Offshore Fatigue

7/28/2019 Offshore Fatigue

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Fatigue AssessmentUsing SESAM program modulesStofat, Framework and Postresp

A White Paper


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Stofat, Framework, PostrespFatigue Assessment

A White Paper


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February 2003

Prepared by DNV Software, an independent business unit of Det Norske Veritas

written by: Heidi Johansen

The information and the software discussed in this document are subject to change without notice and should not beconsidered commitments by DNV Software (DNVS). DNVS assumes no responsibility for any errors in this document.Reproduction, distribution, and transmission of this document by any means photostatic or electronic is restricted without

authorization. 2003, DNV Software. All Rights Reserved.

Including this documentation, and any software and its file formats and audio-visual displays described herein; all rightsreserved; may only be used pursuant to the applicable software license agreement; contains confidential and proprietaryinformation of DNV Software and/or other third parties which is protected by copyright, trade secret, and trademark law and

may not be provided or otherwise made available without prior written authorization.


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INTRODUCTION

Fatigue Loads.........................................................................4

Geometry Tolerances .............................................................5

Structures and analyses .........................................................5

FATIGUE ASSESSMENT APPLYING STOFAT

Analysis Capabilities .............................................................. 7

Environmental Loading.........................................................7

Stochastic Fatigue Calculations ............................................8

SN-curves ...............................................................................8

Stress Concentrations Factors............................................... 9

Structural Model and Fatigue Points.................................... 9

Analysis Results.................................................................... 10

Submodel Analysis ...............................................................10

Uncertainties in Fatigue Life Prediction.............................10

FATIGUE ASSESSMENT APPLYING FRAMEWORK

SCF Factors .........................................................................12

Fatigue Analysis...................................................................13

Structural Model and Fatigue Points.................................. 13

Deterministic Approach.......................................................13

Deterministic Fatigue Applying Framework ...................... 13

Stochastic Approach ............................................................ 14

Stochastic Fatigue Applying Framework............................15

Analysis Results.................................................................... 15

WIND FATIGUE ASSESSMENT IN FRAMEWORK

The Structural Model...........................................................17Overview of Theoretical Basis and Assumptions................17

FATIGUE ASSESSMENT APPLYING POSTRESP

Fatigue Models.....................................................................19

Short-term Fatigue Calculation ..........................................19

Long-term Fatigue Calculation........................................... 19

Results Presentation.............................................................20

TABLE OF CONTENTS


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This paper offers an introduction to the fatigue phenomena and how

to calculate the fatigue damage either by applying deterministic or

stochastic approaches.

To ensure that the structures will fulfil its intended function, fatigue

assessment, supported where appropriate by a detailed fatigue

analysis, should be carried out for each individual type of structural

detail which is subjected to extensive dynamic loading. It should be

noted that every welded joint and attachment or other form of stress

concentration is potentially a source of fatigue cracking and should

be individually considered.

The SESAM suite of programs offers several modules that provide

the opportunity to perform fatigue assessment of various types of

structures. These program modules are

Stofat

Framework

Postresp

Each of the above program modules supports different type of

structures. The below chapters provide a more extensive insightinto what program to use for what type of element model and type

of fatigue analyses. The SESAM fatigue analysis modules support

all the fatigue calculation methods described in DNV classification

note 30.7.

Fatigue Loads

The fatigue life of any member be that a beam, shell or solid should

be calculated considering the repetitive loads, which may lead to

possible significant fatigue damage. The following listed sources of

fatigue loads should, where relevant, be considered:

waves (including those loads caused by slamming andvariable (dynamic) pressures

wind (especially when vortex induced vibrations mayoccur)

currents (especially when vortex induced vibrations mayoccur)

mechanical vibration (e.g. caused by operation ofmachinery)

mechanical loading and unloading (e.g. crane loads)

The effects of both local and global dynamic response shall be

properly accounted for when determining response distribution

related to fatigue loads.

INTRODUCTION


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Geometry Tolerances

In the assessment of fatigue resistance, relevant consideration shall

be given to the effect of stress concentrations, including those

occurring as a result of:

fabrication tolerances, including due regard to tolerances inway of connections involved in mating sequences or

section joints

cut-outs

penetrations

details at connections of structural sections (e.g. cut-outs tofacilitate construction welding)

The DNV rules of classification provide SCFs (stress concentration

factors) for a number of standard details. In other fatigue sensitive

structural areas where predefined SCFs cannot be obtained from

standard tables, e.g. due to different structural arrangement or that

dimensions are out of range of the formula, the need for detailed

finite element analyses arise in order to determine the correct SCF.

Structures and analyses

Any type of offshore structure can be analysed applying SESAM,

be that a FPSO, Spar, Tension Leg Platform, semisubmersible,

jacket, jack-up, any type of top side modules. flare towers or drill

towers.

One of the important strengths within SESAM is the extensive

integration that exists between one program module to another. The

structures are modelled in what is known as pre-processors (e.g.

Patran-Pre and Genie) and the model is then automatically read by

the analysis engines be that environmental or structural analyses. If

both an environmental and structural analysis is performed the

environmental analysis is performed first. The result file from this

analysis is automatically read by the structural analysis engine

alongside the structural model. The result file from the structural

model is then automatically read by the SESAM post-processors

(e.g. Stofat, Framework, Postresp). The model properties and loads

are transferred without the user having to do any manual

transformation or additional load input from one module to the next

in SESAM, and thus reducing the possibility of erroneous input. In

the post-processors all information of model property and loads are

stored and these are readily accessible. Typical result information

stored in the result file is model geometry properties,

displacements, accelerations, forces, moments and different types

of stresses.


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The structures can be modelled in Patran-Pre or Genie or a

combination of the two by applying the superelement technique.

Patran-Pre is a general modeller where there exist extensive options

with respect to element types and loads. Genie is an ingeniousmodeller that is focusing on frame and plane plate structures such

as topside models and frame structures. An analysis model can

readily be built up as a combination of the two as the different

modellers create an interface file that can be merged in Presel by

applying the superelement technique. In Presel the superelement

model parts are assembled through two or more levels to form the

complete model. When running the structural analysis the structural

analysis engine Sestra reduces the equation systems of the

superelements successively until the whole system has been solved.

Results for the complete model or for selected superelements only

may be taken into a SESAM post-processor (e.g. Stofat,

Framework, Xtract) for results presentation or further processing.


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As Stofat is part of the SESAM analysis package you will reap the

benefit of the extensive integration that exists within SESAM and as

such reduce the uncertainties caused by man-made errors, andthereby enhance the quality of the fatigue assessment.

Fatigue cracks and fatigue damages have been known to vessel

designers for several decades. Initially the obvious remedy was to

improve detail design. With the introduction of higher tensile steels

(HTS-steels) in hull structures, at first in deck and bottom to

increase hull girder strength, and later on in local structures, the

fatigue problem became more imminent.

Stofat is an interactive postprocessor performing stochastic fatigue

calculation of welded shell and plate structures. The fatigue

calculations are based on responses given as stress transfer

functions. The stresses are generated by hydrodynamic pressureloads acting on the model. These loads are applied for a number of

wave directions and for a range of wave frequencies covering the

necessary sea states. The loads are applied to a finite element model

of the structure whereupon the finite element calculation produces

results as stresses in the elements. Stofat uses these results to

calculate fatigue damages at given points in the structural model.

Analysis Capabilities

Stofat performs stochastic fatigue analysis on structures modelled

by 2D-shell and solid elements and assesses whether the structure is

likely to suffer failure due to the action of repeated loading. The

assessment is made by an SN-curve based fatigue approachaccumulating partial damages weighted over sea states and wave

directions. The program delivers usage factors representing the

amount of fatigue damage that the structure has suffered during the

specific period. The loads must be computed from a hydrodynamic

analysis using a stochastic approach. A stochastic approach implies

that the computed loads are complex comprising real and

imaginary components.

Environmental Loading

Several wave spectra are available:

Pierson-Moskowitz spectrum

Jonswap spectrum

General Gamma spectrum

Double peaks, six parameters Ochi-Hubble spectrum

The last spectrum can be used to model double peaks present in a

wave energy density, e.g. low frequency swell along with high

frequency wind generated waves, and may represent almost all

stages of development of a sea in storm.

FATIGUE ASSESSMENT

APPLYING STOFAT


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The wave energy spreading functions are used when statistical

calculations are required for short crested sea, i.e. if the user wants

to take into account other directions than the current main wave

direction. The wave energy spreading function may be a cosn().

The wave statistics model describes the sea state conditions during

a long term period and consists of mainly zero up-crossing periods

TZ, significant wave heights HS and their probability of occurrence.

Two scatter diagrams are predefined in Stofat. These are the North

Atlantic scatter diagram and the World Wide scatter diagram, as

given by DNV classification note no 30.7 Fatigue assessment of

ship structures.

Wave direction probability can be specified and this defines the

probability of occurrence for each main wave direction specified in

the hydrodynamic analysis.

For more technical details on load and response modellingreference is made to appendix B of the Stofat user manual.

Stochastic Fatigue Calculations

A stochastic fatigue analysis requires that a linearised frequency

domain hydrodynamic analysis (Wadam) followed by a quasi-static

structural analysis (Sestra) is executed first. The load interface file

generated by Wadam is automatically read into Sestra.

Harmonic waves of unit amplitude at different frequencies and

directions are passed through the structure and generate a set of

stress transfer functions which are read into Stofat through the

Result Interface File and used in the long term stochastic fatiguecalculations.

The long term fatigue calculation is based directly on a scatter

diagram where Rayleigh distributions of the stress ranges are

assumed and takes response spectrum and SN-curves as input.

Usage factors indicating the extent of fatigue damage are calculated

and printed. If a vtf-file is specified the fatigue damage can be

displayed as contour plots in Xtract for better visualisation.

The long term fatigue calculation may also be based on generation

of stress time series by Fast Fourier Transform from stress auto

spectrum, i.e. rainflow cycle counting in the time domain.

Details on the spectral calculation methods applied in Stofat can befound in appendix C of the Stofat user manual.

SN-curves

This is used to define the fatigue characteristics of a material

subjected to repeated cycle of stress of constant magnitude. The

SN-curve delivers the number of cycles required to produce failure

for a given magnitude of stress. The SN-curve may be selected as

one of the pre-defined curves included in the program or it may be

user defined. Different SN-curves may be assigned to individual

elements. Default SN-curve of Stofat is DNVC-I.


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Stress Concentrations Factors

Fatigue computation according to DNV Classification note 30.7

requires use of Stress concentration factors. Stress concentration

factors are dependent upon the level of detail in the model. The

geometrical concentration factor, denoted Kg, is specified when the

structural analysis has calculated nominal stresses in the structural

parts, but for a mesh too coarse to represent local stress gradients.

The geometrical stress concentration factor may be estimated from

the rules by experience, or from a detailed finite element

computation. When the finite element analysis is sufficiently

accurate to simulate the stress gradient caused by the structuraldetail, the geometrical stress concentration factor is omitted. A

stress concentration factor due to the weld itself, denoted Kw, is

usually taken from the rules.

Structural Model and Fatigue Points

Stofat utilizes the structural model information read from the

Results Interface File. Before accessing Stofat, a (.SIN) file

containing a complete model description for the structure and stress

transfer functions of the loadings must have been generated.

Stofat operates on first level superelements and handles one

superelement at the time.

Fatigue assessment may be executed by performing an element

fatigue check or a hotspot fatigue check. The element fatigue check

runs through all elements selected for the fatigue assessment anddelivers one usage factor per element. The hotspot fatigue check

performs fatigue assessment of specific points in the structure

defined by the user and delivers one usage factor per hotspot. The

hotspots may be placed anywhere inside the superelement model

treated by Stofat.

In an element fatigue assessment the fatigue points may be located

at element surfaces

at element corners

at element stress points

at middle planes of the shell elementsThe number of fatigue check points is the same as the number of

stress points for the elements. Fatigue damage is calculated for all

the fatigue points and the usage factor of the point suffering most

damage within an element is taken as the usage factor of the

element.

Calculation of the fatigue damage is based on the maximum

principal stress component (real and imaginary parts) at the fatigue

check point. Stresses are interpolated component by component to

the fatigue check point whereupon the principal stresses are

calculated and applied in the fatigue damage assessment.


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Analysis Results

Stofat produces usage factors expressing the extent of fatigue

damage to the structure as a consequence of the applied loading.

Analysis results are presented to the user in form of tabulated prints

and graphic display of the usage factors. Along with the usage

factors key parameters related to the fatigue check points are

printed. Extended print of detailed results is possible. Such print

includes print of hotspot transfer functions, moments of response

spectrum, damage per sea state, damages per sea directions,

damages per hotspots/elements, exceedence probabilities and stress

range levels.

The fatigue analysis may also be written to file (.VTF) and

displayed as contour plots by Xtract.

Submodel AnalysisIf fatigue sensitive areas in the structure have been identified, but

uncertainties remain about stress concentration factors or stress

gradients, analysis of a submodel may be useful. A submodel

represents a detailed part of the original global analysis. Typical

steps in a submodel analysis are:

Make a finite element model of the area in question whereall relevant detailed geometry is included, e.g. cut-outs,

stiffeners, brackets, welds.

Apply a refined mesh to represent local stress gradients ofthe area with sufficient accuracy

Specify prescribed boundary conditions (displacementsfrom the global analysis) around the perimeter where the

submodel is to be connected to the original model (global

model). The perimeter of the submodel do not have to

match geometric lines in the global model

Run Submod to transfer displacement results from theoriginal global model into prescribed displacement along

the boundary of the submodel

Analyse the submodel in Sestra

Perform fatigue checks in StofatDetailed local models (submodels) can typically be the

column/brace connection of a semisubmersible or the joint of a

jacket structure. Even if the initial global analysis is a framestructure, e.g. created in Genie, the displacements can be

transferred to a local model consisting of shell or solid elements.

The local models perimeter does however need to match geometric

points in the global model if it consists of beams. Within SESAM

the fatigue evaluations of frame structures consisting of beam

elements are performed in Framework, while fatigue assessments of

shell and solid elements are performed in Stofat.

Uncertainties in Fatigue Life Prediction

There are a number of different uncertainties associated with

fatigue life predictions. The calculated loading on the vessel is


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uncertain due to uncertainties in wave heights, periods and

distribution of waves. The resulting stresses in the vessel are

uncertain due to uncertainties in the loading, calculation of response

and calculation of stress concentrations.

Because of the sensitivity of calculated fatigue life to the accuracy

of estimates of stresses, particular care must be taken to ensure that

stresses are realistic. Fatigue damage is proportional to stress raised

to the power of the inverse slope of the SN-curve. I.e. small

changes in stress result in much greater changes in fatigue life.

Special attention should be given to stress raisers like eccentricities

and secondary deformations and stresses due to local restrains. Dueconsiderations should, therefore, be given to the fabrication

tolerances during fatigue design. Furthermore there is a rather large

uncertainty associated with the determination of SN-curves, and

there is also uncertainty associated with the determination of stress

concentration factors.

Model generated applying a SESAM Pre-processor,

e.g. Patran-Pre for individual superelements

and Presel for assembly

Hydrodynamic loads arecalculated in Wadam

Analysis of structure is performedin SESTRA and the result file

*.SIN can be imported by Stofat

Contours of fatigue damagecalculated in Stofat and

presented in Xtract

Stofat is an integrated part of the SESAM system of programs. Shell and solid types of

structures modelled by the SESAM pre-processors and subjected to hydrodynamic loadingmay be analysed using Sestra, which in turn creates a Results Interface File. Stofat reads

this interface file and produces a database file. Model data and element stresses are

transferred to Stofat and used in the calculation of fatigue damages.


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Framework checks the structural integrity of all types of offshore

frame structures: jackets, jack-ups, decks, topsides and flare booms.

All phases throughout the life cycle of the structure are covered:

from the initial design to the re-qualification.

Only the fatigue assessment capabilities of Framework are covered

in this paper.

A fatigue analysis in Framework is performed on a frame structural

member in order to assess whether that member is likely to suffer

failure due to the action of repeated loading. This assessment is

made using Miners rule of cumulative damage, which delivers a

usage factor representing the amount of fatigue damage that a

member has suffered during the specified period.

A fatigue analysis in Framework can be performed using either

a deterministic approach a stochastic approach

SCF Factors

A factor influencing the development of fatigue failure is the

overall geometry of the joint and the detailed geometry of its weld.

For any particular type of loading, the joint geometry governs the

value of the stress concentration in the region where fatigue

cracking is likely to initiate. This region is termed as the hotspot.

In Framework, hotspot stress concentration factors (SCFs) may be

specified by the user. For tubular members only, the user may

alternatively have the SCFs automatically calculated by theprogram using a set of parametric equations based on the joint type

(K, YT, X, etc.).

Each hotspot is associated with 3 concentration factors. These are:

SCF for axial stresses

SCF for in-plane bending stresses

SCF for out-of-plane bending stresses

For tubular members, SCFs are normally assigned at 8 hotspots per

weld side. The hotspots are equally spaced around the pipe

circumference.

A SCF is defined as the factor by which the nominal stress due to

pure axial force or pure in-plane/out-of-plane bending (at the stress

point in question) must be multiplied in order to give the hotspot

stress used in the damage calculation.

In Framework the parametric SCFs are calculated by Kuang,

Wordworth and Smedley, Efthymiou, Smedley and Fisher or

NORSOK depending on the type of joint. Furthermore, Framework

differs between global and local SCFs where the global SCFs are

FATIGUE ASSESSMENT

APPLYING FRAMEWORK

M0: Out-of-plane

moment

M1 :In-plane

moment

Section A - A

Hotspot numbering system for atubular section


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applied to all members and hotspots while the local SCF is applied

to specific members and selected points.

For further details reference is made to the Framework user manual.

Fatigue Analysis

The required model and methods for fatigue analysis for self-

elevating units or jack-ups are dependent on type of operation,

environment and design type of the unit. For units operating at

deeper waters where the first natural periods are in a range with

significant wave energy, e.g. for natural periods higher than 3s, the

dynamic structural response need to be considered in the fatigue

analysis.

Structural Model and Fatigue Points

Framework utilizes the structural model information read from theResults Interface File. Before accessing Framework, a (.SIN) file

containing a complete model description for the structure and stress

transfer functions of the loadings must have been generated.

Deterministic Approach

Fatigue checks can be performed by linear (Weibull) or

piece-wise linear long term distribution of the stress range.

A simplified or deterministic fatigue analysis may be

undertaken in order to establish the general acceptability

of fatigue resistance, or as a screening process to identify

the most critical details to be considered in a stochastic

fatigue analysis. The deterministic fatigue analysis should

be undertaken utilising appropriate conservative design

parameters.

Deterministic Fatigue Applying Framework

A deterministic fatigue analysis requires a deterministic

hydrodynamic analysis (Wajac) followed by a static

structural analysis (Sestra). The frame finite element

model can be generated in Genie. Deterministic loads are

obtained by stepping waves of various heights and

directions through the structure in order to obtain (through

a structural analysis) a stress history for each member ateach of its hotspots.

For each of the wave directions specified in the hydrodynamic

analysis, it is necessary, in Framework, to specify the total number

of waves passing through the structure. A long term distribution of

wave heights is then produced for each of the wave directions in

order to obtain, for each wave height, the associated number of

waves. The long term distribution of wave heights may be obtained

using either a long term Weibull distribution or a piece-wise linear

distribution in H-logN space.


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Usually, the procedure adopted for a deterministic fatigue analysis

in Framework is as follows:

Definition of fatigue constants (target fatigue life, globalSCFs, etc.)

Assignment of chord members

Modelling of local details (assignment of Can and Stubsections, etc.)

Assignment of joint type and joint gap/overlap data

Assignment of SCFs

Assignment of SN curve

Assignment of individual wave data

Execution of fatigue analysis

Printing of results

Stochastic ApproachStochastic fatigue analyses shall be based upon recognised

procedures and principles utilising relevant site specific data or

world wide environment data.

Simplified fatigue analyses should be used as a screening process

to identify locations for which a detailed, stochastic fatigue analysis

should be undertaken.

Fatigue analyses shall include consideration of the directional

probability of the environmental data. Providing that it can be

satisfactorily checked, scatter diagram data may be considered as

being directionally specific. Scatter diagram for world wide

operations (North Atlantic scatter diagram) is given in DNVclassification note 30.5. Relevant wave spectra and energy

spreading shall be utilised. Possible wave spectra to apply in a

stochastic (frequency domain) fatigue analysis may be Jonswap,

Pierson-Moskowitz, Gamma or Ochi-Hubble. Often a Pierson-

Moskowitz spectrum and cos4 spreading function is utilised in the

evaluation of self-elevating or jack-up units.

Structural response shall be determined based upon analysis of an

adequate number of wave directions. Generally a maximum radial

spacing of 15 degrees should be considered. Transfer functions

should be established based upon consideration of a sufficient

number of periods, such that the number and values of the periods

analysed:

Adequately cover the wave data

Satisfactorily describe transfer functions at, and around, thewave cancellation and amplifying periods

(consideration should be given to take account that such

cancellation and amplifying periods may be different for

different elements within the structure)

Satisfactorily describe transfer functions at, and around, therelevant excitation periods of the structure.


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The fatigue damage itself is calculated using a Miners Rule. Stress

concentration factors (SCFs) in tubular joints are automatically

calculated according to Efthymiou or Kuang/Wordsworth-Smedley

or manual input.

Stochastic Fatigue Applying Framework

A stochastic fatigue analysis requires a linearised frequency domain

hydrodynamic analysis (Wajac) followed by a quasi-static or

dynamic structural analysis (Sestra). The frame finite element

model can be generated in Genie. Load transfer functions are

obtained by passing a harmonic wave of unit amplitude at different

frequencies and directions through the structure in order to obtain

(through a structural analysis) a set of stress transfer functions for

each direction for each member at each of its hotspots.

Relevant data required to be defined in Framework are: Short term sea-states and corresponding probabilities in

order to describe the long term distribution of the short

term sea-states. A short term sea-state is characterised by asignificant wave height and a zero up-crossing period.

Probability of occurrence for each of the wave directionsdefined during the hydrodynamic analysis.

The wave spectrum shape used may be either a JONSWAP,Pierson-Moskowitz, Ochi-Hubble or Gamma spectrum.

Sea spreading data in order to define the number ofelementary wave direction and the associated energy

content.

Usually, the procedure adopted for a stochastic fatigue analysis in

Framework is as follows:

Definition of fatigue constants (target fatigue life, globalSCFs, etc.)

Assignment of Chord members

Modelling of local details (assignment of Can and Stubsections, etc.)

Assignment of joint type and joint gap/overlap data

Assignment of SCFs

Assignment of SN curve

Assignment of sea state data

Execution of fatigue analysis

Printing of results

Analysis Results

The fatigue utilizations can be displayed graphically on the screen

or paper and printed in tabulated formats.


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Analysis of structure is performed inSESTRA and the result file *.SIN can be

imported by Framework

Framework is an integrated part of the SESAM system of programs. Frame structures modelled with

beam elements by the SESAM pre-processor Genie and subjected to hydrodynamic loading (Wajac)may be analysed using Sestra, which in turn creates a Results Interface File. Framework reads this

interface file and produces a database file. Model data and element stresses are transferred toFramework and used in the calculation of fatigue damages.

Model generated applying aSESAM Pre-processor, e.g. Genie

Hydrodynamic loads and wind loads are

calculated in Wajac

Results presented in Framework


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Wind fatigue analysis is also supported in Framework as a separate

module. The wind fatigue module has its own internal data storage,

separate from the data base of Framework. Many features of

Framework are thus not available to wind fatigue calculations, Postprocessing facilities are limited to tabulated prints of fatigue

damages of brace/joint intersections.

The Framework wind fatigue module calculates the buffeting and

vortex shedding induced fatigue damage. For details regarding the

assumptions made in Framework reference is given to the

Framework user manual and the Framework theory manual

wind fatigue design.

In this paper only a brief overview of the theoretical basis is

presented.

The Structural ModelStructures modelled by two nodes 3D beam elements with uniform

tubular sections may be analysed for wind fatigue damage. The

fatigue module is primarily intended for fatigue calculations of

frame structures such as flare towers. Similar to the other types of

fatigue calculations in Framework a Result Interface File (.SIN) is

required.

Overview of Theoretical Basis and Assumptions

The wind fatigue module evaluates fatigue damage of frame

structures subjected to wind loading Buffeting loads due to wind

gusts and the vortex shedding effects due to steady state wind are

considered. Wind fatigue due to buffeting loads is treated by thepower spectral density method and the damage is a function of the

overall structural response. The effects of vortex shedding induced

fatigue are treated by evaluation of individual member responses.

The two effects are calculated on the assumption that they are

uncoupled and are summed to give the

overall fatigue damages of joints and

members in the structure.

The fatigue analysis is based on annual wind

data characterized by a set of wind states,

considered to represent the climate for the

year. For each wind state, the response stresspower spectra at local hotspots within a

particular joint are evaluated.

For buffeting fatigue calculations the hotspot

power spectrum response is divided into a

quasi-static response part and a dynamic

response part, see Figure showing the typical

hotspot stress spectrum due to wind loading.

The quasi-static part of the power spectrum

covers the low frequency non-resonant response. This spectrum has

a broad peak at low frequencies but is treated as a narrow band at

its peak frequency with one third of the stress variance of the low

WIND FATIGUE ASSESSMENT

IN FRAMEWORK

Typical hotspot stress spectrum due to wind loading


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frequency broad band stress spectrum. The resulting damage is then

multiplied by 10. This approach assumes that the quasi-static

contribution to damage is small, so that a rigorous evaluation is not

required.

The dynamic response consists of the excited resonant modes. It is

partitioned into separate resonant modal responses; for each of

these an independent damage assessment is made. This assumes

that each response is narrow band and independent of the others,

but sometimes several modes, very close in frequency, are taken as

one.

For each of these dynamic and static partitions a Rayleigh

distribution of the hotspot stress range versus the number of cycles

is assumed. The variance is given by the integral under the power

spectrum. Fatigue damage may then be evaluated by application of

the Palmgren-Miner relationship and use of a recognised SN curve.Vortex shedding from brace members may induce oscillations in

individual braces. These are local modes rather than overall

structural modes. It is assumed that the vortex shedding effects are

only of any significance for fatigue of they induce oscillations in

the first mode of the brace.

The major assumptions of wind fatigue calculation are:

Buffeting damage is dominant by low frequency resonantmodes

The greatest hotspot stresses within a modal response cycleoccur at maximum modal amplitude

The structure is made of welded tubular members Parametric SCF equations or user specified SCFs are used

to evaluate joint stress concentrations

Wind forces are parameterized as linear fluctuatingcomponents superimposed upon mean wind profiles

Wind gust velocities in the mean wind direction and normalto the mean wind both horizontally and vertically are

statistically independent

Member drag coefficients are invariant under thefluctuating wind component and are appropriate to the

mean wind speed

Vortex shedding induced member oscillations and fatigue

are uncoupled from any buffeting induced vibrations anddamage


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A Stochastic fatigue analysis requires that a linearised frequency

domain analysis (Wadam, Wajac or Sestra) is executed first. This

will generate a set of stress transfer functions which can be read

into Postresp through the Hydrodynamic Results Interface File andused in the short or long term stochastic fatigue calculations.

Stress component based stochastic analysis is offered in Postresp.

The load transfer functions calculated by the wave load program

(Wadam) are transferred to stress transfer functions. The load

transfer functions normally include:

Global vertical hull girder bending moments and shearforces

Global horizontal bending moment

Vessel motions in six d.o.f.

Pressures for all panels of the 3-D diffraction model

The stress transfer functions are combined to a total stress transfer

function and a stochastic fatigue evaluation is performed. Hence,

the simultaneous occurrence of the different load effects is

preserved. For further details reference is made to DNV

classification note 30.7.

Fatigue Models

A stochastic fatigue analysis applying Postresp can be done directly

on the result file from a hydrodynamic analysis in which case you

can get short term or long term fatigue calculation on sections

through the hydrodynamic model. This may be useful informationin an early design stage.

A stochastic fatigue analysis applying Postresp can also be done on

the structural analysis results file. As fatigue analyses in Postresp

work on the Hydrodynamic Results Interface File (G*.SIN) the

structural analysis result file (R*.SIN) must be re-formatted. This is

done in Prepost. The fatigue analysis can then be performed on

stresses in specifically selected stress points (hot-spots). While

Stofat calculates the fatigue for all the stress points within a

superelement Postresp only calculates the fatigue for specified

points. Therefore it is important that the engineer is able to specify

the correct fatigue sensitive hot-spots.

Short-term Fatigue Calculation

In the short-term fatigue calculation, the fatigue damage can be

obtained for a short term duration of a given sea state. The shortterm fatigue assumes Rayleigh distribution of the stress ranges and

takes response spectra, SN-curves, and durations as input. The

expected value for failure is then calculated and printed.

Long-term Fatigue Calculation

Long-term fatigue calculation can be calculated either based

directly on a scatter diagram where Rayleigh distributions are

FATIGUE ASSESSMENT

APPLYING POSTRESP


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assumed for each cell in the scatter diagram or based on a Weibull-

fit from a long term response calculation of the significant

responses (stress ranges) of the cells in the scatter diagram.

Both the short term and long term fatigue calculations are based on

the assumption that a single-slope or bi-linear SN-curve is used.

Results Presentation

The total damage and the contribution to damage from each cell in

the scatter diagram and for each direction is printed when

requested.


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1. Det Norske Veritas classification notes 30.7 FatigueAssessment of ship structures September 1998

2. Det Norske Veritas Recommended Practice RP-C203

Fatigue strength analysis of offshore steel structures3. Det Norske Veritas Offshore Standard DNV-OS-

C104, January 2001

4. DNV Software STOFAT User Manual5. DNV Software Framework User Manual6. DNV Software Framework theory manual7. DNV Software Framework theory manual wind

fatigue design

8. DNV Software Postresp User Manual

REFERENCES


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e-mail: [email protected]: www.dnvsoftware.com

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OsloDNV SoftwareVeritasveien 1

NO-1322 Hvik, NorwayTel: +47 67 57 76 50Fax: +47 67 57 72 72

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ShanghaiDet Norske VeritasDNV SoftwareHouse No. 9,No. 1591 Hong Qiao RoadShanghai 200336ChinaTel. +86 21 6278 8076

Fax. +86 21 6278 8090

TaiwanDet Norske Veritas5F-3 No.160 Sec. 6,Minquan E. Rd114 TaipeiTaiwanTel. +886 2 2792 5352Fax. +886 2 2792 5357

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