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Fatigue damage estimation along vessel ’ s voyages

This research outlines fatigue damage estimation for vessels by modeling fatigue in terms of significant wave height (Hs) and applying various methods for accurate estimation. The study delves into the correlation between vessel response and encountered sea states, exploring the use of Rain-flow counting method, Narrow bound approximation (NBA), and theoretical methods based on NBA for precise fatigue estimation. The research also examines the impact of different factors on fatigue modeling validity and compares various estimation techniques to enhance accuracy.

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Fatigue damage estimation along vessel ’ s voyages

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  1. Fatigue damage estimation along vessel’s voyages Chalmers University of Technology Wengang Mao Igor Rychlik

  2. Outline • Background and motivation • Fatigue model in terms of Hs • Application of the fatigue model • Conclusions Smögen Workshop (2008-08)

  3. 1.1, Background • Vessel construction period • Loading period • Shipping period 1, Vessel’s fatigue location 2, Loading condition 3, Shipping condition 4, Encountered sea states Smögen Workshop (2008-08)

  4. 1.1, Background How to estimate vessel’s fatigue damage • Rain-flow counting method (“correct” fatigue estimation) • Narrow bound approximation (NBA) • Theoretical method based on NBA Smögen Workshop (2008-08)

  5. 1.1, Background • Vessel’s response process is correlated with its encountered sea states (5/30 min stationary?) Smögen Workshop (2008-08)

  6. 1.1, Background • 30 minutes’ stationary process check • Rain-flow based on the whole voyage signal • Rain-flow based on 5 minutes’ signal • Rain-flow based on 30 minutes’ signal Smögen Workshop (2008-08)

  7. 1.1, Background • Theoretical fatigue estimation • Hydrodynamic RAO’s depends on heading angles and velocity • Ai means stress caused by each applied load • Hv– transfer function for vertical bending moment • Hh– transfer function for horizontal bending moment • Ht– transfer function for torsional bending moment Smögen Workshop (2008-08)

  8. Linear wave model Directional wave spectrum 1.1, Background Encountered sea states: • Vessel’s response under encountered sea states Smögen Workshop (2008-08)

  9. 1.1, Background • NBA for expected fatigue damage Response zero-crossing frequency Significant response height Smögen Workshop (2008-08)

  10. 1.1, Background • Measurement signal process X(t) Process zero-crossing frequency Method based on the measured signal can be taken as the right fatigue criteria Smögen Workshop (2008-08)

  11. 1.1, Background • Comparing among different methods Note: Rain-flow and NBA method based on measured signal, Standard method based on theoretical simulation. Smögen Workshop (2008-08)

  12. 1.2, Motivation—fatigue model of Hs • Drawbacks of the two typical methods 1, Measured signal is seldom available 2, Theoretical RAO’s need more precision 3, RAO’s (FEM & Hydrodynamic software simulation) • Main motivation 1, Compare different influence factors 2, Simply fatigue model (precise) only in terms of Hs 3, Check model’s validity Smögen Workshop (2008-08)

  13. 2, Fatigue model in terms of Hs 1, Relation between response and wave • Response hs is very correlated with wave Hs, • Significant response height: Severe sea states cause heavy stress response!!! fixed wave Tz Smögen Workshop (2008-08)

  14. 2, Fatigue model in terms of Hs Fixed wave Hs, its associate Tz from 4s to 12s has distribution f(t) Weighted arithmetic average C: Mean C: Smögen Workshop (2008-08)

  15. 2, Fatigue model in terms of Hs Significant response height hs observed vs from model • 1, Constant C works quite well for the model in left figure. • 2, Constant C for these voyages around 19 • 3,C from measurement agrees well with theoretical value Smögen Workshop (2008-08)

  16. 2, Fatigue model in terms of Hs 2, Response zero crossing frequency Assume zero-crossing wave period as: This model is ok for the large Hs, but not in the small Hs area Smögen Workshop (2008-08)

  17. 2, Fatigue model in terms of Hs • Ship’s response frequency should be corresponding to its encountered wave frequency • Encountered wave frequency is also related with shipping velocity and heading angles Note: Here the frequency should be encountered wave frequency Smögen Workshop (2008-08)

  18. 2, Fatigue model in terms of Hs Response zero up-crossing frequency observed vs simplified model with U0=9m/s, HDG=0 Smögen Workshop (2008-08)

  19. 5 voyages from Europe to Canada 2 voyages from Canada to Europe 4 special voyages (strange relation between encountered sea states and response) 3, Application of the fatigue model Voyages between Atlantic ocean travelling in different seasons Smögen Workshop (2008-08)

  20. 3, Application of the fatigue model Fatigue damage distribution along voy080106 from different estimation approaches Smögen Workshop (2008-08)

  21. Fatigue damage distribution of voyages from different estimation approaches 3, Application of the fatigue model Smögen Workshop (2008-08)

  22. 3, Application of the fatigue model • Rain-flow based on signal is “real” fatigue damage • 5 and 30 minutes’ periods both assumed stationary Smögen Workshop (2008-08)

  23. 3, Application of the fatigue model 2 voyages from Canada to EU Smögen Workshop (2008-08)

  24. 3, Application of the fatigue model Constant C and Fatigue damage accumulation Smögen Workshop (2008-08)

  25. 3, Application of the fatigue model 4 special voyages from Canada to EU Smögen Workshop (2008-08)

  26. 3, Application of the fatigue model Constant C and Fatigue damage accumulation Smögen Workshop (2008-08)

  27. 4, Conclusions • Our fatigue estimation model works quite well, and its precision is much better than the theoretical method • For the fatigue estimation location of above vessel, the constant C keeps about 19 • There are a lot of uncertainties in the model (hs, fz)… • Comparing to the other parameters, wave Hs is the most important factor of fatigue damage …(further work) • Need to check wave spectrum measurement by satellite wave model… • Put this model in shipping routing application… Smögen Workshop (2008-08)

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