Page 258 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

9.3 Currents and tether management 247
Edges with angles of 90 or more can form a “tether guide.” These types of structures are handy during operations since they allow the operator to place the tether into the groove for known place- ment and allow for low-friction sliding of the cable along the groove.
Any mission that involves operations in or around structures will require a choice of tether lay and guiding.
9.3.6 Clump weights and usage
Hold a piece of yarn in front of your mouth and blow. The yarn is blown by the air from your mouth due to the cross-section drag of the string. Take that same piece of yarn and tie a weight to the end and repeat the exercise. The yarn does not blow in the wind. The cross-section drag is now absorbed by the weight instead of the yarn itself. This is a close analogy to the need for a clump weight while operating any free-flying ROV. The clump weight serves several purposes in ROV operations:
• It allows for orderly lay of the tether from the water insertion point to the work site on the bottom or mid-water.
• It delivers the tether to a known location that can be measured above the level of the bottom so that only the tether from the clump weight to the submersible requires positive management.
• The submersible is only required to drag the amount of tether from the clump weight to the submersible, thereby freeing the submersible for direct work tasks (Figure 9.31).
When conducting a long bottom search or inspection task, it may be more feasible to use a clump weight to bring the tether to the bottom and then move the work platform, rather than anchor and try to swim the submersible along a transect. Consider the appropriate use of a clump weight to more easily complete the mission.
9.3.7 Rules for deployment/tether management A few rules for tether management are as follows:
• Never pull on the tether to clear a snag.
Clump weight
FIGURE 9.31
Use of a clump weight reduces tether drag to only the length between the clump weight and submersible.
Drag