Research On Time-dependent Reliability Of Jacket Platform Structure Based On Importance Sampling Method

Jacket platform structures operate in harsh offshore environments throughout the year,and safety performance is a primary concern during service.Conventional structural reliability assessment considers that the structural resistance of jacket platform is a constant value within the design reference time.However,the structural resistance of jacket platform will decrease with time.Therefore,the time-dependent reliability assessment method considering the reduction of resistance with time will be more accurate.Therefore,considering the uncertainty of applied load and the degradation mechanism of platform strength with service time,Monte Carlo importance sampling method is used to study the time-dependent reliability of offshore jacket platform structure.(1)Study the environmental load effect of jacket platform structure.Firstly,the finite element model of the jacket platform structure was established,and the load effect value of the jacket platform was determined by considering the action of wave and current.The surface of the load effect value under the action of wave and current was fitted by the plane fitting method,so as to determine the load effect function of wave and current.Secondly,the effect function of wind load is determined separately.Finally,considering the coupling effects of sea breeze,current and wave,the environmental loading effect of jacket platform structure is determined.(2)The ultimate resistance of jacket platform structure is studied.Firstly,the nonlinear finite element model of the jacket platform structure was established,and the pile-soil interaction of the jacket platform structure was considered.Then the environmental load was gradually applied,and the ultimate resistance of the jacket platform structure was studied by using the nonlinear collapse analysis principle.Finally,the initial ultimate resistance of the jacket platform structure was determined.The analysis shows that the strength reserve coefficient of the jacket platform structure is the lowest in the direction of 45° load,which indicates that the resistance of the jacket platform structure in the diagonal direction is weak.(3)Research on time-dependent reliability of jacket platform structure based on importance sampling method.Firstly,the environmental load effect probability model of jacket platform structure is established based on the determined environmental load effect;Secondly,considering the corrosion effect,fatigue effect and the combined effect of corrosion and fatigue respectively,the corresponding damage models were determined,and their resistance attenuation functions were calculated;Finally,Monte Carlo importance sampling method was used to calculate the time-dependent reliability of jacket platform structure,and the decay law of time-dependent reliability index under corrosion,fatigue and corrosion and fatigue combined action was obtained.The results show that compared with the traditional reliability evaluation,the reliability index of time-dependent reliability evaluation is lower,indicating that the time-dependent reliability evaluation method is more consistent with the actual situation;Monte Carlo importance sampling method is efficient and accurate in solving time-dependent reliability of jacket platform structure.