Hybrid simulation and analysis of a dynamic positioning system



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A dynamic positioning system is the most efficient method of maintaining a stationary working platform in deep water areas of the oceans. A system of this type is presently used for deep sea exploration by means of coring. Other capabilities of a dynamically positioned vessel include accurate monitoring of missile trajectories over the oceans and deep sea mining operations. Very little operational data is presently available because of the limited number of dynamically positioned vessels which have been built. An accurate model of a dynamic positioning system is needed to determine parameters which are most important to the operation and design of such a system. The purpose of the research reported here is to develop a valid model and then to use this model to investigate important characteristics of the system. The hybrid computing facility at the University of Houston provides an excellent vehicle for simulation of a dynamic positioning system. The digital computer is used to model the digital control system while the analog computer models the dynamics of the vessel. A specific operating system, the Glomar Challenger, is used to compare model performance with an actual dynamic positioning system. Significant results are obtained for three specific problems. One of these involves finding a digital controller with a frequency response which is similar to a known analog controller. Obtaining the difference equations by means of the z-transform will not always give the desired frequency response. Two procedures are found which still utilize the powerful z- transform, but which give the correct frequency response. A second problem involves coupling between coordinate system axes in the digital controller. Although systems have been built which ignore this coupling, actual system operational data and simulation results indicate the desirability of considering the coupling in the control equations. A set of equations are developed which include the coupling without significantly affecting system complexity. It is important to have accurate data about vessel characteristics when a dynamic positioning system is being designed. The hybrid model is used to monitor vessel response for an accurate and for a less accurate model of the aerodynamic and hydrodynamic drag force curves. It is found that a single Fourier component representation will provide valid results for the design of a dynamic positioning system.