Page 575 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

574 CHAPTER 21 Practical Applications
Deployment platform station-keeping
Drag
Cage
Position of boat with respect to current and target.
Current
Submerged object
FIGURE 21.15
point of a circle of operation, the deployment platform (e.g., the boat, dynamically positioned vessel, or drilling rig) can be moved to within the operational radius of the work site, allowing the work to be easily performed with minimal excess tether (Figure 21.15).
The deployment platform should be placed directly over the work site with the managing plat- form in a position to have good angular access to all points on the work site. If direct access to a location on the job site is obstructed, the best practice, in most cases, will be to move the deploy- ment platform, rather than risk tether entanglement upon the structure.
Perform all work operations with as little tether actually dragging the bottom as possible (to cut down on the possibility of tether hangs). Then recover the submersible before moving the deploy- ment platform or leaving the site.
21.2.3 Enclosed structure penetrations
Probably the most exciting (and nerve wracking) application of ROV technology is the enclosed structure penetration. It is within this application that the small ROV really shines (Figure 21.16). Due to the inherent dangers to divers in underwater structure penetrations (difficulty of rescue, dan- ger of entanglement on the structure, questionable integrity of the structure, etc.), the ROV may be the only way possible to survey the site.
Some rules of thumb gathered during many hours of enclosed structure penetration with small ROVs follow:
• Choose an entry point into the structure that has a smooth transition (to avoid tether chaffing or trapping) and allows for easy tending of the tether as close to the entry point as possible.
• Purposely choose the tether lay along the structure with a constant eye to recovery.
• First pull the entire length of tether into the structure, to the full extent possible in order to lay
out the tether. Then work back toward the entry point, keeping slack in the tether to a minimum.