Dynamics modelling of low-tension tethers for submerged remotely operated vehicles
Date
2011-10-27
Authors
Buckham, Bradley Jason
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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 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 threedimensional
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.
Description
Keywords
ROVs, ROV