Motion Performance And Parameter Sensitivity Analysis Of A Semi-Submersible Platform

In recent years, semi-submersible platforms are increasingly being developed for drilling and production activities in deep and ultra-deep waters. With the rapid demand and progress of drilling deepwater oil and gas in the South China Sea, it should be significant for the designer and the operator to promote the hydrodynamic studies of semi-submersible platforms. Among these studies, we should pay much attention to not only the conventional prediction of hydrodynamic response but also some special hydrodynamic characteristics related to the structures.This thesis investigated the development situation, correlative results and direction of semi-submersible platform in the world. Numerical analysis is performed on motions of a semi-submersible, including surge, sway, heave, roll and pitch motions. The analysis is carried out in frequency domain, aiming at calculating the RAO (Response Amplitude Operator),and in time domain,aiming at calculating motion response. Through comparison with experimental results, it is shown that the satisfied low-frequency motion results can be obtained when employing Newman's approximation; while the viscous drift force should be included when analyzing vertical low-frequency response. Therefore, the method based on 3D potential fluid theory and Morison equations is employed to predict hydrodynamic performance of a semi-submersible platform. The natural frequencies of pitch and roll motions are lower than wave frequencies. As a result the low-frequency response will be significant due to the slowly varying wave drift force. It is much larger than wave-frequency response in operational condition, while it is in the same magnitude with wave-frequency response in survival condition. For heave motion, low-frequency response will depend on environment conditions. In operational condition, the low-frequency response exists with the same level of wave-frequency response. In survival condition, there is no low-frequency response. The low-frequency response of vertical motions should be important and paid much attention to, since the air gap and operating performance of a semi-submersible will be influenced remarkably. The hydrodynamics of a semi-submersible platform is always influenced by the mass and geometry parameter, for instance, the dimension of column and pontoon, the space between columns, the draft etc., mean while it will be infected by mooring system.A typical semi-submersible platform with two pontoons, four columns, and two cylindrical braces is selected during the analysis. Then a number of parameters, including C.G. height, space between center lines of columns; brace shape; cross-section shapes of the column and pontoon; draft; heave plate dimension and depth; pontoon shape, are analyzed. The parameter analysis in this thesis, aiming at reducing the pitch, roll and heave motions, is preliminary and not comprised of the interactive influence between different parameters during the analysis process. Firstly, RAO of modified semi-submersible platform is obtained in the frequency-domain, and then the responses in operational and survival condition are calculated, in the end, all of the results are compared with initial platform's results.Main conclusions includes: increasing the draft can efficiently reduce the response of pitch, roll and heave motions; in some case, heave plate could result in an obvious decline of heave motion, when heave plate is disposed in a proper depth avoiding the influence from waves; wing braces and ring pontoons may deteriorate motion performance of semi-submersible platform;The analysis of low-frequency motion performance and parameter sensitivity could lead to more comprehensive understanding and more accurate prediction of hydrodynamic characteristics of semi-submersible platforms. It is expected to be significant for promoting the development of offshore structures in deep sea areas in China.