Fatigue Crack Growth Life And Reliability Prediction Of Ship And Offshore Structures Based On Fracture Mechanics

Fatigue is one of the main failure modes of ship and offshore structures. Therefore, the fatigue strength assessment is an important part in ship structure design. The traditional fatigue analysis method is based on S-N curve and linear cumulative damage theory and is recommended by major classification societies and widely used in engineering. However, since the traditional fatigue analysis method doesn’t take the effects of initial defects and load sequence into consideration, researchers and classification societies gradually realize that fracture mechanics-based fatigue assessment is the trend of the future, which leads to the subject discussed in this thesis becoming one of the research focus in the field.Fracture mechanics-based fatigue assessment is not applied widely in engineering in recent years because there are still some difficult problems to be solved. In general, it contains four parts for fatigue life prediction based on fracture mechanics, namely: the fatigue load spectrum(FLS), the crack propagation rate, fracture parameters, the failure assessment. Research in this article is all about the four aspects and the main contents are as below.(1) Introducing the background of this study, reviewing the research status of ship fatigue by citing international and domestic literature, especially on the FLS, the crack propagation rate, fracture parameters, reliability. Pointing out some shortcomings of present applications on fracture mechanics-based fatigue assessment.(2) Proposing a SIF calculation method by using the sub-model technology. The method combines the finite element model of PATRAN, frequently used in ship and offshore engineering, and the advantage of ANSYS in SIF calculation. SIF of 3-D crack in ship and offshore structural details can be calculated by the proposed method.(3) Discussing the generation of FLS when the assumption that “the longterm distribution of wave induced alternating stress distribution obeys the two-parameter Weibull distribution” is made. Then the effects on fatigue propagation of shape parameter of Weibull distribution are studied. To compare the empirical formulas of shape parameter recommended by classification societies, the shape parameter of a hot spot in a ship is calculated.(4) Studing the traditional spectral analysis, proposing two methods of fatigue load spectrum(FLS) based on traditional spectral analysis. The first one is by fitting the long-term distribution in the spectral analysis; the second one is to generate load block of every sea-condition based on the short-term distribution in the spectral analysis.(5) Detailly describing the FLS generation method based on short-term distribution in spectral analysis, by predicting the fatigue life of an embedded crack in the deck of a container ship; detailly describing the proposed SIF method by predicting the fatigue life of surface crack at the typical and complex fatigue hotspot of a semi-submersible drilling platform.(6) Proposing a fast numerical calculation method for fatigue life prediction based on the improved Paris model. The limit state equation is derived and the procedure of reliability prediction is given by using Monte Carlo simulation(MCS). Then the fatigue life and fatigue reliability are predicted for the mideship deck with a surface crack in of a large container ship and the uncertainty of parameters is dicussed.Through the works above, the main conclusions are as shown below.(1) Shape parameter of Weibull distribution has an important influence on the crack propagation, namely the larger shape parameter, the shorter fatigue life. The results of shape parameter calculated by empirical formulas recommended by classification society are different, even the discussed hotspots are the same.(2) Since the inherent error of fitting process, the first method of FLS generation, proposed in this article based on traditional spectral analysis, may introduce error for fatigue load and lead to inaccurate results for fatigue life. Therefore, the second FLS generation method is more recommended.(3) The rationality and accuracy of proposed SIF calculation method are verified by comparing with empirical formula. The SIF amplitude of a surface crack in the typical fatigue hotspot of a semi-submersible drilling platform is calculated and the result shows the empirical formula is not suitable for complicate boundary conditions. The proposed SIF calculation method is necessary for the complex wave-induced stress.(4) The proposed fast calculating method is proved correct by comparing with the result predicted by cycle-by-cycle method. In addition, load uncertainty has the largest influence on reliability, followed by Paris constant C. Since the improved Paris model reduces the dispersion of C, the superiority of this model is showed. Effect on reliability of initial crack size is large but the effect of final crack size is small, so we can take the finial crack size as a determined parameter when predicting reliability.Overall, some exploration and try on fatigue assessment based on fracture mechanics are done in this thesis, and some useful conclusions are obtained. This may provide reference for wide application for fatigue assessement of ship and offshore engineering structure.