Research On The Dynamic Positioning Capability Analysis Method And Control Algorithm

As offshore oil drilling and exploration are gradually going to deep sea,dynamic positioning system(DPS)plays an increasingly important role in the marine engineering field.Dynamic positioning system has the characteristics of unlimited by seabed topography,water depth and being able to quickly realize positioning and transferring,therefore,the benefits of its mobility and flexibility are very obvious.However,due to the complexity of the dynamic positioning system,for a newly designed dynamic positioning system,prior to use,it’s positioning performance should be essentially validated.As dynamic positioning technology becomes more and more mature,it has been put forward higher requirements on the efficiency of the means to verify DPS’s performance.Based on these backgrounds,this paper studied the means of simple static dynamic positioning capability analysis(DPCap),time-domain dynamic simulation and model test and dynamic station keeping capability analysis(DynCap)to verify the dynamic positioning system performance.Moreover,in order to design and improve the performance of dynamic positioning system,this paper proposed some new methods,including forbidden sector skipping strategy in thrust allocation and mitigating roll and pitch motion five degrees of freedom control methods.A software program which can perform a DPCap analysis was established as a tool to verify the primary performance of dynamic positioning system.In terms of static thrust allocation logic,a quadratic programming method is used to achieve thrust allocation accurately and quickly,which can handle the thrust system equipped with thrusters for tunnel thrusters,azimuth thrusters,main thruster with a combination of rudder.In order to validate the performance of the program,a failure mode DPCap analysis for a supply vessel was conducted.However,when considering the forbidden sectors for the azimuth thrusters in thrust allocation,there would be non-convex thrust domain,and when using a combination method to solve this problem,it will cause a decline in efficiency to solve.In this paper,a bisection search method has been proposed to investigate the maximum environmental forces.By use of this method,the number of steps needed to be investigated can be significantly reduced,hence the efficiency of the DPCap analysis can be improved.During the DPCap analysis,sometimes it is essential to compare the vessel’s positioning capability in different thruster configurations.To quantify the dynamic positioning capability of the ship,a synthesized criteria was proposed.The synthesized criteria was determined by overall mean positioning capability of the vessel and the stability of positioning capabilities in a given operation heading interval,it can also be applied to vessels with specific offshore operations in specific environmental conditions,tailoring their own synthesized criterion to comprehensively compare positioning capabilities of vessels in different cases.Based on the synthesized positioning capability criteria,a thrust sensitivity analysis was proposed.The study found that the analysis can obtain the highest sensitivity thruster.When considering to improve the synthesized positioning capability of the vessel,the most effective way is to increase the maximum available thrust of the most sensitive thruster.Therefore,the thrust sensitivity analysis can give appropriate reference to the thrust system designer.Based on the synthesized positioning capability criteria,a local optimization method of thruster configuration was also proposed.This optimization method is a supplementary step to optimize the design of the thrust system with the constraint of the thruster position,in order to improve the synthesized positioning capability of the vessel.Time-domain simulation is a relatively easy and reliable means to verify the performance of a newly design dynamic positioning vessel.This paper established a dynamic positioning timedomain simulation program.A model test for a semi-submersible was conducted to validate the performance of the program.It is proved that the dynamic positioning time domain simulation program can be not only used to validate the performance of the dynamic positioning system,but also used as a research tool for dynamic positioning system to study each module separately,and can further improve the performance of dynamic positioning system.Based on the dynamic positioning time domain simulation program,a DynCap analysis was further proposed.Because the time-domain simulation method takes into account the dynamic motion characteristics of marine structures,the dynamic characteristics of thrust systems,control dynamics method,etc.,so the resulting positioning capability is more realistic and adjacent for the real positioning capability of offshore structures.A bisection search method was proposed to search the maximum environmental forces,and the effects of this method was investigated.In addition,as the environmental conditions are constantly changing during the DynCap analysis,a controller gain database method was proposed to automatically assign a gain parameter to the controller.Numerical simulations demonstrated the feasibility of the bisection method and the controller gain database method.Based on the dynamic positioning time domain simulation program,a forbidden sector skipping strategy method was proposed.It can make the azimuth thruster skip the forbidden angle,and ensure that the thruster can achieve a more advantage effect after skipping.Time domain simulations for a barge with 6 thrusters were conducted to demonstrate the performance of the forbidden sector skipping strategy.The study found that the use of forbidden sector skipping strategy can give the azimuth thruster more feasible zones,and thrust allocation can give better allocation scheme among the thrusters.Moreover,this strategy can help the dynamic positioning system to save fuel and improve safety.Based on the dynamic positioning time domain simulation program,a novel roll and pitch control law dedicated to mitigating the roll and pitch motion was proposed.The roll and pitch proportional controller is firstly introduced into the DP control law.The performance of the newly proposed controller was demonstrated by time domain simulations for a dynamically positioned semi-submersible.The results show that the proposed controller can mitigate the roll and pitch motion significantly.However,this control method consumed more power compared with three degrees of freedom control method.Furthermore,the control method reduces the positioning accuracy of the horizontal plane motion,and hence the use of the roll and pitch angle proportional controller is a trade-off considering the requirement of the engineering project in practice.In summary,this paper focuses on the verification and improving of the DPS,and studies certain methods including DPCap analysis,time domain simulation,DynCap analysis,thrust allocation method and control methods.These studies provide useful guidances to verify and improve performance of DPS.