Study On Fatigue Life Of Jack-up Platform Pile Leg Based On S-N Curve And Fracture Mechanics

With less and less land oil recoverable reserves,the mining of ocean oil and gas reservoirs has been paid great attention in the world,a growing number of jack-up platform in use,in order to keep the platform under the harsh Marine environment of Marine oil and gas exploration work properly,the jack-up platform leg structure fatigue life analysis and to forecast the dangerous position of leg structure is particularly important.In this paper,based on the S-N curve method and fracture mechanics method for pile legs structure fatigue life analysis,concrete research content is as follows:(1)Jack-up platform for static analysis,modal analysis and dynamic response analysis.In the static analysis,on the basis of determining the wind load,wave load and flow load,the strength of the pile leg is checked based on the static analysis model,and the dangerous area of the pile leg structure is mainly concentrated in the splash area.In order to avoid the resonance of jack-up platform pile leg structure in deep water zone,the modal analysis is carried out to obtain the modal frequency and Vibration shape.Through dynamic response analysis,the maximum displacement of the platform is obtained,which provides theoretical support for the safety early warning value setting of the platform.(2)Based on the S-Ncurve method,the fatigue life of the jack-up platform truss pile leg is analyzed and the location of the specific hazard node is determined.According to the risk area of jack-up platform structure strength analysis,the fatigue life of the dangerous area of jack-up platform structure is analyzed by using the theory of deterministic analysis and spectral analysis.The fatigue life of pipe joints in the dangerous area of pile leg is analyzed by the deterministic wave theory and the fatigue accumulation theory.The spectral analysis method takes into account the effect of random waves in a particular period of time in the sea area.This method divides random waves into different sea states according to the wave height H_sand the mean span zero period T_z,calculates the stress transfer function of the risk node by combining the wave spectrum of each sea state,and calculates the total fatigue life according to the fatigue accumulation theory.The results of the two methods show that the deterministic method is simple and feasible,but the computational accuracy is not high.The spectral analysis method can better simulate the actual sea state and predict the life of the pile leg more accurately.(3)Based on the fracture mechanics method,the fatigue life of dangerous joints of pile leg structure of jack-up platform was analyzed.The shape of the dangerous joints of the pile leg structure is double K type.At first,an ectopic quantified semi-elliptic initial crack is introduced on the surface of the pipe joint,and then the initial strength factor of each position is calculated.According to the crack propagation theory,the initial crack propagation analysis was carried out on the dangerous pipe joints of pile legs,the relationship between crack length propagation and cycle times at each position,the maximum stress intensity factor and cycle times at different positions were obtained,and the remaining life of pile legs and the most dangerous crack positions were finally determined.Compared with the fatigue accumulation analysis method of S-N curve,the calculation of crack growth life takes into account the inevitable defects and initial cracks in the manufacturing process,so the calculated fatigue life is shorter.