wave analysis-advanced methods for ultimate and fatigue strength of floaters

8/16/2019 Wave Analysis-Advanced Methods for Ultimate and Fatigue Strength of Floaters

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Torbjørn Lindemark, Nauticus Product Manager

Advanced Methods for Ultimate and Fatigue Strength ofFloaters

DNV Software


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© 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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3

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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4

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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5

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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6

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


7/49© Det Norske Veritas AS. All rights reserved.


7

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




8

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


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




9

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


- 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




10

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




11

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




12

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




13

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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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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Shell plating


Local pressure


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Main deck openings and attachments


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


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


Global modelling Make global model for

hydrodynamic and

strength analysis

Ship drawings

Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model


files from HydroD

Calculated fatigue

lives

Spectral ULS

analysis

Calculate long term


method

Global FE model


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


Global modelling Make global model forhydrodynamic and

strength analysis

Ship drawings Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model


files from HydroD

Calculated fatigue

lives

Spectral ULS

analysis

Calculate long term


method

Global FE model


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


Global modelling Make global model forhydrodynamic and

strength analysis

Ship drawings Loading manual

Global FE model

Hydrodynamic

analysis

Calculate loads for


strength

Global FE model

Wave data

Load files for

structural analysis


strength

Global FE model


from HydroD

Ultimate strength

results

Spectral fatigue

analysis


nominal stress


Global FE model


files from HydroD

Calculated fatigue

lives

Spectral ULS

analysis

Calculate long term


method

Global FE model


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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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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wave analysis-advanced methods for ultimate and fatigue strength of floaters

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