Active Control Problems Of Jacket Offshore Platform

In recent years, sustainable development of national economy is limited by scarcity of energy resources. The demand for resources leads to further exploration to marine resources. Offshore platforms, as the base facility, are undergoing the environmental loads such as wind, waves, current, ice and earthquake etc, which may cause fatigue damage of platform components, decreasing the platform's reliability and cause discomfort of staff and the abnormal operation of equipment. Thus, it is of importance, both theoretically and practically, to study vibration control of platforms.Vibration suppression can be divided into two classes: passive control and active control. Passive control does not need external energy. It is to reduce structural vibration by means of energy dissipation of damping components in the structures. Passive control has the characteristics of easy implementation, low cost and simple structure. However, it is short of maneuverability in control. Its control effectiveness highly depends on the characteristics of external excitations and is only available for high-frequency vibration. In modern control systems, more and more attentions are being paid to the vibration suppression using active control strategies. Active control needs external energy input to the structures. It adjusts frequency and damping of the structures actively by inputting external energy to the structures. Active controllers have the advantages that the control performance is hardly affected by the characteristics of external excitations, and evidently superior to that of passive controllers. Therefore, vibration suppression of offshore platform using active controllers has been a very important research topic during the past decades.Based on the advanced technique of active vibration control, the active control of offshore platforms and some related issues are studied extensively in this dissertation. This research was funded by the National Natural Science Foundation of China (Grant Nos. 11072146 and 11002087), the Key Project of Ministry of Education of China (Grant No. 107043) and the Research Project of State Key Laboratory of Ocean Engineering of China (GKZD010807). Some achievements are acquired in both theory and application and the main research and achievements are as follows:(1) The vibration control of platforms is comprehensively reviewed. The research extent and contents of this dissertation are put forward.(2) By introducing the system identification for the offshore platform, this dissertation presents a modeling method of low-order state equation for the offshore platform based on the input-output data of the system, and avtive controller is designed using the low-order model. Firstly, based on the input-output data of the system, the method of Observer/Kalman filter identification (OKID) is used to identify the Markov parameters of the system. Then a low-order state-space model of the system is established by using the eigensystem realization algorithm (ERA). The Linear Quadratic Gaussian (LQG) controller is designed based on the low-order state-space model. Finally, the controller designed is introduced into the finite element model of the system to verify the effectiveness of the controller. Numerical results indicate that the OKID is effective in identifying the Markov parameters of the system, accurate low-order dynamical model can be obtained using the ERA, the LQG controller designed based on the low-order model can reduce the dynamical response of the platform effectively.(3) Load identification and active control of the offshore platform is studied using the discrete variable structure control (DVSC) method. The external load is identified using the disturbance force observer (DFO) and the controller is designed using the DVSC method with considering the identified external load. Simulation results indicate that the external load can be identified effectively by the DFO, and the structural vibration can be restrained by the variable structure controller.(4) The optimal position of actuators on the offshore platform is studied by using the controllable Grarm matrix as optimization criteria and the Particle Swarm Optimizer (PSO) as optimization algorithm. Numerical results demonstrate the effectiveness and feasibility of the presented method.