| Résumé | The use of tethers for surface or subsea intervention, operations or other marine work is an important component in offshore and marine engineering. One of the first usages of such a structural element for marine applications is in the deployment and retrieval of tethered marine subsea bodies, vehicles or systems. For the purpose of this study the subsea body is considered a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The motion of the deployed portion of the tether is considered together with the drum rotation. Stretch in both deployed and underdeployed parts are examined and described. The hydrodynamic effects are considered on both the deployed portion of the tether and tethered mass which are assumed submerged in otherwise still water. The free surface effect is neglected together with the friction on the drum surface, however the drag, buoyancy and added mass of both cable and subsea body are included. The motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. Kane's formalism is used to write the relevant equations of motion which are expressed in matrix form, amenable for algorithm development. The resulting nonlinear system of ordinary differential equations are developed and solved numerically for an example of an almost neutrally buoyant marine tether system. The numerical results and their analysis for the retrieval and deployment of the system are presented. |
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