Simulation Research On Fatigue Life Of Semi-submersible Platform In Polar Environments

The arctic is rich in oil and gas resources,and with the depletion of oil and gas resources in land and offshore areas,the coastal oil and gas giants have put in a lot of effort and money for the research of oil and gas drilling technology and equipment in the polar water area.The semi-submerged platform plays an important role in polar exploitation for its excellent adaptability in deep water.But it is harsh in polar areas and the platform is always subjected to complex environment loading such as wind,wave,flow and floe during the service period,eventually lead it to fatigue failure easily.Therefore,it is very important to study the fatigue related problems of semi-submersible platform structures.In this paper,based on the theory of free floating body’s motion in waves and the crushing theory of brittle materials,time responses of platform structure under wave loads and ice loads were carried out,respectively.Based on the determined numerical simulation method,the platform responses and the strength analysis of the platform structure under wave loads and ice loads were carried out.Based on the Miner linear cumulative damage theory and S-N curve method,the fatigue life evaluation of the key nodes was carried out preliminarily.Analysis results showed that,the stress amplitude of column and the transverse brace connection part was the largest under wave loads,while under ice loads,the stress risk areas mostly were the direct collision area and the adjacent area,as well as the connection of column-transverse brace,so the connection of column-brace and the direct collision area were determined as the key parts of fatigue.Fatigue critical parts of the fine sub-model was established to improve the calculation accuracy and computational efficiency for wave loads,the boundary interpolation of the sub-model was carried out by multi boundary interpolation method,and the stress response transfer function of key nodes under various sea states was calculated by spectral analysis method.Based on the fine sub-model,the fatigue life of key nodes was further evaluated.Analysis results show that the fatigue life of the fatigue critical parts met the design specification requirements,but some safety factors of the key nodes were still insufficient.In the process of fatigue life analysis,there were many uncertain factors,and the interaction between wave and sea ice was one of the most important factors.From stress response and motion response,the paper analyzed the time-history features of semi-submerged platform under the waves loads,ice loads and ice-wave combined loads,and the result showed that although the wave load was more extensive,the fatigue damage caused by sea ice to the platform took up a greater proportion in some polar conditions,and the stress amplitude of structure key nodes increased greatly under ice-wave combined loads,and the amplitude surpassed the stress sum of ice and waves,though the ice could suppressed the wave partly and platform-shock was relieved,which was a threat to the semi-submerged.So,we should take the ice-waves combined effect into consideration when study and design polar deep-water semi-submerged platform.