Page 134 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

122 CHAPTER 6 Thrusters
• What size thrusters, taking into consideration the efficiency of the overall system, are needed to move the vehicle at the necessary speed above the stated environmental conditions?
• What is the total power requirement input to the electric/hydraulic power system?
• What is the size of the umbilical/tether to provide the required power?
Now, the lucky engineer will reach this point and exclaim “voila`” because the estimates for cable and vehicle drag were perfect and the size of the vehicle is sufficient to meet all of the required operational conditions. Unfortunately, this is usually not the case. The size of the cable and/or the vehicle will increase, and thus the necessary buoyancy and then drag, which drives the power required, which requires increased thrust output, which requires (get the picture?).
Once the initial design is finished, that is about the time that the sponsor or supervisor steps in and says, “I’d like to increase the operating current from one to two knots.” No, problem, correct? Double the current, then double the thrust. Right? Wrong!
As discussed in Section 3.5.3, the drag (D) on the vehicle is defined by: D51=2σAV2Cd
where V is the velocity of the vehicle.
Thus, the drag (and the required thrust) is proportional to the square of the velocity. So, that
simple doubling of the velocity actually requires four times the thrust! And do not forget the cable drag, which is also proportional to the square of the velocity. In addition, this change not only affects the vehicle design, but as the cable grow (obviously, the vehicle will need more power and thus a larger umbilical/tether) so grows the size, weight, and power requirements of the cable and handling system on the platform above. It is highly recommended to set the oper- ating requirements properly up front. Do not expect minor changes in those requirements to have a minor impact on the vehicle design because the carry-over effects of simple changes typically magnify exponentially. As an example, see Figure 6.1. The Schilling HD electric motor turns a dual spline shaft driving two pumps—one for the main hydraulic system (for thrusters and manipulators) and one for the auxiliary hydraulics (for tooling). On the auxiliary side, the electric motor powers a Rexroth A10VSO45 Axial Piston Pump with the shaft turning at 1875 rpm. You cannot just switch the pump to the next higher size since the power draw could disable the main hydraulic system. Also, you cannot simply turn the motor faster as this could shear the spline shaft due to excess torque. Changes can result in a significant ripple effect.
So, in reality, the design of an ROV is a complex spiral that will eventually take the engineer to a point where a thruster that supports all requirements (above and below the water) can be chosen. The following sections will provide additional input to those decisions.
6.1 Propulsion and thrust
The propulsion system significantly impacts the vehicle design. The types of thrusters, their config- uration, and the power source to drive them usually take priority over many of the other components.