fatigue of offshore structures: applications and research issues
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Fatigue of Offshore Structures:Applications and Research Issues
Steve Wintersteinstevewinterstein@alum.mit.edu
Fatigue Under Random LoadsMean Damage Rate:
where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
Fatigue Under Random LoadsMean Damage Rate:
where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
Fatigue Under Random LoadsMean Damage Rate:
where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
Assumes:Stresses Gaussian, narrow-band
Fatigue Under Random LoadsMean Damage Rate:
where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
Assumes:Stresses Gaussian, narrow-band
Common errors: Assume Gaussian, narrow-band
Bandwidth & Non-Gaussian EffectsDamage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh]
CBW, CNG = corrections for bandwidth, non-Gaussian effects
Bandwidth & Non-Gaussian EffectsDamage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh]
CBW, CNG = corrections for bandwidth, non-Gaussian effects
Bandwidth Corrections:• Unimodal spectra: Wirsching (1980s)• Bimodal spectra: Jiao and Moan (1990s)• Arbitrary spectra: Simulation (2000s: becoming cheaper)
Bandwidth & Non-Gaussian EffectsDamage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh]
CBW, CNG = corrections for bandwidth, non-Gaussian effects
Bandwidth Corrections:• Unimodal spectra: Wirsching (1980s)• Bimodal spectra: Jiao and Moan (1990s)• Arbitrary spectra: Simulation (2000s: becoming cheaper)• Typically: CBW < 1
Bandwidth & Non-Gaussian EffectsDamage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh]
CBW, CNG = corrections for bandwidth, non-Gaussian effects
Bandwidth Corrections:• Unimodal spectra: Wirsching (1980s)• Bimodal spectra: Jiao and Moan (1990s)• Arbitrary spectra: Simulation (2000s: becoming cheaper)• Typically: CBW < 1
Non-Gaussian Corrections:• Nonlinear transfer functions from hydrodynamics• Moment-based models (Hermite) & simulation or closed-form estimates of CNG
Bandwidth & Non-Gaussian EffectsDamage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh]
CBW, CNG = corrections for bandwidth, non-Gaussian effects
Bandwidth Corrections:• Unimodal spectra: Wirsching (1980s)• Bimodal spectra: Jiao and Moan (1990s)• Arbitrary spectra: Simulation (2000s: becoming cheaper)• Typically: CBW < 1
Non-Gaussian Corrections:• Nonlinear transfer functions from hydrodynamics• Moment-based models (Hermite) & simulation or closed-form estimates of CNG
• Typically: CNG > 1
Can We Even Predict RMS stresses?
Container Ships: Yes (Without Springing)
Can We Even Predict RMS stresses?
Container Ships: Yes (Without Springing)
TLP Tendons: Yes (With Springing)
Can We Even Predict RMS stresses?
Container Ships: Yes (Without Springing)
TLP Tendons: Yes (With Springing)
VIV of Risers: No
Can We Even Predict RMS stresses?
Container Ships: Yes (Without Springing)
TLP Tendons: Yes (With Springing)
VIV of Risers: No
FPSOs: ??
Ship Fatigue: Theory vs Data
Observed Damage (horizontal scale): predicted from measured strains by inferring stresses, fatigue damage. Predicted Damage (vertical scale): linear model based on observed HS
Ref: W. Mao et al, “The Effect of Whipping/Springing on Fatigue Damage and Extreme Response of Ship Structures,” Paper 20124, OMAE 2010, Shanghai.
TLP Tendon Fatigue: 1st-order vs Combined Loads
Water Depth: 300m
One of earliest TLPs (installed 1992)
Ref: “Volterra Models of Ocean Structures: Extremes and Fatigue Reliability,” J.Eng.Mech.,1994
TLP Tendon Fatigue: 1st-order vs Combined Loads
Damage contributionof various Tp
Large damage at Tp = 7s due tofrequency of seastates
Large damage at Tp = 12s due togeometry of platform
Larger non-Gauss effects if TPITCH = 3.5s(resonance when Tp = 7s)
Ref: “Volterra Models of Ocean Structures: Extremes and Fatigue Reliability,” J.Eng.Mech.,1994
VIV: Theory (Shear7) vs Data
Ref: M. Tognarelli et al, “Reliability-Based Factors of Safety for VIV Fatigue Using Field Measurements,” Paper 21001, OMAE 2010, Shanghai.
VIV Factor:
m=3.3,s=1.4
Median:a50=27
LRFD Fatigue Design
LRFD Fatigue Design
LRFD Fatigue Design
LRFD Fatigue Design
Finally: Combined Damage on an FPSO
•High-cycle (low amplitude) loads due to waves… DFAST
•Low-cycle (high amplitude) loads due to other source (e.g., FPSO loading/unloading) -->
DSLOW
•How to combine DFAST and DSLOW?
SRSS: Largest safe region; least conservative
Proposed Combination “Rules”DTOT = [ DSLOW K + DFAST K ] 1/K
•K = 1/m Lotsberg (2005): Effectively adds stress amplitudes
•K= 2/m: Random vibration approach; adds variances
•K = 1: “Linear” damage accumulation
•K = 2: SRSS applied to damage (not rms levels)
Notes: Less conservative rule as K increases; m = S-N slope: Damage = c Sm; D1/m = c’ S
Combined Fatigue:
DNV Approach
Merci beaucoup!
Extra background slides follow…
The Snorre Tension-Leg Platform
Water depth: 300m
One of earliest TLPs (installed 1992)
How important are TN=2.5s cycles?• Important when TWAVE = 2.5s … but this condition has small wave heights
• Important when TWAVE = 5.0s … due to second-order nonlinearity (springing)
• Non-Gaussian effects when TWAVE = 5.0s:
Answer: The Fatiguing
Bookkeeping
Likelihood ofvarious (Hs,Tp)
Answer: The Fatiguing
Bookkeeping
Likelihood ofvarious (Hs,Tp)
Damage contributionof various (Hs,Tp)
Answer: The Fatiguing
Bookkeeping
Likelihood ofvarious (Hs,Tp)
Damage contributionof various Tp
Results:
Damage contributionof various Tp
Large damage at Tp = 7s due tofrequency of seastates
Large damage at Tp = 12s due togeometry of platform
Larger non-Gauss effects if TPITCH = 3.5s(resonance when Tp = 7s)
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