Optimal Vibration Control For Nonlinear Offshore Platform Systems

Vibration control of offshore platform has important practical significance. By one side, the vibration of offshore platform worsens working conditions quality of the staff, by the other side, it also exacerbated the platform mechanical damage and fatigue damage and lower the platform security. Continuous vibration of offshore platforms may also cause to capsize. Offshore platform accidents have occurred at home and abroad for many times, causing major economic losses and adverse social impacts. Therefore, how to reduce platform vibration and improve the reliability and safety of the platform has become a serious problem.Compared to land-based structures, steel jacket offshore platform is a complex pile-soil-water structure. Both its structure and its environment have its own characteristics. Such as the nonlinear dynamic characteristics of pile-soil interaction and fluid-structure interaction have drawn more and more interesting of researchers. However, for vibration control of offshore platforms, the current study is concentrated in linear systems, nonlinear systems for offshore platforms vibration is studied relatively little.The dissertation first reviews the history of the development in optimal control for nonlinear systems and the vibration control of offshore platforms. The latest research tendency and the main methods are also introduced. The major studies are shown as follows.1. Optimal vibration of jacket offshore platforms subjected by irregular waves is studied. A simplified controlled system of offshore platform is employed and an average performance index with exponential decay rate is chosen. Optimal control law with exponential decay rate for offshore platform subjected to irregular wave forces is designed. The simulation results have demonstrated the effectiveness of the control law.2. The vibration semi-active control problem of offshore jacket platforms with semi-active tuned mass damper (SATMD) is studied. Nonlinear self-excited hydrodynamic force effecting on the structure is taken into consideration, a single degree of freedom (SDOF) nonlinear dynamical model of offshore platforms affected by the wave loading disturbances is established. Based on sensitivity approach (SA), an approximate optimal vibration controller is developed for the offshore structures under wave loading for the first time. Numerical simulations illustrate the effectiveness of the proposed controller.3. The vibration control problem of offshore jacket platforms is studied. The model of lumped masses offshore platforms affected by the wave load disturbances with nonlinear interactions of the nonlinear self-excited hydrodynamic force is established. Based on Successive approximation approach, we propose a feedforward and feedback vibration control scheme for the nonlinear offshore jacket platforms. Numerical simulation results illustrate the effectiveness of the proposed approach.4. The vibration control problem of offshore jacket platforms with SATMD is studied. The linearized Morison equation is employed to estimate the wave loads. A coupled multiple-degrees-of-freedom (MDOF) nonlinear dynamical model of offshore structure is established. Based on optimal control theory, an optimal sliding manifold is proposed and a vibration controller is developed for the offshore structures under wave loading for the first time. The controller performance is judged by simulations.5. The vibration control problem of offshore jacket platforms is studied. The model of offshore platforms with nonlinear interactions of the pile-soil affected by the wave load disturbances is established. The wave loads are determined by using the Morison equation and are considered as disturbances with known dynamic characteristics but unknown initial conditions. Based on SAA we propose an approximate optimal vibration control scheme for the nonlinear systems of the offshore jacket platforms. Numerical simulations illustrate the effectiveness of the proposed approach.