Research On Technology Of Fatigue Damage Assessment For Deepwater Semi-submersible Platform

Nowadays, economic development demand for oil remains high, and accelerating the development of offshore oil and gas resources is one important way to ease the pressure.The deep-water floating platform is the main equipment for the deep sea oil and gas resource exploitation which would stay at the marine environment for long time. The structure will accumulate fatigue damage under the influence of cycle load, and may eventually lead to fatigue damage and cause disaster consequences. As one of the most critical forms of damage and principal failure modes for offshore structure, it is therefore essential do some deeper research on the fatigue damage.In this paper, the correlation theories of fatigue analysis and common fatigue assessment methods are summarized. Besides, a state of the art review on the fatigue life assessment in the domestic and overseas is given. Some researches on the structure fatigue assessment and mooring line fatigue assessment are conducted. The main research works of this paper are elucidated as follows:(1) The hydrodynamic model and structure’s finite element model and the mass model of the semi-submersible platform which is study in this paper are established according to the drawing information. The research on structure’s hydrodynamic performance in frequency domain is conducted and the hydrodynamic parameters are acquired. The wave load which applied on the structure are calculated based on the Green function and Morison equation. The design wave elements are obtained according to four kinds of structure’s characteristic goal hydrodynamic response by means of the design wave method and the structure’s strength which under those wave conditions are checked. The research results show that the hydrodynamic performance of the semi-submersible is fine and the strength of the structure satisfy the demand.(2) According to the characteristic global hydrodynamic responses and the stress distribution feature, six key points which are prone to fatigue damage are selected. A higher mesh density is applied to the members which are near the key points. The hot points stress transfer functions are got after the structure’s strength analysis. The fatigue life of the key points under different wave direction are study by means of the spectral based method. The study results show that the fatigue damage of the key points caused by the wave have relationship with the wave direction. The oblique cause the maximum damage and the transverse wave comes in second and the longitudinal wave cause the minimum damage. When the oblique waves action on the structure, the fatigue lives of the connection between column and pontoon and the connection between smaller column and diagonal brace are small.(3) A coupled dynamic analysis of the mooring structure is conducted and the time history of the structure motions and mooring lines tension are acquired. The maximum amplitude of the structure motion and mooring lines tension which under the extreme sea state are checked. The time domain fatigue assessment method is used to study the mooring lines fatigue life. The influence of wave significant height and wave period on the mooring line’s fatigue damage are studied. Investigation results indicate that wave period and wave significant height have different influence on the fatigue damage. If the wave significant height keep unchanged, the mooring lines’fatigue damage increase first and then decrease as increasing of wave period. If the wave period keep unchanged, the mooring lines’fatigue will show exponential growth as the wave significant height increasing. According to the mooring line material, the fatigue life along the mooring line can be divide into three segments. At the similar condition, the fatigue resistance of spiral strand wire is better than the mooring chain and the fatigue resistance of the grade R4 mooring chain is better than the grade R3 mooring chain.(4) In order to improve the computational efficient of time domain method, the method of the sea-states block division is studied. In this paper, four kinds of block division rules are proposed which are named as period equal division principle, wave significant height equal division principle, four sea-states division principle and six sea-states division principle and each kind of the block division principle is studied through the numerical example. Two parameters, assessment deviation parameter and computational efficiency parameter, are designed to compare the principle’s precision and efficiency. The research results show that the ideological of the block division can improve the computational efficient greatly; Compare with the results which acquired through the basic method, the period equal division principle may overestimate the mooring lines fatigue life and the wave significant height equal division principle may underestimate the mooring lines fatigue life and the six sea-states division principle may overestimate the mooring lines fatigue life. While calculation deviation of the four sea-states division principle is small and can be meet the demand. The higher computational efficiency and precision block division principle is the main innovation of this paper.