The Fatigue Strength Of The K - Joints In Offshore Platform Structure Analysis

The tubular joints are the most important but the weakest members of ocean platform. It has been found that tubular joints fatigue cracks are a dominating type of damage in ocean platform, and the economical losses are also considerable. So it is evident that there is a great need for better understanding of tubular joints fatigue behaviors and mechanisms, thus the fatigue lifetimes of tubular joints can be correctly predicted and the effective ways to improve fatigue lifetimes can be found out.The main work of this thesis is to predict the fatigue lifetime of a K-joint in BINGO9000 ocean rig, and do some basic work for the further study of tubular joints fatigue lifetime prediction in future.Firstly, based on the DNV rules, a 3D finite element model of the rig was developed for stress calculation and the strength of structure was checked. To obtain the local stress distribution of the K-tubular joint, sub-model was built and calculated.Then, according to wave long-term distribution of North Sea, stresses of the whole structure subjected to 72 wave loads cases are calculated, which supplies boundary conditions of local tubular joint models in local stress analysis. Then, the fatigue strength analysis of K-joint is carried out. Hot spot stress method is used in the calculation of stress range. With S-N curve selected from DNV rules of fatigue strength analysis for mobile offshore unites, The Miner linear fatigue damage accumulation rule is used to calculate fatigue life of the tubular joints.Finally, some useful conclusions have been drawn, which will give valuable technical support for future design and building of the semi-submersible rig in China.