Page 154 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

142 CHAPTER 7 Power and Telemetry
FIGURE 7.1
They made it—Astronaut Jim Lovell signs co-author Bob Christ’s logbook in Punta Arenas, Chile, 1999.
7.1 Electrical considerations
The entire Apollo 13 team at NASA understood the power budget equation and balanced the power availability with the needs in order to achieve the objective of powering the necessary systems at the appropriate times to complete the mission. This is an excellent segue into a discussion on ROV power delivery as this is a very close metaphor for remote power and data delivery to any subsea vehicle through a tether. Note: this chapter assumes a general knowledge of electricity and electri- cal power delivery.
The following sections discuss specific issues and relationships regarding the tether, power, data, and connectors that bring it all together.
7.1.1 “So you wanna design an ROV—are you sure?”
Imagine a fire hose pumping water to douse a fire. With a given diameter hose, there is a nominal amount of water that can be pumped through that hose at a given pressure. To increase flow, the diam- eter of the hose must be increased (lower resistance), the pipe must be shortened (also lower resis- tance), or the water pressure at the source must be increased (increase pressure). The water requirement (the requirement), of course, depends upon the size of the fire (i.e., the mission driving the requirement). The direct correlation to an ROV system varies the length/size of the conductors in the umbilical/tether (resistance) to carry the electrical flow and the voltage (pressure) at the power source to push the power down the line to satisfy the vehicle power requirements (requirement).
As discussed in Chapter 6, an ROV system is a long series of trade-offs between cost, weight/ buoyancy, energy requirements, and materials. In this chapter, the various components in the power delivery system will be covered with a focus on the power delivered to the vehicle (as that is the power requirement for the system).
7.1.2 Power systems (general)
An ROV power system is requirements driven. The general rule of thumb is to start with the end requirement, and then build back toward the generator to size the power source. The default is to