Page 636 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

636 CHAPTER 22 It’s the Little Things That Matter
degree of certainty. A high-quality GPS receiver with the capability for survey line input, as well as course deviation indication, is very helpful. Complete the survey of the entire area in the survey phase before attempting to switch to the identification phase of the mission (with diver or ROV). Many operations have been inefficiently run by stopping the survey to look at each suspected target (only to discover it was not the proper item). The switchover time between equipment can be con- siderable. Consult the equipment manufacturer’s performance specifications to obtain the proper tow speed and altitude requirements. And do not be afraid to request corrective action of the boat operator if those parameters do not meet the mission requirements.
22.3.5.2 ROV operations
It is paramount to have the ROV deployed over a steady work location. If the boat drifts off loca- tion, the ROV operator will be required to repeatedly reacquire the target (once the submersible is dragged off the target), frustrating efforts to complete the mission. If an ROV operation over a target site is to be performed properly, either multipoint anchoring over the site or dynamic posi- tioning will be necessary. In some situations, a skilled boat operator can keep the vessel steady enough to get the identification done. But as the wind and sea state worsens, the ability to keep the vessel on-station becomes increasingly difficult. For shallow-water operations, a three- or four-point anchoring system directly over the work site (or a jack-up barge) is recommended to complete the work task.
22.3.6 Marking the target(s)
To mark a target for positive identification, the following steps should be performed:
• Complete the side scan sonar survey of the entire area, and then retire to a location where a complete analysis of the data can be made. The targets can then be identified, classified, and prioritized. Once the targets are identified, further investigation can be performed.
• Fabricate a sonar reflector to place next to the target of interest. Sonar targets are highly (sonar) reflective weighted anchors connected to the surface via a line and buoy (Figure 22.11). The
FIGURE 22.11
Sonar reflecting target.
Wrapped in “chicken wire”
Line to buoy on surface
Negatively buoyant frame