Presentation is loading. Please wait.

Presentation is loading. Please wait.

Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein

Similar presentations


Presentation on theme: "Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein"— Presentation transcript:

1 Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein

2 Fatigue Under Random Loads Mean Damage Rate: where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m =

3 Fatigue Under Random Loads Mean Damage Rate: where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m =

4 Fatigue Under Random Loads Mean Damage Rate: where S = stress range; c and m material properties Welded steels: m = 2 - 4; Composites: m = 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 = Assumes: Stresses Gaussian, narrow-band Common errors: Assume Gaussian, narrow-band

6 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

7 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)

8 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

9 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

10 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

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 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.

16 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

17 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

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=1.4 Median:  50 =27

20 LRFD Fatigue Design

21

22

23

24 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 ?

25 SRSS: Largest safe region; least conservative

26 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

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

31

32

33

34 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:

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

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

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

38 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)


Download ppt "Fatigue of Offshore Structures: Applications and Research Issues Steve Winterstein"

Similar presentations


Ads by Google