Download - Offshore Fatigue
-
7/28/2019 Offshore Fatigue
1/23
Fatigue AssessmentUsing SESAM program modulesStofat, Framework and Postresp
A White Paper
-
7/28/2019 Offshore Fatigue
2/23
Stofat, Framework, PostrespFatigue Assessment
A White Paper
-
7/28/2019 Offshore Fatigue
3/23
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.
-
7/28/2019 Offshore Fatigue
4/23
3
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
-
7/28/2019 Offshore Fatigue
5/23
4
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
-
7/28/2019 Offshore Fatigue
6/23
5
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.
-
7/28/2019 Offshore Fatigue
7/23
6
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.
-
7/28/2019 Offshore Fatigue
8/23
7
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
-
7/28/2019 Offshore Fatigue
9/23
8
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.
-
7/28/2019 Offshore Fatigue
10/23
9
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.
-
7/28/2019 Offshore Fatigue
11/23
10
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
-
7/28/2019 Offshore Fatigue
12/23
11
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.
-
7/28/2019 Offshore Fatigue
13/23
12
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
-
7/28/2019 Offshore Fatigue
14/23
13
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.
-
7/28/2019 Offshore Fatigue
15/23
14
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.
-
7/28/2019 Offshore Fatigue
16/23
15
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.
-
7/28/2019 Offshore Fatigue
17/23
16
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
-
7/28/2019 Offshore Fatigue
18/23
17
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
-
7/28/2019 Offshore Fatigue
19/23
18
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
-
7/28/2019 Offshore Fatigue
20/23
19
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
-
7/28/2019 Offshore Fatigue
21/23
20
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.
-
7/28/2019 Offshore Fatigue
22/23
21
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
-
7/28/2019 Offshore Fatigue
23/23
e-mail: [email protected]: www.dnvsoftware.com
Head office:
OsloDNV SoftwareVeritasveien 1
NO-1322 Hvik, NorwayTel: +47 67 57 76 50Fax: +47 67 57 72 72
DNV Software regional offices:
BusanDet Norske VeritasDNV Software
Nambusan P.O. Box 120Busan 613-011Republic of KoreaTel: +82 51 610 7700Fax: +82 51 611 7172
HoustonDNV Software16340 Park Ten PlaceSuite 100Houston, Texas 77084-5132USATel: +1 (281) 721 6700Fax: +1 (281) 721 6880
Kobe
Det Norske VeritasDNV SoftwarePort P.O. Box 77Kobe 651-0191JapanTel: +81 78 291 1305Fax: +81 78 291 1330
Kuala LumpurDet Norske VeritasDNV Software
24th Floor, The Weld TowerJalan Raja Chulan50200 Kuala LumpurMalaysiaTel: +60 3 2050 2888Fax: +60 3 2031 8080
LondonDNV SoftwarePalace House3 Cathedral StreetLondon SE19DEUnited KingdomTel: +44 (0) 20 7716 6525Fax: +44 (0) 20 7716 6738
MarseilleDet Norske VeritasDNV Software16 Impasse Blancard13007 MarseilleFranceTel: +33 (0) 4 91 13 71 66Fax: +33 (0) 4 90 54 46 89
Rio de JaneiroDet Norske VeritasDNV Software
Rua Sete de Setembro111/12 Floor20050006 Rio de JaneiroRio de JaneiroBrazilTel: +55 21 2517 7232Fax: +55 21 2221 8758
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
Visit us at:
DNV Software is the commercial software house of DNV serving more than 3,000 customers in the marine, offshore and process industries./
10-2
005
Design&
production:DNVEGraphicServices
0502-0
30