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Fatigue of Offshore Structures: Applications and Research Issues

Fatigue of Offshore Structures: Applications and Research Issues. Steve Winterstein stevewinterstein@alum.mit.edu. 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 of Offshore Structures: Applications and Research Issues

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  1. Fatigue of Offshore Structures:Applications and Research Issues Steve Winterstein stevewinterstein@alum.mit.edu

  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

  3. 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

  4. 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

  5. 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

  6. Bandwidth & Non-Gaussian Effects Damage Rate: E[DT] = CBW * CNG * E[DT | Rayleigh] CBW, CNG = corrections for bandwidth, non-Gaussian effects

  7. Bandwidth & Non-Gaussian Effects • Damage 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)

  8. Bandwidth & Non-Gaussian Effects • Damage 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

  9. Bandwidth & Non-Gaussian Effects • Damage 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

  10. Bandwidth & Non-Gaussian Effects • Damage 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

  11. Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing)

  12. Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing)

  13. Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing) VIV of Risers: No

  14. Can We Even Predict RMS stresses? Container Ships: Yes (Without Springing) TLP Tendons: Yes (With Springing) VIV of Risers: No FPSOs: ??

  15. 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.

  16. 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

  17. TLP Tendon Fatigue: 1st-order vs Combined Loads 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 TPITCH = 3.5s (resonance when Tp = 7s) Damage contribution of various Tp Ref: “Volterra Models of Ocean Structures: Extremes and Fatigue Reliability,” J.Eng.Mech.,1994

  18. 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.

  19. VIV Factor:m=3.3,s=1.4Median:a50=27

  20. LRFD Fatigue Design

  21. LRFD Fatigue Design

  22. LRFD Fatigue Design

  23. LRFD Fatigue Design

  24. 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?

  25. SRSS: Largest safe region; least conservative

  26. Proposed Combination “Rules” DTOT = [ DSLOWK + DFASTK ] 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

  27. Combined Fatigue: DNV Approach

  28. Merci beaucoup!Extra background slides follow…

  29. The Snorre Tension-Leg Platform Water depth: 300m One of earliest TLPs (installed 1992)

  30. 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:

  31. Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp)

  32. Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp) Damage contribution of various (Hs,Tp)

  33. Answer: The Fatiguing Bookkeeping Likelihood of various (Hs,Tp) Damage contribution of various Tp

  34. 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 TPITCH = 3.5s (resonance when Tp = 7s) Results: Damage contribution of various Tp

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