Research On Hydrodynamic Interference Of Propulsion System And Thrust Allocation Of DP Ship

With the continuous exploration and development of deep sea in our country,marine vessels rely more and more on the dynamic positioning system and put forward higher requirements on their reliability and adaptability under various sea conditions.As an executive agency of dynamic positioning system,the propulsion system distributes thrust reasonably to each thruster according to instructions of the controller in order to resist the effect of external interference,keeping the vessel in the target position for dynamic positioning.Also the certain loss caused by the interference between adjacent thrusters results in deviation between actual output of the propulsion system and controller instructions,which makes vessels unable to achieve high-precision dynamic positioning.So it is very important to study the thruster interference.In this paper,based on the subject of "Large Equipment Motion Simulation System of Marine Engineering" issued by the Ministry of Industry and Information Technology,thrust loss caused by the thruster interference of Offshore Oil Engineering Ship is analyzed and calculated,and design of thrust allocation method considering the thrust loss is studied in depth.Taking the engineering ship as simulation object,three degrees of freedom kinematics,dynamics model and environmental load model of the ship are established.The simulation analysis of the ship's response to environmental disturbances in process of motion control is carried out,which lays foundation for simulation analysis of thrust allocation method.In view of the problem of interference between the propellers,a three-dimensional model of propeller is established.Based on CFD simulation technology of propeller hydrodynamic force,the thrust losses at different pitches,azimuths and speeds are calculated.The results show that the interference between the propellers decreases with the increase of the distance,and can be ignored when it reaches more than 27 D.What's more,the thrust loss is up to 75% when the azimuth of the propeller is [70°,110°] and [250°,290°].The azimuth constraint of thruster is set to [70°,110°] and [250°,290°].The SQP method is optimized based on azimuth constraint,maximum thrust conditions and weight matrix parameters,and an improved thrust allocation method is proposed based on the pseudo-inverse method.The feasibility,effectiveness and characteristics of above thrust allocations method are simulated and analyzed basing on the mathematical model of propulsion system which has been built.On this basis,the DPS model of engineering ship is improved by combining the mathematical model of ship motion,environmental force model and controller model.The reliability and timeliness of improved thrust allocation method in DPS are simulated and analyzed from three aspects,namely azimuth constraint,reset capability under intact mode of thruster and positioning capability under failure mode of thruster.Results show that pseudo-inverse method is more time-efficiency,but it needs to be improved because only single-objective optimization can be proceeded.Sequence quadratic programming method is more accurate,but it depends highly on in initial value,which needs to be simplified.Improved thrust allocation method can take into account the impact of thrust loss and allocates thrust efficiently,which can improve propulsion system efficiency and achieve high-precision dynamic positioning.