The Fundamental Research Of Ice Induced Fuzzy Fatigue Of Ocean Platform

Towards the offshore platform in sea ice zone, sea ice load is the primary control load when ice conditions appear. The structure piece of ocean platform will generate cumulative fatigue damage under the random ocean wave load and sea ice load. At the beginning of platform design and service period, it is necessary to do the cumulative damage and fatigue life evaluation for the primary structure piece of platform in order to gratify the requirement of reliability. And make it a major basis for the fatigue life evaluation.Traditional method of cumulative damage calculation is based on the Miner principal. It considers the fatigue damage as a deterministic matter, that is to say, if fatigue stress amplitudes are greater than the limit of fatigue, the structure will generate fatigue damage, otherwise fatigue damage will not emerge. It neglects the cumulative damages caused by the fatigue stress amplitudes which are less than the fatigue value especially slightly less than the fatigue value. So the cumulative damage and fatigue life from calculation is usually different from the practical situation. Two fuzzy membership functions are raised based on the theories of fuzzy mathematics and the low temperature fuzzy S-N curve which has been improved by considering the fuzzy fatigue damage is given. Through studying of two types of fuzzy S-N curve, the sphere of application and the method of preferences are given.The fatigue damage which is less than the fatigue value is considered in this thesis, and the computed results based on the improved Miner principal are closed to the practice situation. Furthermore, the fatigue life of JZ20-2NW platform is evaluated based on the fuzzy fatigue life estimation method. The computed result illustrate that the fuzzy fatigue analysis for primary structure piece makes the cumulative damage calculation and the estimation of fatigue life more accurate, and more safe and reasonable than tradition method. Therefore this method could be extrapolated to the domain of marine steel structures to design the primary structure piece which requests high reliability and evaluate the fatigue life of the primary structure piece of ocean platform serving in the sea ice zone.