Page 574 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

21.2 Commercial, scientific, and archeological operations 573
Full current
Relatively calm bottom water
Current flow
Turbulent boundary layer
FIGURE 21.13
Bottom conditions relating to current flow.
Low current
Bar
Channel
Low current
High current
High current
(a)
FIGURE 21.14
(b) No current
Examples of channeling on river bends: (a) plan view, (b) cross-section.
Once the hydrodynamic conditions are understood, some other possible solutions to operational problems become apparent. For instance, rivers channel on the outside of a bend and bar on the inside of the turn. If the approach to the object is from the lower current side on the bar, where the vehicle can sneak beneath the boundary layer, the capability is increased to successfully complete an inspection task that otherwise would be unobtainable with a more direct approach (Figure 21.14).
21.2.2 Operations on or near the bottom
By far, the most important working issue in ROV operations is tether management. In operations at or near the bottom, the objective is to reduce the amount of tether the submersible is required to pull to the work site through judicious use of the deployment cage, tether management system, or clump weight. With the weight or managing platform (i.e., the cage or TMS) forming the center