Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein...
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Transcript of Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein...
- Slide 1
- Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein [email protected]
- Slide 2
- Fatigue Under Random Loads Mean Damage Rate: where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
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- Fatigue Under Random Loads Mean Damage Rate: where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 6 - 12
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- Fatigue Under Random Loads Mean 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
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- Fatigue Under Random Loads Mean 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
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- Bandwidth & Non-Gaussian Effects Damage Rate: E[D T ] = C BW * C NG * E[D T | Rayleigh] C BW, C NG = corrections for bandwidth, non-Gaussian effects
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- Bandwidth & Non-Gaussian Effects Damage Rate: E[D T ] = C BW * C NG * E[D T | Rayleigh] C BW, C NG = corrections for bandwidth, non-Gaussian effects Bandwidth Corrections: Unimodal spectra: Wirsching (1980s) Bimodal spectra: Jiao and Moan (1990s) Arbitrary spectra: Simulation (2000s: becoming cheaper)
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- Bandwidth & Non-Gaussian Effects Damage Rate: E[D T ] = C BW * C NG * E[D T | Rayleigh] C BW, C NG = 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: C BW < 1
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- Bandwidth & Non-Gaussian Effects Damage Rate: E[D T ] = C BW * C NG * E[D T | Rayleigh] C BW, C NG = 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: C BW < 1 Non-Gaussian Corrections: Nonlinear transfer functions from hydrodynamics Moment-based models (Hermite) & simulation or closed-form estimates of C NG
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- Bandwidth & Non-Gaussian Effects Damage Rate: E[D T ] = C BW * C NG * E[D T | Rayleigh] C BW, C NG = 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: C BW < 1 Non-Gaussian Corrections: Nonlinear transfer functions from hydrodynamics Moment-based models (Hermite) & simulation or closed-form estimates of C NG Typically: C NG > 1
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- Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing)
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- Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing)
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- Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing) VIV of Risers: No
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- Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing) VIV of Risers: No FPSOs: ??
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- 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 H S Ref: W. Mao et al, The Effect of Whipping/Springing on Fatigue Damage and Extreme Response of Ship Structures, Paper 20124, OMAE 2010, Shanghai.
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- TLP Tendon Fatigue: 1 st -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
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- TLP Tendon Fatigue: 1 st -order vs Combined Loads Damage contribution of various Tp Large damage at Tp = 7s due to frequency of seastates Large damage at Tp = 12s due to geometry of platform Larger non-Gauss effects if T PITCH = 3.5s (resonance when Tp = 7s) Ref: Volterra Models of Ocean Structures: Extremes and Fatigue Reliability, J.Eng.Mech.,1994
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- 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.
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- VIV Factor: m=3.3, s=1.4 Median: 50 =27
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- LRFD Fatigue Design
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- Finally: Combined Damage on an FPSO High-cycle (low amplitude) loads due to waves D FAST Low-cycle (high amplitude) loads due to other source (e.g., FPSO loading/unloading) --> D SLOW How to combine D FAST and D SLOW ?
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- SRSS: Largest safe region; least conservative
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- Proposed Combination Rules D TOT = [ D SLOW K + D FAST 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 S m ; D 1/m = c S
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- Combined Fatigue: DNV Approach
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- Merci beaucoup! Extra background slides follow
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- The Snorre Tension-Leg Platform Water depth: 300m One of earliest TLPs (installed 1992)
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- How important are T N =2.5s cycles? Important when T WAVE = 2.5s but this condition has small wave heights Important when T WAVE = 5.0s due to second-order nonlinearity (springing) Non-Gaussian effects when T WAVE = 5.0s:
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- Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp)
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- Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp) Damage contribution of various (Hs,Tp)
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- Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp) Damage contribution of various Tp
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- Results: Damage contribution of various Tp Large damage at Tp = 7s due to frequency of seastates Large damage at Tp = 12s due to geometry of platform Larger non-Gauss effects if T PITCH = 3.5s (resonance when Tp = 7s)