Device decreases vibrations in marine cables

 

MIT researchers have built a prototype device to absorb the vibrations of underwater cables. The work is important because such shaking causes problems for scientists, fishermen, oil companies and virtually anyone else who relies on some combination of ropes, cables, pipes and casings in marine environments.

The device could also aid researchers who are studying vibrations caused by wind.

A cable in the ocean vibrates because water flows past it and creates a turbulent wake. As vibrations increase, so too does the resisting force-known as drag force-that is exerted on the cable. This can cause a variety of problems. For example, the increased drag force on a shaking cable connecting an anchor, scientific instrument and buoy may pull the buoy under. Increased drag can also make it difficult to tow underwater cameras or vehicles and cause wear and tear on cables, particularly at termination points.

As a result, many attempts have been made to minimize vibrations. "Lots of things have been tried," said J. Kim Vandiver, a professor in the Department of Ocean Engineering and director of the Edgerton Center. "There's a long history of people doing things like adding fuzzy plastic grass to cables." The type of device that works best, he says, has an airfoil-shaped fairing made of plastic, which fits over a cable. However, such casings are also very expensive and won't pass through pulleys or winches.

Armed with a conceptual approach to the problem, Professor Vandiver collaborated with several students to design and build a prototype vibration absorber. The researchers included graduate students Li Li and Ethan Butler, and several students from MIT's Undergraduate Research Opportunity Program. (Dr. Li and Mr. Butler have since graduated.)

The MIT apparatus concentrates on the terminations of cables. Vibration amplitude builds when waves travel down the cable, hit the ends and are reflected back into the center of the cable, creating standing waves. Rather than eliminating the source of vibration-the turbulent wake-this invention diminishes the steady-state standing wave vibration by absorbing waves incident on the terminations.

With the device, each of the cable's ends terminates in a stainless-steel fork commonly used to terminate wire ropes on a yacht. The fork is pinned to a metal flange, which is welded to the side of a foot-long shaft, with the cable and fork roughly perpendicular to the shaft's axis. The shaft is hinged to the boat or towed object with two fixed bearings, so that the cable's tension is transferred from the fork to the shaft and then to the bearings.

The ends of the shafts are encased in non-moving cylindrical sleeves, and an extremely viscous liquid, with a consistency similar to that of very cold honey, is placed between the shaft and the sleeve. This liquid serves as a damping material, absorbing vibrational energy as heat.

Professor Vandiver and Mr. Butler first tested the apparatus in the laboratory, then moved out into Massachusetts Bay. There, they conducted experiments in 100 feet of water, with one cable termination attached to a small boat and the other attached to a towing device known as a V-fin. Resembling an upside-down kite, the V-fin is typically used to tow vehicles or cameras in deep water. The investigators measured vibration and found roughly a 50 percent reduction in vibration amplitude.

Researchers collecting acoustic measurements could also use this instrument. Because cable vibrations can occur at the same low frequencies as those sounds used by the researchers, confusion as to the source of a sound can arise. Reducing vibration lessens the chances of contaminating underwater acoustic measurements.

Applications for the device reach beyond oceanography. "There's a whole group of professionals who worry about wind," muses Professor Vandiver. "Driving down the road occasionally you'll notice a tall smokestack that has funny, spiral things wrapped around it. Those are the smokestack equivalents of grass [around underwater cables], to help prevent it from vibrating in wind."

While Professor Vandiver's cable terminations won't steady smokestacks, the experiments will help researchers understand the underlying physical phenomenon that causes the problem.

The project was funded by MIT Sea Grant and a consortium of 11 oil companies.

A version of this article appeared in MIT Tech Talk on March 13, 1996.