Study On Crack Propagation Of Jacket Platform Pile Leg Under Extreme Storm Condition

Jacket platform is the infrastructure in the process of offshore oil and gas exploitation,and the pile leg is the main structure supporting the platform operation.Due to the long-term interaction of environmental loads such as wind,wave and current,as well as the strong corrosiveness of sea water,the pile leg is prone to crack and other defects.Under the action of extreme environmental loads,the crack will continue to expand,which will eventually lead to the decline of the platform’s carrying capacity.Therefore,it is necessary to evaluate the remaining life of the platform to provide reference for the subsequent platform life extension,so as to ensure the safety of the platform in service.The purpose of this paper is to simulate the crack propagation at the key parts of the platform and predict their remaining life based on the fracture mechanics theory.The specific research contents are as follows:Based on the drawing of a jacket platform,the finite element model of the platform was established by ABAQUS software,and the dangerous parts of the platform were obtained through static analysis under extreme storm conditions.The first three order modal frequencies and mode shapes of the platform were obtained based on modal analysis.Through dynamic analysis,the changing process of displacement,velocity and acceleration of the platform under extreme storm conditions is obtained,which provides a reference for the subsequent safety assessment of the platform.Establish the solid model of the dangerous parts,and divide the sub-model from the solid model.Using Franc3 D software,single crack and multiple crack were introduced into the sub-model,respectively.The propagation direction of the crack was determined by the maximum tensile stress criterion,and the appropriate propagation step size was selected to simulate the propagation process of single crack and multiple crack.The relationship between the crack propagation length and the crack propagation length is obtained.The results show that the growth velocity at both ends of the crack is greater than that at the crack tip.When multiple cracks fuse,the stress intensity factor(SIF)will mutate.The propagation morphology and stress variation trend of single crack and multiple crack are obtained through the propagation process of single crack and multiple crack.The relationship between the stress intensity factor(SIF)and the crack propagation distance was obtained through the propagation process of single crack and multiple crack.Paris and NASGRO crack growth rate formulas were used to calculate the load cycles of single crack and multiple crack respectively,and the propagation lives of single crack and multiple crack were calculated by environmental load cycles.The single crack life and multiple crack life calculated by Paris formula and NASGRO formula are compared and analyzed.Based on the influence of the corrosion environment on the crack growth,the single crack growth process of the hazardous site under the corrosion condition was simulated,and the relationship between the stress intensity factor and the crack growth distance was obtained.The residual life of the pile leg under the corrosion condition was calculated by using the formula of the crack growth rate.The results show that the material parameters C and m have a great influence on the crack growth life,and the corrosion environment will accelerate the crack growth and reduce the crack life.