Study On Fatigue Reliability Of In-service Offshore Jacket Platform Under Storm Surge

With the changing global climate, the marine environment has become very complex.Especially in recent years, typhoon occurs frequently in coastal areas of china, and the stormsurge brings the safety of offshore oil platform a severe test. It is easy to cause fatigueproblem of structure under the long-term effect of high amplitude random wave forces. Theinfluence factors of the fatigue life of structure are uncertain. We must start from thereliability theory to predict the fatigue life of structure. Stress amplitude and stressconcentration factor are two important factors that influence the fatigue life of structure.Taking the bottom of pile legs and parts of tubular joints of each layer beam as the researchobject, the fatigue property and reliability of the platform structure under random waveforces were investigated. This paper does research on the fatigue reliability of the structureunder storm surge—taking a jacket platform as example. The main research contentsinclude:(1) Based on2D model of random wave—Longuest-Higgins model, with the improvedJONSWAP spectrum as the ideal wave spectrum, the numerical simulation of wave stormsurge environment through the MATLAB programming. Based on the basic principle ofWelch algorithm, pwelch function in MATLAB is used to estimate the power spectrum ofthe simulation results. Compared with the original JONSWAP spectrum, it is shown that thesimulation effect is good. On this basis, the main parameters and its standard value thatinfluence the simulation results and the power spectrum estimation is proposed.(2) According to the application scope of different wave theory, the second-orderStokes wave theory and liner wave theory are seclected. Combined with Morison equation,the formula for random wave forces in time domain are given, respectively. On this basis,numerical simulation of the random wave force can be realized by using MATLAB. Thefinite element model of structure was established by using finite element software ANSYS.Through the imposition of random wave forces and the transient dynamic analysis of thestructure, the nominal stress of the platform section nodes are resulted.(3) Considering the influence of stress concentration factor on the structural fatiguedamage, the local finite element models of the tube node are established with SHELL63element in ANSYS. By applying the displacement boundary conditions of the whole finiteelement model, we solve the hot spot stress of pipe node and get the stress concentrationfactor of the hot spot. Then, we use the the rain flow counting method to count the equivalent the stress cycles of the hot spot stress schedule. Finally, we estimate the fatiguedamage drgree D and the fatigue life Nfof each hot spots based on the S-N curve and Minerlinear fatigue cumulative damage theory.(4) This paper introduces the structural fatigue damage calculation method thatconsidering the uncertain factors under the action of random fatigue loads, and gives theparameter expression of the structure fatigue life. By definition of random variables, theuncertain factors that influence the structural fatigue life are described, and the fatigue lifereliability calculation formula of the structure on the conditions of uncertainty are given.Thus we can get the fatigue life reliability Prand the failure probability Pfof each point ofthe platform structure.