Torsional Mechanics in Dynamics Simulation of Low-tension Marine Tethers

摘要

A finite element model of a ROV tether is presented that makes use of a twisted cubic spline element form with torsional stiffness and a lumped mass approximation. In existing works, the torsional contributions to the low-frequency tether motion are disregarded. The primary objective of this work was to explore the role of torsional stiffness in the simulated low-tension tether dynamics during typical ROV maneuvers. Thus, a representation of the tether's torsional rigidity was included in the model to capture the complete dynamics of the low-tension tether. The motion of a small ROV was experimentally captured during operation in tank trials. Using the measured towpoint motion to drive the tether dynamics simulation, it was readily apparent that tether twist significantly affects the disturbances predicted at the ROV. For the purposes of virtual-reality ROV pilot training, the torsional effects are thus necessary to ensure fidelity of the model. The small elements necessary to capture the tether curvature induce high-frequency motions that limit the step size of any numerical integration schemes used. Two integration methods were implemented in attempts to eliminate these unwanted high-frequency components and garner the associated improvement in the model execution time: a variable step size explicit Runge-Kutta method and an implicit Generalized-α technique. The large number of small cable elements required for convergence created a practical limit on the execution speed of the Generalized-α approach.