| With the exploration of oil and gas in deepwater, the problems related tofloating platform are becoming the research hotspot at home and abroad due to thesignificant role of deepwater filoating platform serving as drilling and productionfacility in deepwater oil and gas exploitation. Air gap is one of the key issues thatneed to be considered in the design of floating platform, which has been widespreadconcern. Air gap of floating platform is not only related to the free wave surface, butalso the vertical movement of platform. Because of the coupled interaction betweenplatform and wave, the incident wave is superimposed with diffracted and radiatedwaves, which lead to complex variation of wave surface. Meanwhile, the movementof platform also has a significant influence on air gap performance of floatingplatform. The air gap performance of a deep draft multi-spar platform (DDMS) hasbeen investigated under the sea environment of Gulf of Mexico with100-returnperiod in this dissertation. The main works are as follow:(1) A numerical model of interaction between wave and DDMS platform isestablished under potential flow theory based on diffraction and radiation. Based onsome observed points under platform deck, the comparative analysis of thediffracted and radiated wave surface variation under different incident direction iscarried out. Meanwhile, the influence of DDMS’s structural geometry on the waveelevation is analyzed. The contribution of diffracted and radiated waves to the totalwave elevation is investigated under irregular wave based on JONSWAP spectrum.The results show that a strong near-trapping phenomenon induced by the diffractedand radiated waves is observed due to novel five-column structure of DDMS, whichleads to a large local wave elevation. In specified sea condition, diffracted andradiated effects are able to magnify the total wave elevation.(2) The first-order hydrodynamics and motion response of DDMS platform aresolved in frequence domain and time domain respectively under high-orderboundary element method based on potential flow theory. The results indicate thatDDMS platform has excellent motion performance and seakeeping ability owed tothe low natural periods of movements which are away from the frequence ofincident wave. Furthermore, the statistic analysis of the vertical motion responsehistory of platform is conducted and the results show that the heave response has asizable contribution on vertical displacement of platform, while that of roll and pitchresponse is relatively small.(3) The history of air gap response of DDMS platform is synthetized using thehistory of absolute wave and vertical movement of platform. The results manifest that the air gap of DDMS platform meets the requirement and the relative waveelevation is mainly affected by the absolute wave while is less influenecd by themovement of platform. The numerical prediction of air gap is carried out base on thefirst-order results in frequence domain and compare with the results derived fromthe time domain. The results show that the prediction of air gap can reflect the airgap performance of DDMS platform.(4) The influence of column of platform on Air gap performance is analyzed bychanging the cholumn shape and spacing respectively. The research shows that thecolumn shape has less effects on the air gap performance of platform, but thecolumn spacing make that a big diffrence which has a overall manifestation that thebigger of the column spacing, the better of the airgap performance. |
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