Numerical simulation of motion response control of multibody floating systems
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Significant hydrodynamic interactions occur when transferring cargo at sea between a large cargo ship and a smaller amphibious. The motion response calculation between multiple bodies as well as the response control is a great concern for US Navy. The Deep Water Stable Craneship (DWSC) spar allows safe cargo transfer under a wide range of sea environments, because the DWSC spar is a very stable platform with a small waterplane. Further, the Dynamic Positioning (DP) system may be installed to minimize the motions of this spar. In this paper, the hydrodynamic coefficients of the multibody floating systems are firstly calculated using the hydrodynamic software WAMIT. The motion Response Amplitude Operators (RAOs) for the multiple bodies are obtained and compared with the RAOs of the single body. Then, a new scheme to calculate the motion responses of multibody floating systems is proposed, in which the equations of motion (EOM) of multibody floating system are re-organized into standard state-space format, using the constant coefficient approximation and the impulse response function (IRF) method. In the IRF method, use the trapezoidal rule to expand the convolution term. Further, the Ordinary Differential Equation (ODE) solvers in MATLAB can be directly employed to solve this state-space model. The ideal DP system has been incorporated in the motion response control of multibody floating system. Especially, the innovative representation of EOM in the state-space format makes it easy to apply various controllers. In this example, the modified Linear Quadratic Regulator (LQR) Method is utilized to calculate the feedback force due to the thrusters of DP system. Copyright 2013 by the International Society of Offshore and Polar Engineers (ISOPE).
