ACTIVE CONTROL OF OFFSHORE JACKET PLATFORMS

Abstract

The wave-induced dynamic force is one of the most important excitations to be dealt with in the design of offshore structures. In order to perform a reliable design of an offshore structure, it is important to obtain an exact evaluation of its dynamic response but also to examine the ways of reducing the response. This paper presents the response of offshore jacket platforms with an active tuned mass damper (TMD) under wave loading. The TMD is installed at the top of the structure and the control force is evaluated using the classical linear optimal control algorithm. The governing equations of motion of the jacket platform with an active TMD are derived and their solution in the frequency domain is presented. The uni-directional random wave loading is expressed by the Pierson-Muskowitz spectrum. The response of the jacket platform with active TMD is compared with the corresponding response without a control system in order to investigate the effectiveness of the active control system. It is observed that active TMD can reduce the response of the jacket platform significantly. In addition, a parametric study had also been conducted to investigate the effects of important parameters on the efficiency of the active TMD. The important parameters considered include: the ratio of TMD mass to the structure mass, damping of the TMD, tuning frequency of the TMD and flexibility of the jacket platform. It had been observed that the above parameters significantly influence the response of the jacket platform with active TMD.