Both specially designed apparatus and off-the-shelf equipment – including three women’s regulation softballs – were part of a suite of devices used successfully to cage and lift four sulfide chimneys from the seafloor off the coast of Washington and British Columbia.
Engineers and machine-shop workers at the University of Washington’s Applied Physics Laboratory designed and fabricated equipment – much of it unique – for the endeavor, says Le Olson. He’s a principal engineer at the lab and the lead engineer for the chimney-recovery project carried out June 26 to July 18 and sponsored by the University of Washington and the American Museum of Natural History in New York.
First things first
Before retrievals were attempted, transponders that allow operators at the surface to determine the location of structures and equipment at the seafloor, cages to go over the top of sulfide chimneys and baskets containing 11/2 miles of retrieval line had to be deployed.
Cages were lowered to the seafloor attached to the ROPOS garage. Three cylindrical cages with hoops of two-inch aluminum tubing were fabricated with inside diameters of 36, 47 and 52 1/2 inches and ranging in height from 7 to 9 feet. Each cage included a trio of “nooses” made of flexible five-eighths-inch steel cable. Once ROPOS maneuvered a cage over a chimney, ROPOS tightened the nooses around the chimney and one-way latches kept the cables as snug as possible.
Three women’s regulation softballs bolted on each frame gave the claws at the ends of the ROPOS arms something to grip while positioning the cage and making the nooses snug.
The wire-mesh baskets, each 48-by-42-by-66 inches, were filled with 8,000 feet (11/2 miles) of line that was painstakingly layered by hand and secured with plastic tie wraps in and around the basket. Each basket was lowered on a cable from the Thompson prior to the recovery attempts. The goal was to land within about 100 feet of a targeted chimney.
“The operation is like flying a helicopter a mile over Mount Rainier and trying to lower a basket to the summit while the wind is blowing,” Olson says. “Now imagine doing it with your eyes closed and relying on interpretive acoustic chirps to know its location – something similar to the way dolphins navigate.”
Once on the seafloor, the ship sent a coded sound signal to release the cable just above the basket so the cable could be brought back on board the ship.
Saw used on three of the four chimneys
A saw was used to partially cut three of the four chimneys once ROPOS had placed cages over them but before the retrieval line was in place and the winch on board the Tully did any lifting. UW Applied Physics Laboratory engineers constructed a retractable plunger arm and attached it under ROPOS. The arm carried a commercial underwater demolition saw with an 18-inch bar. When extended, the arm and the saw together stretched out 2 feet.
When it was time to lift the chimney, ROPOS would hook one end of the retrieval line to the cable slings on the cage. (The 150 feet of line closest to the chimney was rated to withstand temperatures of about 400 degrees F before weakening so it could withstand hot vent waters seeping or spewing from the more active chimneys.)
Once ROPOS surfaced and was out of the water, the ship sent a coded sound signal to the basket to release six glass-sphere floats attached to the end of the retrieval line still in the basket. The floats smoothly tugged the line out of the basket, layer by layer, until they reached the surface sometimes within 35 minutes of when they had been released.
The line was then connected to a specialized salvage winch on loan from the U.S. Navy. The beauty of this 30,000-pound winch system is that it automatically reels in and lets out up to 25 feet of line to compensate for the ship moving up and down and lessening the chances of the line snapping, Olson says.
Slicing sulfide structures
Once on board the Tully, scientists began time-critical sampling and engineers used a standard beaded-wire rock-cutting saw to slice the chimneys as required for future museum displays and research.
For more information:
Le Olson, (206) 543-1374, firstname.lastname@example.org
Vern Miller, (206) 543-4715, email@example.com