Framework Wave Induced Fatigue
description
Transcript of Framework Wave Induced Fatigue
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DNV Software Sesam User Course Framework Wave Induced Fatigue Analysis on Revised: November 1, 2013
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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.
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Interaction with other Sesam programs
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Interaction with other Sesam programs (1/2)
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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.
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Interaction with other Sesam programs (2/2)
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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
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Fatigue principles in Framework
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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
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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.
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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
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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
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stress
S1 S2 S3 S4
log S
Si
log N ni Ni
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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
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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
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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
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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.
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Input to Framework
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Input to Framework (1/4)
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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
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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
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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
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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
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Input to Framework (4/4)
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Deterministic fatigue analysis
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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
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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
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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.
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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
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Stochastic fatigue analysis
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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?
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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.
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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.
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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
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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.
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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
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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
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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
Run fatigue analysis
View/print results
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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
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Stochastic fatigue analysis (3/6)
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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
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Stochastic fatigue analysis (4/6)
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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
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Stochastic fatigue analysis (5/6)
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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
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Run fatigue analysis
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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.
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Output from Framework
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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
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Summary
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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