utilised in case WSD design method defined in Sec.3 C is used for the evaluation.

Sec.5
E 300   Analysis model

Sec.5 E
301   Local models may be included directly in FE model used for the part ship analysis or by separate sub-models with prescribed boundary conditions, displacements and forces.

Sec.5 E
302   If sub-model is used, the extent of the local FE model is to be such that the calculated stresses at the areas of interest are not significantly affected by the imposed boundary conditions and application of loads. The boundary of the FE model should be coincided with primary support members, such as girders, stringers and floors, in the part ship model.

Sec.5 E
303   Local details at the areas of interest should be generally modelled with element mesh size of either 50 ´ 50 mm or 100 ´ 100 mm or 200 ´ 200 mm depending on the actual thickness and geometrical complexity of local details to be checked. Proper attention should be paid to transition of mesh density. Abrupt changes of mesh density should be avoided and transition area should be well off the stress concentration.

Sec.5
E 400   Design loading condition

Sec.5 E
401   The most onerous loading condition among those relevant for the hull girder longitudinal strength analysis or transverse strength analysis shall be applied for the local areas to be assessed.

Sec.5 E
402   If the local fine mesh analysis is run as a sub-model, prescribed boundary deformations or forces taken from the hull girder longitudinal strength analysis or the transverse strength analysis shall be applied. Local loads acting on the structure shall be applied to the model.

Sec.5
E 500   Acceptance criteria

Sec.5 E
501   Permissible peak usage factors given in Table E1 are defined according to the structural components, design method, load combination and applied mesh size.

Sec.5 E
502   The calculated usage factor based on the von Mises equivalent membrane stress at centre of a plane element (shell or membrane) shall not exceed the permissible peak usage factor given in Table E1.

raster

Sec.5 E
Table E1 Permissible peak usage factor (hPeak) for fine mesh FE analysis 
Structural
component 1) 		Design
method 		Load combination
 		Mesh size 
50 ´ 50 mm 100 ´ 100 mm 200 ´ 200 mm 
Hull in general Ship Rules Static+Dyn.
(10-4 level) 2) 		1.53 1.33 1.13 
Hull and topside interface structures WSD method
 		Static 1.36 1.18 1.0 
Static+Dyn.
(10-8 or 10-8.7 level) 2) 		1.7 1.48 1.25 
  1. For details to be checked, see unit specific provisions Sec.11 and Sec.12
  2. See Sec.1 C212 for definition.

Note:
  • Load level is to be consistent according to the applied design method and load combination 

    • Sec.5
    F. Fatigue Strength

    Sec.5
    F 100   Application

    Sec.5 F
    101       Fatigue strength should be evaluated based on fine element mesh models made for the critical stress concentration details for hull and topside supporting structures which are not sufficiently covered by stress concentration factor given in DNV Classification Notes No.30.7 or DNV-RP-C203.

    Sec.5 F
    102       The correlation between different loads such as global wave bending, external and internal dynamic pressure and acceleration of the topside should be considered in the fatigue assessment.

    Sec.5 F
    103       Areas to be checked are given in unit specific provisions Sec.11 and Sec.12.

    Sec.5 F
    104       Low cycle fatigue due to the repetitive effects of loading and unloading for structure with oil storage shall be checked according to principles given in DNV-RP-C206.

    Sec.5
    F 200   Analysis model

    Sec.5 F
    201       The size of the model should be of such extent that the calculated stresses in the hot spots are not significantly affected by the assumptions made for the boundary conditions.

    Sec.5 F
    202       Element size for stress concentration analyses is to be in the order of the plate thickness. Normally, shell elements may be used for the analysis.

    Sec.5
    F 300   Design loading conditions

    Sec.5 F
    301       Design loading conditions to be applied for fatigue analysis are given in unit specific provisions Sec.11 and Sec.12.

    Sec.5
    F 400   Design criteria

    Sec.5 F
    401       Design criteria are given in Sec.8.
    Sec.4: Ships Pt.3 [Table of Contents] Sec.6: Strength of Topside Structures