advanced methods for uls and fls
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Torbjørn Lindemark, Nauticus Product Manager
Advanced Methods for Ultimate and Fatigue Strength of Floaters
DNV Software
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
2
Agenda Strength assessment of FPSOs and related software from DNV
Introduction to direct load and strength calculations
Deterministic vs. spectral analysis
Fatigue loading and critical details for FPSOs
Case study and software demo on direct strength calculations of a ship shaped FPSO
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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FPSO - What is required?
FPSO - Complex design process - Ships and Offshore Rule requirements - Regulatory requirements - Seakeeping, Hydrodynamic analysis - Long operation life without docking - Topside & Topside/Hull interaction - Turret area - Risers & Moorings - Deep water
Tools for assessment of - Conversion of tanker to FPSO - FPSO newbuilding
Tools for maintenance of FPSO’s in operation
We deliver a package that ties it all together and provide a complete, integrated toolkit, tailor made for FPSOs
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Challenge of FPSO New Build and Conversion Conversions
- Increase certainty that the chosen vessel is suitable for conversion,
- Determine how much steel should be replaced during conversion/maintenance,
- Identify where to focus surveys.
New Builds
- Selection corrosion protection strategy to determine a rational material thickness
- Identify comprehensive analysis requirements for design
- Develop Inspection Plans - Choice of turret design
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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FPSO Package for design and analysis
Proven solutions in use by major companies
around the world
Topside Genie
Main scantlings Nauticus Hull
Risers DeepC
Turret Local analysis
GeniE
Hydrodynamics • Seakeeping • Wave loads
HydroD
Fatigue Simplified,
Spectral Nauticus Hull Sesam/Stofat
Mooring Mimosa
3D Hull modelling
GeniE
Risk Analysis Safeti
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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FE analysis
4. Global stress and deflection & fatigue screening
Direct Calculations in an Integrated Analysis System
1. Stability and wave load analysis
Wave scatter diagram
2. Pressure loads and accelerations
Loa
d tr
ansf
er
3. Structural model loads (internal + external pressure)
Local FE analysis
5. Local stress and deflection & fatigue
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Wave Load Analysis Input
- Models - Panel &/or Morrison model - Mass model - Compartments - Structural model for load transfer
- Loading conditions - Compartment fillings, draught and trim
- Wave and environmental data - Scatter diagram - Wave spectrum - Directionality and spreading - Current - Water depth
Output - Load transfer functions (Response Amplitude
Operators – RAOs) - Motions in 6 dof (+ derived velocities and
accelerations) - External wave pressures - Internal tank pressures - Morrison forces - Sectional loads
- Load statistics - Derived by combining the load RAOs with wave data - Design values for ULS/ALS - Long term load distribution for simplified fatigue
calculations - Load files for transfer to structural model
- Design waves for deterministic ULS and/or FLS analysis
- Load RAOs for stochastic ULS and FLS analysis - Both containing accelerations, external and internal
pressures
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Finite Element Analysis
Input - Global and local FE models - Design wave load transfer files (or long term
loads by manual input)
Output - Stress response for a given design wave/load
Input - Global and local FE models - RAO based load transfer files - Wave and environmental data
- Scatter diagram - Wave spectrum - Directionality and spreading
Output - Stress transfer functions (Response Amplitude
Operators – RAOs) - Stress statistics
- Derived by combining the stress RAOs with wave data
- Short and long term distribution - Design values for specified probability level/return
period
Deterministic Analysis Spectral Analysis
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Fatigue Analysis by Cumulative Damage
Input - Long term stress distribution
- Described by Weibull distribution or stress histogram - The Weibull distribution is described by
- Stress at a given probability level - Weibull parameter - Zero crossing frequency
- S-N curves
Output - Calculated fatigue life or damage
Input - Stress transfer functions (Response Amplitude
Operators – RAOs) - Wave and environmental data
- Scatter diagram - Wave spectrum - Directionality and spreading
- S-N curves
Output - Calculated fatigue life or damage
- Fatigue calculations performed based on short term statistics by summing up part damage for each cell in the scatter diagram the uncertainties involved in Weibull fitting are avoided
Deterministic Analysis Spectral Analysis
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Simplified vs. direct fatigue calculations
Wave Load Analysis:
Stress analysis:
Environment
Long term Weibull distribution by rule formulas
Direct calculated loads - 3D potential theory
Fatigue damage analysis:
Wave scatter diagram and energy spectrum
Accelerations, pressure and moments on 10^-4 or 10^-8 probability level by rule formulas
Load transfer to FE model. Stress transfer function implicit in FE model
Rule formulations for stresses and correlation of different loads
Based on expected largest stress among 10^4 cycles of a rule long term Weibull distribution
Based on summation of part damage from each Rayleigh distributed sea state in scatter diagram.
Simplified Spectral Analysis
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Fatigue loads and stress components Global wave bending moments Hull girder stress Stress in topside supports due to global hull
deflections Stress in turret and moonpool areas due to hull
deflections
Wave pressure Shell plate local bending stress Local stiffener bending stress Secondary stiffener bending due to deflection
of main girder system Local peak stresses in knuckles due to
deflection of main girder system
Vessel motions (accelerations) Liquid pressure in tanks Stress in topside support from inertia forces Mooring and riser fastenings
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Moonpool areas
Long. stress in deck (noshear lag effect)
CL
Nominal stresslevel
Actual stressdistribution
Long. stress in deckuniform deck thickness
Long. stress in deckwhen plates near side
are increased
Increased platethickness
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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In-service Experience on Fatigue Critical Details Stiffener end connections
Root source of cracking Global hull girder bending Local dynamic pressures Relative deflections caused by bending of
girder system Stress concentration at stiffener toe and
heel
Longitudinal
Stiffener Web-plating
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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In-service Experience on Fatigue Critical Details
Cracks under development
Repair example
Knuckles in inner structure (hopper knuckle)
Root source of cracking: Deflection on main girder system High stress concentration
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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In-service Experience on Fatigue Critical Details Shell plating
Root source of cracking Local pressure
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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In-service Experience on Fatigue Critical Details Main deck openings and attachments
Root source of cracking Global hull girder stress Stress due to hull girder deflection and stiff topside
lattice construction Stress from topside inertia forces Local stress concentrations
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Summary Fatigue Critical Details Main deck openings, attachments and topside support
Moonpool area
Knuckles and discontinuities in the main girder system
Stiffener end connections
Side shell plating
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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A few useful ratios Ratio Stress factor
(equivalent stress reduction)
Fatigue Damage factor
Base / Weld - SN curve
(10^12.89) / (10^12.65)
0.83 1.74
World wide / North Atlantic ocean
0.8 / 1.0 0.8 2.0
Non-corrosive / corrosive environment
(10^12.65) / (10^12.38)
0.81 2.0
Mean / Design SN curve
(10^12.09) / (10^11.63)
0.7 3.0
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Part 2 – Case Study and Demos
Direct strength ULS and FLS calculations of a ship shaped FPSO
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Why direct load and strength calculations Rule loads are not always the truth Modern
calculation tools give more accurate loads - Ultimate strength loads - Fatigue loads - Phasing and simultaneity of different load effects
Design and strength optimizations based on analysis closer to actual operating conditions
Improved decision basis for - In-service structural integrity management - Life extension evaluation
0
500000
1000000
1500000
2000000
0 0.2 0.4 0.6 0.8 1
[kN
m]
VBM (linear)
0
50000
100000
150000
0 0.2 0.4 0.6 0.8 1
[kN
]
VSF (linear)
Pressure
Rule −−−
Direct −−−
Time
Stre
ss
Vertical BendingMomentSea Pressure
Double Hull Bending
Total Stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Direct calculated loads vs. rule loads Fatigue loads:
0.00
0.20
0.40
0.60
0.80
1.00
1.20
VerticalBending
HorizontalBending
Pressure WL Vert. Acc.
DirectDNV RuleCSR
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Spectral vs Simplified Fatigue Analysis Comparison of fatigue damage by DNV rules and Common Scantling Rules relative
to spectral fatigue calculations:
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Bottom atB/4
Side atT/2
Side at T TrunkDeck
Comp. Stoch.DNV RuleCSR
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Analysis Overview Task Purpose Input Output
Global modelling Make global model for hydrodynamic and strength analysis
Ship drawings Loading manual
Global FE model
Hydrodynamic analysis
Calculate loads for fatigue and ultimate strength
Global FE model Wave data
Load files for structural analysis
ULS analysis Calculate hull girder strength
Global FE model Snap shot load files from HydroD
Ultimate strength results
Spectral fatigue analysis
Fatigue screening on nominal stress Local fatigue analysis
Global FE model Frequency domain load files from HydroD
Calculated fatigue lives
Spectral ULS analysis
Calculate long term stress based on spectral method
Global FE model Frequency domain load files from HydroD
Long term stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Creating the Global Model
The global model is used to calculate loads and strength and must represent the actual properties of the ship
For direct strength calculations essential properties are - Buoyancy and weight distribution - Compartment loads - Structural stiffness and strength
Modelling of hull form
Creating compartment and loads
Mass tuning
Challenges Model requirements
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Demo – Global Modelling with GeniE
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Benefits of GeniE for Global Modelling
One common model for hydrodynamic and structural analysis
Geometry modelling - Advanced surface modelling functions - Re-use data from CAD - Parametric modelling using JavaScript - Use of units
Compartment and loads - Compartments are created automatically - GeniE calculates tank volumes and COG - Loads are generated from compartment
fillings and automatically applied to tank boundaries
Mass tuning - Scaling mass density to target mass
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Analysis Overview Task Purpose Input Output
Global modelling Make global model for hydrodynamic and strength analysis
Ship drawings Loading manual
Global FE model
Hydrodynamic analysis
Calculate loads for fatigue and ultimate strength
Global FE model Wave data
Load files for structural analysis
ULS analysis Calculate hull girder strength
Global FE model Snap shot load files from HydroD
Ultimate strength results
Spectral fatigue analysis
Fatigue screening on nominal stress Local fatigue analysis
Global FE model Frequency domain load files from HydroD
Calculated fatigue lives
Spectral ULS analysis
Calculate long term stress based on spectral method
Global FE model Frequency domain load files from HydroD
Long term stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Hydrodynamic Analysis
Hull shape as real ship
Correct draft and trim
Weight and buoyancy distribution according to loading manual
Mass and buoyancy in balance
Obtain correct weight and mass distribution
Balance of loading conditions
Challenges Model requirements
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Demo – HydroD
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Benefits of HydroD One common model for
- Stability calculations - Linear hydrodynamic analysis - Non-linear hydrodynamic analysis - With or without forward speed
Supports composite panel & Morrison models
Model shared with structural analysis
Loading conditions - Multiple loading conditions by changing compartment
contents
Balancing the model - Auto balance of loading conditions by draft and trim or
compartment fillings
Built in roll damping module - Stochastic linearization - Quadratic damping
Strong postprocessing and graphical results presentation
Load transfer to FE analysis - Snap shot or frequency domain - With splash zone correction for fatigue
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
33
Analysis Overview Task Purpose Input Output
Global modelling Make global model for hydrodynamic and strength analysis
Ship drawings Loading manual
Global FE model
Hydrodynamic analysis
Calculate loads for fatigue and ultimate strength
Global FE model Wave data
Load files for structural analysis
ULS analysis Calculate hull girder strength
Global FE model Snap shot load files from HydroD
Ultimate strength results
Spectral fatigue analysis
Fatigue screening on nominal stress Local fatigue analysis
Global FE model Frequency domain load files from HydroD
Calculated fatigue lives
Spectral ULS analysis
Calculate long term stress based on spectral method
Global FE model Frequency domain load files from HydroD
Long term stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Ultimate Strength Analysis Global structural analysis with load
transfer from hydrodynamic analysis
Snap shot load transfer of non linear loads for selected design conditions
Yield and buckling check with PULS
Benefits of global analysis with direct load transfer Eliminate effect of boundary conditions Loads applied as a simultaneous set of sea
and tank pressures according to the calculated design wave No need for conservative and/or uncertain assumptions Integrated buckling check
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
35
Cutres - Verification of Applied Loads
0 50 100 150 200 250 300 350
Distance from AP
Ver
tical
she
ar fo
rce
WASIMCUTRES
Vertical shear force distribution
0 50 100 150 200 250 300 350
Distance from AP
Vert
ical
ben
ding
mom
ent
WASIMCUTRES
Vertical bending moment distribution
Cutres calculates and integrates the force distribution of cross sections and is ideal to evaluate the hull girder structural response
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
36
PULS – Advanced Buckling & Panel Ultimate Limit State
PULS is a code for buckling and ULS assessments
of stiffened and unstiffened panels
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Benefits of PULS Characteristics
- Higher accuracy than traditional rule formulations and classic buckling theory
- Quick and easy-to-use design tool for calculation of ULS capacity
- Valuable information about failure mode and buckling pattern
- Effective to evaluate
Benefits - Design optimization with increased control of safety
margins
0
50
100
150
200
250
0 20 40 60 80 100 120 140
σ2 (MPa)
τ12 (
MPa
)
AbaqusPULSDNV RulesGL Rules
Py
Px
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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PULS - Element library Un-stiffened plate element
Stiffened plate element (S3)
Corrugated plate element (K3)
Stiffened plate element (T1)
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Demo – PULS Code Check in GeniE
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
40
Analysis Overview Task Purpose Input Output
Global modelling Make global model for hydrodynamic and strength analysis
Ship drawings Loading manual
Global FE model
Hydrodynamic analysis
Calculate loads for fatigue and ultimate strength
Global FE model Wave data
Load files for structural analysis
ULS analysis Calculate hull girder strength
Global FE model Snap shot load files from HydroD
Ultimate strength results
Spectral fatigue analysis
Fatigue screening on nominal stress Local fatigue analysis
Global FE model Frequency domain load files from HydroD
Calculated fatigue lives
Spectral ULS analysis
Calculate long term stress based on spectral method
Global FE model Frequency domain load files from HydroD
Long term stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
41
Stochastic Fatigue Analysis
Fatigue Life
Wave Load Analysis - Input: Global model, wave headings and frequencies - Output: Load transfer functions (RAOs)
Stress Response Analysis - Input: FE models and load file from wave load analysis - Output: FE results file with load cases describing complex
(real and imaginary) stress transfer functions (RAOs)
Direct Load Transfer
S-N Fatigue Curves
Wave scatter diagram
Stress Transfer Functions Fatigue Damage Calculation
- Input: Stress transfer functions (FE results file), wave data - Output: Calculated fatigue life
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
42
Global Frequency Domain Analysis Loads from HydroD
Static load case - For verification of load balance and static shear
and bending compared to loading manual - Enables automatic calculation of mean stress
effect in fatigue calculartions - Enables possibility for to calculate long term
extreme loads including static stress
Dynamic load cases - Number of complex dynamic load cases =
number of wave headings x number of wave periods (e.g. 12 x 25 = 300)
Head Sea
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
43
Demo - Stofat
Calculated fatigue damage by nominal stress and user defined SCF for an LNG carrier
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
44
Global Screening Analysis Fatigue calculations based on nominal
stress from global analysis and stress concentration factors
Typical use - Identify fatigue sensitive areas - Determine critical stress concentration factors
for deck attachment and topside supports - Determine location of local models and fine
mesh areas - Decide extent of reinforcements based on SCF
from local analysis
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
45
Local Fatigue Analysis Local fine mesh model created
from global GeniE model by changing the mesh density in the location of the crack
Hot spot stress RAOs at the location of the crack established by spectral FE calculation
Submodelling techniques is used to transfer the results from the global FE analysis to the boarders of the local model
Fatigue damage/life calculated using Stofat
Concept model with mesh densities
Local fine mesh model
Calculated fatigue life
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
46
Fatigue Strengthening and Screening of Extent Soft bracket added in the local
model of the stringer at crack location
Re-run sub-model analysis and fatigue calculation to check effect of strengthening proposal
Necessary extent of repair evaluated by fatigue screening of global
Stress concentration factor used in global screening calculated by the ratio of long term stress from local and global analysis
Local model with new bracket
Results from fatigue screening of global model to evaluate extent of repair
Fatigue results
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
47
Analysis Overview Task Purpose Input Output
Global modelling Make global model for hydrodynamic and strength analysis
Ship drawings Loading manual
Global FE model
Hydrodynamic analysis
Calculate loads for fatigue and ultimate strength
Global FE model Wave data
Load files for structural analysis
ULS analysis Calculate hull girder strength
Global FE model Snap shot load files from HydroD
Ultimate strength results
Spectral fatigue analysis
Fatigue screening on nominal stress Local fatigue analysis
Global FE model Frequency domain load files from HydroD
Calculated fatigue lives
Spectral ULS analysis
Calculate long term stress based on spectral method
Global FE model Frequency domain load files from HydroD
Long term stress
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
48
Stochastic ULS Analysis
Long term stress
Wave Load Analysis - Input: Global model, wave headings and frequencies - Output: Load transfer functions (RAOs)
Stress Response Analysis - Input: FE models and load file from wave load analysis - Output: FE results file with load cases describing complex (real and
imaginary) stress transfer functions (RAOs)
Direct Load Transfer
Wave scatter diagram
Stress Transfer Functions Long Term ULS Load Calculation
- Input: Stress transfer functions (FE results file), wave data - Output: Calculated long term stress
Challenge: Determine ULS design wave for areas subjected to a combination of different load effects (e.g. turret area)
Typical way: Selection of one or several design waves Uncertainties New solution with Stofat: Spectral stress analysis to determine long term stress distribution directly
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
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Stofat – Features and Benefits Features
- Stochastic fatigue calculations based on wave statistics - Supports all common wave models - Predefined and user defined S-N curves - Option for implicit mean stress correction (by static
load case) - Statistical stress response calculations
- Calculation of long term stress and extreme response including static loads
- Graphical presentation of fatigue results and long term stress directly on FE model
Benefits - Unique functionality for spectral fatigue and
stochastic long term stress and extreme response calculations
- Flexible – support all your needs - Transparent – all calculation steps can be
documented
Calculated fatigue damage by nominal stress and user defined SCF for an LNG carrier
Calculated long term stress amplitude (left) and fatigue damage (right) for the hopper knuckle in an oil tanker
© Det Norske Veritas AS. All rights reserved.
Advanced Methods for Ultimate and Fatigue Strength of Floaters
50
Benefits of Sesam for Advanced Analysis Complete system – Proven Solution
- Cover your needs for strength assessment of ship and offshore structures
- 40 years of DNV experience and research put into software tools
Concept modelling - Minimize modelling effort by re-use of models for various
analysis - Same concept model for global & local strength analysis and for
hydrodynamic analysis - Same model basis for hydrostatics and frequency and time domain
hydrodynamic analysis
Same system for offshore and maritime structures - Minimizes the learning period and maximizes the utilisation of
your staff
Process, file and analysis management by Sesam Explorer
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Advanced Methods for Ultimate and Fatigue Strength of Floaters
51
Safeguarding life, property and the environment
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