| Oil&gas resources are the most concerned topics to many countries for theirnational economy security&sustainable development against heavy oil consumption.As is known that oil exploitation both on land and offshore regions has been on for acertain history, more oil resources still hide in deepwater even in ultra deepwater areas,which results in the challenges to gas exploitation, as well as to opportunities to solvethe energy crises at present.In order to solve the challenges in gas exploitation, attention is placed on designand hydrodynamic-performance investigations of the floating structures. As the SouthChina Sea is a potential area being rich in oil&gas resources, relentless effort iscontributed to new floating concept with better performances, methods to improveperformance of the floating structures, and etc.The deepwater semi-submersibles are widely used due to their superioradvantages of high wind-resistance ability, adaptation to a wide range of water depths,broad deck and strong load-carrying capacity and drilling ability. Besides hullterminal installation and overall towage can be conveniently implemented for thedeepwater semi-submersibles and reasonable arrangement of production facilities areeasier to realize. However, traditional deepwater semi-submersible platforms havepoor performance of heave motion and thus cannot serve as the host of the dry treewell system. To integrate the advantages of both the dry-tree well system and thesemi-submersibles, many innovative ideas and methods are proposed to improve theperformance of heave motion for deepwater semi-submersibles.Jacket combined deep-draft semi-submersible platform (JCDDS) is a newconcept of deep-draft semi-submersible platforms and is designed to support the drytree well system in the harsh environment for the South China Sea. The underwaterstructure of this conceptual platform includes not only the underwater structure of thetraditional deep-draft semi-submersibles, but also the jacket part, which are connected in the inner diagonal areas at the bottom of columns. The jacket part is introduced toimprove the stability as well as increase the air gas on the condition that its negativeinfluence on hydrodynamic performance of the platform is minor. As a new platformconcept, it is of significant importance to comprehensively investigate itshydrodynamic performance, dynamic characteristics when coupling with mooringsystem, and characteristics of the VIM. Moreover, the hydrodynamic performances ofthe platform and its mooring system in different environmental conditions influencedirectly the quality of platform concept, and thus call for deeper understanding andcontinuous improvement.For an improvement of the heave motion performance, the draft is increased andVIM as a negative effect for the platform is thus induced. An increase of the draftbrings the increase of underwater height of the columns and thus enhances theinfluences from sea currents. VIM is a transverse motion occurring when fluid passesa blunt obstacle accompanying with periodical vortex shedding. Its maincharacteristics include long periodicity of motion, large transverse response amplitude,and etc. VIM is widely investigated for its negative effect on the fatigue life of themooring and riser systems. This concept platform adopts the strategy of increasingdraft to improve heave motion performance, and therefore it is crucial to analyze thevortex shedding performance for a better understanding and restraining of VIM.A new platform concept is chosen as the object in this master thesis and acomprehensive analysis is carried out on hydrodynamic performance andcharacteristics of VIM phenomenon. Major works are as follows:(1) Inverstigations on hydrodynamic performance of the new deep-draftsemi-submersible concept and its mooring systemModel tests are carried out, the results of which are compared to results of thenumerical simulations to verify its accuracy. A series of hydrodynamic analysis of theconcept are conducted covering different environmental conditions and cases whenthe mooring system is intact or one mooring line fractures. The results can providereferences for the design, performance improvement and safety assessment of theplatform.(2) Parametric sensitivity analysis of the new floating conceptMajor emphasis is placed on the influences of vertical height of gravity center,jacket part and the two methods improving heave motion performance of the floatingsystem. The influence of gravity height on motions of the platform in six degrees offreedom is analyzed in frequency domain. Subsequently, based on the consideration in engineering, influences of the jacket part are analyzed on motion in the vertical plane.Lastly, a comparison is made between two methods to further improve the heavemotion performance: increasing the draft and increasing area of certain local structure.(3) Characteristics of the VIM phenomenonThis paper uses numerical method to simulate the flow around fixed squarecolumn and VIM of square column to investigate the phenomenon of VIM frommechanism perspective and to explain the internal mechanism of VIM phenomenon.Re is set as50000~765000for the flow around square column and the resultsare compared to the existing results of flow around a cylinder.Major differences are discussed, and transvers motion amplitude, motiontrajectory and the flow field characteristics are investigated. This proves a solid basisfor further study of the VIM characteristics of flow around a four-column deep-draftsemi-submersible.A deep research is conducted on VIM characteristics of semi-submersibleplatform for the flow coming from0degrees and45degrees. Main scope of Reynoldsnumber is within10000~100000. Loading and vertex shedding characteristics areanalyzed in such conditions. At the same time, vertex shedding characteristic analysisis carried out in single degree of freedom under mooring stiffness, which providesreferences for further researches on platform VIM characteristics under sea currents inthe South China Sea. |
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