Originally Posted by SLFguy
Quote:
[snip] Every movement of the vessel at the surface is translated to the Remora's umbilical cable with a delay, said Brennan Phillips, manager of ROV operations at the University of Rhode Island in the U.S.
"If the ship moves, it takes half an hour for the vehicle to feel it," he said. "You need an extremely stable ship."
[end quote]
I wonder why they don't use a compensating system (hydraulic-air) like used on the drill rigs (semi) or DS for the risers tensionners and on the derrick travelling block
This is odd. Why no Tether Management System?
The quote from Phillips refers to the horizontal movement of the ship. The delay is related to the depth of the operation (and various cable parameters), but the half-hour figure is applicable at 4000m.
The heave of the ship, or payin/payout of cable, is transmitted down the cable quickly (at a speed determined by the elastic and mass properties of the cable). That is where, as you suggest, heave compensation can be applied. There are various methods possible:
Gas or active hydraulic rams/sheaves, of the sort you suggest,
Active winch (constant tension),
Bobbing crane (active or passive),
Attachment of floats to the lower tether to form an "S" section for (partial) decoupling,
Two-body deployment: depressor weight, with shorter neutral tether to the ROV,
And probably others. The latter two also decouple horizontal movement of the ship; and the last is sometimes implemented with a free length of tether, and sometimes from a second, smaller tether management system (reel) as part of the clump weight (what you referred to, right?). One reason not to use the first three methods for this job is that the constant flexing, under tension, of the same part of the cable, while deployed at a near-constant depth, can weaken the cable or damage the conductors or fibers.
I don't see any place where Phoenix says they won't use one of these methods, but I agree that their standard data sheet for the Remora 6000 does not show or claim any. Their web site does list an available ram-type motion compensator. I have no clue as to what they plan to use, and agree that delicate work with no decoupling of wave motion would be tricky.