Control of Dynamic Responses of Towerlike Offshore Structures in Waves

Abstract

This paper presents a preliminary attempt to control the dynamic response of a towerlike offshore structure subjected to regular waves. The structures are modeled in two ways. One is a vertical rigid pipe supported at the lower end by a pin joint. The other is a vertical flexible pipe fixed at the lower end. The formulation of the optimal control shows that the control consists of a feedback control and a feedforward control based on the disturbance. In this research, two types of feedforward control are employed apart from the optimality. One is to compensate the entire wave forces acting on the structure. The other is on-off control to compensate the principal Fourier component of the wave forces by using the three states of the thruster, forward, stop and backward. The displacement and deformation of the structures were measured by an ultrasonic measurement system. The surface elevation was measured by a capacitance-type wave height meter. These data were sampled and processed by a 16-bit microprocessor, and the thrust was applied by a propeller-type thruster. The performance of the control was satisfactory, and the responses of the controlled structure were reduced to about 30 percent of those of the uncontrolled system.