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Torbjørn Lindemark, Nauticus Product Manager
Advanced Methods for Ultimate and Fatigue Strength ofFloaters
DNV Software
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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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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
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Challenge of FPSO New Build and Conversion
Conversions
- Increase certainty that thechosen vessel is suitable for
conversion,
- Determine how much steel
should be replaced during
conversion/maintenance,
- Identify where to focus surveys.
New Builds
- Selection corrosion protectionstrategy to determine a rational
material thickness
- Identify comprehensive
analysis requirements for design
- Develop Inspection Plans
- Choice of turret design
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FPSO Package for design and analysis
Proven solutions in use
by major companies
around the world
TopsideGenie
Main scantlings
Nauticus Hull
RisersDeepC
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
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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
L o a d
t r a n s f e r
3. Structural model loads(internal + external pressure)
Local FE analysis
5. Local stress and
deflection & fatigue
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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 AmplitudeOperators – 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
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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 AmplitudeOperators – RAOs)
- Stress statistics
- Derived by combining the stress RAOs with wavedata
- Short and long term distribution
- Design values for specified probability level/return
period
Deterministic Analysis Spectral Analysis
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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 AmplitudeOperators – 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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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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Fatigue loads and stress components
Global wave bending moments
Hull girder stress
Stress in topside supports due to global hulldeflections
Stress in turret and moonpool areas due to hulldeflections
Wave pressure
Shell plate local bending stress Local stiffener bending stress
Secondary stiffener bending due to deflectionof main girder system
Local peak stresses in knuckles due todeflection 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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Moonpool areas
Long. stress in deck (no
shear lag effect)
CL
Nominal stress
level
Actual stress
distribution
Long. stress in deck
uniform deck thickness
Long. stress in deck
when plates near side
are increased
Increased platethickness
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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 andheel
Longitudinal
StiffenerWeb-plating
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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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In-service Experience on Fatigue Critical Details
Shell plating
Root source of cracking
Local pressure
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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
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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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A few useful ratios
Ratio Stress factor
(equivalent stressreduction)
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
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Part 2 – Case Study and Demos
Direct strength ULS and FLS calculations of a shipshaped FPSO
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Why direct load and strength calculations
Rule loads are not always the truth Moderncalculation tools give more accurate loads
- Ultimate strength loads
- Fatigue loads
- Phasing and simultaneity of different load effects
Design and strength optimizations based on analysiscloser 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
[ k N m ]
VBM (linear)
0
50000
100000
150000
0 0.2 0.4 0.6 0.8 1
[ k N ]
VSF (linear)
Pressure
Rule
Direct
Time
S t r e s s
Vertical BendingMomentSea Pressure
Double Hull Bending
Total Stress
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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
Vertical
Bending
Horizontal
Bending
Pressure WL Vert. Acc.
Direct
DNV Rule
CSR
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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 at
B/4
Side at
T/2
Side at T Trunk
Deck
Comp. Stoch.
DNV Rule
CSR
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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
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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
ChallengesModel requirements
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Demo – Global Modelling with GeniE
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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 tankboundaries
Mass tuning
- Scaling mass density to target mass
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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
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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
ChallengesModel requirements
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Demo – HydroD
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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 resultspresentation
Load transfer to FE analysis- Snap shot or frequency domain
- With splash zone correction for fatigue
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Analysis Overview
Task Purpose Input Output
Global modelling Make global model forhydrodynamic 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
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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 forconservative and/or uncertain assumptions
Integrated buckling check
http://c/Data/Presentasjoner/Video/Fatigue%20loads%20from%20waves_0006.avi
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Cutres - Verification of Applied Loads
0 50 100 150 200 250 300 350
Distance from AP
V e r t i c a l s h e a r f o r c e
WASIM
CUTRES
Vertical shear force distribution
0 50 100 150 200 250 300 350
Distance from AP
V e r t i c a l b e n d i n g m o m e n t
WASIM
CUTRES
Vertical bending moment distribution
Cutres calculates and integrates the force distribution of cross sections and is ideal
to evaluate the hull girder structural response
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PULS – Advanced Buckling & Panel Ultimate Limit State
PULS is a code for bucklingand ULS assessments
of stiffened and unstiffenedpanels
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Benefits of PULS
Characteristics
- Higher accuracy than traditional rule formulationsand 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 safetymargins
0
50
100
150
200
250
0 20 40 60 80 100 120 140
2 (MPa)
1 2 ( M P a )
Abaq us
PULS
DNV Rules
GL Rules
Py
Px
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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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Demo – PULS Code Check in GeniE
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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 scatterdiagram
Stress Transfer Functions
Fatigue Damage Calculation
- Input: Stress transfer functions (FE results file), wave data- Output: Calculated fatigue life
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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
http://dnv/Workspaces/Advanced_methods_for_ULS_and_FLS/60_Global_FLS_analysis/20_Xtract/HeadSea.AVIhttp://dnv/Workspaces/Advanced_methods_for_ULS_and_FLS/60_Global_FLS_analysis/20_Xtract/HeadSea.AVI
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Demo - Stofat
Calculated fatigue damage by nominal stress and user defined SCFfor an LNG carrier
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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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Local Fatigue Analysis
Local fine mesh model created
from global GeniE model by
changing the mesh density inthe 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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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 screeningof 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
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Analysis Overview
Task Purpose Input Output
Global modelling Make global model forhydrodynamic 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
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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 LoadTransfer
Wave scatterdiagram
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
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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 staticload case)
- Statistical stress response calculations
- Calculation of long term stress and extreme responseincluding static loads
- Graphical presentation of fatigue results and longterm stress directly on FE model
Benefits
- Unique functionality for spectral fatigue andstochastic long term stress and extreme
response calculations- Flexible – support all your needs
- Transparent – all calculation steps can bedocumented
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
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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 offshorestructures
- 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
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