A Study On Random Vibration Explicit Time-domain Method For Jacket Platforms Under Wave Loads

Offshore oil and gas exploitation are going forward into deep waters,where jacket platforms are subjected to harsher wave environment.As wave loads are typical random dynamic loads in nature,one of the most important subject in offshore engineering is the random wave responses analysis and the reliability analysis of jacket platforms based on random vibration theory.Considering the inherent nonlinearity of wave loads,it is required to conduct the statistical linearization of nonlinear wave loads within the framework of traditional power spectrum method(PSM)for random vibration analysis of jacket platforms.Besideds,the extreme value distribution assumption needs to be considered in the prediction of peak values of structural responses,and the level-crossing process assumption is needed to be considered in the dynamic reliability analysis.Obviously,significant errors exist within the solution procedure of random vibration of jacket platforms based on PSM.In view of this,the random simulation method(Monte-Carlo simulation)is directly adopted to conduct the analysis of random vibration of jacket platforms in the time domain in this dissertation,without the influence of forementioned assumptions.However,the extremely low efficiency obstructs the application of the traditional random simulation method to engineering practice,because of the large computational cost due to repeatedly solving the equation of motion for numerous time-history analyses of the jacket platform under different samples of wave loads.To tackle the difficulties involved in traditional methods,this dissertation proposes an efficient time-domain approach with random simulation based on the explicit time-domain method(ETDM)for the random vibration analysis of jacket platforms subjected to wave loads.On this basis,efficient approaches are devoted to the development of the dynamic reliability analysis of jacket platforms subjected to some level of short-term sea state based on the first-passage failure criterion,and the dynamic reliability analysis of jacket platforms subjected to the long-term sea state within the design life based on the fatigue failure criterion,respectively.It is expected to promote the engineering application of random vibration analysis methods in design project of jacket platforms.The main contents of this dissertation can be described as follows:(1)The literature review is conducted on the traditional methods for the random vibration analysis of jacket platforms under nonlinear wave loads,the first-passage dynamic reliability analysis of jacket platfroms under some level of sea state and the fatigue reliability analysis of jacket platforms within the design life,respectively.The development of the traditional methods is first presented,and then their advantages and disadvantages are summarized.(2)The research on the random vibration analysis approach of jacket platforms subjected to nonlinear wave loads is conducted.The nonlinear Morison equation is first adopted to depict wave loads.The Newmark-β integration scheme is then employed to construct the explicit expressions for dynamic responses of jacket platforms in terms of the discretized wave loading vectors at different time instants.On this basis,an efficient time-domain approach which is termed ETDM-based random simulation is proposed for the random vibration analysis of jacket platforms under nonlinear wave loads.Owing to the explicit formulation of the dynamic responses,there is no need to repeatedly solve the equation of motion of the structure for different sample analyses,which greatly enhances the computational efficiency of random simulation.Furthermore,as the present approach is conducted through random simulation in essence,nonlinear wave loads can be readily taken into consideration rather than using the statistical linearization theory,and accurate evaluation of the mean peak values of structural responses can be obtained without additional assumption regarding the distribution probability regarding the extreme responses.(3)The research on the dynamic reliability analysis of jacket platforms based on the firstpassage failure criterion is conducted.Owing to the dimension-reduced analysis and the loadtruncated mechanism,a hybrid approach is proposed with ETDM-based random simulation for first-passage dynamic reliability analysis of jacket platforms under some level of sea state,in which only certain critical responses regarding structural dynamic reliability need to be focused on,and the loading terms with little influence exerted onto the structural responses at different instants can be truncated.The use of truncated ETDM greatly enhances the computational efficiency of random simulation involving large sample size and long duration time of wave loads,making it feasible for the dynamic reliability analysis of jacket platforms under any level of sea state.Furthermore,as the present approach is conducted through random simulation in essense,the exact solution to the component reliability can be well approximated without additional assumption regarding the probability distrubtion of level crossings,and accurate result of the system reliability can be achieved without the assumption involving partial correlation properties among different failure modes.(4)The research on the dynamic reliability analysis of jacket platforms based on the fatigue failure criterion is conducted.Within the framework of Miner’s cumulative damage rule,the fatigue damge analysis under each level of sea state for one fatigue detail of a component is first obtained through the ETDM-based random simulation for stress responses at fatigue details in conjunction with the rainflow counting method for statistics of stress cycles.On this basis,in combination with random simulation for the reoccurrence numbers of all levels of sea states within the design life,a hybrid approach is proposed with ETDMbased random simulation for fatigue dynamic reliability analysis of jacket platforms within the design life.The random samples of the fatigue damge at the fatigue detail of the component within the design life can be obtained by the present approach,and then the probability distribution of fatigue life can be determined.Significant enhancement of computational accuracy can be achieved for the fatigue dynamic reliability analysis of components,without additional assumption regarding the probability distrubtion of stress ranges or the average zero up-crossing rate.Within the solution procedure of random vibration of jacket platforms based on PSM,computational errors are mainly caused by the statistical linearization of nonlinear wave loads,the use of the extreme value distribution assumption of structural responses and the use of the level-crossing process assumption of structural responses,respectively.Engineering examples involving jacket platforms subjected to random wave loads illustrate the ideal accuracy and efficiency of ETDM-based random simulation for the prediction of peak values of structural responses,the first-passage dynamic reliability analysis and the fatigue dynamic reliability analysis.The proposed approaches addresses the bottleneck of the low accuracy of traditional methods based on PSM and the low efficiency of traditional random simulation methods.A wider prospect for engineering application can be expected for random vibration analysis and reliability analysis of offshore structures.