Research On The Stability Of Unmanned Underwater Vehicle During Mooring Lurking Procedure

Unmanned Underwater Vehicle(UUV)lurking on seabed offers a potentially new pattern in marine resources development and utilization and is earnestly expected by numerous scholars.According to the lurking strategy,these UUVs can be divided into two categories,lurking by supporting bar or mooring mechanism,each of these lurking strategies has its own advantages and defects,for instance,lurking by supporting bar requires high flatness of the lurking location,has poor sediment resistance but also guarantees high stability during the process of resident,On the contrary the mooring resident strategy has poor lurking stability and lurking location accuracy,has low requirements of the seabed flatness and doesn't need to consider the sediment burial phenomenon.In order to extend the application of mooring lurking strategy,a UUV's two-point mooring lurking strategy was proposed,based on a literature review of the present theories and technologies,the key technologies of the two-point mooring lurking strategy,the mathematical model of the whole mooring system,UUV's stability analysis during mooring procedure and the mooring attitudes' regulate strategy were investigated in this dissertation and the focus of the work can be summarized as the following aspects:1.Research on the key technologies of UUV's two-point mooring lurking strategy(1)The steady-state analysis method was applied to obtain the effects of various parameters to system's mooring attitudes and tension distribution along the cable,such as the elastic modulus,the drag coefficient and the density of the cable,based on the analysis results,material of the cable which connects the UUV and anchor was selected;(2)In addition,the mutual influence amongst the length of catenary cable,the UUV's mooring depth and negative buoyant was simulated under two ultimate mooring conditions,taking the simulated results into comprehensive consideration,an optimum length of entire cable,the vehicle's negative buoyancy and the specific type of the anchor were selected to guarantee security of the mooring system,under the premise of avoid the case of anchor dragging and prevent the collision between the UUV and seabed;(3)On the basis of these previous work,issues about the cable and anchor fix during UUV's cruise process,anchor and cable release when the UUV reside,cable release length adjustment when regulating UUV's lurking attitude and cable separation when the vehicle restart sailing were studied to ensure UUV lurking on the seabed by this strategy could autonomously achieve all functions,then a three-dimensional virtual prototype was established.2.Mathematical model of UUV's two-point mooring lurking system(1)The translational and rotational motion equations of cable element were deduced,take factors like extendibility and usability into consideration,the continuous cable was discrete as a series of differential element to facilitate the application of lumped mass method to substitute a series of ordinary differential equations of motion for the continuous equation;(2)A viscoelastic physical model which is more approximate with the actual properties of the ocean floor was established,then the cable-seabed interaction was added into the cable's mathematical model;(3)Improved the discretization method of the underwater cable,the previous discretization method is not suitable to describe the bending moment and torque along the cable,a large number of discrete elements both in time and space domain are needed to obtain accurate results,which brings a considerable amount of computation and increases the calculation truncation error and rounding error,a series of non-linear elements,which was approximated by cubic spline interpolation method were constructed to replace the linear element in lumped mass method,then the Galerkin's method was selected to establish the cable's weighted residual equations to obtain cable's ranslational and rotational motion results;(4)A 6 degree of freedom mathematical model of the lurk UUV,which contains the variable buoyancy system function,auxiliary thruster force and underwater cable's pulling effect was developed,the kinematic equation of anchor was also proposed;(5)The Mathematical models of UUV,the cable and the anchor were coupled together through the dynamic and kinematic boundary conditions.3.Stability analysis of UUV's two-point mooring lurking system(1)The dynamic response,such as centroid offset dimension of the UUV,the variation of the UUV's attitudes,the tension along the cable and the anchor dragging distance,from drop anchor to stable lurking stage were simulated to analyze UUV's stability when the angle between UUV axis and flow direction is changing;(2)The influence of cable's tangential drag coefficient,normal drag coefficient and the density,the space between the UUV's barycenter and buoyant center,the residual velocity of the UUV to UUV's stability were also simulated,the analysis above will provide meaningful references to UUV's mooring lurking system designing.4.Research on attitude regulation of UUV's two-point mooring lurking system(1)Three practical and feasible adjustment strategies were proposed to make up the potential risk of the mooring lurking procedure,namely auxiliary thruster regulation scheme,the rudder angle regulation scheme,the cable length regulation scheme,the effect and efficiency of these strategies were compared;(2)The auxiliary thruster and the cable length regulation mechanism were rounded up to restrict UUV's pitching angle in allowable range,avoid the occurrence of dragging anchor and grounding phenomenon,this adjustment strategy brings a wider application range for the mooring lurking system designed in this dissertation.