Page 236 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

(a) (b)
FIGURE 9.5
Cage deployed ROV for both (a) OCROV and (b) MSROV.
9.1 Free-flying vehicle deployment techniques 225
directly above the work site, thus locating the center point of an operational circle at the clump weight. In short, the vehicle only needs to drag the tether length between the clump weight and the vehicle for operations on the bottom.
Cage deployed
Cages are used for small and large ROVs to protect the vehicle against abrasions and deploy- ment damage due to the instability of most vessels of opportunity while under way. When operating from a vessel with dynamic positioning (DP), the vessel’s thrusters are operating at all times while on DP requiring positive control of the tether from the surface to depth. For a free-flying ROV with neutrally buoyant tether from the surface to the vehicle, the tether naturally drifts with the water flow—possibly straight into the operating vessel’s thrusters! Cage deployment systems assist with the positive management of the tether. In Figure 9.5, a cage deployment system without a TMS is displayed. With this type system, the vehicle is overboarded through the splash zone enclosed within the cage with the tether hand tended (or winch tended) through an access guide at the rear of the cage. Once through the splash zone, the vehicle swims out of the cage with the tether controlled/guided through the cage. The benefit of a cage system without a TMS is its sim- plicity. With a cage enclosed TMS, many operators consider the complexity of that TMS similar to having a second ROV concurrently in the water.
Cages also function as a negatively buoyant anchor to overcome the drag imposed from the cross-section of the cable presented to the current (between the platform and the cage) at shallower depths (Figure 9.4). This allows the weight of the cage to fight the current instead of the vehicle fighting the current. The cage further provides room for a TMS to meter the softer tether in small amounts (as discussed below), thus lowering the risk of tether entanglement. For deployment
(Courtesy SeaTrepid.)