Page 64 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

52 CHAPTER 2 The Ocean Environment
FIGURE 2.17
Vehicle suspended from vessel over the side.
ROV operations manuals should set the maximum sea state in which to deploy the submersible. In a high sea state on a rocking vessel, the submersible becomes a wrecker ball suspended in mid- air, waiting to destroy the vessel structure as well as the vehicle, deployment system, and any bod- ies unfortunate enough to be in the wrecker ball’s path. If the sea state ramps up unexpectedly while the vehicle is still in the water, most operations manuals specify to leave the vehicle sub- merged until a sea state that allows safe recovery.
Another important wave propagation factor affecting ROV operations includes the heave com- ponent of the vessel while the vehicle is in the water. Particularly affected is the umbilical or tether length between the hard point of the launching platform and the clump weight, cage deployment platform, tether management system, or the vehicle itself. Of particular concern is the snap loading of the tether due to tether/umbilical pull with the vessel heaving. This rapid loading of the tether/ umbilical can easily exceed the structural limitation of the tether and/or part conductors and communications components within the line. If this snap load parts the line, the vehicle could be lost. Larger and more sophisticated launch systems have a stress limit adjustable to the tether or umbilical loading limit. The system will slack or pull based upon the given parameters to maintain tension while avoiding overstressing the line.