Page 576 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

21.2 Commercial, scientific, and archeological operations 575
FIGURE 21.16
Small ROV, with a diver tending the cable, performing internal wreck penetration of the USS Arizona.
(Courtesy Brett Seymour, National Park Service.)
• As discussed in Section 9.3, tether traps are the vehicle’s biggest danger within enclosed structures. A tether naturally channels to juncture points within the structure, possibly trapping the tether in a less-than-90 wedge point. Plan the tether lay to avoid the tether being pulled into one of these traps, possibly trapping the expensive vehicle irretrievably within the structure.
• It is best to assure a neutrally buoyant tether is available for structure penetrations since tethers can be fouled on either the ceiling or the floor of the structure.
• Enclosed structures (especially old archeological sites) normally have significant silt buildup within the structure. To assure the least stirring of silt (thus obscuring visibility), maintain the vehicle close to neutral buoyancy (cheating on the positive side so that any thrusting needed is away from the bottom silt).
• When exiting the structure, fly the vehicle out adjacent to the tether while the tether handler slowly pulls the tether out of the structure.
• Never pull the tether to free a snagged vehicle. If there is any resistance to pulling the tether from the structure, slack and then reassess.
• Plan on getting into some minor snags and plan the time budget with this in mind.