Model Anlysis And Control Strategy Of Wave Compensation Platforms

In recent years,offshore wind energy has developed rapidly in our country.In order to obtain more favorable wind resources,the distance of the future offshore wind farms to shore will continue to increase.This practical application requirement brings greater challenges to the installation,operation and maintenance(O&M)of the wind farms.The main difficulties in far-offshore O&M are extreme wind and wave conditions,inclement weather,low visibility,which results in the traditional O&M hard to work(work-boats,helicopters and personnel basket,etc.).In order to overcome the above extreme environment and meet the requirements of heavy load and multi-degreeof-freedom ship motion,the Stewart platform is installed on the O&M vessel as a wave compensation platform,the top platform will not be not affected by the movement of the O&M vessel,and maintenance personnel and equipment can access the wind turbine through the top platform,then the O&M of offshore wind farms is simpler and safer,the time window for offshore operation can be widened.Since the wave compensation platform can ensure offshore operations safely and smoothly,it is of great significance to the development of ocean industry in our country.The working process of the Stewart platform installed on the O&M vessel is a wave compensation process.The Stewart platform works in the non-inertial frame,and the vessel motion input inevitably affects the dynamics process of the Stewart platform.This influence can be expressed as ship motion disturbance.This problem needs to be addressed by improving the dynamic characteristics of the Stewart platform.However,the development of offshore wind farms to the far sea and deep sea brings heavy load requirements on the wave compensation platform.These requirements are in contradiction with the dynamic characteristics of the Stewart platform,and it is difficult to guarantee both.Moreover,blindly raising the dynamic characteristics also leads to an increase in the structural size of the Stewart platform and an increase in the overturning moment on the O&M vessel,threatens to the safety of the O&M vessel.Since the dynamic characteristics of the Stewart platform is determined by the system natural frequency,and the better dynamic characteristics indicates the higher system natural frequency.Therefore,this dissertation aims to improve the natural frequency,takes the dexterity of the Stewart platform and mechanical structure as the constraints,optimizes the structural parameters of the Stewart platform and improves the dynamic characteristics of the Stewart platform while ensuring the compensation space.Based on the system frequency characteristic model,the main factor of the timevarying disturbance of the top platform is the ship motion velocity through the investigation on the mechanism of ship motion disturbance,a multi-degree-of-freedom velocity feedforward compensator is proposed,which alleviates the ship motion disturbance and improves the dynamic accuracy of the wave compensation platform.The wave compensation platform works under heavy load,and the parameter optimization can only improve the dynamic characteristics to a certain extent.The motion decoupling effect of the proposed velocity feedforward compensator is also limited by the dynamic characteristics of the Stewart platform.In order to ensure the dynamic accuracy of wave compensation,it is necessary to further improve the dynamic characteristics from the perspective of the control method.Based on the system model of the ship-mounted Stewart platform,an adaptive robust dual-loop control strategy is proposed to further improve the dynamic characteristics through dynamics compensation.The stability of the proposed control strategy is proved based on Lyapunov theory.The control performance of the traditional control method and the proposed control strategy is compared and analyzed.Finally,experimental system for the wave compensation platform is built,and the proposed multi-degree-of-freedom velocity feedforward compensator and the adaptive dual-loop control strategy are experimentally investigated.The comparative analysis of the experimental results verifies the effectiveness of the proposed control strategy.