Dynamic Response And Control Of A Multi-modular Very Large Floating Plamforms

A very large floating platform is composed of multiple floating modules.It can be used for deep sea resource development,tourism,biochemical processing,transportation,storage and mobile military base.It is a kind of forward-looking and high-end marine engineering equipment.The multi-modular floating platform has the characteristics of large structural scale,complex configuration,fluid-solid coupling,rigid-flexible coupling and strong nonlinearity.Because of the importance of the very large floating platform,scholars from various countries,especially for the United States and Japan,have done a lot of researches about the dynamic characteristics.However,the research on the dynamic stability control of the very large floating platforms is still in the blank.In this paper,a network method is innovatively used to build the mathematical model of a very large floating platform and the researches about their motion stability control are developed.The research begins with two-dimensional model towards three-dimensional model.The modules include the typical pontoon and semi-submersible types.The control methods contain the passive control based on the amplitude death mechanism,nonlinear active control based on the Backstepping method,optimal control and random control methods.The sea conditions include regular waves,irregular waves,random waves,unknown waves.Saturation constraints on the controllers are also considered to make the proposed control methods more suitable for engineering practices.Combining with control algorithms,the corresponding connector designs are proposed to stabilize the motions of modularized floating platforms in specified or all directions.The innovations of this paper are as follow.Firstly,the network method is introduced to model a very large floating platform with control.Secondly,passive and active control methods are developed for the motion control of multi-modular floating platforms,which greatly improves the stability of the floating platform.Thirdly,new connector designs for the multi-modular floating platform are proposed.The specific content of the paper is as follow.Build the two-dimensional mathematical model of a multi-modular floating platform based on the linear wave theory and network theory.Build the hydrodynamic model of each module,wave excitation model,anchor chain model,air spring connector model and then introduce a topological matrix to build up the two-dimensional model of a pontoon-type floating platform.Analyze the nonlinear network characteristics of the floating system,obtain the semi-analytical solution of responses by harmonic average method and then get the parameter boundary of the amplitude death state.In the parameter domain corresponding to amplitude death state,semi-control can be activated by tuning the stiffness of the air spring connector to minimize the vibration of the VLFS.Control all the motions of the two-dimensional multi-modular floating platform.The arrangements of the controllers are described in consideration of controllability.Based on the Lyapunov stability theorem,the backstepping method is used to control all the motions of the floating platform.In engineering applications,the output of the controllers cannot be infinite,and therefore a saturation constraint stabilizer is introduced into the control process to cope with the truncated boundaries.In the numerical simulations,the performance of the nonlinear control method is estimated in comparing with PID control method.This work is the first attempt to use nonlinear control method to stabilize the motions of a multi-modular floating platform.Considering when the floating platform operates in uncertain sea environments,a layout of thrusters is proposed for control.An optimal control algorithm is proposed by combining a wave load observer.In the numerical simulations,surge,sway and yaw motions of a multi-modular floating platform are controlled.The method allows the system to achieve smallest motions with minimal energy consumption.Aiming to control all the motions of a three-dimensional multi-modular floating platform,propeller layout scheme for three directions is proposed,along with its mathematical model.Combining the nonlinear control method with uncertain load observer and optimal algorithm,the outputs of the propellers are determined.Numerical simulations show that the control process can successfully suppress all the motions of the floating platform and the load estimator can also trace the unknown wave excitation.Propose a new design for connector.Combining with an annular rubber spring with a hydraulic actuator,the active connector can achieve the dual functions of connection and control.The novel connector as flexible element can be arbitrarily configured to form different connections in order to control the motions in specific directions.In this paper,aiming to weaken the surge,heave,pitch and yaw motions,a group of novel connectors is designed and the mathematical model is built.Based on the random vibration theory and the frequency domain analysis method,a random control method is developed.Comparing with flexible and hinged connectors,the active novel connector outperforms for suppressing the module motions and the connector loads.