Research On Delay-dependent H_∞Control For Offshore Steel Jacket Platforms

Offshore platform is the basic facilities of developing andusing the offshore resources. Due to located in the hostile ocean environ-ment,offshoreplatformareinevitablyinfluencedbyloadings,suchaswaveforces, earthquake, and ice. The loadings caused the vibration of the plat-form, resulting in the problems of the safety and structural damage. There-fore, the study of offshore platform vibration control system is particularlyimportant.In order to ensure the safety and reliability of offshore platform, theplatform system modeling and control problem of vibration reduction hasbeen one of the focus topic. In this paper, we study delay-dependent H∞control in accordance with the vibration system of the offshore platform.By considering the unmodeled dynamics and control delay, a nonlineartime-delay system of the simplified offshore platform is given, and we alsostudy the sliding mode H∞control for a vibration system of offshore steeljacket platforms. At the same time, the effect the properties of the offshoreplatform is shown through the simulation.Firstly, we study delay-dependent robust H∞control for an offshoresteel jacket platform subject to wave force. By taking into account thevariations of the structural natural frequency and damping ratio, and time-varying control delay, an uncertain time-delay dynamic modal of the off-shore platform is established first. Then a delay-dependent robust H∞con-trolscheme is proposed to attenuate thewave-inducedvibration oftheplat-form. A delay-dependent robust H∞controller is obtained in terms of thesolvabilityofanonlinearminimizationproblem. Finally,simulationresultsshow that though there exist time-varying control delay and parameter un-certainties in the system, the designed controller can effectively attenuate the vibration of the offshore platform and keep the stability of the system.Secondly, we study the sliding mode H∞control for a vibration systemof offshore steel jacket platforms subject to the external wave force. First,by taking the unmodeled dynamics and control delay into account, a non-linear time-delay system of the simplified offshore platform is established.Then, an active integral sliding mode control scheme is investigated, and adelay independent sliding mode H∞controller is proposed. The controllergaincanbeobtainedbynumericallysolvinganonlinearminimizationopti-mal problem. The effectiveness of the proposed control scheme is demon-strated by simulation results.