Research On Time Domain Simulation And Model Test Of Dynamic Positioning System Assisted By Mooring System

Positioning is needed for the offshore structures under the working conditions in deep sea.The traditional positioning method is mooring positioning.Depending on restoring forces provided by mooring lines to balance disturbing force excited by environmental conditions,the platform is fixed in a certain range.With development into the deep sea,increase of chain length and strength of mooring system leads to increase of weight.Mooring system layout is complicated and expensive,which limits its application more and more.Therefore,dynamic positioning system,which is not affected by water depth,has been developed rapidly.And in recent years,the dynamic positioning system assisted by mooring system which combinatesadvantages of mooring system and dynamic positioning system has been developed rapidly.There are some advantages and disadvantages for two positioning methods which are widely used at present: mooring system and dynamic positioning system.In shallow water,it can achieve excellent positioning performance with good localization accuracy using mooring system;in deep sea or harsh environmental conditions,mooring system is unavailable or the positioning accuracy was significantly decreased.The accuracy of dynamic positioning system is high,not affected by water depth,and system response speed is fast.But its disadvantage is huge energy consumption,composed by complicated devices with not very high system stability.Therefore,the combinated use of mooring system and dynamic positioning system is chosen in engineering industry,and named the dynamic positioning system assisted by mooring system,It can not only guarantee the positioning operation of deep sea platforms in deep water,but also reduce the power consumption,energy saving and emission reduction,and greatly improve the economic performance.In design of semi-submersible drilling platform dynamic positioning system assisted by mooring system,the general use of multi-point catenary anchor leg mooring system and dynamic positioning system are combinated.Mooring system resistsmain part of environmental loads and ensuresthat marine structures can always maintain at a predetermined position and keep heading while dynamic positioning system provides thrust and torque to resist another part of environmental loads,controllinglocation and heading with a high precision.Dynamic positioning system can effectively prevent mooring line exceeding fracture strength limit,and ensure the safety of mooring system.In operation conditions,dynamic positioning system can effectively reduce fatigue and wear of mooring line,extend service life of the mooring system.In the first chapter,history of dynamic positioning system assisted by mooring system is introduced,and three positioning methods includingmooring system,dynamic positioning system and dynamic positioning system assisted by mooring system are researched and given an overview of domestic and international research situations.In the second chapter,basic theoretical formulas of the dynamic positioning system assisted by mooring system is constructed and method of numerical simulation of time-domain coupled calculation of combinated effects of mooring system and dynamic positioning is presented.The theory of dynamic positioning system assisted by mooring system time domain simulation are studied including the establishment of the equations of motion,mooring system numerical model,environment loads calculation,control system and thrust allocation algorithm.The specific steps of solving the equations of motion are:semi submersible platform's motion is divided into two parts which include low frequency motions and wave frequency motions,calculating hydrodynamic coefficients including the added mass,damping coefficient,RAO(response amplitude operator)and QTF(Quadratic Transfer Function)etc.,taking into account mooring forces of mooring system,andthe equations of motion are solvedfinally.The wave frequency motion is filtered and the low frequency movementsaretaken into control circuit of the system,and the controller calculates total thrusts,and thrust allocation algorithm will be assigned to each thruster,and the semi-submersible platform is getting positioned in the closed loop control.The time domain simulation programwith self intellectual property rights has been developed which is coupled calculated,taking inaccount dynamic impacts of mooring line forces,the motions and dynamic positioning system.The lumped mass method establishes the simplified mooring system model,constructs the control equations for the movement of the nodes on the mooring chains,and the finite difference method is used to solve the equations,and completethe time domain simulation ofthedynamic positioning system assisted by mooring system.In the third chapter,cases of time domain simulation calculation of dynamic positioning system assisted by mooring system are carried out based on theoretical formula which is construted in the second chapter.The casesare concerning with one semi-submersible platform with intact mooring system and intact dynamic positioning thruster working modes which are divided into 180 degrees,135 degrees and 90 degrees in the three degree directions of wind,current and wave in 1500 meters water depth.The dynamic positioning system assisted by mooring system simulation and analysis is based on the coupled calculation of the lumped mass method for mooring system and dynamic positioning time domain simulation program.In the fourth chapter,the basic theory of model test are studied,test equipments are designed,model making is finished,mooring system model tests anddynamic positioning model tests are carried out separately for use of calibration.And then the model tests of dynamic positioning system assisted by mooring system of semi-submersible drilling platformare carried out to verify the time domain simulation method.All model tests are based on similarity theory which includes three aspects: geometry similarity,motion similarity and dynamic similarity,and carried out in ocean engineering basin.Firstly,the scale ratio is chosen to be 50 according to simulated wind,wave and current generation in basin,dynamic position system simulation,mooring system simulation etc.Because of basin dimension limit of the simulated water depth,equivalent truncation method is used to simulate the mooring system.Comparing to actual mooring system,dynamic response of truncated mooring system will be different,but horizontal stiffness will be adjusted to remain the same.The model should be adjusted according to the weight of the model,the center of gravity and inertia parameters in simulative regulation,to ensure the similarity of the model mass and the mass distribution which will directly affect the motion and forces of floating structure model in wind,wave and current.Model test content includes environmental load measurement tests,hydrostatic decay tests,and mooring system model tests and dynamic positioning model tests.Every mode of experiments,respectively,is adjusted for different environmental parameters,control mode and positioning position.The mooring system model tests and dynamic positioning model tests are carried out just as a calibration for the model tests of dynamic positioning system assisted by mooring system.Model tests of the semi-submersible platform with the dynamic positioning system assisted by mooring system are carried out in various working conditions.The semi submersible platform,dynamic positioning system assisted by mooring system,thrust system and connection mechanisms of each part are designed and simulated together in 1500 meters water depth with intact mooring system and intact dynamic positioning thruster working modes.The time domain simulation is used to compare the numerical results with model test results.The data are including the average positioning localization;drift radius and propeller power consumption.The reliability of program system with self intellectual property rights is verifiedIn the fifth chapter,the set point positioning problem which is important to dynamic positioning system assisted by mooring system isstudied.Using of dynamic positioning system assisted by mooring system,positioning location is mainly chosen and set by the dynamic positioning system operator.If the location is not set properly,it will cause a lot of problemsincluding precision and power consumption.The location of the balance position without environmental force is defined as the zero point.If the environmental load is not zero,the semi-submersible platform will be moved to one external enviromental load balance point.In the case of non-zero environmental load,if the distance from the point to the origin is close to the equilibrium position,the thruster system resists some part of the environmental loads,and the mooring system resists other part of the environmental loads.This means that mooring system does not resist all the environmental loads which will cause the utilization rate of mooring system not to be used maximized,and dynamic positioning system consuming some extra energy is used for positioning which shouldbe avoided.In the sixth chapter,the whole paper and the prospects for future researches are summarized.To sum up,fully coupled time domain simulation method for dynamic positioning system assisted by mooring system of the deepwater semi-submersible platform in wind,current and waves is presented in this paper,and validated by model tests,and the optimization problem of the set point of positioning is studied.