Page 610 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

610 CHAPTER 22 It’s the Little Things That Matter
If a 50% reduction in drag due to the tether being less than perpendicular is assumed, Table 22.2 can be derived.
If a 75% tether drag reduction is assumed, Table 22.3 is derived.
With the above assumptions, the approach to an ROV underwater port security task will vary depending upon the conditions to be encountered during that task.
Negligible drag
Vessel movement
Buoy (no drag)
Tether drag
Vehicle Negligible drag
Clump
weight (no drag)
Vehicle drag
FIGURE 22.1
Vehicle and tether drag at 100% tether drag due to perpendicular water presentation.
Table 22.2 Net Thrust Crossover Points by ROV Size Category with 50% Reduction of Tether Drag Due to Less Than Perpendicular Tether Presentation to the Water Flow
Velocity (knots) Small (ft) Medium (ft) Large (ft)
.250 .250 .250 125 200 250 40 75 110
0.5
1.0
1.5
2.0 5 10 50
Table 22.3 Net Thrust Crossover Points by ROV Size Category with 75% Reduction of Tether Drag Due to Less Than Perpendicular Tether Presentation to the Water Flow
Velocity (knots) Small (ft) Medium (ft) Large (ft)
.250 .250 .250 200 .250 .250 75 100 175
0.5
1.0
1.5
2.0 10 15 75
1000' length