Research On Robust Safe Control For Thruster-Assisted Position Mooring System Of FPSO

The oil and gas resources on land are decreasing year by year,and the abundant resources contained in the ocean have become the competitive focus of all countries in the world.When working in harsh sea conditions,the deep-sea platform is often not effectively guaranteed to be safe.Hence,the deep-sea development capacity of our country is seriously constrained.Floating Production Storage and Offloading vessels(FPSO)are very popular for the oil and gas industry of deep sea.Based on the background above,taking a deep-sea FPSO as the study object,this dissertation carries out deep research on the modeling of FPSO's buoy mooring system and the robust safe positioning control theory.The main research contents are as follows:(1)To lay a foundation for the following positioning control theory research,the kinematics and dynamics models of FPSO hull,the model of marine environment and the model of buoy mooring system are established respectively.Firstly,according to the kinematics and dynamics principle,the kinematics and dynamics models of the internal turret FPSO are established based on the North-East-Down and Body-fixed reference frames.Secondly,the marine environmental model is described by first order Markov process.Then,the catenary method is used to analyze the equilibrium forces of mooring line infinitesimal element and the mathematical model of buoyancy mooring system is established.Finally,simulations are conducted to verify the rationality of the established model and the necessity of introducing the dynamic positioning system to assist the mooring system.(2)Considering the safety of mooring lines under harsh sea conditions,the structural reliability-based backstepping and dynamic surface positioning controllers are designed respectively.Firstly,in order to facilitate the design of positioning controller,the structural reliability factor is given to quantify the safety degree of mooring line.Secondly,a matrix based on the structural reliability derivative is proposed and the structural reliability-based backstepping and dynamic surface positioning controllers are designed respectively based on the designed matrix.Then,the stability of the closed-loop systems is proved according to the Lyapunov stability theory.Finally,simulation experiments verify that the designed structural reliability-based positioning controllers can not only effectively assist the mooring system of the FPSO under harsh sea conditions,but also fully use the capability of mooring system on the premise of ensuring the safety of mooring lines.However,the safety performance of the structural reliability-based dynamic surface positioning controller is demonstrated better.(3)Considering the system uncertainty of FPSO,robust structural reliability-based positioning controllers are proposed based on the disturbance observer and the online constructive fuzzy approximator respectively.Firstly,the analysis shows that the system uncertainty of FPSO mainly includes the parameters uncertainty and the unknown time-varying marine environment disturbances.Then,a nonlinear disturbance observer is used to estimate the unknown time-varying disturbances;by combining the disturbance observer with the designed structural reliability-based dynamic surface positioning controller,a robust structural reliability-based positioning controller based on the disturbance observer is designed;the stability of the closed-loop system is proved by Lyapunov stability theory and the effectiveness of the proposed robust controller is verified by simulations.Finally,an online constructive fuzzy approximator is designed,which can not only estimate the system uncertainty of FPSO but also adjust the structure of the fuzzy system according to the current positions and velocity information of FPSO;based on online constructive fuzzy system,a robust structural reliability-based online constructive fuzzy(RSROCF)positioning controller is designed;the stability of the entire system is proved by Lyapunov stability theory and the effectiveness of the designed RSROCF positioning controller is verified by simulations.(4)Considering the input saturation of FPSO,a dynamic auxiliary system is designed to quantify the effect of input saturation on the system.Based on this proposed auxiliary system,a RSROCF anti-saturation positioning controller is designed,which can simultaneously handle the system uncertainty and input saturation of FPSO.Firstly,the input saturation of FPSO is analyzed theoretically,and the influence of input saturation on the FPSO positioning in harsh sea conditions is verified by simulations.Then,a dynamic auxiliary system which can quantify the effect of input saturation is designed;the structure reliability-based anti-saturation positioning controller is designed based on the proposed dynamic auxiliary system;the stability of the whole closed-loop system is proved by Lyapunov stability theory,and the validity of the structure reliability-based anti-saturation positioning controller is verified by simulations.Finally,according to the RSROCF positioning controller,a RSROCF anti-saturation positioning controller is designed,which can handle the system uncertainty and input saturation of FPSO simultaneously;the stability of the whole system is proved by Lyapunov stability theory,and the effectiveness of the proposed RSROCF anti-saturation positioning control algorithm is verified by simulation experiments.