CAMBRIDGE, Mass.--Researchers at the Massachusetts Institute of
Technology 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 them. Those are
the smokestack equivalents of grass [around underwater cables], to help
prevent them 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 eleven
oil companies.
