Seismic vibration control of offshore jacket platforms using decentralized sliding mode algorithm

A semi active control algorithm, using MR dampers, is developed for reduction of responses of offshore jacket platforms induced by earthquake ground motions and the responses are obtained for both nonlinear and linearized drag forces. The MR dampers are placed at different levels of the jacket structure and the coupled structure-damper system is modelled in Simulink. Decentralized sliding mode controllers are designed to drive the response trajectories into the sliding surfaces and the command voltage to the MR dampers are generated with the help of clipped-optimal control algorithm. The results of the numerical simulations show that the designed semi active sliding mode controller is effective in reducing the structural vibrations caused by earthquake ground motions. As a result of linearization of the drag force, the effectiveness of the controllers is reduced and responses obtained are conservative in nature, i.e., the increase in controlled responses are more when linearized drag force is considered. The control system is found to be stable and robust; however, the positions and the number of MR dampers have quite significant impact on the performance of the controller.