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Failsafe Slips On Synthetic Rope
Solution for Rope Internal Heating in Motion Compensation Operations

AUTHOR:


Benton F. Baugh



Motion compensation system for using steel cables to lift a synthetic rope.
Smooth-faced failsafe slips are a potential solution for providing motion compensation characteristics with synthetic rope. Conventional synthetic rope winches generate substantial friction heat to the point of damaging the rope.

Baugh Consulting Engineers, Inc. (Houston, Texas) was asked by a client to investigate a solution to the problem of potentially destructive internal heating within synthetic rope during motion compensation operations.

A handling setup was created, consisting of synthetic rope on a reel, with the load taken by a traction winch. The synthetic rope we used was 2.5 inches in diameter, and the load rating was 90,000 pounds. The reel was primarily for holding the synthetic rope, rather than being used as a winch. The traction winch carried the load from rope to pulley friction with a total of 20 pulleys, or 4,500 pounds per pulley. During motion compensation operations, the load cycled back and forth over the pulleys under 90,000 pounds of load on the first one and progressively lower load on each subsequent pulley. The last pulley presumably had 4,500 pounds of load on it.

A primary reason for the heat generation was that the 20 pulleys all traveled at the same speed. As each pulley imparted more load into the synthetic rope, it became a little longer, so only one of the pulleys could be traveling at the speed of the rope. The rope was required to slide on the other pulleys to maintain tension. The friction of this sliding on the pulleys as it moved back and forth generated heat. A potential solution was to run water over the synthetic rope, but this was not considered satisfactory by the client.

The reason for considering the synthetic rope for this service is that when you lower a heavy steel cable to a 10,000-foot water depth, a large percentage of the winch cable strength and winch capacity is consumed in supporting the winch cable. As synthetic ropes are near neutrally buoyant, the lifting capacity at the surface is retained at water depths of 10,000 feet.


Developing Solutions
The solution attempted involved a yellow synthetic rope that can be stored in a bin on a vessel or, at the most, would require a light-duty reel. The synthetic rope goes over a central sheave on a mast and down to the payload.

Two gripper units are around the synthetic rope, and each is tied to a winch across a sheave on the mast. These winches can have relatively short stroke and take turns gripping the synthetic rope for lowering and then returning to a higher position for gripping once again.

There may or may not be economies in handling with two short-stroke winches instead of the alternatives, but this does allow the support of the synthetic rope during motion compensation to be done with a steel cable, which is not sensitive to heat-generation problems.

After seeing the damage that can happen to an oil platform at sea if the injector head begins to slip and the coiled tubing runs away into the hole, we undertook making a slip assembly that would stop the coiled tubing. Stopping was of primary importance, but preventing damage was also a serious concern.

Ideally, the stopping would happen immediately, before high velocities are encountered. This means that the coiled tubing must be engaged where it is and not at some connection point (which doesn’t normally exist). This requires engaging the smooth outer diameter of the coiled tubing. Conventional sharp teeth would destroy the usefulness of the coiled tubing, even if stopped immediately. To continue this article please click here.



Dr. Benton F. Baugh, Ph.D., P.E., is president of Baugh Consulting Engineers, Inc. and has previously worked with Radoil, Beta, Vetco, Cameron, Camco and Bowen Tool Co. He is a member of the National Academy of Engineering and a fellow of the National Academy of Inventors, American Society of Mechanical Engineers and Marine Technology Society. He holds more than 100 U.S. patents.







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