Page 86 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

74 CHAPTER 3 Design Theory and Standards
The function of an ROV is to act as a delivery platform (for sensors and tooling) to a remote work site. All items and subsystems of the vehicle support this function. The vehicle must have some type of locomotion to take it to the work site and perform the work. In order to achieve the locomotion objective, the vehicle must power itself and overcome the fluid drag of the vehicle/ tether combination to travel to and remain at the work site. This sounds simple, but the devil is in the details.
3.5.3.1 The drag equation
Nestled in the Appalachian Mountains of central Pennsylvania is the Applied Research Lab at Penn State University. The facility houses a closed-circuit water tunnel for inducing all of the character- istics for identifying a theoretical fluid flow equation, namely:
1. a fully enclosed fluid
2. a blunt form factor
3. a large enough Reynolds number to induce turbulence downstream from the item.
There is no better place to isolate the components of the drag equation than in this controlled environment. Imagine the item (let us call it a cube for now) stationary in the tunnel with no water flow. There would be no fluid drag. However, turn on the pump and make the water flow. As the water flow through the tunnel is slowly increased (with the cube still stationary in the tunnel), the drag will increase in an amount proportional to the density of the fluid (in this case either 62.43 lb/ft3 (1000 kg/m3) for fresh water or 64.62 lb/ft3 (1035 kg/m3) for seawater at maximum density) as well as the square of the object’s speed relative to the fluid. This may sound complicated, but it is quite simple (double the speed/quadruple the drag). Also, with a constant object volume, the shape of the object will directly affect its drag force—this factor is referred to as the “coefficient of drag” (Cd).
Let us first examine Cd. In Figure 3.10, various shapes are depicted along with their value of Cd (often called “shape or form drag”). The value of Cd, as will be shown shortly, will play a signifi- cant role in determining the overall drag of the ROV system.
3.5.3.2 Vehicle stability
As with a child’s seesaw, the further a weight is placed from the fulcrum point, the higher the mechanical force, or moment, needed to “upset” that weight (the term “moment” is computed by the product of the weight times the arm or distance from the fulcrum). It is called “positive stabil- ity” when an upset object inherently rights itself to a steady state. When adapting this to a submers- ible, positive longitudinal and lateral stability can be readily achieved by having weight low and buoyancy high on the vehicle. This technique produces an intrinsically stable vehicle on the pitch and roll axis. In most observation-class ROV systems, the higher the stability the easier it is to con- trol the vehicle. With lower static stability, expect control problems (Figure 3.11).
External forces, however, do act upon the vehicle when it is in the water, which can produce apparent reductions in stability. For example, the force of the vertical thruster when thrusting down appears to the vehicle as an added weight high on the vehicle and, in turn, makes the cen- ter of gravity appear to rise, which destabilizes the vehicle in pitch and roll. The center of buoy- ancy and center of gravity can be calculated by taking moments about some arbitrarily selected point.