As the human race has not evolved to live or work in the underwater environment, conducting operations subsea has always proved difficult. This is mainly due to the low levels of oxygen and the high pressures.

One could argue that it is simpler for humans to operate in space than subsea, as one only has to create a suitable atmosphere at a differential pressure of one bar, whereas in the subsea environment, the differential pressures are often hundreds of bar. Creating a subsea working environment is therefore very expensive and carries high risk. In shallow waters, up to 150m, divers can operate for limited time periods, or with specialised gas mixtures and use of compression chambers. However, at greater depths, it is not possible for routine operation outside a pressure chamber or submarine.

The use of remote operated vehicles (ROVs) increases the time and depth at which subsea operations can be undertaken, and they are being used to conduct increasingly complex tasks. There are, however, several aspects which make operating ROVs costly.

Communication with the undersea vehicle is difficult. WiFi and Bluetooth communication, which we now take for granted, cannot be used in water. Acoustic and light-based data transmission can be used underwater, but data transfer rates are very low, typically measured in bits per second as opposed to the megabits per second that are common in typical WiFi communication. These issues necessitate wire-based communications and are one of the main reasons why underwater vehicles are tethered. However, it should be noted that high bandwidth, wire-based communication over long umbilicals is not without complications. Normal ethernet connections deteriorate rapidly when umbilical lengths exceed 100m. Specialised communication equipment is therefore required. The use of fibre optics is increasingly being used, but has proved unreliable in the harsh offshore environment.

The use of Autonomous Underwater Vehicles (AUV) has developed as a way of reducing the requirement for underwater communication. However, AUVs require a clearly defined work scope and/or some kind of onboard intelligence to be effective and in an operational environment.

Power requirements are also a limiting factor in the underwater vehicle. Operating underwater vehicles in even limited sea currents requires a lot of power to hold the vehicle stable. Another draw on subsea power consumption is the requirement for lighting. Light attenuates very quickly with distance, so artificial lighting is usually required. The use of high-efficiency LED lighting has reduced the power requirements. The development and use of sonar has also improved subsea ‘visibility’.

Transmitting large amounts of electrical current through an umbilical results in excessive voltage drop. In all but the smallest ROVs the operating voltage is increased to reduce the electrical current requirement. Typically, small ROVs (less than 400 mm wide) operate at voltages below 75V, while larger vehicles operate at voltages ranging from 300V to thousands of volts. Obviously, working with these increased voltages in a marine environment introduces significant risk and needs to be appropriately managed. The use of batteries is becoming increasingly popular, particularly in smaller and autonomous underwater vehicles. However, a trade-off exists between the size of the battery and the time of the underwater mission. The use of high power density lithium batteries also introduces an additional safety risk as they are highly unstable when damaged. Transport of batteries has also become more difficult with the introduction of additional legislation.
The depth that the vehicle is designed to operate at also has a big influence on the size of the underwater vehicle. Pressure increases with depth, and with each 10 m of additional depth, an additional bar of pressure must be resisted. In coastal waters at say 100 m depth, the pressure is 10 bar (just higher than the typical pressure of most domestic water supplies) and on the seabed away from the continental shelf, the depth is typically around 3000 m which is approximately 300 bar (the pressure of typical hydraulic power system).

As depth increases, the size of the pressure vessels housing the components that need to be protected from these high pressures increases. Correspondingly, the amount of buoyancy needed to support this additional weight, to make the underwater vehicle neutrally buoyant also increases. In turn, this increases the power requirements and hence the size of the umbilical and/or batteries. Clearly a design compromise! This compromise can be mitigated by defining clear operational requirements, reducing the size of pressure vessels by using microelectronics, and using equipment that is designed to operate at these pressures.

Reliability can be problematic, especially since it is often not possible to include redundancy owing to the size constraints. The marine environment is harsh. Besides the high loads imposed by adverse weather conditions, corrosion, growth of marine organisms, and abrasive nature of sediment all contribute to poor reliability of subsea tools. This can sometimes be mitigated by regular preventative maintenance. However, the complex nature of these vehicles makes maintenance difficult. Determining the underlying cause of failures is difficult and time-consuming. However, with careful disassembly, inspection and testing, faulty electromechanical, or electrical components can be identified. Failures are typically related to i) leakage or partial failure of seals, ii) corrosion due to a breakdown of conformal coatings, insulation of wiring and galvanic attack of dissimilar materials, iii) contamination by sediments and organism growth and iv) overload related to higher than design loads being applied in adverse conditions.

Although there are nuances associated with these specialised vehicles, the underlying modes of failure are typical of many components in the marine environment, and the skill set required to investigate the modes of failure on remote vehicles is similar to those employed on these components.