Page 256 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

9.3 Currents and tether management 245
experienced by the tether at any moment. As experience is gathered, a situational “lay of the land” orientation should be gained, allowing a mental picture of the location/lay of the tether. From this feel, the ROV pilot can direct the tether handler for the best tether management.
In general, the submersible should pull the tether to the work site (as opposed to the tether push- ing the vehicle). The tether handler should allow just enough slack in the water so that the submers- ible is not wasting thruster power pulling the tether into the water. Any excess tether in the water, other than that absolutely necessary to accomplish the job, invites tether entanglements and wastes thruster power due to the drag of the excess tether.
Direct ingress/egress routes to the work site are generally preferable to multiple turn navigation due to tether friction and the higher likelihood of snags. The intended task should be planned in advance so that the best route can be chosen. The extra preparation time to choose the best route could save time later unfouling the tether.
In every ROV pilot/operator’s tool kit should be additional floats and weights to compensate for the varying density of water. If an ingress route to the work site takes the submersible over a wall and then under an overhang, a weight at the proper location, followed by a float near the top, will lay the tether properly and avoid unnecessary tether hangs.
A neutrally buoyant tether is best for practically all ROV operating situations—but neutrally buoyant in what water type? Usually, ROV manufacturers specify neutral buoyancy of the tether based upon fresh water at average temperature (i.e., zero salinity and 60F/15C water). As the tem- perature of the water moves down and the salinity of the water moves up, the tether becomes increas- ingly buoyant. This factor must be taken into consideration and compensation made in the field.
While operating ROV equipment, a negatively buoyant tether may be useful in situations such as an under-hull ship hull inspection (to keep the tether clear of hull obstructions). But a negatively buoyant tether operated near the bottom will drag across any item protruding from the bottom much like a dragline.
Clear communications need to be established between the tether handler and the ROV pilot to avoid misunderstanding under critical situations. Standard terminology includes:
• “Launch vehicle”
• [for predive inspection] (Pilot) “Lights” (Tether Handler) “Check,” etc.
• “Pull tether 3 feet (meters)” for pulling in tether
• “Slack tether 3 feet (meters)” for slacking tether
• “Pull tether 3 pounds (kilograms)”
• “All Stop”
• “Hold”
• “Pull until tension”
• “Pull then slack in [frequency] succession”
• “Recover vehicle”
9.3.4 Tether snags
Tether snags range from friction drag across an obstacle to total entanglement. The steps to clear
tether entanglements are fairly universal on all ROV platforms and include the following:
1. When the submersible stops moving, the tether is snagged somewhere behind the vehicle.