Page 165 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

Further consideration to the makeup of cables and connectors is discussed in detail in Chapter 8.
7.1.7 Power source
7.1.7.1 Electrical power
The ROV system is made up of a series of compromises. The type of power delivered to the sub- mersible is a trade-off of cost, safety, and needed performance. Direct current (DC) allows for lower cost and weight of tether components; since inductance noise is minimal, it allows for less shielding of conductors in close proximity to the power line. Alternating current (AC) allows longer transmission distances than that available to DC while using smaller conductors.
Most operators of ROV systems specify a power source independent of the vessel of opportu- nity. The reason for this separation of supply is that the time the vessel is in most need of its power is normally the time when the submersible is most in need of its power. Submersible systems attempting to escape a hazardous bottom condition have been known to lose power at critical moments while the vessel is making power-draining repositioning thrusts on its engines. This can cause entanglement of the vehicle. With a separate power source, submersible maneuvering power is separated from the power needs of the vessel.
With the advent of the lightweight microgenerators for use with small ROVs, the portability of the ROV system is significantly enhanced. Some operators prefer usage of the battery/inverter com- bination for systems requiring AC power. Also, some smaller systems use only DC as their power source. Either method should have the power source capable of supplying uninterrupted power to the system at its maximum sustained current draw for the length of the anticipated operation.
7.1.7.1.1 AC versus DC considerations
Electrical power transmission techniques are an important factor in ROV system design due to their effect upon component weights, electrical noise propagation, and safety considerations. The DC method of power transmission predominates the observation-class ROV systems due to the lack of need for shielding of components, weight considerations for portability, and the expense of power transmission devices. On larger ROV systems, AC power is used for the umbilical due to its long power transmission distances, which are typically not seen by the smaller systems. AC power in close proximity to video conductors could cause electrical noise to propagate due to electromotive force (emf) conditions. The shielding necessary to lower this emf effect could cause the otherwise neutrally buoyant tether to become negatively buoyant, resulting in vehicle control problems. And the heavy and bulky transformers are a nuisance during travel to a job site or as checked baggage aboard aircraft.
Larger work-class systems normally use AC power transmission from the surface down the umbilical to the TMS (the umbilical normally uses fiber-optic transmission, lowering the emf noise through the video) since the umbilical does not require neutral buoyancy. At the TMS (if provided), the AC power is then rectified to DC to run the DC-powered components through the neutrally buoyant tether that runs between the TMS and the vehicle.
7.1.7.1.2 Data throughput
The wider the data pipeline from the submersible to the surface, the greater the ability for the vehi- cle to deliver to the operator the necessary job-specific data as well as sensory feedback needed to
7.1 Electrical considerations 153