Page 257 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

246 CHAPTER 9 LARS and TMS
2. Thrust forward slowly to bring the tether to “taut” condition to isolate the tether between the location of the snag and the vehicle.
3. Reverse slightly to bring slack in the tether line behind the vehicle.
4. Make a 180 turn in place to locate the tether between the vehicle and the snag.
5. Follow the tether to the location of the snag.
6. Work the tether snag out visually in coordination with the tether handler.
Once the tether snag is located, the general procedure for clearing the snag will present itself.
There are four general ways of clearing a snag:
1. The preferable choice is to move the submersible to the snag and clear the foul using positive control.
2. If the above is not possible, move the operations platform (i.e., move the tether from the operator’s side).
3. It may be possible to move the snag or the item upon which the tether is snagged.
4. The last choice is to cut the tether either physically or via a connection point.
If it becomes necessary to leave the operations area before a snagged tether or stuck vehicle can
be freed, the vehicle can be powered down, left in the water, and retrieved at a later time. Steps to accomplish this task are as follows:
1. Power down the vehicle.
2. Pay out all of the tether.
3. Unplug the top-end or intermediate tether connection point and wrap in plastic or some other
water-resistant wrapping.
4. Note the location with GPS.
5. Attach a buoy to the tether for easy retrieval upon return.
6. Advise operations that there is a tether in the water, which may pose a hazard to navigation, so
that advisories can be made.
9.3.5 Tether guides and ROV traps
There are objects that, due to their shape and size, are more likely to snag a loose tether. Some can be used as tether guides, but some are considered “tether traps.”

The typical tether trap has an edge with any converging angle less than 90 . This type of arrangement will allow the tether to slide naturally into the groove made by the angle and, once the tether is tensioned, form a friction lock on the tether (Figure 9.30). At that point, it is likely the submersible is lost.
FIGURE 9.30
Tether guides and traps.
Tether Tether Tether guide guide trap