Study On Vibration Control Of The Flexible Marine Riser System

With the gradual broadening of production field and continued increase of energy demand,more and more countries focus on the exploitation of offshore oil and gas resources.As the connection structure between the subsea well head and the floating platform on the ocean surface,flexible riser plays an indispensable role in the offshore oil and gas development process.Besides,it is also the most vulnerable component.In the special ocean environment,the riser is always influenced by various loads.Thus the vibration will occur,which can produce the premature fatigue problem,degrade the performance of the system and even lead to oil and gas leakage,resulting in serious environmental pollution.Thus,vibration control of riser system has become a problem to be solved imperatively in the process of oil and gas development.Considering the large length to diameter ratio,the flexible riser system is usually simplified as a typical Euler-Bernoulli beam model,and its dynamics is represented by the infinite-dimensional partial differential equation(PDE)and finite-dimensional ordinary differential equations(ODEs).This paper takes the flexible riser as the research object,and designs controller directly based on the original infinite dimension characteristics of the riser system.When there are unknown environmental disturbances,unknown parameters,unmeasurable states,backlash input constraint and output constraint,the vibration abatement of the riser system is deeply discussed by the integration between the finite dimensional backstepping technique,adaptive technique,robust control strategy,highgain observer,observer backstepping technique,barrier Lyapunov function,disturbance observer,signum function and boundary control method.The main research contents are as follows:1.The vibration abatement issue of the flexible marine riser system with unknown parameters under the condition of unknown environmental disturbances is discussed.For reducing the vibration displacement of the whole riser system,in view of the original partial differential equation-ordinary differential equations model,the finite dimensional backstepping technique is integrated with robust adaptive control and boundary control to design the controller,which is placed on the top boundary of the riser system.Moreover,parameter adaptive law is designed to compensate for uncertain parameters,and disturbance adaptation law is employed to eliminate the influence on system performance by estimating the upper bound of unknown boundary disturbance.Then the stability of closed-loop riser system is theoretically proved by mathematical derivation.Finally,numerical simulations based on Matlab platform verify the effectiveness of designed controller.2.The output feedback control issue of the flexible riser system is studied.In the process of designing the controller,the measurement noise from sensors in practical project and the noise amplification caused by the backward difference algorithm for obtaining the unmeasurable states are both taken into account.The observer backstepping technique is served for refactoring the state available for feedback which can not be measured directly.The high-gain observers are served for estimating unmeasurable states in the designed control law.Combining with boundary control method,an output feedback boundary control algorithm with disturbance observer is constructed.Subsequently,Lyapunov's direct method is used to prove the uniformly bounded stability of the closed-loop system.Finally,the finite difference method is used to carry out the numerical simulation and the results verify the effectiveness of the constructed control algorithm.3.The vibration control and output constraint issues of the flexible riser system are studied under the environmental loads.By combining boundary control method,Lyapunov's direct method,finite dimensional backstepping technique and barrier Lyapunov function,a barrier Lyapunov function-based boundary controller is constructed for ensuring that the vibration displacement of riser system is reduced and the output constraint is satisfied.Besides,a disturbance observer with barrier term is designed to avoid the chattering caused by the signum function under the precondition of dealing with the boundary disturbance.Then the stability of closed-loop riser system is analyzed based on the Lyapunov theory.Finally,the simulation results show the effectiveness of the designed boundary controller.4.Considering the mechanical structure of actuating equipment and production safety,the control design problem of flexible riser system under multiple uncertainties and restrictive conditions of unknown parameters,unknown disturbances,input backlash and output constraint is studied.The input backlash is decomposed into a desired control input and a bounded nonlinear error,the nonlinear error is added to the unknown timevarying disturbance,and the result is regarded as a new disturbance-like item.Then,a barrier Lyapunov function-based adaptive boundary control scheme is proposed through a combination of the barrier Lyapunov function,Lyapunov's direct method,adaptive technique and boundary control method.Besides,a disturbance adaptation law is proposed to estimate the unknown bound of the disturbance-like item.Then the uniform boundedness of state of closed-loop riser system is proved.Finally,the simulation results show that the designed control scheme can reduce riser's vibration displacement and handle the input and output constraint problems simultaneously.