Research On The Motion Control Methods For Dynamic Positioning Of Sandglass-type FDPSO

The Floating,Drilling,Production.Storage and Offloading unit(FDPSO)is a new type of equipment for deepwater oilfield development,which integrates drilling,production,storage and offloading modules,Among them,although the cylindrical FDPSO floating body overcomes the shortcomings of ship-type floating body such as sensitivity to wave direction,it still has some performance limitations,such as large heave motion under wave action.In view of the above problems,a new sandglass-type floating body with excellent hydrodynamic performance has been proposed.On the basis of sandglass-type floating body,the sandglass-type FDPSO owns strong adaptability to extreme sea conditions and provides favorable support for for oil and gas exploration in ultra-shallow water,deep sea and ultra-deep sea.When the sandglass-type FDPSO operates in deep water and ultra-deep water with complex marine environment-the drilling process has high requirements for the positioning accuracy of the floating body.Therefore,it is necessary to adopt the dynamic positioning technology with high positioning accuracy and without limitation of working depth for motion control.Among them,the positioning accuracy and fuel consumption are the key issues that limit the application of dynamic positioning system.Based on the shape characteristics of the sandglass-type FDPSO floating boy,in this thesis,the motion control methods are studied in depth for improving the dynamic positioning accuracy and reducing fuel consumption.The main work of this thesis can be concluded as:(1)Study on the motion performance and fundamental motion control method for the dynamic positioning sandglass-type FDPSO.Firstly,based on the physical model pool test,it is found that the pitch natural frequency of sandglass-type FDPSO floating body is small.which is close to the natural frequency of surge motion with dynamic positioning system.Then,the coupled surge-heave-pitch motion model is established.From the model,it is found that the dynamic positioning sandglass-type FDPSO floating body has obvious coupled surge-pitch motion response.Furthermore,from the perspective of controlling the initial stability to weaken the coupling effect,a surge controller considering the pitch restoring stiffness(Linear PID(Proportion-Integration-Differentiation)control method)is studied,which will serve as the foundation for the control method in the dynamic positioning system.(2)Study on the motion control method for the dynamic positioning sandglass-type FDPSO and establish the adaptive fuzzy PID control method based on piecewise design.Firstly,based on the surge controller considering the pitch restoring stiffness,the piecewise PID control method is proposed which considers the position and directionality of surge motion.Moreover,by introducing the fuzzy control method,an adaptive fuzzy PID control method with the surge proportional control term as the output(Single output fuzzy PID control method)and an adaptive fuzzy PID control method with dual outputs of both surge and pitch proportional control terms(Dual output fuzzy PID control method)are designed.Among the above four control methods,each of them meets the requirement of the positioning accuracy.However,in terms of the fuel consumption,the Dual output fuzzy PID control method has the minimum energy consumption,the Single output fuzzy PID control method has the medium energy consumption while the traditional linear PID control method is the most energy-consuming.(3)Study on the average pitch problem and the related dynamic positioning control method for the sandglass-type FDPSO.The theoretical studies indicate that when the wind load acts on the sandglass-type FDPSO,the wind heeling moment will make the floating body incline in the pitch degree,that is.leads to the average pitch problem.The current dynamic positioning system,which only supplies the horizontal control force cannot solve the average pitch problem.In order to cope with the problem,this paper proposes and studies the control method with wind force and wind moment feedforward.Besides,considering the functional requirements for eliminating the average pitch problem,two kinds of novel DP thrusters,namely,the orthogonal two-direction azimuth thruster and all-direction thruster,are proposed.Furthermore,the thrust allocation algorithm considering wind moment feedforward control is formed.The results show that compared with the traditional azimuth thruster and retractable azimuth thruster,two new dynamic positioning thrusters proposed in this thesis can not only solve the large mean pitch angle problem of the sandglass-type FDPSO under wind heeling moment,but also consume low fuel consumption.Among them,the novel all-direction thruster requires minimal fuel consumption and has the best energy-efficient performance and the orthogonal two-direction azimuth thruster has the second highest energy consumption.(4)Study on the unstable yaw motion of the dynamic positioning sandglass-type FDPSO.The deviation of the installation position of the dynamic positioning thruster will cause the controlled thrust force in surge degree of freedom(DOF)to deviate from the center of gravity.thereby forming the excitation of the yaw motion.Firstly,based on the installation deviation limit of the dynamic positioning thruster group allowed by the specification,the 1.5-DOF nonlinear yaw motion equation of the dynamic positioning sandglass-type FDPSO floating body is established.Through investigating the yaw motion performance under controlled thrust force in surge degree of freedom,it is found that shape of the revolving body of the sandglass-type floater makes its damping small.As a result,the excitation moment which is caused by the deviation of the installation position of the dynamic positioning thruster,may result in the unstable yaw motion and cause large yaw motion response.Then,the multi-scale method,the LP(Lindstedt-Poincare)method and the Runge-Kutta numerical method are used to discuss the yaw motion responses of the optimal heading mode and the heading control mode for the sandglass-type FDPSO floating body.The main factors are discussed.By comparing the two positioning modes,it can be found that when the incident direction of wind,wave and current is relatively stable,the thrust required by the optimal heading mode is smaller than that of the heading control mode.However,the yaw angle of the floating body varies greatly with the former mode.Thus when the positioning requirement for the heading motion is high,the heading control mode should be adopted.