Page 642 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

Vehicle power
The biggest question for the vehicle within the future subsea oilfield is the location of the ser- vice vehicle power source. And that is directly related to the enabling technology of battery power density. Placement of the power source folds into the following categories:
1. Surface power: Current ROV technology for larger vehicles has the vehicle sleeping at the surface (aboard the deck of a multiservice vessel (MSV), drilling rig, or production platform). Future vehicles will “sleep at the bottom” yet could be powered from the surface. The power is routed via a remote docking station powered from an umbilical to the controlling node or platform. This allows for unlimited power but retains some type of tether to the surface for electrical connection/power.
2. Onboard power: This is clearly the preferred method of power as it frees the vehicle of the parasitic drag and complexities of a tether. The problem remains one of power density to achieve the locomotion to the work site along with the tasking requirements.
3. Hybrid: There is a possible third solution whereby the vehicle can sleep at the bottom mated to a docking station where the battery is charged from the surface. The vehicle uses this onboard power (once undocked) to achieve the goals of its tasking.
With that preamble, the ultimate question to be addressed in this chapter is: “Where is ROV technology today and where will it be in the future?” This question will be answered by consider- ing the three methods of power and control available, that is, via a powered tether mode (with cop- per or fiber telemetry/control), fiber-optic tether (for remote telemetry without remote power) and tetherless operating mode.
23.1 Standard ROVs
Today’s standard ROVs, with power and communications down the tether or umbilical (or both), have reached the level of sophistication that the market required. When they first entered the oil patch in the 1970s and 1980s they were considered more of a nuisance to the divers (who ruled the waters) than an asset. That soon changed as the capability and reliability of the vehicles increased and the operating depth requirements went beyond diver depths. Once the oil companies understood the inevitable, they finally met the ROV manufacturers half way, actually more than half, and worked together to develop underwater equipment that could be installed, inspected, maintained, and repaired by an ROV.
There are no limits to the depths that an ROV can support. US Navy-funded vehicles broke the magic 20,000 ft (6096 m) barrier in 1990—first by the CURV III, which reached a depth of 20,106 ft (6128 m). This was followed less than a week later by the Advanced Tethered Vehicle (ATV), which reached a depth of 20,601 ft (6279 m). This record was short-lived when in 1995 Japan’s KAIKO ROV reached the deepest point in the ocean—the Challenger Deep in the Mariana Trench—setting a record that can only be tied at 35,800 ft (10,911.4 m). That tie—we will call it a tie—happened when the Nereus vehicle reached the bottom of the Challenger Deep in 2009, recording a depth of 35,773 ft (10,903 m). The Nereus will be discussed more in the next section.
23.1 Standard ROVs 643