Active, Passive And Their Joint Control For The Motion Responses Of Semi-Submersible Platforms

In the deep water offshore engineering, traditional mooring system is not suitable and economic for the floating structure in more than 1000m water depth, such as the mooring length and total weight increased dramatically, layout difficulty, soared cost and installation fees. Instead of mooring system, the dynamic positioning system (DP) is widely used in deep water offshore engineering, because its positioning capacity is independent on the water depth. And the thruster-assisted mooring system benefits to improve the position capacity and protect mooring lines'safety.Dynamic positioning system composes of a series of propellers to provide thrust. The number of propellers is more than the controlled degrees of freedom, so it is an over-actuated system. Considering the propellers' thrust direction and response rate, only the surge, sway and yaw low frequency motions are controlled by DP. So the general DP controller is designed based on the low frequency model. And then, the control forces are allocated through the optimal method to each propeller. Due to the small water plate, the coupled effects between the semi-submersible platform vertical plane and horizontal plane DOF cannot be neglected, these should be considered to design the DP controller. Moreover, the large amplitude roll, pitch and heave motions will affect the operation safety and air-gap. So the platform vertical plane DOF dynamic performance should be gotten attention. In order to achieve a better dynamic performance, several control methods are adapted in this dissertation, and the man works are as follow:1 Analyze the semi-submersible platform's fully coupled dynamic response under the DP. The dynamic surface control based on the radial basis function is used to design the DP control, and the low frequency model only considering horizontal plane DOF is adopted in the controller design process. The control forces are imposed to the Cummins equation which is used to describe the platform fully coupled motion. And the dynamic states used in the controller are directly gained from the Cummins equation.2 For the over-actuated system, the control forces need to be allocated to each propeller through the optimal method. In the allocation process, the thrust-thrust interference due to the propeller wake needs to be considered avoid the thrust loss. In this dissertation, the fuel consumption is set to the optimization goal. And the dynamic setting feasible region and minimal thrust angle are designed to avoid the thrust loss. Considering the propeller wear and tear, the thrust angle change ?? and thrust change AT are set to be optimal parameters. The position capacity and the real time control force allocation are all given to demonstrate these strategies.3 Due to the small water plate, the semi-submersible platform roll, pitch and heave DOFs' stiffness and damping are small compared with the inertia force. Besides these, the coupled effects also exist by the hydrodynamic. So it is needed to design the DP controller based on the fully coupled model. Aiming for the Cummins equation, the retardation term can be considered to be the impulse function, and it can be replaced by the state space model. After this operation, the linear mathematical model is derived from the Cummins equation. L_ control is designed to reject persistent environment disturbance.4 To improve the position capacity and protect the mooring, the thrust assisted mooring system is analyzed. The mooring finite element model is built, and then the single mooring line's dynamic performance and the mooring platform are all simulated. The path follow control based on dynamic surface control combined with the Bang-Bang control is applied for the thrust assisted mooring system. The advantage is that the maxim mooring tension is limited, and the mooring system is adequately used in the permissible range to improve the economy.5 The tuned heave plate system is designed to improve the semi-submersible platform vertical plane dynamic performance. When the tuned ratio is set to be 1, the response amplitudes of the heave plates will be amplified. As a result, the amplitudes of the control forces offered by the heave plate system will be doubled. Like the traditional multi tuned mass dampers, the anti-roll control objective can be achieved through the four plates' cooperative works. And when the generators and the tuned heave plate system are installed together, it can provide power for the platform. The program is demonstrated through the numerical simulation and model experiment.