classification notes no. 34.1 csa - direct analysis of ship structures

CLASSIFICATION NOTESNo. 34.1CSA - DIRECT ANALYSIS OF SHIP STRUCTURES

JANUARY 2011DET NORSKE VERITASVeritasveien 1, NO-1322 Hvik, Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, andcarries out research in relation to these functions.Classification NotesClassification Notes are publications that give practical information on classification of ships and other objects. Examplesof design solutions, calculation methods, specifications of test procedures, as well as acceptable repair methods for somecomponents are given as interpretations of the more general rule requirements.All publications may be downloaded from the Societys Web site http://www.dnv.com/.The Society reserves the exclusive right to interpret, decide equivalence or make exemptions to this Classification Note.Main changesThe main changes are: New class notation CSA-1 and CSA-FLS1 included. CSA-FLS1 has reduced fatigue scope compared to existing class notation CSA-FLS. CSA-1 includes requirements for ULS and CSA-FLS1. Existing notations CSA-FLS and CSA-2 are kept, but CSA-FLS is renamed CSA-FLS2. Include experience from recent project on ore carrier.The electronic pdf version of this document found through http://www.dnv.com is the officially binding version Det Norske Veritas

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Classification Notes - No. 34.1, January 2011

Page 3CONTENTS

1. Introduction............................................................................................................................................ 41.1 Objective ...................................................................................................................................................41.2 General......................................................................................................................................................41.3 Definitions.................................................................................................................................................41.4 Programs ...................................................................................................................................................52. Overview of CSA Analysis ................................................................................................................... 62.1 General......................................................................................................................................................62.2 Scope and acceptance criteria ...................................................................................................................62.3 Procedures and analysis ............................................................................................................................62.4 Documentation and verification overview................................................................................................83. Hydrodynamic Analysis ........................................................................................................................ 83.1 Introduction...............................................................................................................................................83.2 Hydrodynamic model................................................................................................................................93.3 Roll damping...........................................................................................................................................113.4 Hydrodynamic analysis...........................................................................................................................113.5 Design waves for ULS............................................................................................................................123.6 Load Transfer..........................................................................................................................................134. Fatigue Limit State Assessment.......................................................................................................... 154.1 General principles ...................................................................................................................................154.2 Locations for fatigue analysis .................................................................................................................164.3 Corrosion model......................................................................................................................................204.4 Loads.......................................................................................................................................................204.5 Component stochastic fatigue analysis ...................................................................................................214.6 Full stochastic fatigue analysis ...............................................................................................................244.7 Damage calculation.................................................................................................................................275. Ultimate Limit State Assessment........................................................................................................ 295.1 Principle overview ..................................................................................................................................295.2 Global FE analyses local ULS .............................................................................................................295.3 Hull girder collapse - global ULS...........................................................................................................376. Structural Modelling Principles ......................................................................................................... 406.1 Overview.................................................................................................................................................406.2 General ...................................................................................................................................................426.3 Global structural FE-model.....................................................................................................................436.4 Sub models..............................................................................................................................................456.5 Mass modelling and load application .....................................................................................................467. Documentation and Verification ........................................................................................................ 487.1 General....................................................................................................................................................487.2 Documentation........................................................................................................................................487.3 Verification .............................................................................................................................................498. References............................................................................................................................................. 55Appendix A.Relative Deflection Analysis .......................................................................................................................... 56

Appendix B.DNV Program Specific Items ........................................................................................................................ 59

Appendix C.Simplified Hull Girder Capacity Model - MU ............................................................................................. 62

Appendix D.Hull Girder Capacity Assessment Using Non-linear FE Analysis............................................................. 65

Appendix E.PULS Buckling Code Design Principles Stiffened Panels .................................................................... 66DET NORSKE VERITAS


Page 41. Introduction1.1 ObjectiveThis Classification Note for Computational Ship Analysis, CSA, provides guidance on how to perform anddocument analyses required for compliance with the classification notations CSA-FLS1, CSA-FLS2, CSA-1and CSA-2 as described in the DNV Rules for Classification of Ships, Pt.3 Ch.1. The aim of the class notationsis to ensure that all critical structural details are adequately designed to meet specified fatigue and strengthrequirements.

1.2 GeneralCSA-FLS1, CSA-FLS2, CSA-1 and CSA-2 are optional class notations for enhanced structural calculations ofships. All calculations are based on direct calculation of load and response. CSA-FLS1 and CSA-FLS2 coverfatigue analyses, while CSA-1 and CSA-2 additionally covers fatigue and ultimate strength analyses. The CSA notations have requirements for the structural parts and details of the ship hull. Tank systems andtheir supports are not a part of the scope for CSA. Likewise, structural details connected to moorings or offshoreloading systems are outside the scope of CSA. Loads caused by slamming, sloshing and vibration are not included in the CSA notations. This Classification Note describes the following steps of the CSA analyses:

scope of analysis (areas/details to be considered) procedures for:

- modelling- hydrodynamic analyses- structural analysis- ULS post processing- FLS post processing.

acceptance criteria documentation and verification of the analyses.

The CSA notations are applicable to all ship types. Details to be analysed is specified for the following shiptypes:

Tankers LNG carriers (Moss type and membrane type) LPG carriers Container ships Ore carrier.

For other ship types the details are selected on case by case basis. The notations are especially relevant for vessels fulfilling one or more of the following criteria:

novel vessel design increased size compared to existing vessel design operating in harsh environment operational challenges different from similar ships high requirements for minimizing off-hire.

1.3 Definitions

1.3.1 AbbreviationsThe following abbreviations and definitions are used in this Classification Note.

FLS Fatigue Limit StateULS Ultimate Limit StateDNV Det Norske VeritasCSA Computational Ship AnalysisCSA-FLS1 Computational Ship Analysis - Fatigue Limit State with limited scopeCSA-FLS2 Computational Ship Analysis Fatigue Limit State with full scopeCSA-1 Computational Ship Analysis - Fatigue Limit State with limited scope and Ultimate Limit StateCSA-2 Computational Ship Analysis Fatigue Limit State with full scope and Ultimate Limit StateDET NORSKE VERITAS


Page 51.3.2 SymbolsThe following symbols are used in this Classification Note:

1.4 ProgramsThe CSA procedure requires programs with possibility for direct application of pressures and inertia from a 3Dnon-linear hydrodynamic program to a finite element (FE) analysis program with suitable applications and

CSR Common Structural RulesPLUS Class Notation covering additional fatigue requirements based on rule loadsCN Classification NoteSCF Stress concentration factor

D Moulded depthB Moulded breadthTact Actual draughtK Stress concentration factorhot spot Stress at hotspotnominal Nominal stress in structure Roll-angle Wave amplituderp Correction factor for external pressure in waterline regionpd Dynamic pressure amplitudezwl Water head due to external wave pressure at waterlineN Number of cyclesa constant related to mean S-N curvem S-N fatigue parameter Stress rangefm Factor taking into account mean stress ratiof Yield stress of materialf1 Material factore Nominal Von Mises stress Nominal stressg Nominal stress from global bending/axial force2 Nominal stress from secondary bending (e.g. double bottom bending) Nominal shear stress Usage factorAW Effective shear area AWmod Modelled shear areat thicknessp Pressure Densityav Vertical accelerationpn Fraction of time at sea in the different loading conditionsg Gravitational constantMS is the still water vertical bending momentMW is the wave vertical bending momentMUI is the ultimate moment capacity of the intact hull girderMUD is the ultimate moment capacity of the damaged hull girder S Partial safety factors reflecting uncertainties and ensuring the overall required target safety margin

with respect to the still water vertical bending moment D Partial safety factors reflecting uncertainties and ensuring the overall required target safety margin

with respect to the wave vertical bending moment M Partial safety factors reflecting uncertainties and ensuring the overall required target safety margin

with respect to the ultimate moment capacityV maximum service speed in knots, defined as the greatest speed which the ship is designed to main-

tain in service at her deepest seagoing draughtDET NORSKE VERITAS


Page 6post-processing tools to ensure good documentation and verification possibilities for a third party to review.The Nauticus programs provided by DNV are well suited for these analyses. Relevant Nauticus applicationsare described in Section 8. Other programs may also be accepted.

2. Overview of CSA Analysis 2.1 GeneralThe requirements for the CSA notations are given in the Rules for Classification of Ships, Pt.3 Ch.1.CSA notations require compliance with NAUTICUS (Newbuilding) or CSR, whichever is applicable.For class notation CSR this implies that all CSR requirements are to be complied with and documented.For NAUTICUS (Newbuilding) the ULS analysis are to be complied with independent of CSA. Howeverrequirements for FLS need not be performed if compliance with CSA is documented and confirmed. All details except the stiffener-frame connections as defined by the PLUS notation shall also be included inCSA-FLS2 but only the details in 2.2 are to be included in the scope of CSA-FLS1.In case PLUS notation in addition to CSA is specified, calculations for stiffener frame connections have to beperformed according to the procedure specified by the PLUS notation including low cycle fatiguerequirements, while other requirements are documented and confirmed as part of CSA.

2.2 Scope and acceptance criteriaThe CSA procedure includes the following analysis and checks:CSA-FLS1

Fatigue of critical details in cargo hold area:- knuckles- discontinuities- deck openings and penetrations.

CSA-FLS2

Fatigue of longitudinal end connections and frame connection in cargo hold area. Fatigue of bottom and side-shell plating connection to frame/stiffener in the cargo hold area. Fatigue of critical details in cargo hold area:

- knuckles- discontinuities- deck openings and penetrations.

CSA-1

FLS - Fatigue requirements as for CSA-FLS1. Local ULS - Yield and buckling strength of structure in the cargo hold area. Global ULS - Hull girder capacity of the midship section in intact and two damaged conditions.

CSA-2

FLS - Fatigue requirements as for CSA-FLS2. Local ULS - Yield and buckling strength of structure in the cargo hold area. Global ULS - Hull girder capacity of the midship section in intact and two damaged conditions.

Each project should together with the Society define the total scope of the calculations. Note that fatigue andstrength analyses may also be required outside the cargo hold area if deemed necessary by the Society. Somedetails outside the cargo hold area are already specified in the Rules.The design life basis for CSA-analysis, is the minimum design life as defined by class notation NAUTICUS(Newbuilding) or CSR whichever is relevant, as defined in the Rules for Classification of Ships, Pt.3 Ch.1. Theacceptance criteria for fatigue is stated in Section 4.7.1, while the acceptance criteria for Local-ULS andGlobal-ULS is given in Section 5.2.5 and Section 5.3.4 respectively.

2.3 Procedures and analysisThe flowchart in Figure 2-2 shows the typical analysis procedure for a typical CSA.DET NORSKE VERITAS


Page 7Figure 2-1CSA calculation procedure

All calculations shall be based on direct calculated wave loads using a 3D hydrodynamic program includingeffect of forward speed. The pressures and inertia loads from the hydrodynamic analysis shall be transferred tothe FE-models maintaining the phasing definitions.For FLS two principal fatigue calculation methodologies are used to comply with CSA requirements:

full stochastic (spectral) fatigue analysis (Section 4.6) DNV component stochastic method (Section 4.7).

CSA-FLS1 require analysis with full stochastic analysis, while for CSA-FLS2 both analysis procedures areneeded. Two types of ULS analyses are to be carried out, i.e.

1) Global FE analyses local ULS (Section 5.3)Is required for all structural members in the cargo hold area. Linear FE stress analyses are performed for verification of plating, stiffeners, girders etc. against bucklingand material yield. The buckling and ultimate strength limits are evaluated using PULS buckling code. Thisis required for all structural members in the cargo hold area; however buckling is in general only performedfor longitudinal members.

2) Hull girder collapse global ULS (Section 5.4)This ULS assessment is based on separate hull girder strength models accounting for buckling and non-linear structural behaviour of plating, stiffeners, girders etc. in the cross-section. The purpose is to controland ensure sufficient overall hull girder strength preventing global collapse and loss of vessel. Simplifiedstructural models (HULS) or advanced non-linear FE analyses may be used. Both intact and damaged hullsections are to be assessed.DET NORSKE VERITAS


Page 8The CSA analysis is based on a set of different structural FE-models, (Section 6). A global FE-model isrequired for the analyses in addition to models with element definition applicable for evaluation of yield/buckling strength and fatigue strength respectively.

2.4 Documentation and verification overviewThe analysis shall be verified in order to ensure accuracy of the results. Verification shall be documented andenclosed with the analysis report. The documentation shall be adequate to enable third parties to follow each step of the calculations. For thispurpose, the following should, as a minimum, be documented or referenced:

basic input (drawings, loading manual, weather conditions, etc.), assumptions and simplifications made in modelling/analysis, models, loads and load transfer, analysis, results (including quality control), discussion, and conclusion.

Checklists for quality assurance shall also be developed before the analysis work commences. It is suggestedthat project-specific checklists be defined before the start of the project and to be included in the project qualityplan. These checklists will depend on the engineering practices of the party carrying out the analysis, andassociated software.

3. Hydrodynamic Analysis3.1 IntroductionSea keeping and hydrodynamic load analysis for CSA-FLS1, CSA-FLS2, CSA-1 and CSA-2 shall be carriedout using 3-D potential theory, with possibility of forward speed, with a recognized computer program. Non-linear theory needs to be used for design waves for ULS assessment, where non-linear effects are consideredimportant. The program shall calculate response amplitude operators (RAOs, transfer functions) and timehistories for motions and loads in regular waves. The inertia loads and external and internal pressures calculatedin the hydrodynamic analysis are directly transferred to the structural model.For FLS the reference loads shall represent the stresses that contribute the most to the fatigue damage, e.g.typical loading conditions with forward speed in typical trading routes. It is assumed that the loads contributingmost to fatigue damage have short return periods and are therefore small but frequent waves. It is thereforesufficient to use linear analysis for fatigue assessments, since the linear wave loads give sufficientapproximation of the loads for waves with small amplitudes or when ship sides are vertical. For linearizationand documentation purposes, a reference load level of 10-4 is to be used, representing a daily load level.For ULS the loads representing the condition that leads to the most critical response of the vessel shall be found.Normally a design wave, representing the most critical response (load or stress), is applied, and thesimultaneous acting loads (inertia and pressures) at the moment when maximum response is achieved, istransferred to the structural model. Several design waves are defined, representing different structuralresponses. In general the hydrodynamic loads should be represented by non-linear theory for design waveswhere the response is dominated by vertical bending moment and shear force. Other design waves may bebased on linear theory, since the non-linear effects are negligible, or difficult to capture.Figure 3-1 shows a schematic overview of the work flow for the hydrodynamic analysis as part of the CSA-FLS1, CSA-FLS2, CSA-1 and CSA-2 calculations.Section 4.4 and Section 5.2.2 defines loading conditions, environment conditions, etc. applicable for FLS andULS hydrodynamic analysis, respectively.DET NORSKE VERITAS


Page 9Figure 3-1Flow chart of a hydrodynamic analysis for CSA

This section describes the procedure for the hydrodynamic analysis.

3.2 Hydrodynamic model

3.2.1 GeneralThere should be adequate correlation between hydrodynamic and structural models, i.e. both models shouldhave:

equal buoyancy and geometry equal mass, balance and centre of gravity. DET NORSKE VERITAS


Page 10The hydrodynamic model and the mass model should be in proper balance, giving still water shear forcedistribution with zero value at FP and AP. Any imbalance between the mass model and hydrodynamic modelshould be corrected by modification of the mass model.

3.2.2 Hydrodynamic panel modelThe element size of the panels for the 3-D hydrodynamic analysis shall be sufficiently small to avoid numericalinaccuracies. The mesh should provide a good representation of areas with large transitions in shape, hence thebow and aft areas are normally modelled with a higher element density than the parallel midship area. Thehydrodynamic model should not include skewed panels. The number of elements near the surface needs to besufficient in order to represent the change of pressure amplitude and phasing, since the dynamic wave loadsincreases exponentially towards the surface. This is particularly important when the loads are to be used forfatigue assessment. In order to verify that the number of elements is sufficient, it is recommended to double thenumber of elements and run a head sea analysis for comparison of pressure time series. The number of panelsneeded to converge differs from code to code. Figure 3-2 shows an example of a panel model for the hydrodynamic code WASIM.

Figure 3-2Example of a panel model

The panels should, as far as possible, be vertical oriented as indicated to the right in Figure 3-3. This is to easethe load transfer. For component stochastic fatigue analysis transverse sections with pressures are input to theassessment, which is easier with the model to the right.

Figure 3-3Schematic mesh model

3.2.3 Mass modelThe mass of the FE-model and hydrodynamic model has to be identical in order to obtain balance in thestructural analysis. Therefore the hydrodynamic analysis shall use a mass-model based on the global FEstructural model. In many cases, however, the hydrodynamic analysis will be performed prior to the completionof the structural model. A simplified mass model may then be used in the initial phase of the hydrodynamicanalysis. The structural mass model shall be used in the hydrodynamic analysis that establishes the pressureloads and inertia loads for the load transfer.

3.2.3.1 Simplified Mass modelIf the structural model is not available a simplified mass model shall be made. The mass model shall ensure aproper description of local and global moments of inertia around the longitudinal, transverse and vertical globalship axes. The determination of sectional loads can be particularly sensitive to the accuracy and refinement ofthe mass model. Mass points at every meter should be sufficient.

3.2.3.2 FE-based Mass modelThe FE-based mass model is described in Section 6.5.DET NORSKE VERITAS


Page 113.3 Roll dampingThe roll damping computed by 3-D linear potential theory includes moments acting on the vessel hull as a resultof the waves created when the vessel rolls. At roll resonance, however, the 3-D potential theory will under-predict the total roll damping. The roll motion will, consequently, be grossly over-predicted. To adequatelypredict total roll damping at roll resonance, the effect from damping mechanisms not related to wave-making,such as vortex-induced damping (eddy-making) near sharp bilges, drag of the hull (skin friction), skegs andbilge keels (normal forces and flow separation), should be included. Such non-linear roll damping models havetypically been developed based on empirical methods, using numerical fitting to model test data. Example ofnon-linear roll damping methods for ship hulls includes those published by Tanaka /6/ and Kato /9//10/.Results from experiments indicate that non-linear roll damping on a ship hull is a function of roll angle, wavefrequency and forward speed. As the roll angle is generally unknown and depends on the scatter diagramconsidered, an iteration process is required to derive the non-linear roll damping. The following 4-step iteration procedure may be used for guidance:

a) Input a roll angle, xinput, to compute non-linear roll damping.b) Perform vessel motion analysis including damping from a).c) Calculate long-term roll motion, xupdate, with probability level 10-4 for FLS or 10-8 for ULS, using design

wave scatter diagram.d) If xupdate from c) is close to xinput in step a), stop the iteration. Otherwise, set xinput as the mean value

of xupdate and xinput, and go back to a).

Viscous effects due to roll are to be included in cases where it influences the result. Roll motion can affectresponses such as acceleration, pressure and torsion. Viscous damping should be evaluated for beam andquartering seas. The viscous roll damping has little influence in cases where the natural period of the roll modeis far away from the exciting frequencies. For fatigue it is sufficient to calibrate the viscous damping for beamsea and use the same damping for all headings.

3.4 Hydrodynamic analysis3.4.1 Wave headingsA spacing of 30 degree or less should be used for the analysis, i.e. at least twelve headings.

3.4.2 Wave periodsThe hydrodynamic load analysis shall consider a sufficient range of regular wave periods (frequencies) so asto provide an accurate representation of wave energies and structural response.The following general requirements apply with respect to wave periods:

The range of wave periods shall be selected in order to ensure a proper representation of all relevantresponse transfer functions (motions, sectional loads, pressures, drift forces) for the wave period range ofthe applicable scatter diagram. Typically wave periods in the range of 5-40 seconds can be used.

A proper wave period density should be selected to ensure a good representation of all relevant responsetransfer functions (motions, sectional loads, pressures, drift forces), including peak values. Typically 25-30 wave periods are used for a smooth description of transfer functions.

Figure 3-4 shows an example of a poor and a good representation of a transfer function. For the transferfunction with a poor representation, the range of periods does not cover the high frequency part of the transferfunction and the period density is not high enough to capture the peak.DET NORSKE VERITAS


Page 12Figure 3-4Poor representation of a transfer function on the left and on the right a transfer function where peak and shorterwave periods are well represented

3.5 Design waves for ULS3.5.1 GeneralA design wave is a wave which results in a design load at a given reference value (e.g. return period). Using adesign wave, the phasing between motions and loads will be maintained giving a realistic load picture. Normally it is assumed that maximising the load will result in also the maximised stress response. However some responses are correlated and the combined effect may give higher stresses than if each load ismaximised. In such cases it is recommended to transfer the load RAOs and perform a full stochastic analysis. Thestress RAOs of the most critical regions can then be used as basis for design waves.In case of linear design waves the response of the response variable shall be the same as the long term responsedescribed in Section 3.5.2. For non-linear design waves, e.g. for vertical bending moment, the non-linear maximum response is notnecessarily at the same location as the maximum linear response. Several locations need to be evaluated inorder to locate the non-linear maximum response. The linear and non-linear dynamic response shall becompared, including the non-linear factor defined as the ratio between the maximum non-linear and lineardynamic response.Water on deck, also called green water, might occur during ULS design conditions. If the software does nothandle water on deck in a physical way it is conservative to remove the elements and pressures from the deck.In a sagging wave the bow will be planted into a wave crest. Applying deck pressures in such case will reducethe sagging moment.There are several ways of generating design waves. The following presents two acceptable ways:

regular design wave conditioned irregular extreme wave.

3.5.2 Regular design waveA regular design wave can be made such that a linear simulation results in a dynamic response equal to the longterm response. The wave period for the regular wave shall be chosen as the period corresponding to the maximumvalue of the transfer function, see Figure 3-5. The wave amplitude shall be chosen as:

Transfer Function for Vertical Bending Moment

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

7.00E+05

8.00E+05

9.00E+05

0 10 20 30 40 50 60Wave Period

VBM

/ W

ave

Am

plitu

de


0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

7.00E+05

8.00E+05

9.00E+05

0 10 20 30 40 50Wave Period

VBM

/ W

ave

Am

plitu

de

[ ] [ ]

=

mNmNm

peakfunctionTransferresponseermtLongmDET NORSKE VERITAS


Page 13Figure 3-5Example of transfer function

The wave steepness shall be less than the steepness criterion given in DNV-RP-205 /3/. If the steepness is toolarge, a different wave period combined with the corresponding wave amplitude should be chosen. The regularresponse shall converge before results can be used.

3.5.3 Conditioned irregular extreme wavesDifferent methods exist to make a conditioned irregular extreme wave (ref. /11/, /12/, /13/). In principle anirregular wave train which in linear simulations returns the long term response after short time is created. Thesame wave train can be used for non linear simulations in order to study the non-linear effects.

3.6 Load Transfer

3.6.1 GeneralThe hydrodynamic loads are to be taken from the hydrodynamic load analysis. To ensure that phasing of allloads is included in a proper way for further post processing, direct load transfer from the hydrodynamic loadanalysis to the structural analysis is the only practical option. The following loads should be transferred to thestructural model:

inertia loads for both structural and non-structural members external hydro pressure loads, internal pressure loads from liquid cargo, ballast 1) viscous damping forces (see below).1) The internal pressure loads may be exchanged with mass of the liquid (with correct center of gravity)

provided that this exchange does not significantly change stresses in areas of interest (the mass must beconnected to the structural model).

Inertia loads will normally be applied as acceleration or gravity components. The roll and pitch induced fluctuatinggravity component (g sin() g ) in sway and surge shall be included.Pressure loads are normally applied as normal pressure loads to the structural model. If stresses influenced bythe pressure in the waterline region are calculated, pressure correction according to the procedure described inSection 3.6.2.2 need to be performed for each wave period and heading.Viscous damping forces can be important for some vessels, particularly those vessels where roll resonance isin an area with substantial wave energy, i.e. roll resonance periods of 6-15 seconds. The roll damping may,depending on Metocean criteria, be neglected when the roll resonance period is above 20-25 seconds. If torsionis an important load component for the ship, the effect of neglecting the viscous damping force should beinvestigated.


0.00E+ 00

1.00E+ 05

2.00E+ 05

3.00E+ 05

4.00E+ 05

5.00E+ 05

6.00E+ 05

7.00E+ 05

8.00E+ 05

9.00E+ 05

0 10 20 30 40 50 60Wa ve Period

VBM

/ W

ave

Am

plit

udeDET NORSKE VERITAS


Page 143.6.2 Load transfer FLSThe loads from the hydrodynamic analysis are used in the fatigue analysis. For the full stochastic analysis the inertia is applied to the FE model and the inertia pressure of tank liquids andwave-pressures are transferred to the global FE model for all frequencies and headings of the hydrodynamicanalysis. For the component stochastic analysis the load transfer functions at the applicable sections and locations arecombined with nominal stress per unit load, giving nominal stress transfer functions. The loads of interest arethe inertia pressures in the tanks, the sea-pressures, and the global hull girder loads, i.e. vertical and horizontalbending moment and axial elongation.

3.6.2.1 Inertia tank pressuresThe transfer functions for internal cargo and ballast pressures due to acceleration in x-, y- and z-direction arederived from the vessel motions. The acceleration transfer functions are to be determined at the tank centre ofgravity and include the gravity component due to pitch and roll motions. Based on the free surface and filling level in the tank, the pressure heads to the load point in question isestablished, and the total internal transfer function is found by linear summation of pressure due to accelerationin x, y and z-direction for the load point in question (FE pressure panel for full stochastic and load point forcomponent stochastic.)

3.6.2.2 Effect of intermittent wet surfaces in waterline regionThe wave pressure in the waterline region is corrected due to intermittent wet and dry surfaces, see Figure 3-6. This is mainly applicable for details where the local pressure in this region is important for the fatigue life,e.g. longitudinal end connections and plate connections at the ship side.

Figure 3-6Correction due to intermittent wetting in the waterline region

Since panel pressures refer to the midpoint of the panel, the value at waterline is found from extrapolating thevalues for the two panels closest to the waterline. Above the waterline the pressure should be stretched usingthe pressure transfer function for the panel pressure at the waterline combined with the rp-factor.Using the wave-pressure at waterline, with corresponding water-head, at 10-4 probability level as basis, thewave-pressure in the region limited by the water-head below the waterline, is given linear correction, see Figure3-6. The dynamic external pressure amplitude (half pressure range), pe, for each loading condition, may betaken as:

where:

pd is dynamic pressure amplitude below the waterlinerp is reduction of pressure amplitude in the surface zone

Pressures at 10-4 probability

Extrapolated t

Water head f

Water head f Corrected

p r pe p d =DET NORSKE VERITAS


Page 15In the area of side shell above z = Tact + zwl it is assumed that the external sea pressure will not contribute tofatigue damage.Above waterline the wave-pressure is linearly reduced from the waterline to the water-head from the wave-pressure.

3.6.3 Load transfer ULSIn case of load transfer for ULS, the pressure and inertia forces are transferred at a snapshot in time. Everywetted pressure panel on the structural FE model shall have one corresponding pressure value while inertiaforces in six degrees of freedoms are transferred to the complete model.

4. Fatigue Limit State Assessment4.1 General principles4.1.1 Methodology overviewThe following defines fatigue strength analysis based on spectral fatigue calculations. Spectral fatiguecalculations are based on complex stress transfer functions established through direct wave load calculationscombined with subsequent stress response analyses. Stress transfer functions then express the relation betweenthe wave heading and frequency and the stress response at a specific location and may be determined by either:

component stochastic analysis full stochastic analysis.

Component stochastic calculations may in general be employed for stiffeners and plating and other details witha well defined principal stress direction mainly subjected to axial loading due to hull girder bending and localbending due to lateral pressures. Full stochastic calculations can be applied to any kind of structural details.Spectral fatigue calculations imply that the simultaneous occurrence of the different load effects are preservedthrough the calculations and the uncertainties are significantly reduced compared to simplified calculations.The calculation procedure includes the following assumptions for calculation of fatigue damage:

wave climate is represented by a scatter diagram Rayleigh distribution applies for the response within each short term condition (sea state) cycle count is according to zero crossing period of short term stress response linear cumulative summation of damage contributions from each sea state in the wave scatter diagram, as

well as for each heading and load condition.

The spectral calculation method assumes linear load effects and responses. Non-linear effects due to largeamplitude motions and large waves are neglected, assuming that the stress ranges at lower load levels(intermediate wave amplitudes) contribute relatively more to the cumulative fatigue damage. Wherelinearization is required, e.g. in order to determine the roll damping or intermittent wet and dry surfaces in thesplash zone, the linearization should be performed at the load level representing stress ranges giving the largestcontribution to the fatigue damage. In general a reference load or stress range at 10-4 probability of exceedanceshould be used.Low cycle fatigue and vibrations are not included in the fatigue calculations described in this ClassificationNote.

4.1.2 Classification Note No. 30.7Fatigue calculations for the CSA notations are based on the calculation procedures as described inClassification Note No. 30.7 /4/. This Classification Note describes details and procedures relevant for the

= 1.0 for z < Tact zwl

= for Tact zwl < z < Tact+ zwl

= 0.0 for Tact+ zwl < zzwl is distance in m measured from actual water line to the level of zero pressure, taken equal to water-head

from pressure at waterline. =

pdT is dynamic pressure at waterline Tact

T z zz

act wl

wl

+ 2

gpdT4

3

DET NORSKE VERITAS


Page 16CSA-notation. For further details reference is made to CN 30.7. In case of conflicting procedure, the procedureas given in CN 30.7 has precedence.

4.2 Locations for fatigue analysis4.2.1 GeneralFatigue calculations should in general be performed for all locations that are fatigue sensitive and that may haveconsequences for the structural integrity of the ship. The locations defined by NAUTICUS (Newbuilding) orCSR, whichever is relevant, and PLUS, shall be documented by CSA fatigue calculations. The generallocations are shown in Table 4-1 with some typical examples given in Figure 4-1 to Figure 4-7.

For the stiffener end connections and shell plate connection to stiffeners and frames it is normally sufficient toperform component stochastic fatigue analysis using predefined load/stress factors and stress concentrationfactors. All other details, including those required by ship type, need full-stochastic analysis with use of stressconcentration models with txt mesh (element size equal to plate thickness).

Figure 4-1Longitudinal end connection

Table 4-1 General overview of fatigue critical detailsDetail Location Selection criteria

Stiffener end connection one frame amidships one bulkhead amidships one frame in fwd. tank one frame in aft tank*)

All stiffeners included

Bottom and side shell plating connection to stiffener and frames

one frame amidships one frame in fwd. tank one frame in aft tank*)

All plating to be included

Stringer heels and toes one location amidships one location in fwd hold*) other locations*)

Based on global screening analysis and evaluation of details

Panel knuckles one lower hopper knuckle amidships other locations identified*)


Discontinuous plating structure between hold no. 1 and 2*) between Machinery space and cargo

region*)Based on global screening analysis and evaluation of details

Deck plating, including stress concentrations from openings, scallops, pipe penetrations and attachments.


*) Global screening and evaluation of design in discussion with the Society to be basis for selectionDET NORSKE VERITAS


Page 17Figure 4-2Plate connection to stiffener and frame

Figure 4-3Stringer heel and toe

DET NORSKE VERITAS


Page 18Figure 4-4Example of panel knuckles

Figure 4-5Example of discontinuous plating structureDET NORSKE VERITAS


Page 19Figure 4-6Example of discontinuous plating structure

Figure 4-7Hotspots in deck-plating

4.2.2 Details for fine mesh analysisIn addition to the general positions as described in Section 4.2.1, fine mesh full stochastic fatigue analysis fordefined ship specific details also need to be performed, see the Rules for Classification of Ships, Pt.3 Ch.1. Theship specific details are details either found to be specially fatigue sensitive and/or where fatigue cracks mayhave an especially large impact on the structural integrity. Typical vessel specific locations that require fine mesh full stochastic analysis are specified in the following.In the following the mandatory locations in need of fine mesh full stochastic analysis are listed for differentvessel types. For vessel-types not listed, details to be checked need to be evaluated for each design.Tankers

lower hopper knuckle upper hopper knuckle stringer heels and toes one additional critical location found on transverse web-frame from global screening of midship area.DET NORSKE VERITAS


Page 20Membrane type LNG carriers

lower hopper knuckle upper hopper knuckle stringer heels and toes dome opening and coaming lower and upper chamfer knuckles longitudinal girders at transverse bulkhead trunk deck at transverse bulkhead termination of tank no. 1 longitudinal bulkhead aft trunk deck scarfing.

Moss type LNG carriers

lower hopper knuckle stringer heels and toes tank cover to deck connection tank skirt connection to foundation deck inner side connection to foundation deck in the middle of the tank web frame longitudinal girder at transverse bulkhead.

LPG carriers

dome opening and coaming lower and upper side bracket longitudinal girder at transverse bulkhead.

Container vessel

top of hatch coaming corner (amidships, in way of E/R front bulkhead, and fore-ship) upper deck hatch corner (amidships in way of E/R front bulkhead, and fore-ship hatch side coaming bracket in way of E/R front bulkhead scarfing brackets on longitudinal bulkhead in way of E/R critical stringer heels in fore-ship stringer heel in way of HFO deep tank structure (where applicable).

Ore carrier

inner bottom and longitudinal bulkhead connection horizontal stringer toe and heel in ballast tank cross-tie connection in ballast tank hatch corner hatch coaming brackets upper stool connection to transverse bulkhead additional critical locations found from screening of midship frame.

4.3 Corrosion model4.3.1 ScantlingsAll structural calculations are to be carried out based on the net-scantlings methodology as described by therelevant class notation. This yields for both global and local stresses. E.g. for oil tankers with class notationCSR 50% of the corrosion addition is to be deducted for local stress and 25% of the corrosion addition is to bededucted for global stress. For other class notations the full corrosion addition is to be deducted.

4.4 Loads4.4.1 Loading conditionsVessel response may differ significantly between loading conditions. Therefore the basis of the calculationsshould include the response for actual and realistic seagoing loading conditions. Only the most frequent loadingconditions should be included in the fatigue analysis, normally the ballast and full load condition, which shouldbe taken as specified in the loading manual. Under certain circumstances, other loading conditions may beconsidered.

4.4.2 Time at seaFor vessels intended for normal, world wide trading, the fraction of the total design life spent at sea, should notbe taken less than 0.85. The fraction of design life in the fully loaded and ballast conditions, pn, may be takenDET NORSKE VERITAS


Page 21according to the Rules for Classification of Ships, Pt.3 Ch.1, summarised in Table 4-2.

Other fractions may be considered for individual projects or on owners request.

4.4.3 Wave environmentThe wave data should not be less severe than world wide or North Atlantic for vessels with NAUTICUS(Newbuilding) notation or CSR notation, respectively. The scatter-diagrams for World Wide and NorthAtlantic are defined in CN 30.7. Other wave data may also be considered in addition, if requested by owner.This could typically be a sailing route typical for the specific ship.Fatigue is governed by the daily loads experienced by the vessel, hence the reference probability level forfatigue loads and responses shall be based on 10-4 probability level. Weibull fitting parameters are normallytaken as 1, 2, 3 and 4.A Pierson-Moskowitz wave spectrum with a cos2 wave spreading shall be used.If a different wave data is specified, it is recommended to perform a comparative analysis to advice which ofthe scatter diagram gives worse fatigue life. If one yields worse results, this scatter diagram may be used for allanalysis. If the results are comparative, fatigue life from both wave environments may need to be established.

4.4.4 Hydrodynamic analysisA vessel speed equal to 2/3 of design speed should be used, as an approximation of average ship speed over thelifetime of the vessel.All wave headings (0 to 360) should be assumed to have an equal probability of occurrence and maximum30 spacing between headings should be applied.Linear wave load theory is sufficient for hydrodynamic loads for FLS, since the daily loads contribute most tothe fatigue damage.Reference is made to Section 3 for hydrodynamic analysis procedure.

4.4.5 Load applicationThe loads from the hydrodynamic analysis are used in the fatigue analysis. For the full stochastic analysis the following hydrodynamic loads are applied to the global structural model forall headings and frequencies:

external panel pressures internal tank pressures inertia loads due to rigid body accelerations.

For the component stochastic analysis the loads at the applicable sections and locations are combined withstress transfer functions representing the stress per unit load. The loads to be considered are:

inertial loads (e.g. liquid pressure in the tanks), sea-pressure global hull girder loads:

- vertical bending moment - horizontal bending moment and - axial elongation.

Details are described in Section 3.

4.5 Component stochastic fatigue analysisComponent stochastic fatigue analysis is used for stiffener end connections and plate connection to stiffenersand frames, see Section 4.2.1. The component stochastic fatigue calculation procedure is based on linear combination of load transferfunctions calculated in the hydrodynamic analysis and stress response factors representing the stress per unitload. The nominal stress transfer functions for each load component is combined with stress concentrationfactors before being added together to one hot spot transfer function for the given detail. The flowchart shown in Figure 4-8 gives an overview of the component stochastic calculation procedure givinga hot-spot stress transfer function used in subsequent fatigue calculations. If the geometry and dimensions of

Table 4-2 Fraction of time at sea in loaded and ballast conditionVessel type Tanker Gas carrier Bulk carrier Container vessel Ore carrierLoaded condition 0.425 0.45 0.50 0.65 0.50Ballast condition 0.425 0.40 0.35 0.20 0.35DET NORSKE VERITAS


Page 22the given detail does not have predefined SCFs, the stress concentration factor need to be found through a stressanalysis using a stress concentration model for the detail, see CN 30.7 /4/. In such cases the procedure andresults shall be documented together with the results from the fatigue analysis.A short overview of the procedure for stiffener end connections and plate connections is given in Section 4.5.2and Section 4.5.3, respectively.

Figure 4-8DNV component stochastic fatigue analysis procedure

4.5.1 Considered loadsThe loads considered normally include:

vertical hull girder bending moment horizontal hull girder bending moment hull girder axial force internal tank pressure external (panel) pressures.

In the surface region the transfer function for external pressures should be corrected by the rp factor asexplained in Section 3.6.2.2, and as given in CN 30.7 /4/, to account for intermittent wet and dry surfaces. Thetank pressures are based on the procedure given in Section 3.6.2.1.DET NORSKE VERITAS


Page 234.5.2 Stiffener end connectionsFatigue calculations for stiffener end connections are to be carried out for end connections at ordinary framesand at transverse bulkheads. Note that the web-connection of longitudinals (cracks of web-plating) is not covered by the CSA-notations.This is covered by PLUS notation only, and shall follow the PLUS procedure.

4.5.2.1 Nominal stress per unit loadThe stresses considered are stress due to:

global bending and elongation local bending due to internal and external pressure relative deflections due to internal and external pressure.

Stress from double side or double bottom bending may be neglected in the CSA analyses since these stresses arerelative small and varies for each frame. The stress due to relative deflection is only assessed for the bulkheadconnections, where the stress due to relative deflection will add on to the stress due to local bending and hencereduce the fatigue life. A description of the relative deflection procedure is given in Appendix A.Formulas for nominal stress per unit load are given in CN 30.7. They may alternatively be found from FE-analysis.

4.5.2.2 Hotspot stressThe nominal stress transfer function is further multiplied with stress concentration factors as defined in CN 30.7.For end connections of longitudinals, they are typically defined for axial elongation and local bending.The total hotspot stress transfer function is determined by linear complex summation of the stresses due to eachload component.

4.5.3 PlatingFatigue calculations for plating are carried out for the plate welds towards stiffeners/longitudinals and framesas illustrated in Figure 4-3. The stress in the weld for a plate/frame connections consist of the following responses:

local plate bending due to external/internal pressure global bending and elongation.

For a plate/longitudinal connection the global effects may be disregarded and only the contributions fromstresses in transverse directions are included. The total stress in the welds for a plate/longitudinal connectionis mainly caused by the following responses:

local plate bending relative deflection between a stringer/girder and the nearby stiffener rotation of asymmetrical stiffeners due to local bending of stiffener.

These three effects are illustrated in Figure 4-9.

Figure 4-9Nominal stress components due to local bending (left), relative deflection between stiffener and stringers/girders(middle) and rotation of asymmetrical stiffeners (right)

The local plate bending is the dominating effect, but relative deflection and skew bending may increase thestresses with up to 20%. This effect should be considered and investigated case by case. As guidance, thefollowing factors can be used to correct the stress calculations for a plate/longitudinal connection:

plate weld towards stringer/girder 1.15plate weld towards L-stiffener 1.1DET NORSKE VERITAS


Page 24The combined nominal stress transfer function is determined by linear complex summation of the stresses dueto each load component.

4.5.3.1 Hotspot stress The nominal stress transfer function is further multiplied with stress concentration factors as defined in CN30.7. The total hotspot stress transfer function is determined by linear complex summation of the stresses dueto applicable load components.

4.6 Full stochastic fatigue analysis4.6.1 GeneralA full stochastic fatigue analysis is performed using a global structural model and local fine-mesh sub-models.This method requires that the wave loads are transferred directly from the hydrodynamic analysis to thestructural model. The hydrodynamic loads include panel pressures, internal tank pressures and inertia loads dueto rigid body accelerations. By direct load transfer the stress response transfer functions are implicitly describedby the FE analysis results, and the load transfer ensures that the loads are applied consistently, maintainingload-equilibrium.Quality assurance is important when executing the full stochastic method. The structural and hydrodynamicanalysis results should have equal shape and magnitude for the bending moment and shear force diagrams.Also, the reaction forces due to unbalanced loads in the structural analysis should be minimal. Figure 4-10 shows a flow chart for the full stochastic fatigue analysis using a global model. References torelevant sections in this CN are given for each step.

Figure 4-10Full stochastic fatigue analysis procedure

The analysis is based on a global finite element model including the entire vessel in addition to local modelsof specified critical details in the hull. Local models are treated as sub models to the global model and theDET NORSKE VERITAS


Page 25displacements from the analysis are transferred to the local model as boundary displacements. From local stressconcentration models the geometric stress transfer functions at the hot spots are determined by the t x t elementsthat pick up the stress increase towards the hotspot.The hotspot transfer functions are combined with the wave scatter diagram and S-N data and the fatiguedamage is summarised from each heading for all sea states in the scatter diagram (wave period and waveheight).

4.6.2 Global screening analysisThe global screening analysis is a full stochastic fatigue analysis performed on the global model, or parts of theglobal model, using a SCF typical for the details investigated. The global screening analysis generally has fourdifferent purposes:

calculate allowable stress concentrations in deck find the most fatigue critical detail from a number of similar or equal details establish a fatigue ratio between identical details evaluate if there are fatigue critical details that are not covered in the specification.

Note that the global screening analysis only includes global effects as global bending and double bottombending. Local effects from stiffener bending, etc. are not included.

4.6.2.1 Allowable stress concentration in deckA significant part of the total fatigue cracks occur in the deck region. This is mainly due to the large nominalstresses in parts of this area and the fact that there are many cut-outs, attachments, etc. leading to local stressincreases.A crack in the deck is considered critical since a crack propagating in the deck will reduce the effective hullgirder cross section. Even if a crack in the deck will be discovered at an early stage due to easy inspection andhigh personnel activity, it is important to control the fatigue of the deck area. The nominal stress level in the deck varies along the ship, normally with a maximum close to amidships. Largeropenings, structural discontinuities, change in scantlings or additional structure will change the stress flow andlead to a variation of stress flow both longitudinally and transversely. The information from the fatigue screening analysis may be used together with drawing information aboutdetails in the deck. Typical details that need to be taken into consideration are:

deck openings butt weld in the deck (including effect of eccentricity and misalignment) scallops cut outs, pipe-penetrations and doubling plates.

The stress concentrations for each of these details need to be compared to the results from the global screeninganalysis in order to show that the required fatigue life is obtained for all parts of the deck area.

4.6.2.2 Finding the most critical location for a detailA ship will have many identical or similar details. It is not always evident which ones are more critical, sincethey are subject to the same loads, but with different amplitudes and combinations. Through a global screeninganalysis, the most critical location might be identified, by comparing the global effects. Local effects, which may be of major importance for the fatigue damage, are not captured in the globalscreening analysis. Element mesh must be identical for the positions that are compared; otherwise the effect ofchanging the mesh may override the actual changes in loads.An example of the result from a global screening for one detail type is shown in Figure 4-11 where relativedamage between different positions in a ship is shown for three different tanks.DET NORSKE VERITAS


Page 26Figure 4-11Fatigue screening example relative damage between different positions

4.6.2.3 Fatigue ratio between different positionsThe fatigue calculations used for relative damage between different positions for identical details helpsevaluate where reinforcements are necessary. E.g. if local reinforcements are necessary in the middle of thecargo hold for the example shown in Figure 4-11, it may not be needed towards the ends of the cargo hold. New detailed fatigue calculations should be performed in order to verify fatigue lives if different reinforcementmethods are selected.

4.6.2.4 Finding critical locations not specified for the vesselBy specifying a critical level for relative damage the model can be scanned for elements that exceed the givenlimit, indicating that it may be a fatigue critical region. Since not all effects are included the results are notreliable, but will give an overview of potential problem areas. This exercise will also help confirm assumedcritical areas from the specifications stage of the project in addition to point at new critical areas.

4.6.3 Local fatigue analysis The full stochastic detailed analysis is used to calculate fatigue damages for given details. The analysis isnormally performed either for details where the stress concentration is unknown, or where it is not possible toestablish a ratio between the load and stress. Full stochastic calculations may also be used for stiffener endconnections and bottom/side shell plating, and will in that case overrule the calculations from the componentstochastic analysis.Several types of models can be used for this purpose:

local model as a part of the global model local shell element sub-model local solid element model.

If sub-models are used, the solution (displacements) of the global analysis is transferred to the local models.The idea of sub-modelling is in general that a particular portion of a global model is separated from the rest ofthe structure, re-meshed and analysed in greater detail. The calculated deformations from the global analysisare applied as boundary conditions on the borders of the sub-models, represented by cuts through the globalmodel. Wave loads corresponding to the global results are directly transferred from the wave load analysis tothe local FE models as for the global analysis.It is not always easy to predefine the exact location of the hotspot, or the worst combination of stress

Lower Chamfer Knuckle

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

100425 120425 140425 160425 180425 200425 220425

Distance from AP [mm]

Fatig

ue D

amag

e [-]

Screening ResultsTBHD Pos.Local Model ResultDET NORSKE VERITAS


Page 27concentration factor and load level, and therefore the fine-mesh model frequently does not include fine meshin all necessary locations. The local model shall be screened outside the already specified hotspot to evaluateif other locations in close proximity may be prone to fatigue damage, requiring evaluation with mesh size inthe order of t t. This can be performed according to the procedure shown in Section 4.6.2.

4.6.4 Determination of hotspot stress

4.6.4.1 GeneralFrom the results of the local structural analysis, principal stress transfer functions at the notch are calculatedfor each wave heading. In general, quadratic shaped elements with length equal to the plate thickness areapplied at the investigated details, and the geometry of the weld is not represented in the model. Since thestresses are derived in the element gauss points, it is necessary to extrapolate the stresses to the consideredpoint. The extrapolation procedure is given in CN30.7 /4/.Alternatively to the extrapolation procedure, the stress at t/2 multiplied with 1.12 is also appropriate for thestress evaluation at the hotspot.

4.6.4.2 Cruciform connectionsAt web stiffened cruciform connections the following fatigue crack growth is not linear across the plate, andthe stresses need to be specially considered. The procedures for the cruciform joints and extrapolation to theweld toe are described in CN 30.7 /4/.

4.6.4.3 Stress concentration factorThe total stress concentration K is defined as:

Also other effects, like eccentricity of plate connections, need to be considered together with the stress-resultsfrom the fine-mesh analysis.This needs to be included in the post-processing.

4.7 Damage calculation4.7.1 Acceptance criteriaCalculated fatigue damage shall not be above 1.0 for the design life of the vessel. Owner may require loweracceptable damage for parts of the vessel.The fatigue strength evaluation shall be carried out based on the target fatigue life and service area specifiedfor the vessel, but minimum 20 years world wide, for vessels with Nauticus (Newbuilding), or 25 years NorthAtlantic, for vessels with CSR notation. The owner may require increased fatigue life compared to theminimum requirement.

4.7.2 Cumulative damageFatigue damage is calculated on basis of the Palmgrens-Miner rule, assuming linear cumulative damage. Thedamage from each short term sea state in the scatter diagram is added together, as well as the damage fromheading and load condition.

4.7.3 S-N curvesThe fatigue accumulation is based on use of S-N curves that are obtained from fatigue tests. The design S-Ncurves are based on the mean-minus-two-standard-deviation curves for relevant experimental data. The S-Ncurves are thus associated with a 97.6% probability of survival.Relevant S-N curves according to CN 30.7 /4/ should be used.It is important that consistency between S-N curves and calculated stresses is ensured.

4.7.3.1 Effect of corrosive environmentCorrosion has a negative effect on the fatigue life. For details located in corrosive environment (as water ballastor corrosive cargo) this has to be taken into account in the calculations.For details located in water ballast tanks with protection against corrosion or where the corrosive effect is small,the total fatigue damage can be calculated using S-N curve for non-corrosive environment for parts of the designlife and S-N curve for corrosive environment for the remaining part of the design life. Guidelines on which S-Ncurve to use and the fraction in corrosive and non-corrosive environment are specified by CN 30.7 /4/.

alno

spothotKmin

=

DET NORSKE VERITAS


Page 28For details without corrosion protection, a S-N curve for corrosive environment has to be used in thecalculations for the entire lifetime.

4.7.3.2 Thickness effectThe fatigue strength of welded joints is to some extent dependent on plate thickness and on the stress gradientover the thickness. Thus for thickness larger than 25 mm, the S-N curve in air reads

where t is thickness (mm) through which the potential fatigue crack will grow. This S-N curve in generalapplies to all types of welds except butt-welds with the weld surface dressed flush and with small local bendingstress across the plate thickness. The thickness effect is less for butt welds that are dressed flush by grinding ormachining.The above expression is equivalent with an increase of the response with.

4.7.4 Mean stress effectThe procedure for the fatigue analysis is based on the assumption that it is only necessary to consider the rangesof cyclic principal stresses in determining the fatigue endurance. However, some reduction in the fatiguedamage accumulation can be credited when parts of the stress cycle are in compression.A factor, fm, accounting for the mean stress effect can be calculated based on a comparison of static hotspotstresses and dynamic hotspot stresses at a 10-4 probability level.

4.7.4.1 Base materialFor base material, fm varies linearly between 0.6 when stresses are in compression through the entire load cycleto 1.0 when stresses are in tension through the entire load cycle.

4.7.4.2 Welded materialFor welded material, fm varies between 0.7 and 1.0.

4.7.5 Improvement of fatigue life by fabricationIt should be noted that improvement of the toe will not improve the fatigue life if fatigue cracking from the rootis the most likely failure mode. The considerations made in the following are for conditions where the root isnot considered to be a critical initiation point for fatigue cracks.Experience indicates that it may be a good design practice to exclude this factor at the design stage. Thedesigner is advised to improve the details locally by other means, or to reduce the stress range through designand keep the possibility of fatigue life improvement as a reserve to allow for possible increase in fatigue loadingduring the design and fabrication process.It should also be noted that if grinding is required to achieve a specified fatigue life, the hot spot stress is ratherhigh. Due to grinding a larger fraction of the fatigue life is spent during the initiation of fatigue cracks, and thecrack grows faster after initiation. This implies use of shorter inspection intervals during service life in orderto detect the cracks before they become dangerous for the integrity of the structure.The benefit of weld improvement may be claimed only for welded joints which are adequately protected fromcorrosion.The following methods for fatigue improvement are considered:

weld toe grinding (and profiling) TIG dressing hammer peening.

Among these three, weld toe grinding is regarded as the most appropriate method, due to uncertaintiesregarding quality assurance of the other processes.The different fatigue improvements by welding are described in CN 30.7 /4/.

= log

25log

4loglog mtmN a

41

25

= trespDET NORSKE VERITAS


Page 295. Ultimate Limit State Assessment

5.1 Principle overview

5.1.1 GeneralThe Ultimate Limit State (ULS) analyses shall cover necessary assessments for dimensioning against materialyield, buckling and ultimate capacity limits of the hull structural elements like plating, stiffeners, girders,stringers, brackets, etc. in the cargo region. ULS assessments shall also ensure sufficient global strength in order to prevent hull girder collapse, ductile hullskin fracture and compartment flooding. Two levels of ULS assessments are to be carried out, i.e.

global FE analyses - local ULS hull girder collapse - global ULS.

The basic principles behind the two types of assessments are described in more detail in the following.

5.1.2 Global FE analyses local ULSThe local ULS design assessment is based on a linear global FE model with automatic load transfer fromhydrodynamic wave load programs. The design of the structural elements in different areas of the ship, arecovered by different design conditions. Each design condition is defined by a loading condition and a governingsea state/wave condition, which together are dimensioning for the structural element. For each design condition the calculation procedure follows the flow chart in Figure 5-1, i.e. the static andhydrodynamic wave loads for the loading condition are transferred to the structural FE model for a linearnominal stress assessment. The nominal stresses are to be measured against material yield, buckling andultimate capacity criteria of individual stiffened panels, girders etc.The material yield checks cover von Mises stress control using a cargo hold model, and for high peak stressedareas using local fine-mesh models.The local ULS buckling control follow two different principles, allowing and not allowing elastic buckling,depending on the elements main function in the global structure, using PULS /8/. The procedure for local ULS assessment is further described in Section 5.2.

5.1.3 Hull girder collapse - global ULS The hull girder collapse criteria are used to check the total hull section capacity against the correspondingextreme global loads. This is to be carried out for the mid-ship area for one intact and two damaged hullconditions. Specially developed hull girder capacity models based on simplified non-linear theory or full-blown FE analyses are to be used for assessing the hull capacity. The extreme loads are to be based on directcalculations and the static + dynamic load combination giving the highest total hull girder moment shall beused, including both the extreme sagging and hogging condition. For some ship types other sections than the mid-ship area may be relevant to be checked, if deemed necessaryby the Society. This applies in particular to hull sections which are transversely stiffened, e.g. engine room ofcontainer ships etc.The procedure for the global ULS assessment is further described in Section 5.3.

5.1.4 Scantlings/corrosion modelAll FE calculations shall be based on the net scantlings methodology as defined by the relevant class notationsNAUTICUS (Newbuilding) or CSR. The buckling calculations are to be carried out on net scantlings.

5.2 Global FE analyses local ULS

5.2.1 GeneralThe local ULS design assessment is based on a linear global FE analysis with automatic load transfer fromhydrodynamic programs, as schematically illustrated in Figure 5-1.DET NORSKE VERITAS


Page 30Figure 5-1Flowchart for ULS analysis: Load transfer: Hydro Global FE model

Selection of design loads, and procedures for selection of stress and application of the yield and bucklingcriteria is described in the following.

5.2.2 Designloads

5.2.2.1 GeneralThis section is closely linked to Section 3, which explains how hydrodynamic analyses are to be performed.

5.2.2.2 Design condition and selection of critical loading conditionsThe design loading conditions are to be based on the vessels loading manual and shall include ballast, full loadand part load conditions as relevant for the specific ship type. The loading conditions and dynamic loads areselected such that they together define the most critical structural response. Depending on the purpose of thedesign condition, e.g. the region to be analysed and failure mode (yield/buckling) for the structural elements,different loading conditions and design waves are required to ensure that the relevant response is at itsmaximum. Any loading condition in the loading manual that, combined with its hydrodynamic extreme loads,may result in the design loads should be evaluated.For each loading condition, hydrodynamic analysis shall be performed, forming the basis for selection ofdesign waves and stress assessment. For areas where non-linear effects are not necessary to consider (e.g. fortransverse structural members) a design wave need not be defined. The design stress is then based on long-termstress, where the stress at 10-8 probability level for the loading condition is found. A design wave is requiredif non-linear effects need to be considered. The design wave may be defined based on structural response, orwave load, depending on the purpose of the design condition. DET NORSKE VERITAS


Page 31Table 5-1 gives an overview of the design conditions that need to be evaluated, and should at a minimum becovered. Additional design conditions need to be evaluated case by case, depending on the ships structuralconfiguration, trading/operational conditions, etc, which may require several design conditions to ensure thatall the structures critical failure modes are covered.

5.2.2.3 Hydrodynamic analysisThe hydrodynamic analyses are to be performed for the selected critical loading conditions. A vessel speed of5 knots is to be used for application of loads that are dominated by head seas. For design conditions where thedriving response is dominated by beam or quartering seas, the speed is to be taken as 2/3 of design speed.

5.2.2.4 Design life and wave environmentWave environment is minimum to be the North Atlantic wave environment as defined in the CN 30.7 /4/. Ifother wave environment is required by design, it should not be less severe than the North Atlantic waveenvironment.The hydrodynamic loads are to be taken as 10-8 probability of exceedance according to Pt.3 Ch.1 Sec.3 B300and Pt.8 Ch.1 Sec.2 for Nauticus (Newbuilding) and CSR respectively, using a cos2 wave spreading functionand equal probability of all headings.

5.2.2.5 Design wavesThe design waves used in the hydrodynamic analysis should basically cover the entire cargo hold area.Different design waves are used to check the capacity of different parts of the ship. It is important that thedesign waves are not used outside the area for which the design wave is valid, i.e. a design wave made for tankno.1 must not be used amidships.An overview of the relation between the design loads and areas they are applicable for should be checkedagainst the different design loads is given in Table 5-1. The design conditions together with its applicableloading condition and design load need to be reviewed on project basis. It can be agreed with ClassificationSociety that some design conditions can be removed based on review of design together with loadingconditions and operational profile. It is considered that only design waves which represents vertical bending moment and vertical shear force needto be performed with non-linear hydrodynamic analysis.

5.2.2.6 Load transferA load transfer (snap-shot) from the hydrodynamic analysis to the structural analysis shall be performed whenthe total load/response from the hydrodynamic time-series is at its maximum/minimum. The load transfer shallinclude both gravitational and inertial loads, and the still water and wave pressures, see Section 3.6.

Table 5-1 Guidance on loading condition selectionDesign Condition Loading condition & design loads

IDReference

load/response(Dominant or max

load/response)

Design area Loading condition Typical loading pattern

Design wave(maximised re-sponse/load)

1A hogging bending moment Midship (global hull)Max/large hogging bending moment

Max hogging wave moment

1B Sagging bending moment Midship (global hull)Max/large sagging bending moment

Max sagging wave moment

2A Hogging + doublebottom bending

Midship double bot-tomTransverse bulk-heads

Large hogging com-bined with deep draft

Tanks/hold empty across with adjacent tanks/hold full

Max hogging wave moment

2B Sagging + double bottom bendingMidship double bot-tom

Large sagging com-bined with shallow draft

Tanks/hold full across with adjacent tanks/hold empty

Max sagging wave moment

3A Shear force at aft quarter lengthAft hold shear ele-ments Max shear force aft

Max wave shear force at aft quarter-length

3B Shear force at fwd quarter lengthFwd hold shear ele-ments Max shear force fwd

Max wave shear force at fwd quarter lengthDET NORSKE VERITAS


Page 325.2.3 Design stress

5.2.3.1 GeneralBased on the global FE analysis a nominal stress flow in the hull structure is available. This nominal stress flowshall be checked against material yield and acceptable buckling criteria (PULS). The nominal stresses produced from the FE analysis will be a combination of the stress components fromseveral response effects, which in a simplistic manner can be categorized as follows:

hull girder bending moment hull girder shear force hull girder axial loads (small) hull girder torsion and warping effects (if relevant) double side/bottom bending local bending of stiffener local bending of plates transverse stresses from cargo and sea pressure transverse and shear stresses from double hull bending other stress effects due to local design issues; knuckles, cut-outs etc.

Guidelines for determining design stresses are given in the following.

5.2.3.2 Material yield assessmentIn the material yield control all effects are to be included apart from local bending stress across the thicknessof the plating. This means that the yield check involves the von Mises stress based on membrane stresses andshear stresses in the structure evaluated in the middle plane of plating, stiffener webs and stiffener flanges. For cases where large openings are not modelled in the FE-analysis, either as cut-outs or by reduced thickness,see Section 6.3.2.2, the von Mises stress should be corrected to account for this.In areas with high peaked stress, where the von Mises stress exceeds the acceptance criteria, the structureshould be evaluated using a stress concentration model (t x t mesh). Frame and girder models (stiffener spacingmesh or equivalent) that reflect nominal stresses should not be used for evaluation of strain response in yieldareas. Areas above yield from the linear element analysis may give an indication of the actual area ofplastification. Non-linear FE analysis may be used to trace the full extent of plastic zones, large deformations,low cycle fatigue etc. but such analyses are normally not required.For evaluation of large brackets, the stress calculated at the middle of a brackets free edge is of the samemagnitude for models with stiffener spacing mesh size as for models with a finer mesh. Evaluation of bracketsof well-documented designs, may be limited to a check of the stress at the free edge. When 4-node elementsare used, fictitious bar elements are to be applied at the free edge to give a straightforward read-out of thecritical edge stress. For brackets where the design needs to be verified, a fine mesh model needs to be used.

4A Internal pressure/load in no.1 tank/holdTank no 1 double bottom

Loaded at shallow draft fwd

No.1 tanks/hold full across with no.2 tanks/hold empty

Maximum vertical accelerations at no.1 tanks/hold in head sea

4B External pressure at no.1 tanks/holdTank no.1 double bottom

Loaded at deep draft fwd

No.1 tanks/hold emp-ty across with no.2 tanks/hold full

Maximum bottom wave pressure at no.1 tanks/hold in head seas

5Combined vertical, horizontal and tor-sional bending

Entire cargo region

Loaded condition with large GM com-bined with large hog-ging for hogging vessels or large sag-ging for sagging ves-sels

Design wave(s) in quartering/beam sea condition: maximised torsion maximised

horizontal bending maximised stress

at hatch corners/large openings

6 Maximum transverse loading Entire cargo regionLoaded with maxi-mum GM

Maximum transverse acceleration

Table 5-1 Guidance on loading condition selection (Continued)Design Condition Loading condition & design loads

IDReference

load/response(Dominant or max

load/response)

Design area Loading condition Typical loading pattern

Design wave(maximised re-sponse/load)DET NORSKE VERITAS


Page 33Figure 5-2Bracket stress to be used

5.2.3.3 Buckling assessmentIn order to be consistent with available buckling codes the nominal stress pattern has to be simplified, i.e. stressgradients has to be averaged and the local bending stress due to lateral pressure effects has to be eliminated.The membrane stress components used for buckling control shall include all effects listed in Section 5.2.3.1,except for the stresses due to local stiffener and plate bending, since these effects are included in the bucklingcode itself. When carrying out the local ULS-buckling checks the nominal FE stress flow has to be simplified to a formconsistent with the local co-ordinate system of the standard buckling codes. In the PULS buckling code the bi-axial and shear stress input reads (see Figure 5-3):

1 axial nominal stress in primary stiffener and plating (normally uniform*) (sign convention in bucklingcode (PULS): positive stress in compression, negative stress in tension)

2 transverse nominal stress in plating. Normally uniform stress distribution, but it can vary linearly acrossthe plate length in the PULS code, also into the tension range; 2,1 2,2 at plate ends)

12 nominal in-plane shear stress in plating (uniform and as assessed by Section 5.3.3.3p net uniform (average) lateral pressure from sea or cargo (positive pressure acting on flat plate side).

Figure 5-3PULS nominal stress input for uni-axially or orthogonally stiffened panels (bi-axial + shear stresses)

=

Primary stiffeners direction1 x -

Secondary stiffeners any) x2- direction (ifDET NORSKE VERITAS


Page 34Note: Varying stress along the plate edge can be considered by checking each stiffener for the stress acting at thatposition. Since the PULS buckling model only consider uniform stresses, a fictive PULS model have to beused with the actual number of stiffener between rigid lateral supports (girders etc.) or limited by maximum5 stiffeners)

The local plate bending stress is easily excluded by using membrane stresses in the plating. The stiffenerbending stress can not directly be excluded from the stress results unless stresses are visualised in the combinedpanel neutral axis. This is, for most program systems, not feasible.

Figure 5-4Stiffener bending stress - mesh variations

The magnitude of the stiffener bending stress included in the stress results depends on the mesh division andthe element type that is used. This is shown in Figure 5-4 where the stiffener bending stress, as calculated bythe FE-model, is shown dependent on the mesh size for 4-node shell elements. One element between floorsresults in zero stiffener bending. Two elements between floors result in a linear distribution with approximatelyzero bending in the middle of the elements. When a relatively fine mesh is used in the longitudinal direction the effect of stiffener bending stresses shouldbe isolated from the girder bending stresses for buckling assessment.For the buckling capacity check of a plate, the mean shear stress, mean is to be used. This may be defined asthe shear force divided on the effective shear area. The mean shear stress may be taken as the average shearstress in elements located within the actual plate field, and corrected with a factor describing the actual sheararea compared to the modelled shear area when this is relevant. For a plate field with n elements the followingapply:

where:

AW = effective shear area according to the Rules for Classification of Ships, Pt.3 Ch.1 Sec.3 C503AWmod = shear area as represented in the FE model.

5.2.4 Local buckling assessment - plates, stiffeners, girders etc.

5.2.4.1 GeneralBuckling control of plating, stiffeners and girders/floors shall be carried out according to acceptable designprinciples. All relevant failure modes and effects are to be considered such as

plate buckling local buckling of stiffener and girder web plating torsional/sideways buckling and global (overall) buckling of both st

classification notes no. 34.1 csa - direct analysis of ship structures

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