Page 148 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

136 CHAPTER 6 Thrusters
today have become extremely reliable. Such reliability is a critical aspect of ROV operations since any vehicle downtime that prevents the user’s equipment from getting back on line can be extremely costly. When the vehicle has to be repaired, the fewer parts necessary for the functioning of the vehicle, the faster they can be replaced/installed.
One of the selling points by the all-electric vehicle manufacturers is that their electric thrusters have fewer parts and a simpler overall system. A vehicle with hydraulic thrusters has substantially more parts including the hydraulic power system, compensators, valve packages, and (potentially) leaky lines that could land one in trouble with environmental regulators. With a hydraulically pow- ered vehicle, the electricity being supplied down the umbilical has to be converted to hydraulic power with the consequent loss of energy due to the conversion efficiency. Exacerbating this are the additional losses in the hydraulic lines and components. However, the conversion at the prime mover (e.g., a hydraulic pump) is only done once and the hydraulic power is connected directly to whichever component requires it. For an all-electric vehicle, a prime mover is required at each point of use and the electric actuators are usually larger than their hydraulic counterparts. Therefore, this can also lead to a heavier and bulkier system.
On the other hand, the electrical components will not necessarily be ruined should there be a failure in the system. The high-power hydraulic systems on WCROVs are worked hard and should they not be maintained adequately could experience a pump failure—with the subsequent contami- nation and damage to the entire hydraulic system. Even with environmentally acceptable hydraulic fluids, a leak in the system would not be politically correct. The use of seawater hydraulics has been on the engineer’s shelf for some time and may see a role in the future.
With the power required to run tools for construction, installation, and field R&M work reach- ing 75-plus horsepower, having a single prime mover may be a weight savings. If the electric vehi- cle has to have a hydraulically powered skid added to support the tooling, then a lot of the electric vehicle benefit would be lost in the heavy-duty WCROVs.
Regardless of the system trade-offs, as efficiency goes down at the working end of the umbilical, the same overall thrust and work system output is still required at the thrusters and tools. The conversion losses from electrical to mechanical must be compensated for. Therefore, the top- side power system will have to increase and possibly the size of the umbilical and tether, which eventually impact the overall spread.
One may not think that the diameter of the umbilical or tether has that much of an impact on the system design. But as discussed earlier, the drag on the umbilical/tether is proportional to the projected area of that same umbilical/tether. As an example, consider the umbilical for Schilling’s earlier all-electric Quest ROV. As a comparison, the Quest’s umbilical diameter is 1.06 in (27 mm) as compared to their present UHD ROV, with an umbilical diameter of 1.56 in. (39.7 mm). The projected area of the umbilical, and thus the drag, increases by about 50%. But the cross-sectional area of the umbilical, and thus the umbilical volume for a given length, increases by over 100%, which one can consider (assuming a similar effect on the weight) reflects the increase in the weight of the umbilical. Reportedly, this reduction in the diameter of the umbilical reduced the total Quest system weight by 20,000 lb (9066 kg). The ROV will not be pulling the umbilical around if it has a heavy TMS at the end. But the same factors apply to the tether between the TMS and the vehicle. For the smaller vehicles without a TMS, the entire tether in the water will see increased drag as the diameter increases.