The use of a measure of horsepower to describe an ROV’s capability has been used in the offshore hydraulic workclass industry to try to provide a metric to compare one ROV against another. It has never been a very good measure even for systems using like power distribution methods (electo/hydraulic) since the efficiency of thrusters varies so much from one machine to the next. To try to compare ROV’s using brushless DC thrusters to Hydraulic ROV’s using horsepower is even less meaningful.
The problem with using this method to compare is that it assumes similar efficiencies between the two. The fact is, using hydraulics to provide the means of driving thrusters is relatively in efficient. It is generally the preferred choice with work class vehicles (Shilling Robotics Quest ROV system being a notable exception and an example of an extremely efficient ROV) because using hydraulics is a more practical method of conduction energy in the rugged conditions of the offshore work environment.
Efficiency is of a lesser concern with the large workclass vehicles since they are able to carry the necessary Hydraulic Power Units (HPUs) and haul the large diameter umbilicals needed to carry the large amount of power. What they win in ease of use in reconfiguring with different tooling packages more then compensates for their lack of efficiency. With smaller inspection class and light intervention vehicles, the benefit of efficiency becomes more pronounced. By driving the ROV using brushless DC thrusters we are able to achieve energy efficiencies in excess of 100% of hydraulic systems. This means that we are able to use much smaller diameter umbilicals (resulting in the need for less thrust). In addition, by using High Voltage DC transmission rather then AC and using sophisticated DC/DC converters to step down the voltage, we are able to do away with heavy transformers on the ROV. The result of all this is a much smaller ROV volume and mass. Drag on an ROV increases roughly as a cube of its dimensions. i.e. double the Length, Width, Height of the ROV and you cube the drag requiring the larger ROV to have a cube of thrust to achieve the same maneuverability.
So, back to the question of horsepower “How do you compare capability of two drastically different vehicles?”
There is not one answer to this question though comparing thrust to surface area seems to us to be the closest and most informative way of performing a rule of thumb comparison. If you calculate the bollard pull of the ROV in each plane of freedom (vertical, longitudinal and lateral) and divide that area by the surface area of that vehicle, you will come up with a figure that can be roughly used to compare two vehicles.
We have prepared a graph comparing the following vehicles:
|Perry Slingsby||XLS||Perry Slingsby||Electro/hydraulic||One of most powerful workclass ROV’s on the market|
|Saab Seaeye||Tiger||Seaeye Tiger||Brushless DC||Industry standard offshore inspection class ROV|
|Seaeye/SeaView Systems||Raptor||SeaView Systems Raptor||Brushless DC||SeaView in-house developed modification to the highly efficient Seaeye Falcon DR|