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

摘要

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.{09}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 higher-order 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 ROV tether, it is shown that the choice of element form introduced in this second approach halves the number of variables required to define the tether state as compared to these existing techniques.; Applying the higher order lumped mass model to the simulation of a typical three-dimensional ROV maneuver, the importance of torsional effects in the discrete motion equations is evident. Inclusion of a non-zero torsional stiffness produced a resolution of significant tether motions and disturbances on a small ROV that, previous to this work, was not possible with existing cable models. In addition to providing improved bending effects and new torsional considerations, the higher order element was shown to be an important prerequisite for shorter simulation execution times. Small bends that develop during ROV operation require relatively small elements compared to other marine cable applications. The smaller elements, regardless of the integration technique adopted, constrain allowable time step sizes. By allowing for slightly longer element sizes, the higher order approach mitigates this negative characteristic of the low-tension tether dynamics. Execution times were reduced by up to 70% over the times incurred when using the element sizes necessary in the linear approach.