| The unavoidable motion in six degrees of freedom of semi-submersible platform can be induced by the action of environmental disturbances such as winds,waves and currents,the platform can't generate restoring forces in horizontal direction,which will lead to the offset in surge.sway and yaw direction,and then the platform's security and working performance will be damaged,so it is very important to design the positioning system of platform.Mooring positioning and dynamic positioning have been considered as the most widely used positioning system,however,mooring positioning system has low positioning accuracy,and dynamic positioning system has poor economy,so an automatic mooring positioning system is presented which is base on mooring line switching control.The new positioning system not only improves the positioning precision,but also guarantees the economy,meanwhile,it can optimize the tension distribution of the mooring lines to avoid their breakage caused by uneven tension distribution,and the security of the positioning system is improved.In order to study the dynamic characteristics of automatic mooring positioning semi-submersible platform,the platform's low frequency motion model is firstly analyzed in this paper.To simplify the design of the control system and facilitate analysis of the plant,the system equations are linearized in a condition of low speed,and the platform low frequency linear equations in state form is presented.Mooring line damping has direct influence in predicting the motion response of semi-submersible platform and mooring system,so the quasi-static analysis method is used to estimate mooring line damping,and the damping coefficients from the calculation are added to the damping matrix of the platform motion equation in order to calculate the motion responses of platform more accurately.Through the theoretical analysis of marine disturbance nature that the platform suffered,the wind force?current force?first order and second order wave force model are established,and the numerical simulation aiming at the target platform is executed to calculate the disturbing forces under different sea conditions.The necessity of dynamic analysis in mooring lines is explained,and the elastic rod theory which ignores flexural rigidity is used to establish the mooring lines' motion equation.Nonlinear factors are considered in elastic rod theory,and the elastic rod theory has rigorous deducing process.It is hard to solve the nonlinear differential equation which deduced form mooring line control equation,so the finite element method is used to transform the differential equation into the weak form,and the numerical integral method is used to solve the equation to analyze the dynamic performances of the mooring line.Several integration methods are used to solve the integral equation to obtain the dynamic characteristics of mooring lines.Several common numerical integration methods are introduced,improved generalized-a algorithm which has the characteristics of second order accuracy?controllable numerical damping and unconditional stability is chosen to conduct the dynamic analysis of mooring line after analyzing the advantages and disadvantages of each integration method.Furthermore,indirect time-domain method is applied to conduct coupled analysis between platform and mooring lines,hydrodynamic parameters of platform are first solved by three dimensional potential flow theory in frequency domain,then hydrodynamic parameters are transformed into time domain and the time domain coupled equation is established to solve motion responses of platform and mooring lines.The characteristics of the automatic mooring positioning system are explained including the input time delay and uncertainty,and the essential drawbacks of LQR control are shown.H? control strategy with range input delay and the guaranteed cost control strategy with theory of linear matrix inequality,robust control and Lyapunov stability are developed to calculate control forces that can guarantee the platform in desired position under environmental disturbances.The non-zero condition of the lower bound in time delay is considered in Hx control strategy with range input delay,and the lower bound information is also considered during the construction of Lyapunov function.In the process of designing the guaranteed cost control strategy with input delay,the form based on the error vector is extended to realize the set-point positioning of the platform.The characteristics of the automatic mooring positioning system are explained including the input time delay and uncertainty,and the essential drawbacks of LQR control are shown.Two different control strategies with theory of linear matrix inequality,robust control and Lyapunov stability is developed to calculate control forces that can guarantee the platform in desired position under environmental disturbances.The designed controllers have solved the uncertainty problem induced by uncertain hydrodynamic parameters and the input delay problem caused by mooring lines' switching.The tension distribution is optimized after the target position of the platform is guaranteed.The uniform tension distribution is achieved by defining the objective function which has the minimum quadratic sum of the tension differences,and the position of the target platform is satisfied by the constraint conditions.Referring to the theory of 'Memetic,method,a hybrid algorithm which takes both advantages of genetic algorithm and simulated annealing algorithm is proposed to distribute resultant forces which guarantees the platform is in desired position to mooring lines evenly,and the optimization results are regarded as switching threshold of mooring lines.In the end,semi-physical simulation experiment is established,and the experimental results show that the positioning scheme is reasonable and the control strategy has good performance. |
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