Advanced Methods for ULS and FLS

Advanced Methods for Ultimate and Fatigue Strength of
Floaters
DNV Software

Torbjørn Lindemark, Nauticus Product Manager

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

Advanced Methods for Ultimate and Fatigue Strength of Floaters

© Det Norske Veritas AS. All rights reserved. 2

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



Challenge of FPSO New Build and Conversion
 New Builds  Conversions
- Selection corrosion protection - Increase certainty that the
strategy to determine a rational chosen vessel is suitable for
material thickness conversion,
- Identify comprehensive - Determine how much steel
analysis requirements for design should be replaced during
- Develop Inspection Plans conversion/maintenance,
- Choice of turret design - Identify where to focus surveys.



FPSO Package for design and analysis
Risk Analysis
Hydrodynamics Safeti
Topside
• Seakeeping Genie
• Wave loads
HydroD
Main scantlings
3D Hull Nauticus Hull
modelling
GeniE
Fatigue
Turret Simplified,
Local analysis Spectral
GeniE Nauticus Hull
Sesam/Stofat
Risers
DeepC Mooring
Proven solutions in use Mimosa
by major companies
around the world


Direct Calculations in an Integrated Analysis System
1. Stability and wave load 2. Pressure loads and
analysis accelerations

Wave
scatter diagram

Load transfer
Local FE analysis

5. Local stress and
deflection & fatigue

FE analysis

4. Global stress and 3. Structural model loads
(internal + external pressure)
deflection & fatigue
screening


Wave Load Analysis
 Input  Output
- Models - Load transfer functions (Response Amplitude
- Panel &/or Morrison model Operators – RAOs)
- Mass model - Motions in 6 dof (+ derived velocities and
accelerations)
- Compartments
- External wave pressures
- Structural model for load transfer
- Internal tank pressures
- Loading conditions
- Morrison forces
- Compartment fillings, draught and trim
- Sectional loads
- Wave and environmental data
- Load statistics
- Scatter diagram
- Derived by combining the load RAOs with wave data
- Wave spectrum
- Design values for ULS/ALS
- Directionality and spreading
- Long term load distribution for simplified fatigue
- Current calculations
- Water depth
- 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



Finite Element Analysis
Deterministic Analysis Spectral Analysis
 Input  Input
- Global and local FE models - Global and local FE models
- Design wave load transfer files (or long term - RAO based load transfer files
loads by manual input) - Wave and environmental data
- Scatter diagram
- Wave spectrum

 Output  Output
- Stress response for a given design wave/load - 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



Fatigue Analysis by Cumulative Damage
Deterministic Analysis Spectral Analysis
 Input  Input
- Long term stress distribution - Stress transfer functions (Response Amplitude
- Described by Weibull distribution or stress histogram Operators – RAOs)
- The Weibull distribution is described by - Wave and environmental data
- Stress at a given probability level
- Scatter diagram
- Weibull parameter
- Wave spectrum
- Zero crossing frequency
- S-N curves - S-N curves

 Output  Output
- Calculated fatigue life or damage - 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



Simplified vs. direct fatigue calculations
Environment Simplified Spectral Analysis

Long term Weibull Wave scatter diagram and
distribution by rule energy spectrum
formulas

Wave Load Analysis: Accelerations, pressure and
Direct calculated loads -
moments on 10^-4 or 10^-8 3D potential theory
probability level by rule
formulas

Stress analysis:
Rule formulations for Load transfer to FE model.
stresses and correlation of Stress transfer function implicit
different loads in FE model

Based on expected largest stress Based on summation of part damage
Fatigue damage among 10^4 cycles of a rule long from each Rayleigh distributed sea
analysis: term Weibull distribution state in scatter diagram.



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



Moonpool areas

Increased plate
thickness

Nominal stress
level

Actual stress
distribution

CL

Long. stress in deck (no Long. stress in deck
Long. stress in deck
shear lag effect) when plates near side
uniform deck thickness
are increased



In-service Experience on Fatigue Critical Details
 Stiffener end connections
Web-plating
 Root source of cracking Stiffener
Global hull girder bending Longitudinal
Local dynamic pressures
Relative deflections caused by bending of
girder system
Stress concentration at stiffener toe and
heel



 Knuckles in inner structure (hopper knuckle)
 Root source of cracking:
Deflection on main girder system
High stress concentration

Cracks under development

Repair example



 Shell plating
 Root source of cracking
Local pressure



 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



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



A few useful ratios
Ratio Stress factor Fatigue Damage
factor
(equivalent stress
reduction)
Base / Weld - SN (10^12.89) / 0.83 1.74
curve (10^12.65)
World wide / North 0.8 / 1.0 0.8 2.0
Atlantic ocean

Non-corrosive / (10^12.65) / 0.81 2.0
corrosive environment (10^12.38)

Mean / Design SN (10^12.09) / 0.7 3.0
curve (10^11.63)



Part 2 – Case Study and Demos

Direct strength ULS and FLS calculations of a ship
shaped FPSO



Why direct load and strength calculations
 Rule loads are not always the truth  Modern 2000000

calculation tools give more accurate loads 1500000

[kNm]
- Ultimate strength loads 1000000

- Fatigue loads 500000

- Phasing and simultaneity of different load effects 0
0 0.2 0.4 0.6 0.8 1
 Design and strength optimizations based on analysis VBM (linear)
closer to actual operating conditions
150000

 Improved decision basis for 100000

[kN]
- In-service structural integrity management
50000
- Life extension evaluation
0
0 0.2 0.4 0.6 0.8 1
Vertical Bending
Moment VSF (linear)
Sea Pressure

Double Hull Bending

Total Stress

−−−
Stress

Rule

Direct −−−
Pressure

Time



Direct calculated loads vs. rule loads
 Fatigue loads:

1.20

1.00

0.80
Direct
0.60 DNV Rule
CSR
0.40

0.20

0.00
Vertical Horizontal Pressure WL Vert. Acc.
Bending Bending



Spectral vs Simplified Fatigue Analysis
 Comparison of fatigue damage by DNV rules and Common Scantling Rules relative
to spectral fatigue calculations:

1.20

1.00

0.80
Comp. Stoch.
0.60 DNV Rule
CSR
0.40

0.20

0.00
Bottom at Side at Side at T Trunk
B/4 T/2 Deck



Analysis Overview
Task Purpose Input Output

Global modelling  Make global model for  Ship drawings Global FE model
hydrodynamic and  Loading manual
strength analysis
Hydrodynamic Calculate loads for Global FE model Load files for
analysis fatigue and ultimate Wave data structural analysis
strength
ULS analysis Calculate hull girder Global FE model Ultimate strength
strength Snap shot load files results
from HydroD
Spectral fatigue Fatigue screening on Global FE model Calculated fatigue
analysis nominal stress Frequency domain load lives
Local fatigue analysis files from HydroD
Spectral ULS Calculate long term Global FE model Long term stress
analysis stress based on spectral Frequency domain load
method files from HydroD



Creating the Global Model
Model requirements Challenges

 The global model is used to calculate  Modelling of hull form
loads and strength and must represent
the actual properties of the ship  Creating compartment and loads

 For direct strength calculations  Mass tuning
essential properties are
- Buoyancy and weight distribution
- Compartment loads
- Structural stiffness and strength



Demo – Global Modelling with GeniE



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



Analysis Overview

strength analysis
strength
from HydroD



Hydrodynamic Analysis
Model requirements Challenges

 Hull shape as real ship  Obtain correct weight and mass
distribution
 Correct draft and trim
 Balance of loading conditions
 Weight and buoyancy distribution
according to loading manual
 Mass and buoyancy in balance



Demo – HydroD



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



Analysis Overview

strength analysis
strength
from HydroD



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



Cutres - Verification of Applied Loads
 Cutres calculates and integrates the force distribution of cross sections and is ideal
to evaluate the hull girder structural response

Vertical shear force distribution Vertical bending moment distribution

0 50 100 150 200 250 300 350

Vertical bending moment
Vertical shear force

WASIM WASIM
CUTRES CUTRES
0 50 100 150 200 250 300 350

Distance from AP Distance from AP



PULS – Advanced Buckling & Panel Ultimate Limit State

PULS is a code for buckling
and ULS assessments
of stiffened and unstiffened
panels



Benefits of PULS
Py

 Characteristics
- Higher accuracy than traditional rule formulations
and classic buckling theory
- Quick and easy-to-use design tool for calculation of
ULS capacity Px

- Valuable information about failure mode and
buckling pattern
- Effective to evaluate

 Benefits 250
Abaqus
PULS
DNV Rules

- Design optimization with increased control of safety
200 GL Rules

margins 150

τ 12 (MPa)
100

50

0
0 20 40 60 80 100 120 140

σ 2 (MPa)



PULS - Element library

 Un-stiffened plate element

 Stiffened plate element (S3)

 Corrugated plate element (K3)

 Stiffened plate element (T1)



Demo – PULS Code Check in GeniE



Analysis Overview

strength analysis
strength
from HydroD



Stochastic Fatigue Analysis
 Wave Load Analysis
- Input: Global model, wave headings and frequencies
- Output: Load transfer functions (RAOs)
Direct Load
Transfer

 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)

Stress Transfer Functions
S-N Fatigue
 Fatigue Damage Calculation Curves
- Input: Stress transfer functions (FE results file), wave data Wave scatter
- Output: Calculated fatigue life diagram

Fatigue Life



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
Head Sea
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)



Demo - Stofat

Calculated fatigue damage by nominal stress and user defined SCF
for an LNG carrier



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



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 Local fine mesh model
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

Calculated fatigue life



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 Local model with new bracket Fatigue results

 Stress concentration factor used
in global screening calculated by
the ratio of long term stress from
local and global analysis

Results from fatigue screening of global model to evaluate extent of repair



Analysis Overview

strength analysis
strength
from HydroD



Stochastic ULS Analysis
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

 Wave Load Analysis
- Input: Global model, wave headings and frequencies
- Output: Load transfer functions (RAOs)

Direct Load
 Stress Response Analysis Transfer
- 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)

Stress Transfer Functions
 Long Term ULS Load Calculation Wave scatter
diagram
- Input: Stress transfer functions (FE results file), wave data
- Output: Calculated long term stress

Long term stress



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
Calculated fatigue damage by nominal stress
- Graphical presentation of fatigue results and long and user defined SCF for an LNG carrier
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 long term stress amplitude (left) and fatigue
damage (right) for the hopper knuckle in an oil tanker



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



Safeguarding life, property
and the environment

www.dnv.com



Advanced Methods for ULS and FLS

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