Development and validation of a lumped-mass dynamics model of a deep-sea ROV system

A one-dimensional finite-element lumped-mass model of a vertically tethered caged ROV system subject to surface excitation is presented. Data acquired during normal operation at sea are used with a least-squares technique to estimate the coefficients required by the model. The model correctly predicts: (a) the motion of the cage and the tension in the tether at the ship; (b) the spectrum of cage acceleration in the wave-band; (c) the transfer function between the vertical motion of the ship and cage; and (d) the natural frequency of the system and its harmonics. A simple model of the wake of the cage was added to the model simulation and this reduced the error in the calculated motion and tension by almost a factor of two and brought the calculated transfer function within the 95% confidence interval of the measurements. By increasing ship motion slightly, the model accurately reproduced eight snap loads and their non-linear characteristics-a regularly spaced series of rapid increases in the records of the acceleration of the cage and tension at the ship-that occurred during the measurements used for validation. (C) 2000 Elsevier Science Ltd. All rights reserved.