Page 255 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

244 CHAPTER 9 LARS and TMS
counter the excessive umbilical in the water—grab hold of something on the bottom while the ten- der takes up the slack. The same technique can be applied to ROVs by placing the vehicle on a sta- tionary item on the bottom and then having the tender pull the excessive tether back on deck.
9.3.2 Currents
As stated previously, the objective of operating an ROV is to deliver a camera, instrumentation package, or work system and tools to a place where it can be of use. Under most situations, the item of interest will be submerged in a fixed location. Currents can cause difficulty in flying the submersible to the work site.
The resistance to delivery of the submersible to the work site is directly related to the total drag upon the submersible and tether. As stated mathematically in Chapter 3, hydrodynamic drag upon the wetted surfaces of the submersible system (vehicle plus tether) is affected by the following:
• Density of the seawater
• Characteristic area for both the submersible and the tether
• Velocity of the submersible system through the water
• Nondimensional drag coefficient of the system (essentially the hydrodynamic shape of the
system through the water).
Increase any of these items and the submersible will have more difficulty fighting the drag. Drag caused by current can be mitigated by the following:
• Maintain as little tether in the water as possible to accomplish the task.
• When pulling the tether to the work site, attempt to pull the tether into the current to present the
least cross-section drag possible.
• If possible, lay the tether across some obstruction (while assuring an easy egress) between the
work site and the pilot’s operating station, so that the submersible is not required to continually
fight the drag while station keeping.
• Approach a structure from the lee side to allow operation in the turbulent area downstream of
the structure of interest.
• Operate at slack tide, or consider delaying the operation until slack tide, if currents prove to be
above the submersible’s power capability. Or invest in a more powerful ROV system.
• Make liberal use of clump weights while working on or near the bottom so that the weight takes
the cross-section drag on the tether instead of the submersible’s thrusters.
• When operating on the bottom, make use of the boundary layer near the bottom to stay in the
calmer waters below the boundary layer.
9.3.3 Teamwork and proper tether management
Proper coordination between the tether handler and the submersible pilot will do more to prevent tether management problems, as well as entanglements, than any technological solution.
An ROV pilot’s success as an operator of robotic equipment will directly depend upon his abil- ity to figuratively place his head inside the submersible. As part of that “feel,” the ROV pilot is in a position to gain a “feel” for the amount of tether needed as well as the degree of pull being