Most ROVs require a cable to transfer the mechanical loads, power, and communications to and from the vehicle. Alternatives to this would be vehicles under autonomous or semi-autonomous control (such as an acoustic link), or vehicles with expendable cables such as fiber optic microcables. The vehicle size, weight and operating depth, as well as the vehicle motors, subsystems, and payload, all combine to determine the ROV’s cable design. For the standard ROV, which uses an electro-mechanical cable, there are two general categories for cable: umbilical cable (ship to the ROV or tether management system (TMS)) and tether cable (TMS to the ROV). Initial cable design considerations include, power, signal and strength requirements

The power requirements translate into amperes. For each ampere it is necessary to have enough material to conduct the power to the far end. Most conductors have resistance to electrical energy flow, which creates a voltage drop. Therefore, it is necessary to use material with as low a resistance as possible such as copper, which is the most common.

Another consideration is insulation on the conductors to contain the electrical energy. ROV cables usually use thermoplastic materials for insulation such as TEFLON™. However, because thermoplastics soften or melt with heat, it is important to know both the operating environment and the current requirements.

The operating voltage is another consideration in the cable design. It is important to limit voltage stress on the insulation. If this is too high it can cause the insulation to fail and the electrical energy to exit the conductor before it reaches its objective, which can create a hazardous condition. Therefore, it is important for the cable design to address the insulation voltage stress. Also, a separate conductor for an emergency ground is common as a safeguard in case there is a breakdown in the insulation.

The signal requirements translate to attenuation losses. The signal, whether electrical or optical, attenuates through both the conductor and the insulator. This loss varies with both the signal transmission media and frequency.

Signal transmission can be either analog or digital, and either electrical or optical. Copper conductors with thermoplastic insulation are also common for electrical signals. Signal transmission wires frequently require a shield from electro-magnetic interference (EMI) and radio-frequency interference (RFI). Also, it is common to group the signal transmission wires separate from the power conductors. There are both balanced and unbalanced electrical transmission schemes, and the system determines this requirement. Typical balanced lines are twisted-pairs, and unbalanced lines are coaxial. Other parameters to consider for signal transmission include impedance, capacitance and frequency.

You can also transmit signals over multi-mode and single mode optical fibers.

Some parameters to consider in any type fiber optic are: attenuation, bandwidth and wavelength

The strength-member provides the mechanical link to the ROV. It usually has to support the cable weight, the ROV and any additional payload, and handle any dynamic-loads. Also, the cable size can influence the load on the cable due to drag. Therefore, there are many variables to consider when choosing the cable strength.

Steel is the most common strength-member material for umbilical cables; usually a carbon steel wire with a galvanizing coating on the outside to protect the steel from corrosion. This material’s tensile strength, modulus, and abrasion-resistance protect the cable from damage in service.

Synthetic fibers, such as KEVLAR™ from DuPont, can reduce weight. Synthetic fibers are frequently necessary in tether cables, and also in umbilical cables for deep-water systems. Synthetic fiber strength-members usually require an overall jacket for abrasion resistance. A synthetic strength-member is generally more expensive than steel, but the weight difference can be significant. For ultra deep systems, using synthetic fiber is the only way to get to the necessary depth.

Overall, the design of an ROV umbilical or tether is critical to the successful operation of the system. However, the technology has advanced to the point that it is indeed a design problem and excellent cables are available for virtually any application, whether for a low cost ROV inspecting a dam or the KAIKO, searching the bottom of the Mariana Trench.