Fatigue Strength Of Key Area Of Ultra-Deep Water Drillship

With the good mobility and loading capacity,ultra-deep water drillship plays an important role in deep-sea oil and gas exploration and development.Due to the terrible working condition and long-term wave alternating load,fatigue failure is one of the main failure modes of hull structure.For the function of drilling,the moonpool will be arranged in the midship of the ship.The existence of the moonpool opening destroys the continuity of the ship structure,which makes the fatigue problem in this area particularly prominent.Therefore,it is necessary to carry out the fatigue strength assessment for the drillship.In this thesis,the seventh generation of multi-functional ultra-deep water drillship is taken as the research object,the global finite element model of ship is established,the wave loads are calculated by the three-dimensional potential flow theory,then the stress response of the ship structure under transit,drilling and extraction loading conditions are solved,and the fatigue assessment of the key areas of the drillship are carried out by the spectral-based analysis method.Finally,the simplified fatigue assessment is used to obtain the fatigue damage of each joint under different load conditions,and the results are compared with the spectrum analysis results to determine the fatigue leading load condition of different joints.The main study work of this thesis are as follows.(1)The theories of fatigue spectrum analysis of ship and offshore structure are summarized,including wave load calculation method,random wave theory and spectrum analysis theory.The requirements of fatigue calculation model of drillship and the related process of spectrum analysis are also introduced.(2)Fatigue screening for the key areas of the target drillship is carried out to determine the joints that need to be detailed analysis.Then fine grid models are establishes to calculate the hot spot stress range,and the fatigue damage and life of typical joints are calculated in combination with relevant environmental parameters.(3)The sensitivity research of different loading conditions,wave direction and sea conditions are carried out for the typical joints in the moonpool area of drillship,the contribution of each parameter to the fatigue damage of joints are analyzed to clarify the fatigue load forms of different joints.(4)According to the results of spectrum analysis,the parameters of long-term stress distribution in the moonpool area are fitted by the least square method,and then different load cases are selected based on the characteristics of the joints to the simplified fatigue analysis,and the results are compared with the spectrum analysis,the fatigue load conditions of the typical joints of the drilling ship are determined.The research in this thesis can provide some advices for the structural design and fatigue strength assessment of typical joints of drillship.