The underwater robot transmits the mechanical load, power, and signal to the submersible through a zero-buoyancy cable. There are two main types of cables for standard ROV use umbilical zero-buoyancy cables (from ship to ROV or umbilical cable management system TMS) and tethered cables (TMS to ROV). Electric power, signal, and strength conditions need to be considered when designing underwater rov cables.
1. Power requirements
The power is generally calculated in amperes. Most conductors have resistance to the flow of current and will cause a drop in voltage. Therefore, it is necessary to use a low-resistance material such as copper. In addition, the insulating layer on the conductor needs to be able to maintain electrical energy. ROV cables usually use thermoplastic materials such as polytetrafluoroethylene as the insulator.
The cable design should also consider the operating voltage. If the voltage is too high, the insulating layer will fail and the electrical energy will leave the conductor before reaching the target, which will cause a dangerous situation. Therefore, how to deal with voltage stress in the design of zero-buoyancy cables is very important.
2. Signal requirements
The signal requirement needs to be converted into attenuation loss. Both electrical and optical signals will attenuate as they pass through conductors and insulators. These attenuations vary according to the signal transmission medium and frequency. Copper conductors and thermoplastic materials are usually used to transmit electrical signals. Signal transmission lines often need to be shielded from electromagnetic interference (EMI) and radio frequency interference (RFI).
The common way is to separate the signal transmission cable from the power transmission conductor. There are two types of power transmission schemes: balanced and unbalanced. Balanced cables are twisted-pair cables, and unbalanced cables are coaxial cables. Other factors to consider include impedance, capacitance, and frequency. Signal transmission can also be carried out through multi-mode and single-mode fiber optic cables. No matter what kind of fiber optic cable needs to consider: attenuation, bandwidth, and wavelength.
3. Strength requirements
Usually need to support zero buoyancy cable weight, ROV, and additional load to cope with the dynamic load. The size of the cable will affect the load on the cable due to tension. Therefore, many variables need to be considered when selecting cable strength.
Steel is a commonly used material for umbilical cables, and usually includes a layer of carbon steel wire and a galvanized coating on the outside of the steel to prevent it from being corroded. The tensile strength, modulus, and abrasion resistance of this material will protect the cable from damage during use.
Synthetic fibers, such as Kevlar, can reduce cable weight. The strength of synthetic fibers requires a protective layer to increase abrasion resistance. Synthetic fibers are usually much more expensive than steel cables, but the difference in weight is quite noticeable. For ultra-deepwater, the use of synthetic fibers is the way to reach the necessary depth.
