Page 646 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

runs about $2.25/yard ($2.06/m) for a 0.055 inch (1.38 mm) micro-cable spooled for use. If the fiber is expendable the costs mount up. Also, if a method is not employed to recover the fiber after it is expended, then the result is an entanglement hazard if future operations are planned for that area. If the fiber is a reinforced cable, then the vehicle can be operated like a standard ROV and the fiber recovered onto a winch, thus keeping it from becoming a hazard. Should the communica- tion link become severed, the vehicle will require some type of fail-safe auto-homing or recovery system for the vehicle to be reliably retrieved.
For the smaller vehicles, with less onboard energy, their operational time/area will be limited. However, the inspection requirements for offshore structures should be able to keep a fleet of inspection vehicles very busy. A benefit of an expendable fiber is the freedom that the vehicle has to maneuver, which can allow it to conduct inspections in more confined spaces.
One of the best examples of the application of an expendable fiber used by an ROV is the Nereus vehicle (Figure 23.4) developed by Woods Hole Oceanographic Institution (WHOI). In May 2009, the Nereus vehicle reached a depth of 35,773 ft (10,903 m) in the Challenger Deep of the Mariana Trench. The 26 hour dive required a descent of 8.5 hours and resulted in nearly 11 hours on the bottom. The vehicle’s high-bandwidth tether provided the link for video and com- mand/control as it surveyed the ocean floor. The Nereus is a unique vehicle in that it can operate in either the ROV or AUV mode. When the bottom operations are completed, the vehicle severs its optical tether and returns autonomously to the surface for recovery.
The Nereus vehicle, with its ROV/AUV capabilities, provides the perfect segue for the book’s final section that includes AUVs and the emerging hybrid vehicles.
FIGURE 23.3
Bluefin’s swappable lithium polymer battery.
23.2 Fiber-optic linked ROVs 647
(Courtesy Bluefin Robotics.)