| As the exploitation of marine oil and gas resources gradually extends to the deep sea, design technology of deepwater platforms has been developing fast in the recent years. Semi-submersible platforms have been widely applied for their various advantages, such as great performances of stability, wider ranges of operational water depth, and strong ability of drilling. Thus, it will be of great significance to carry out comprehensive analyses on hydrodnamic characteristics of deepwater semi-submersible platforms, to find coupled kinetic and dynamic response characteristics of the platform and its mooring system in various sea conditions, to study the effects of environmental changes on the hydrodynamic performances of platform, and to carry out optimization analysis on crucial parameters of a deepwater semi-submersible.In this paper, kinetic and dynamic characteristics of a deepwater semi-submersible platform and its mooring system in the South China Sea are comprehensively explored after the present research situation and development trend of hydrodynamic characteristics of semi-submersible platforms have been sufficiently investigated. On this basis, further study on hydrodynamic characteristics of semi-submersibles is carried out in this paper as well. The main work of this paper can be concluded as follows:(1) Analysis of hydrodynamic characteristics of a deepwater drilling semi-submersible platform and its mooring systemBy comparing the results of model tests and numerical simulation, hydrodynamic characteristics of a deepwater drilling semi-submersible platform and its mooring system are comprehensively analyzed in this paper. Numerical simulation consists of frequency-domain calculation and time-domain simulation. According to 3-D potential flow theory, the first step is frequency-domain calculation of the semi-submersible in different conditions, to obtain the added mass and damping coefficients, as well as response amplitude operators (RAO) of motions in various frequency and wave directions. Then, a database of frequency-domain hydrodynamic parameters is built up. After that, considering different typical wave directions, time-domain coupled analyses of the semi-submersible and its mooring system are carried out in both operating and survival sea conditions, to obtain motion responses in 6 degree of freedom and tensions of mooring system. Meanwhile, study on effects of different drafts and sea conditions, unsteady wind and an extreme condition that one mooring line is broken, on the kinetic and dynamic characteristics of the platform and its mooring system is implemented, which will be useful to ensure operating safety of the platform in real sea.Model tests are completed in the deepwater basin of State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University, including free decay test, regular wave and white noise tests as well as irregular wave tests in both operating and survival conditions. Motion responses in the 6 degree of freedom and tensions of mooring system can be obtained from model tests, which are further analyzed by comparing with corresponding numerical results.(2) Parameter sensitivity analysis of the deepwater drilling semi-submersible platform systemSensitivity analyses of various crucial parameters in the deepwater drilling semi-submersible platform system are conducted by comparison of corresponding numerical simulation results. The effects of different gravity center heights and drafts on the hydrodynamic performances of whole system are explored respectively. Besides, a heave plate is arranged under the platform to improve its heave motion. Through a series calculation of different shaped heave plates in different water depths, the design optimization of heave plate in the platform system is carried out. Conclusions obtained in the study will be useful in further design and application of heave plate in a semi-submersible platform system.(3) Design and characteristic analysis of Typhoon Prevention Drilling Riser (TPDR) in the deepwater drilling semi-submersible platform system Effects of new type Typhoon Prevention Drilling Riser on the hydrodynamic performances of a platform and its mooring system are studied by numerical analysis in this paper, which consist of the case when the riser stands freely, as well as the case when riser is connected to the platform. The minimum net buoyancy needed to ensure the safety of freely-standing riser is determined in Typhoon condition. Besides, effects of TPDR on hydrodynamic performances of platform and its mooring system are analyzed with comparison of a regular riser's case. Internal wave effects in the South China Sea are considered in these analyses as well. All the study above will be the basis of design optimization of the new type Typhoon Prevention Drilling Riser.(4) Study on hydrodynamic characteristics of 3-D adjacent multiple bodiesBy using programming language FORTRAN, the Higher-Order Boundary Element Method and the wave interaction theory are respectively adopted to study the 3-D problem of adjacent multiple bodies. The applicabilities of these two methods in conditions when distance between bodies varies are explored by analyzing the results of wave exciting forces and pressure distribution on the body surface. Conclusions in this study will promote the further application of Higher-Order Boundary Element Method and the wave interaction theory in the hydrodynamic problems of various kinds of deepwater semi-submersible platforms.This paper mainly studies on hydrodynamic characteristics of a deepwater drilling semi-submersible platform, including the coupled numerical simulation methods, model tests, parameter sensitivity, characteristics of new type riser, and the application of Higher-Order Boundary Element Method and the wave interaction theory in 3-D problem of adjacent multiple bodies. All these conclusions obtained can be applied in the further research on design and optimization of semi-submersible platforms, risers and mooring systems, which will be very significant to promote the technical development of deepwater semi-submersible platforms.
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