Page 206 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

8.8 Underwater cable design 195
8.8 Underwater cable design
ROVs require a cable to handle both electrical and mechanical functions. The EM cable transfers the mechanical loads, power, and/or communications between the ship and the vehicle. It must be possible to spool the EM cable out and in over and around sheaves and drums, be directed by a cable lay system, and operate in the intended environment. Interconnect cables inside the vehicle transfer power and signal between components.
It is important to characterize the operating environment of the cable with respect to:
• Depth
• Duration
• Temperature
• Salinity
• Suspended particulate matter
• Water chemistry
• Anaerobic condition
• Current profile
Some advanced ROVs carry their own power sources and only require a communication link to the surface vessel through an expendable fiber-optic microcable.
Most ROVs therefore have three general categories of cable to consider: umbilical cable, tether cable, and interconnect cable. Umbilical and tether cables are EM cables, containing the strength, power, and signal components. Interconnect cables seldom carry mechanical load and have only power and signal components in their construction.
The diameter of the cable is the dominant factor in overall vehicle drag. Therefore, minimizing cable diameter is an important part of ROV design and operation.
8.8.1 Umbilical and tether cables
The umbilical cable connects the ship to the ROV or TMS, while the tether cable connects the TMS to the ROV. The umbilical cable is generally steel jacketed, while the tether cable uses syn- thetic fibers to maintain neutral buoyancy. Use of a low-density jacket may help offset the negative weight of copper wire.
Initial cable design considerations include: (i) power requirements, (ii) signal requirements, and (iii) strength and weight requirements.
The power and signal requirements are usually the most important considerations because the cable’s primary purpose is to transmit power to the ROV and return signals from it. However, the strength and weight are also important considerations. Therefore, a proper solution to designing an ROV cable requires a concurrent view at these multiple variables.
8.8.2 Power requirements
Transferring electrical power through a cable involves four factors:
• Voltage • Phase