Framework Wave Induced Fatigue

DNV Software Sesam User Course Framework Wave Induced Fatigue Analysis on Revised: November 1, 2013

Det Norske Veritas AS. All rights reserved.

Purpose and Goals Whats in it for you?

To be able to perform a deterministic or stochastic fatigue analysis in Framework. - Based on wave loads.

Learning objectives

Understand interaction with other Sesam programs.

Understand the principles on which fatigue analysis in Framework is based.

Know how to input data.

Know which data to enter required for fatigue analysis.

Know how to enter environmental data for deterministic or stochastic fatigue.

Know how to run the analysis and output results.


Interaction with other Sesam programs


Interaction with other Sesam programs (1/2)

4

Start from GeniE

Tools > Analysis > Frame Code Check (Framework) - Automatically transfers structure

and results. - Choose whether to transfer

load case and concept names.

Start from Sesam Manager

As part of workflow or separately.

Input is given interactively or through command input file.


Interaction with other Sesam programs (2/2)

5

Deterministic fatigue:

Deterministic wave analysis Calculation of loads from waves stepped through the structure.

Static analysis Structural response to wave loading.

Deterministic fatigue analysis According to American Welding Society (AWS).

GeniE/Sesam Manager Structural model

Wajac Wave loads

Sestra Structural response

Stochastic fatigue:

Frequency domain wave analysis Calculation of load transfer

functions.

Quasi-static / dynamic analysis Calculation of stress transfer

functions.

Stochastic fatigue analysis According to Vugts & Kinra.

Framework Fatigue analysis


Fatigue principles in Framework


Fatigue principles in Framework (1/8) Four types of fatigue analysis possible in Framework:

Deterministic fatigue analysis

Stochastic fatigue analysis

Time history fatigue analysis not covered here

Wind fatigue analysis not covered here

7


Fatigue principles in Framework (2/8) Model requirements

Only 2 node beams (i.e. 1st order elements) - All other elements neglected

- 3 node beams (i.e. 2nd order elements) - Plates, shells

Possible cross sections: - Pipe section - General section - Other sections converted to general section:

- Bar, box, I, L, channel, etc.

8


Fatigue principles in Framework (3/8) Stress range

Stress variation contributes to fatigue, constant stress does not.

Nominal stress from beam forces and moments.

Nominal stress: = Fx/A + My/Wy + Mz/Wz

Slide 9


Fatigue principles in Framework (4/8) Fatigue calculation based on Miners rule

Summation of partial damages due to cycles with different stress ranges.

Fatigue failure if ni / Ni > 1 - ni is number of cycles of stress range Si - Ni is number of cycles of stress range Si that will

result in failure

Miners rule gives a usage factor

Fatigue life = target fatigue life / usage factor

10

stress

S1 S2 S3 S4

log S

Si

log N ni Ni


Fatigue principles in Framework (5/8) Stress concentration factors

Presence of joint increases stress in hotspot.

Accounted for by stress concentration factors (SCFs).

3 SCFs for each hotspot - Axial stress: SCFax - In-plane bending stress: SCFby - Out-of-plane bending stress: SCFbz

hotspot S = Fx/A SCFax + My/Wy SCFby + Mz/Wz SCFbz n = number of cycles

Slide 11

in-plane bending

out-of-plane bending

axial force

x z

local beam coordinate system


Fatigue principles in Framework (6/8) Stress concentration factors

SCFs can be assigned globally (for the whole structure), or locally.

SCFs can be given a constant value, or can be calculated parametrically. - Parametric: Efthymiou, Lloyds,

Kuang, Wordsworth - Minimum SCFs can be assigned

when parametric formulae are used.

Slide 12

hotspot S = Fx/A SCFax + My/Wy SCFby + Mz/Wz SCFbz n = number of cycles

in-plane bending

out-of-plane bending

axial force

x z

local beam coordinate system


Fatigue principles in Framework (7/8) Hotspots for stress computation

Fatigue is based on stresses obtained from forces and moments.

Stresses are computed in stress points (hotspots) distributed around the section.

Fatigue analysis for selected hotspots: - 8 hotspots per weld side for tubes. - 4 hotspots per weld side for general section.

y

z

1

4 7

10

13

16 19

22

pipe

y

z

general

Slide 13


Fatigue principles in Framework (8/8) Overview of involved quantities

Nominal stress ranges Fx/A, My/Wy and Mz/Wz. - Fx, My and Mz computed by Sestra.

Stress concentration factors SCF - Direct input to or computed by formulae in Framework.

Number of cycles ni for each stress range Si. - Input to Framework.

Slide 14


Input to Framework


Input to Framework (1/4)

16

Commands can be given in 3 different ways

1. Use Menu commands

2. Use command input line - Activate command line mode by button in

the Framework toolbar.

3. Import a command file - Import command file by button in the

Framework toolbar.

Commands correspond to menu entries: Command: SELECT FATIGUE-CHECK-TYPE DETERMINISTIC Menu: Select > Fatigue check type > Deterministic


Input to Framework (2/4) Frequently used menus / commands

ASSIGN To assign data to the model and environment, e.g.: - Joint types and gaps - SCFs - SN curves to joints and members - Deterministic fatigue: long-term wave height distribution - Stochastic fatigue: wave direction probability, wave statistics, spectrum and spreading

CREATE To create new data, e.g.: - New section or material - SN curve (when not available in included library of SN curves) - Stochastic fatigue: wave statistics (scatter diagram) and spreading

DEFINE To define fatigue constants, e.g.: - Target fatigue life - Parametric SCFs

Slide 17


Input to Framework (3/4) SN curves

Describes materials resistance to fatigue.

Select/create SN curves: - Library of SN curves. - User defined by command:

CREATE SN-CURVE name - Assign thickness correction to SN curves

(incorporated in some curves).

Assign SN curve by command: ASSIGN SN-CURVE

log S

log N

N = S-m K

Typical shape of SN curve

Slide 18


Possibility to start with non-zero initial partial damage

Manually - Damage obtained from other analysis or inspection. - Use command ASSIGN FATIGUE-PART-DAMAGE.

Automatic - Used for analysis over different phases, e.g. transportation and in-place. - Framework calculates initial damage for subsequent fatigue analyses.

- Worst damage over position (all hotspots in section) applied to all hotspots. - Use command DEFINE FATIGUE-CONSTANTS ACCUMULATE-FATIGUE-RUN.

Possibility to scale stress ranges

Stress ranges at each hotspot multiplied with load factor.

Use command ASSIGN WAVE-LOAD-FACTOR

Slide 19

Input to Framework (4/4)


Deterministic fatigue analysis


Deterministic fatigue analysis (1/3) Interaction with Wajac and Sestra

Wajac - Several wave directions - Several waves (any theory) for each

direction - Each wave stepped through structure (non-

linear drag)

Sestra - Structural analysis - Number of loads =

directions waves steps

Framework - Maximum stress difference stress range

- Environmental data: long term wave height distribution number of cycles

stress

stress range

wave directions

waves: theory + height + length steps

H

Hi

log N Ni

Slide 21


Deterministic fatigue analysis (2/3) Steps in Framework

Select Fatigue check type: Deterministic

Specify input data: - Define target fatigue life - Assign joint type and gap data - Assign stress concentration factors - Assign SN-curve(s) to members and/or joints - Assign individual wave data

- Number of waves per direction

Run fatigue analysis

View/print results

22


Deterministic fatigue analysis (3/3) Environmental data

Assign wave height distribution to wave directions: - Linear - Piecewise

- Number of waves n

Specify time period for which number of waves is specified.

Slide 23

Wave height distribution for logarithmic scaled N

Wave height distribution, points actually given as input: ni = Ntot Ni

N1 Ntot

H

logN

H1

H2

N2

H3

N3 N4 H4

N

n3 n2 n1

H

H3 H4

H1

H2

n4


Stochastic fatigue analysis


Stochastic fatigue: preparations (1/6) Modify model and do Eigenvalue analysis

Structural modelling: - No static loads may be defined. - Convert any equipment and appurtenance loads to masses. - Linearize model: idealise piles by linear spring stiffness matrices.

Compute added mass and mass of internal water using Wajac.

Eigenvalue analysis using Sestra to determine Eigen frequencies.

Examine mode shapes in Framework or Xtract: real or false?

Slide 25


Stochastic fatigue: preparations (2/6) Compute wave loads

Compute hydrodynamic loads using Wajac / Wadam. - Select wave frequencies based on:

- Eigen frequencies found. - Cancellation / attenuation of forces. - Environmental statistics.

- Select method for linearization of drag, see next page.

Slide 26


Stochastic fatigue: preparations (3/6) Drag linearisation

Hydrodynamic drag [FD = (D/2) Cd vn |vn|] in Wajac must be linearized.

Two methods available: 1. Wave height linearization

- Based on steepness criterion (< 1/7). - Based on qualified guessing. - Assumes loading applied up to wave crest through whole wave cycle. - Over-estimates drag for low wave heights. - Under-estimates drag for high wave heights.

2. Spectral linearization - Based on a design sea state (Hs, Tz). - Selection should be reviewed after fatigue analysis.

- Choice of method is important when dynamic effects are significant and when drag becomes dominant.

Slide 27


Stochastic fatigue: preparations (4/6) Base shear and overturning moment

Store transfer functions for base shear and overturning moments on G1.SIF file while computing hydrodynamic loads in Wajac. - Use Wajac command OPTI, parameter

OPT3=2

Postresp presents transfer functions, use command: DISPLAY RESPONSE-VARIABLE

Slide 28


Stochastic fatigue: preparations (5/6) Structural analysis in Sestra

Selection of analysis method: - Quasi-static (neglecting inertia and damping effects in the structure). - Dynamic forced response by:

- Modal Superposition. - Direct Frequency Response.

Reduction methods for dynamic analysis: - Master-Slave. - Component Mode Synthesis.

Selection of damping model for dynamic analysis: - Dashpots (only if Direct Frequency Response). - Modal damping (only if Modal Superposition). - Rayleigh damping (= proportional damping), see next page. - Structural damping.

Slide 29


Stochastic fatigue: preparations (6/6) Rayleigh damping in structural analysis

Rayleigh damping coefficients in structural dynamic analysis: C = M + K where: = 2 1 i (i 1 - 1 i) / = 2 (1 1 - i i) / and = (12 - i2)

Select damping as fraction of critical for two selected frequencies.

i 1

i 1

response

/n

1.0

1.0

critical

2% = 0.02

Typical damping for a single DOF system

Slide 30


Stochastic fatigue analysis (1/6) Interaction with Wajac and Sestra

Wajac - Several wave directions - Several frequencies (linear harmonic

waves) for each direction - Linearization of drag

Sestra - Quasi-static or dynamic analysis

- Number of loads = directions wave frequencies

- Complex loads and complex results

Framework - Each wave direction given probability - Wave statistics defined and assigned to directions:

- Create scatter diagram long term distribution of wave heights vs. zero up-crossing - Assign wave spectrum to scatter diagram - Create wave spreading function and assign to scatter diagram

wave directions

waves: harmonic, unit amplitude

1

Slide 31


Stochastic fatigue analysis (2/6) Steps in Framework

Select Fatigue check type: Stochastic

Specify input data: - Define target fatigue life - Assign joint type and gap data - Assign stress concentration factors - Assign SN-curve(s) to members and/or joints - Assign sea state data

- Wave scatter diagram - Wave spreading - Wave spectrum - Wave direction probability


View/print results

32


Environmental data: Wave scatter diagram

Create one or multiple scatter diagram(s). - P(Hs,Tz)

- Discretised into < 200 cells. - Simplification of scatter diagram will reduce

computation time. - Use command:

CREATE WAVE-STATISTICS stat-name - Probability or occurrence - Ochi-Hubble (includes spectrum)

Assign scatter diagram to wave direction. - Use command:

ASSIGN WAVE-STATISTICS wave-dir stat-name

Tz

Hs Graphical illustration of a typical scatter diagram

Slide 33

Stochastic fatigue analysis (3/6)


Environmental data: Wave spreading

Create wave spreading function. - Use commands:

CREATE WAVE-SPREADING-FUNCTION spread-name - COSINE-POWERED

(analytical f() = cos2) - USER-DEFINED (discretised)

Sum over function: E() = 1.0

Assign wave spreading function to scatter diagram. - Use commands:

ASSIGN WAVE-SPREADING-FUNCTION stat-name spread-name - ALL - PART

E()

main

+22.

5

+45

+67.

5

+90

-90

-67.

5

-45

-22.

5

cos2

Slide 34



Environmental data: Wave spectrum

Assign wave spectrum shape to scatter diagram. - (Not for Ochi-Hubble scatter diagram) - Use command:

ASSIGN WAVE-SPECTRUM-SHAPE stat-name - PIERSON-MOSKOWITZ - JONSWAP - GENERAL-GAMMA

Different shapes may be assigned to different parts of the scatter diagram.

H

p

JONSWAP

Pierson-Moskowitz

Slide 35



Stochastic fatigue analysis (6/6) Environmental data: Wave probabilities

Assign probabilities associated with wave directions. - Probability p must be given for all main

wave directions . - Zero probability involves omitting

corresponding direction. - Use command:

ASSIGN WAVE-DIRECTION-PROBABILITY wave-dir p

Sum of probabilities: p() = 1.0

p()

0 45 90 135 180 225 270 315

Slide 36


Run fatigue analysis Fatigue analysis is run via

Run > Fatigue check - Specify name and description of run. - Specify which part of the structure to

include.

Framework will indicate which part of the structure is being analysed while the fatigue analysis is running.

Slide 38


Output from Framework


Output from Framework Print results to screen or file.

- Use command: PRINT FATIGUE-CHECK-RESULTS

Display results on screen. - Use command:

DISPLAY FATIGUE-CHECK-RESULTS

Dump (print) intermediate fatigue results for in-depth study. - Use command (prior to RUN FATIGUE):

DEFINE FATIGUE-DUMP

Slide 40


Summary


Summary Stresses found from Wajac and Sestra.

Fatigue analysis based on Miners rule, and only for beams.

Three different ways to give commands.

Two inherently different types of fatigue analysis: 1. Deterministic 2. Stochastic (spectral) - Choice of method influences model, wave loads, analysis. - Similar input for target fatigue life, joint data, SN curves, SCFs, etc. - Different input for environmental data. - The two methods have their strengths and weaknesses:

- Deterministic: More accurate wave loads (any wave theory and non-linear drag included). - Stochastic: Better coverage of structural dynamics and environmental data.

Proper stress concentration factors (SCFs) important.

Output results graphically, textual on screen or to file.

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DNV SoftwarePurpose and Goals Interaction with other Sesam programs (1/2)Interaction with other Sesam programs (2/2) Fatigue principles in Framework (1/8)Fatigue principles in Framework (2/8)Fatigue principles in Framework (3/8)Fatigue principles in Framework (4/8)Fatigue principles in Framework (5/8)Fatigue principles in Framework (6/8)Fatigue principles in Framework (7/8)Fatigue principles in Framework (8/8) Input to Framework (1/4)Input to Framework (2/4)Input to Framework (3/4)Input to Framework (4/4) Deterministic fatigue analysis (1/3)Deterministic fatigue analysis (2/3)Deterministic fatigue analysis (3/3) Stochastic fatigue: preparations (1/6)Stochastic fatigue: preparations (2/6)Stochastic fatigue: preparations (3/6)Stochastic fatigue: preparations (4/6)Stochastic fatigue: preparations (5/6)Stochastic fatigue: preparations (6/6)Stochastic fatigue analysis (1/6)Stochastic fatigue analysis (2/6)Stochastic fatigue analysis (3/6)Stochastic fatigue analysis (4/6)Stochastic fatigue analysis (5/6)Slide Number 36 Run fatigue analysis Output from Framework SummarySlide Number 43

Documents

Framework Wave Induced Fatigue