Page 245 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

234 CHAPTER 9 LARS and TMS
FIGURE 9.14
Dynacon’s Model 501 heave compensator.
direct the AHC unit to pay out or take up cable based upon those active readings. With passive heave compensation (PHC), the cable tension is held constant, thus varying the weight to maintain compensation (instead of position). AHC is much more accurate for achieving a constant load posi- tion over the bottom as it works on a position reference frame (rather than weight which has no direct reference to motion).
Heave compensation systems can be incorporated directly with overboarding cranes, but most of the larger systems will be using stand-alone or integrated heave compensators with the winches as discussed below.
Stand-alone heave compensator
For use of a stand-alone heave compensator to counter higher sea states and/or ship dynamics, units such as Dynacon’s Model 501 heave compensator (Figure 9.14) can be added to the system; however, be sure to factor in the additional footprint, hydraulic power requirements, and acceptable umbilical cable runs.
Integrated winch and heave compensator
If a mechanically cleaner, albeit larger and more complex, winch is desired, there are options for winches that have an integrated active heave compensation system such as the one produced by MacArtney (Figure 9.15).
9.2.2 Tether management systems
In most offshore applications, the ROV umbilical will be handled by a winch system, whether heave compensated or not, that connects to a TMS. The usually neutrally buoyant tether connects the ROV to the TMS, which pays out or takes in the tether as the operator dictates. Control of the