Dynamics of a vertically tethered marine platform

Date

2017-10-26

Authors

Driscoll, Frederick Ralph

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Abstract

Rapid and high resolution motion and tension measurements were made of a typical vertically tethered system, a caged deep-sea ROV, while it operated at sea. The system is essentially one-dimensional because only the vertical motions of the underwater platform and the ship were coherent, while horizontal motions of the platform were weak and incoherent with any component of motion of the ship. The natural frequency of the system is found to be within the frequency band of ship motion for most of its operating range and the platform response is weakly non-linear. This results in a vertical acceleration of the platform that is up to 2.2 times larger than that of the ship. Large vertical excursions of the ship produce momentary slack in the tether near the platform. At the instant prior to re-tensioning, the tether and platform are moving apart and upon re-tensioning, the inertia of the platform imparts a large strain—a snap load—in the tether. The resulting strain wave propagates to the surface with the characteristic speed (3870 ms⁻¹) of tensile waves in the tether. An extremely repeatable pattern of echoes is detectable at each end. Two models, a continuous (closed form) non-dimensional frequency domain model and a discrete finite-element time domain model are developed to represent vertically tethered systems subject to surface excitation. Both models accurately predicts the measured response, with slightly better accuracy in the discrete version. The continuous model shows that the response is governed by only two non-dimensional parameters. The continuous model is invalid for slack tether and inherently unable to predict snap loads. By slightly increasing the ship motion, the discrete model accurately reproduces the observed snap loads and their characteristics. Discrepancies between the predicted and measured response of the platform bring into question the concepts of a constant drag coefficient and a constant added mass for oscillatory flow around the platform. By adding a simple wake model to account for flow history, the error in the calculated platform motion and tension in the tether were reduced by almost a factor of 2. Passive ship-mounted and cage-mounted heave compensation systems were investigated with a view to reducing the cage motion and tension in the tether. Both systems were found to be effective and for reasonable parameters, they can reduce the motion of the cage and the tension in the tether by a factor of 2. Addition of either compensation system reduced the natural frequency of the system and extended the operating sea state of a cage ROV system. However, the characteristics of the compensation systems must be carefully chosen or the operational problems will be exacerbated. In particular, the natural frequency of higher modes may enter the waveband for deeper operating depths. During extreme sea states, the cage compensated system eliminated all snap loads.

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Keywords

Remote submersibles, Remote sensing

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