Page 234 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 9.2
Free-fly crane-launched MSROV.
9.1 Free-flying vehicle deployment techniques 223
the use of a davit or crane (Figure 9.2). Notice in both Figures 9.1 and 9.2 that there is a soft neu- trally buoyant “flying” tether between the surface and the vehicle. Larger vehicles can be directly deployed, but the risk of damage increases as the weight of the vehicle increases. This is due to the vehicle’s momentum building through vessel sway while the vehicle is suspended in air between the launch point on the deck and water insertion point in the splash zone (i.e., the vehicle becomes a “wrecker’s ball”). Directly deployed vehicles are more vulnerable to any currents prevalent from the surface to the vehicle’s operating depth. This is due to the neutrally buoyant flying tether being more subject to current drag than the heavy metal umbilical (as discussed later in this chapter).
9.1.2 Tether management system
Within the ROV industry, there is some confusion as to the exact definition of a TMS. Technically, the TMS is the subsea tether-handling mechanism (only) allowing the soft flying tether to be payed out or taken up from the junction between the clump/depressor weight and the tether. But by com- mon convention, the TMS is typically described as the entire subsea mechanism from the end of the umbilical (umbilical termination to the clump/depressor weight, cage or top hat) to the begin- ning of the tether. The vehicle handling system (subsea cage or top hat) houses the tether-handling mechanism as well as the vehicle itself and is launched with the vehicle either within the cage or attached to the top hat mechanism. With a clump/depressor weight system (without a TMS), the vehicle is launched first, and then the clump weight is launched once the vehicle’s tether is stretched (due to travel distance away from the launch platform).
Simplistically, the TMS can be attached to the so-called clump weight (Figure 9.3) or be part of a cage or top hat deployment system (Figure 9.4). The main function for the TMS is to manage a soft neutrally buoyant tether cable—the link from the TMS to the ROV for electrical power and sensors, including video and telemetry. The tether cable allows the ROV to make excursions at
(Courtesy SeaTrepid.)