Analysis On Fatigue Life Of Platform Structure Based On The Critical Plane-Energy Method

In coastal areas, the jacket platform is the main offshore platform structure style, the design of which is based on the wave loading. Platform in the sea under the influence of wave loading and current loading, offshore riser and its legs get fatigue damage by accumulate over a long period of microscopic damage. Fatigue becomes one of the important forms of failure of offshore platform structure, so the fatigue theory study of offshore platform and fatigue life prediction is particularly important. Offshore platform fatigue issue has been the difficulty in marine engineering. It contains multiaxial fatigue, high cycle fatigue, variable amplitude loading, material nonlinear trait and so on. All sorts of problems relate to each other, so for many years, the related fatigue theory development is slow.Based on critical plane-energy method, this paper establishes a application of high cycle fatigue theory model of jacket platform under multiaxial loading. Taking cyclic loading with different amplitude on specimens with sample material analysis to analysis by ANSYS, get the corresponding relation between strain energy density and stress amplitude, the relationship only depends on the material constitutive features; Under random wave loading and current loading, dynamic analysis was carried out on platform legs, take coordinate transformation to stress and strain tensor of dangerous point, in order to determine the critical plane position; Taking plastic energy density on critical plane as fatigue damage parameter, the single cyclic stress is equivalent to symmetrical cyclic stress, according to s-n curve for different cyclic stress generated by the damage; By Miner damage accumulation theory, cumulative damage generated by the circulation is obtained, get high cycle fatigue life of the platform leg; Through simulation to fatigue life of Workbench and extremum counting method, comparing with results of critical plane-energy method, deviations are19.0%and33.7%respectively.Critical plane-equivalent energy method takes less undetermined parameters and energy as fatigue damage parameters, critical plane as a basis for the crack extension, overcomes the traditional energy method as a scalar to describe the disadvantages of the extending direction of crack; makes the model clear physical meaning, and with energy as the medium, complex cyclic loadings is equivalent to symmetric cyclic loadings, to calculate damage caused by cyclic loadings, so this method apply to calculate multiaxial high cycle fatigue life of offshore platform, the research has theoretical significance and engineering value.