Wave Induced Response Analysis And Heave Motion Control Of A Semi Submersible Platform

Semi-submersible offshore platforms are the best choice for oil production in deep water of the South China Sea, because they have many advantages such as: less total investment, high mobility, multi-function, and good motion performance, etc. The processes of drilling and oil production allow very small motions of the platform and the heave motion of semi-submersible platform is a limiting factor. In this paper, the platform hydrodynamic analysis and model test is performed on the motion of a semi-submersible platform in the South China Sea under the wave loads, and a model which can calculate the first order motion of heave, pitch and roll freedoms is constructed. By analyzing the factors which affect the heave motion performance of the semi-submersible platform, an active heave plate is designed to reduce the heave motion of semi-submersible, based on the principle of Turned Mass Damper (TMD) and using Magnetorheological Fluid Damper (MRFD).First, the hydrodynamic parameters in the frequency domain of the semi-submersible platform is analyzed, and the motions of the semi-submersible platform under the wave loads is analyzed in the time domain using AQWA software; the viscous damping of numerical model is corrected according to the results of free decay tests in each degree of freedom ; the heave, pitch and roll motion in the range of wave frequency is analyzed by comparing the RAOs from regular wave tests and numerical simulation; the global motion performance under the work ocean condition and 100-y-survive ocean condition is analyzed in frequency domain by comparing the results from irregular wave tests and numerical simulation.Then, according to the hydrodynamic parameters calculated with AQWA software in the frequency domain, a numerical model is built with Matlab to calculate the first-order motion of heave, pitch and roll freedom degrees. In this model, the wave surface time curve and the first-order wave excited force is simulated using Jonswap spectrum as target spectrum; the first order radiation force is predicted by Convolution integral method; the motion of the platform under different ocean conditions is analyzed in Matlab, which is compared with the calculation from AQWA-Drift software.Finally, the impact of the draft depth and the depth of heave plate on heave motion performance of a semi-submersible platform is analyzed; a kind of active heave plate designed based on the principle of turned mass dampers is proposed be installed to the traditional semi-submersible to reduce the heave motion; the connection parameters between heave plate structure and platform is optimized; MRFD is used between the active heave plate and the platform to improve the control effect. As the offshore oil industry goes towards deeper waters, it will request more stringent on the motion performance of deep water platforms. In this paper, the active heave plate can improve the motion performance of the traditional platform, which is expected to be significant for bringing the potential economic effects.