Research On Collision Avoidance Control Methods For Multiple Dynamic Positioning Ships

With the increasing complexity of the operational tasks at sea,cooperative operation is a common working mode for multiple Dynamic Positioning(DP)ships in modern marine engineering.Although multiple DP ships cooperation can improve work efficiency and perform complex tasks,the problem of collision avoidance isn't considered.Collision avoidance control is related to the safety of the operation,which is the key to the success of marine operation.Due to the contradiction between the objective of collision avoidance and other coordination objectives,coordinated collision avoidance becomes more complicated.In addition,hydrodynamic interference,the form of communication and the shape of the collision avoidance area could also affect the effect of collision avoidance.This paper focuses on the collision avoidance of dynamic positioning ships,and studies the collision avoidance control methods of multi dynamic positioning ships under different coordination tasks.The contents in this paper are presented as following:Firstly,to eliminate the phenomenon of "ship suction",a DP collision avoidance control model with interaction of hydrodynamic forces is established.On the basis of the mathematical model of single DP ship,the theoretical calculation process of the hydrodynamic interference between ships is studied based on the three-dimensional potential flow theory.In order to simplify the calculation process,a calculation model under the conditions of meeting and overtaking is proposed by combining the test data of the pool with the coefficient regression method.The calculated hydrodynamic interference is feeded to the model of a single ship,and then the calculation error of the hydrodynamic calculation is estimated.The DP collision avoidance control model with nonlinear second order form is obtained through model transformation.Secondly,based on the passive synchronization theory and the navigation function method,a collision avoidance control method of passive navigation function is proposed for multi DP ships formation under balanced communication topology.For the kinematic model of DP,a navigation function avoidance algorithm without local minimum is designed,and its convergence and control input boundedness are analyzed.Considering the nonlinear dynamic characteristics of the ship,the passive navigation function formation framework is designed,and a passive collision avoidance formation error function is proposed based on the decentralized navigation function,so that the DP ship can converge to the desired formation.Simulation results show that the proposed control algorithm can make the ship converge to the desired formation without collision.Thirdly,a continuous collision avoidance control law based on smooth step function is proposed to solve the problem of non-circular collision avoidance area for DP ships coordination around platform.The calculation model of the nearest distance between ellipse and circle is established.The relationship between the closest distance between ellipses and the closest distance between ellipses and circles is analyzed,and the calculation model of the nearest distance between DP ships with elliptical collision avoidance region is obtained.The collision avoidance function is constructed through the smooth step function,and the collision avoidance control law of each ship is obtained by using the properties of the smooth step function and the backstepping method.And the effectiveness of the proposed coordinated collision avoidance control algorithm in the elliptical collision avoidance area is verified by simulation.Finally,a decentralized NSB coordinated collision avoidance control method is proposed to solve the DP Ship search and rescue problem with unstable communication topology and multiple control targets switching in the operation process.Aiming at the priority invariant multi-agent system,a zero space projection based behavior control method is designed,and the stability conditions of the system are analyzed.In view of the fact that there is no stable communication in the process of search and rescue,a decentralized control framework,including the supervisory layer,action layer and control layer is proposed.The finite state machine,NSB coordination method and the integral sliding mode control method are synthesized to form a coordinated collision avoidance controller according to different stages of the search and rescue.And the effectiveness of the proposed decentralized control framework is verified by simulation.