Dynamics modeling and control of a variable length remotely operated vehicle tether

A computational model is developed to simulate the dynamics of variable length tether in a tethered underwater vehicle system. The system is comprised of a surface ship and winch, a slender armoured cable that links the surface ship and the remotely operated vehicle (ROV), and the ROV itself. The cable is considered to be variable length to facilitate paying out and reeling in maneuvers. The motion equation for variable length tether is obtained from Newton's second law of motion for variable mass systems. Unlike many existing formulations, the model can treat the rapid deployment of tether accurately. The weighted residual finite element technique is applied to the continuous motion equation to obtain a system of spatially discrete nonlinear second order differential equations. Time domain simulation of simple payout maneuvers is used to validate the variable length tether model: conservation of internal strain energy is shown to hold over a deployment of the tethered system. The model is applied to the development of a dynamic positioning system. A decoupled controller incorporating a Dahlin compensator for positioning in the longitudinal plane and a PD controller for depth regulation, produces ship motion and winch activity to position an underwater point on the tether, called the control node. It is shown that the use of the control system to regulate the position of the control node brings about significant reduction in the disturbance force exerted by the tether on the ROV during a station-keeping maneuver.