Nonlinear Design Wave Approach For The Fatigue Assessment Of Hull Structure

The fatigue life prediction of ship structures has been the hot spot of the study ofship mechanics. Currently, the fatigue life prediction method based on spectrumanalysis is considered as the accurate one. However, the experiences show that theworkload of this method is very large and it brings much inconvenience for practicalapplication,besides, the theory of spectrum analysis is based on the linear systemtransformation, but in recent years, many studies have shown that influence cased bythe nonlinear wave loads to the hull structure fatigue strength can not be ignored, sothe method has its limitations. The design wave method can not only get reasonableresults, but also consider effect of th the nonlinear loads on the fatigue strength, whileat the same time greatly reducing the amount of calculation, so it has a goodapplication prospect in engineering projects.The aim of this paper is to do a research on the design wave approach of fatigueassessment for hull structures, and then consider the influence cased by the nonlinearwave loads to fatigue strength.The main contents are as follows:1) Several dominant load parameters of the structural detail are determinedaccording to the structure form of the refined nodes, in terms of the identifieddominant load parameters, a group of design waves are obtained in accordance withthe principle of design wave. Basing on combination factors obtained by regressionanalysis, the stress ranges can be calculated under the four loading conditions, andthen the fatigue damage of the refined nodes can be calculated;2) Referring to the common structure rules, the dominant load parameters arereasonably selected, the equivalent stress ranges of the refined nodes are calculatedaccording to the principle of damage equivalence, so the design waves can bedetermined. Considering the impact of the nonlinear design wave on the fatiguestrength, the fatigue loads of the nonlinear design waves are calculated, the fatiguedamage of the hull structure under the nonlinear design waves are calculated by finiteelement method, and the results of liner and nonlinear design wave approach arecompared;3) The wave loads and motion response of the ship induced by irregular waves in the time domain are calculated. Basing on the stress time history of hull structure,statistical samples values of the stress range can be obtained from rain-flow counting,the distribution function of the stress range are fitted, and then, the fatigue damageunder the irregular waves are calculated, finally the design waves can be determinedaccording to the principle of damage equivalence.