Research On Dynamic Positioning Control For Moored Ships

As human demand for resources is growing, Marine resources development technology has now become a hot research field. For platforms or ships at sea for a long time, under moderate sea state, how to ensure the safety of mooring lines and to save fuel consumption;under harsh sea state, how to ensure the safety of mooring lines and to achieve positioning.These are the difficulties of the research. The study object of this paper is the turret mooring floating production storage and offloading (Floating Production, Storage and Offloading Unit,FPSO). Dynamic positioning technology was studied for the single point of internal turret mooring FPSO.Firstly, the kinematics and dynamics mathematical models of the FPSO are established.Mathematical models of marine environmental interference are also established. The establishment of the North-East coordinate system and the body-fixed coordinate system is considered before modeling. Based on the kinematics and mechanics, the motion mathematical model of FPSO is established. The model validation experiments for FPSO mathematical model is carried out to prove that the model meet future research requirements.In addition, this paper describes in details about the modeling process of interference models under marine environment, like wind, wave and current, and gives the relevant simulation curves.Secondly, a single point mooring system of FPSO internal turret is analyzed and the mechanical model of the mooring system is established. Applying lumped mass method, finite element method and catenary method to analyze the mooring line forces, and mathematical model of the single mooring cable line based on the catenary equation is set up. Eventually,the mathematical model for turret mooring system is established. Under three sea states, the mooring positioning simulation experiments is carried out and relevant results are given.Finally, considering the FPSO fuel consumption and operational safety under extreme sea conditions, dynamic positioning controller of moored FPSO is designed based on structural reliability. In order to filter out high-frequency waves and measurement noise, the unscented Kalman filter is designed. The structural reliability index is derived. The PID controller and backstepping state feedback controller based on structural reliability are furthermore designed. In order to verify the controller is based on structural reliability, the simulation experiments are carried out. And the experimental results were compared with ordinary PID controller. The results verify that the former can effectively reduce the FPSO mooring line tension, ensure safety of mooring lines and achieve positioning, meanwhile saving thruster fuel consumption.In conclusion,on the basis of internal turret moored FPSO modeling,high energy consumption problem for deep-sea FPSO mooring positioning the safety of long-term offshore operations under harsh sea conditions should be considered. PID controller and state feedback backstepping controller based on mooring structural reliability are proposed. The control methods are verified to resolve mooring cables safety in harsh sea conditions and energy saving issues by simulation experiment. So, the subject of theoretical study and practical engineering applications,has a good theoretical reference value and application prospect.