Dynamics modelling of low-tension tethers for submerged remotely operated vehicles

Abstract

Continuing efforts to establish a more continual human presence in the deep ocean are requiring a drastic increase in the number of remotely operated vehicle (ROV) deployments to the ocean floor. Through real-time telemetry afforded by the ROV tether, a human operator can control the ROV, and the vehicle’s robotic manipulators, through haptic and visual interfaces. Given the need for a human presence in the control loop, and the lack of any wireless alternative, the tether is a necessity for ROV operation. While the tether generally maintains a slack or low-tension state, environmental forces that accumulate over the tether can significantly affect ROV motion and complicate the job of the human pilot. The focus of the work presented in this dissertation is the development of a low-tension tether dynamics model for application in the simulation of ROVs. Two methods for modelling the low-tension ROV tether are presented. Both developments include representations of bending and torsional stiffness and are based on a lumped mass approximation to the tether continuum, an approach that has been widely applied in the simulation of taut underwater cables. The first approach appends a bending model to the standard linear lumped mass formulation by applying a discretization scheme to only the bending terms of the governing motion equations. The resulting discrete bending effects are then inserted into the classical linear lumped mass model. Simulated results and an experimental validation showed that the revised linear model captures planar low-tension tether motion very well. In the second approach, a higher-order element geometry is applied that allows the full continuous equations of motion to be discretized producing a new lumped mass formulation. By using a higherorder geometric form for the tether element, a better approximation to the bending terms and a new representation of torsional effects are achieved. The improved bending model is shown to allow element size increases of 35% to 50% over the revised linear lumped mass method. While existing higher-order finite elements could be used to model the