Combined Dynamic Positioning and Optimal Wave Frequency Motion Damping of Surface Effect Ship
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- Institutt for marin teknikk 
The offshore wind industry is requesting high annual accessibility to wind-turbines. The turbines are located increasingly further from the shore. Simultaneously, operations & maintenance costs are to be minimized, therefore innovation is needed for the turbine service vessels which transfer crew and equipment. Surface Effect Ships (SESs) are fast and fuel-efficient when sailing long distances. Active damping of vertical motions means that low motion levels can be achieved even for small vessels in high seas. However, safe interaction with fixed offshore installations necessitates automatic control of the horizontal vessel motions. Such control has never been implemented on a SES before, and was therefore investigated for the work of this thesis. We derived the necessary dynamics, describing both the horizontal- and vertical states of the plant. Since conventional bow thrusters are hard to fit in SESs we derived a model for the lateral thrust capabilities obtainable by controlling the direction of the out flow from the SES air cushion. To successfully simulate the derived plant for somewhat realistic conditions, the model was augmented to include the effects of environmental disturbances. The control problem was dual: While the main objective was to investigate the possibilities for dynamic positioning of the plant. Due to the fast dynamics of the air cushion actuators, we also wanted to check the potential for damping of horizontal and vertical, 1st order wave induced motions. The latter controller was derived by an augmentation and slight alteration of an already existing control scheme based on optimal control, while the former was a simple PID controller solely intended to prove the potential of dynamic positioning of the derived plant. Due to large levels of saturation and mutually inflicting control desires, much care was given in ensuring that the phases of the wave frequency motion damping control signals where tuned to minimize the degree of infliction. The simulations show strong performance of the controllers, while the derived model seems to provide accurate indications regarding the behaviour of the real plant. For moderate sea states, we obtained almost 80% damping of the heave motions while we reduced the wave frequency motions in sway by as much as 50%. The two wave frequency controllers was also run simultaneously, where the controller achieved a damping in sway and heave of 32- and 60%, respectively, for 0.5m high waves. The DP controller also performed well, and the vessel seemed to maintain position, by the means of water jets and airflow thrust, in 15m/s wind, 1m/s current and regular waves of 2m. We also performed simultaneous station keeping and vertical motion damping, which revealed a strong dependency between the airflow thrust demand and heave compensation capacity of the vessel, but still indicated that such simultaneous operation was indeed possible.