Study On Vibration Control Of Jacket Offshore Platforms Using Intelligent Adaptive Inverse Methods

With the development of ocean exploitation, more and more large-scale flexible offshore platforms are used for drilling operations and oil exploration. In a hostile ocean environment, offshore platforms usually experience various environmental loads such as wind, wave, flow and tidal loads, which will induce continuous vibration of the offshore platforms. Excessive vibrations will result in injury to human health, capacity degradation or even halt of the equipments and apparatus. Also, the vibrations can cause structural fatigue damages, decrease the reliability and threat the structure safety of the platforms. Hence, it is important to find effective control schemes for mitigating the vibrational response of the offshore platforms. When passive control methods are used to control vibrations in jacket offshore platforms, the cotrolled frequency band range is narrow. Active control methods using accurate mathematical models cannot achieve ideal purposes, because the jacket offshore platform is a system with complex structure, undetermined structural parameters and stochastic uncertain external load. Moreover, the delay of control signal transmission may cause instability and surge problems of the system. Therefore, in vibration control of jacket offshore platform, it is urgent to find methods of reducing the effects of the delay. The problems in vibration control can be solved effectively by intelligent methods. To better control the vibration of the jacket offshore platform under the wave and wind load, this paper applies intelligent adaptive inverse control methods. The main work includes:(1) Dynamic response calculation of jacket offshore platforms based on dynamic stiffness matrix methodIn this dissertation, we carried out the simulation of wave load and wind load that act on jacket offshore platforms via numerical methods. Dynamic stiffness matrix method is a quite effective method of dynamic structural analysis, which can increase the computation speed and precision. Therefore, we apply the dynamic stiffness matrix method to analysize dynamic response of jacket offshore platforms under wave and wind load. This has provided precondition for control vibration of jacket offshore platforms.(2) Based on dynamic stiffness matrix method, a model is established for vibration intelligent adaptive inverse control of jacket offshore platformCombining the advantages of dynamic stiffness matrix method, intelligent method and adaptive inverse control method, we proposed a new model for intelligent adaptive inverse control of jacket offshore platform based on dynamic stiffness matrix method. The adaptive inverse control method identifies the inverse model of the controlled system, then the inverse model is used as a controller that utilize feed-forward controller and disturbance eliminator controller to control the dynamic performance and disturbance of the controlled object. In this research, the intelligent method is adopted to identify the inverse model of the jacket offshore platform, because it has strong capability of handling uncertain and nonlinear information. Vibration responses of the offshore platform under random wave loads are calculated by the dynamic stiffness matrix method, the calculated responses are taken as the input signals of feed-forward controller, which controls the vibration responses correspondingly. The responses of feed-forward control are taken as the input signals of disturbance eliminator controller, which hinders the influence of the above factors on the control system performance.(3) Study on the intelligent methods and vibration intelligent adaptive inverse control of jacket offshore platform(a) Fuzzy neural network was used to identify the predictive inverse model of the jacket offshore platform in this paper. The inverse model is used as the adaptive inverse controller, which controls the vibration of jacket offshore platform under random wave and wind load. Simulation results show that our model is effective and practical.(b) Support vector machine was firstly applied to control vibration of the jacket offshore platform. Inverse control method based on a support vector machine can be adopted to estimate the inverse plant model. The inverse model is used as the adaptive inverse controller, which controls the vibration of jacket offshore platform. Simulation results indicate that the method is effective for control vibration responses of jacket offshore platform.(c) A novel rough neural network was proposed in this paper by combining rough set and neural network. In our study, the rough set theory is used to simplify network structure and increase the speed of training and calculation. Rough neural network is implemented in the active control vibration of jacket offshore platform. Simulation results show that the obtained rough neural network has advantages of fast calculation, better identification and generalization capacity, the adaptive inverse control method based on rough neural network is effective to control vibrations of jacket offshore platform.(d) Combining the grey situation decision theory with the neural network, we propose a new structure for construction of the neural network. Neural network based on quondam effect measure has weak capacity of identification and generalization, so we provide a new effect measure. Theoretical proofs and simulation results show the new measure is effective and reasonable. A novel grey neural network was proposed by combining neural network and grey situation decision theory using the new effect measure. The grey neural network has advantages such as clear structure, simple calculation. The grey neural network takes full advantages of grey situation decision theory and neural network, and it is used as the adaptive inverse controller, which controls vibrations of jacket offshore platform. Simulation results show that the adaptive inverse control method based on grey neural network can effectively decrease the harmful vibration of jacket offshore platform under the wave and wind load.(e) Combining the advantages of grey forecasting theory, dynamic stiffness matrix method, intelligent methods and adaptive inverse control method, we proposed a new model for intelligent adaptive inverse control of jacket offshore platform based on grey forecasting theory and dynamic stiffness matrix method. In addition, the problem of time delay during the control process was solved using the grey predictive control method. Simulation results show that our method is effective.(4) Design of vibration active mass damper control system of jacket offshore platformFurthermore, we designed an active mass damper control system, which provided a theoretical basis for physical realization of jacket offshore platform adaptive inverse control system based on intelligent methods.