Wave Analysis-Advanced Methods for Ultimate and Fatigue Strength of Floaters

Advanced Methods for Ultimate and Fatigue Strength of Floaters DNV Software Torbjørn Lindemark, Nauticus Prod uct 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 de tails for FPSOs



Case study and software demo on direct strength calculations of a ship shaped FPSO

Advanced Adva nced Methods for Ultimate and Fatigue Strength Strength of Floaters

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 de tails for FPSOs



Case study and software demo on direct strength calculations of a ship shaped FPSO


FPSO - What is required? 

FPSO - Complex design process - Ships and Offshore Rule requi rements - 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 Advanced Adva nced Methods for Ultimate and Fatigue Strength Strength of Floaters

Challenge of FPSO New Build and Conversion 

New Builds



Conversions

- Selection corrosion protection strategy to determine a rational material thickness

- Increase certainty that the chosen vessel is suitable for conversion,

- Identify comprehensive analysis requirements for design - Develop Inspection Plans

- Determine how much steel should be replaced during conversion/maintenance,

- Choice of turret design

- Identify where to focus surveys.


FPSO Package for design and analysis Hydrodynamics • Seakeeping • Wave loads HydroD

Risk Analysis Safeti

Main scantlings Nauticus Hull

3D Hull modelling GeniE

Fatigue Simplified, Spectral Nauticus Hull Sesam/Stofat

Turret Local analysis GeniE Risers DeepC

Advanced Methods for Ultimate and Fatigue Strength of Floaters

Topside Genie

Proven solutions in use by major companies around the world

Mooring Mimosa

Direct Calculations in an Integrated Analysis System 1. Stability and wave load analysis

2. Pressure loads and accelerations Wave scatter diagram

Local FE analysis

5. Local stress and deflection & fatigue

r e f s n a r t d a o L

FE analysis 4. Global stress and deflection & fatigue screening Advanced Methods for Ultimate and Fatigue Strength of Floaters

3. Structural model loads (internal + external pressure)

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 distributi on 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 ana lysis - Both containing accelerations, external and internal pressures


Finite Element Analysis Deterministic Analysis 

Input

Spectral Analysis 

Input

- Global and local FE models

- Global and local FE models

- Design wave load transfer files (or long term loads by manual input)

- RAO based load transfer files - Wave and environmental data - Scatter diagram - Wave spectrum - Directionality and spreading



Output - Stress response for a given design wave/load



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


Fatigue Analysis by Cumulative Damage Deterministic Analysis 

Input

Spectral Analysis 

- 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

- Directionality and spreading

- S-N curves



Output - Calculated fatigue life or damage

Input

- 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


Simplified vs. direct fatigue calculations Environment

Wave Load Analysis:

Stress analysis:

Fatigue damage analysis:

Simplified

Spectral Analysis

Long term Weibull distribution by rule formulas

Wave scatter diagram and energy spectrum

Accelerations, pressure and moments on 10^-4 or 10^-8 probability level by rule formulas

Direct calculated loads 3D potential theory

Rule formulations for stresses and correlation of different loads

Load transfer to FE model. Stress transfer function implicit in FE model

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.


Fatigue loads and stress components 

Global wave bending moments   



Wave pressure    



Hull girder stress Stress in topside supports due to global hull deflections Stress in turret and moonpool areas due to hull deflections

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 shear lag effect)


Long. stress in deck uniform deck thickness

Long. stress in deck when plates near side are increased

In-service Experience on Fatigue Critical Details 

Stiffener end connections



Root source of cracking Global Local

hull girder bending

dynamic pressures

Relative

deflections caused by bending of girder system

Stress

concentration at stiffener toe and

heel


Web-plating

Stiffener Longitudinal


Knuckles in inner structure (hopper knuckle)



Root source of cracking: Deflection High

on main girder system

stress concentration Cracks under development

Repair example Advanced Methods for Ultimate and Fatigue Strength of Floaters


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 Local

from topside inertia forces

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 (equi valent 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


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 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

2000000 1500000 ] m N 1000000 k [

500000 0 0

0.2

0.4

0.6

0.8

1

0.2

0.4

0.6

0.8

1

VBM (linear)

150000 100000

- In-service structural integrity management - Life extension evaluation

] N k [

50000 0

Vertical Bending Moment Sea Pressure

0

VSF (linear)

Double Hull Bending Total Stress s s e r t S

Rule Direct

Pressure Time


Direct calculated loads vs. rule loads 

Fatigue loads: 1.20 1.00 0.80 Direct DNV Rule CSR

0.60 0.40 0.20 0.00 Vertical Bending


Horizontal Bending

Pressure WL

Vert. Acc.

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. DNV Rule CSR

0.60 0.40 0.20 0.00 Bottom at B/4 Advanced Methods for Ultimate and Fatigue Strength of Floaters

Side at T/2

Side at T

Trunk Deck

Analysis Overview Task

Purpose

Global modelling



Input

Make global model for hydrodynamic and strength analysis

 

Ship drawings Loading manual

Output Global

FE model

Hydrodynamic analysis

Calculate

loads for fatigue and ultimate strength

Global

FE model Wave data

Load

ULS analysis

Calculate

Global

Ultimate

strength

FE model Snap shot load files from HydroD

Spectral fatigue analysis

Fatigue

screening on nominal stress Local fatigue analysis

Global

FE model Frequency domain load files from HydroD

Calculated

Spectral ULS analysis

Calculate

Global

Long

hull girder

long term stress based on spectral method



files for structural analysis strength

results fatigue

lives term stress

Creating the Global Model Model requirements 



Challenges

The global model is used to calculate loads and strength and must represent the actual properties of the ship



Modelling of hull form



Creating compartment and loads

For direct strength calculations essential properties are



Mass tuning

- 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



Purpose

Global modelling



Input


 


Output Global

FE model


Calculate


Global


Load

ULS analysis

Calculate

Global

Ultimate

strength



Fatigue


Global


Calculated


Calculate

Global

Long

hull girder





results fatigue

lives term stress

Hydrodynamic Analysis Model requirements 

Hull shape as real ship



Correct draft and trim



Weight and buoyancy distribution according to loading manual



Mass and buoyancy in balance


Challenges 

Obtain correct weight and mass distribution



Balance of loading conditions

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



Purpose

Global modelling



Input


 


Output Global

FE model


Calculate


Global


Load

ULS analysis

Calculate

Global

Ultimate

strength



Fatigue


Global


Calculated


Calculate

Global

Long

hull girder





results fatigue

lives term stress

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

e c r o f r a e h s l a c i t r e V

0

50

100

150

200

250

Distance from AP


300

Vertical bending moment distribution

350

WASIM CUTRES

t n e m o m g n i d n e b l a c i t r e V

0

50

100

150

200

250

300

350

WASIM CUTRES

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 

Characteristics

Py

- 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 Abaq us PULS DNV Rules

- Design optimization with increased control of safety margins

GL Rules

200

) a P M (

150

2 1

100

50

0 0

20

40

60

80 2 (MPa)


100

120

140

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



Purpose

Global modelling



Input


 


Output Global

FE model


Calculate


Global


Load

ULS analysis

Calculate

Global

Ultimate

strength



Fatigue


Global


Calculated


Calculate

Global

Long

hull girder





results fatigue

lives term stress

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)



Fatigue Damage Calculation

Stress Transfer Functions

- Input: Stress transfer functions (FE results file), wave data

Wave scatter diagram

- Output: Calculated fatigue life

Fatigue Life


S-N Fatigue Curves

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

Demo - Stofat

Calculated fatigue damage by nominal stress and user defi ned 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 used to transfer the results from the global FE analysis to the boarders of the local model



Fatigue damage/life calculated using Stofat

Local fine mesh model

Concept model with mesh densities Calculated fatigue life Advanced Methods for Ultimate and Fatigue Strength of Floaters

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 Advanced Methods for Ultimate and Fatigue Strength of Floaters


Purpose

Global modelling



Input


 


Output Global

FE model


Calculate


Global


Load

ULS analysis

Calculate

Global

Ultimate

strength



Fatigue


Global


Calculated


Calculate

Global

Long

hull girder





results fatigue

lives term stress

Stochastic ULS Analysis Challenge: Typical way:

Determine ULS design wave for areas subjected to a combination of different load effects (e.g. turret area) 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 frequencie s - Output: Load transfer functions (RAOs)



Direct Load Transfer

Stress Response Analysis - Input: FE models and load file from wave loa d analysis - Output: FE results file with load cases describing complex (real and imaginary) stress transfer functions (RAOs)



Long Term ULS Load Calculation - Input: Stress transfer functions (FE results file), wave data

Stress Transfer Functions

Wave scatter diagram

- Output: Calculated long term stress

Long term stress Advanced Methods for Ultimate and Fatigue Strength of Floaters

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



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

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


Wave Analysis-Advanced Methods for Ultimate and Fatigue Strength of Floaters

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