May 3, 2011
New device helps oceanographers get their instruments back from the depths
Getting heavy objects down to the seafloor is pretty easy—in a best-case scenario, you just drop them over the side of a ship, and hope that they land right side up on the ocean bottom. However, getting those same objects back to the surface can be problematic. MBARI Marine Operations Technician Mike Conway recently developed a new device called a “line elevator” that will make this process easier. One result will be fewer anchors and other heavy objects sitting on the seafloor of Monterey Bay.
When NASA sends space probes to the moon or Mars, these multi-million-dollar probes don’t usually come back. They are considered disposable. In most cases, oceanographers do not have that “luxury.” For one thing, seawater blocks most radio waves, so most oceanographic instruments have to be brought back to the surface before scientists can download the data they have collected.
MBARI researchers often use our remotely operated vehicles (ROVs) to place instruments on the seafloor and then bring them back to the surface. However, the ROVs cannot bring back instruments that weigh more than a few hundred pounds in air. This means that such instruments must be hauled back to the surface using a crane on board a ship.
Unfortunately, attaching a line from a ship to an instrument 900 meters (3,000 feet) below is a tricky business. At that depth, you can’t simply lower a line and hope to snag the instrument. And you can’t send an ROV down with a line because of the danger that the ROV’s control tether could get tangled in the line.
Sometimes a researcher will attach large floats and a heavy weight to an instrument. The heavy weight causes the instrument to sink to the seafloor. To get the instrument back, the researcher sends a sound signal to the instrument. This triggers the instrument to release the weight, which allows the instrument to float back to the sea surface.
This process seems straightforward, but is fraught with risks. Both the number of floats and the weight the instrument have to be very carefully balanced, or the instrument will refuse to sink, or (worse yet) refuse to rise back to the surface. Another challenge is that the ocean is a surprisingly noisy place, so acoustic releases don’t always release or may be released accidentally. Even after the instrument reaches the surface, it can be very difficult to locate in the open ocean, particularly in rough conditions.
Finally, there’s the issue of the heavy “drop weight” left sitting on the seafloor. In the old days, these iron weights were typically left on the seafloor, where they would gradually rust away. However, both researchers and public agencies are becoming increasingly concerned about the amount of trash on the seafloor. Thus, MBARI researchers now try very hard not to leave any objects such as drop weights or anchors on the seafloor.
To address these issues, in 2005 MBARI partnered with the Monterey Bay National Marine Sanctuary to open a design competition for an environmentally-friendly anchor system. More than a dozen entries were received from across the United States, including one from a 16-year-old high school student. In the end, two conceptual designs won awards. However, at the time of the contest, neither of these had actually been built or tested in the ocean.
Over the last year, Mike Conway has been addressing a related task—how to get heavy instruments back to the surface without the use of cumbersome floats and drop weights. Conway’s system is called a “line elevator,” because its key element is a platform that can descend into the ocean bottom to deliver a spool of line, and then rise back to the surface to bring the free end of the line back to a ship.
Conway was able to build this device quickly and cheaply by using the frame of an older elevator used to lift heavy sediment cores to the surface. In between his other projects at MBARI, he somehow found time to design and fabricate all the additional parts he needed.
Here’s how Conway’s new design works (see illustration above):
- The line elevator is released from a ship and allowed to sink to the seafloor.
- After the line elevator reaches the seafloor, an ROV is launched from the ship.
- The ROV moves the line elevator close to the instrument and attaches the lifting line from the elevator to the instrument.
- After the ROV returns to the ship, an acoustic signal is sent down to the line elevator. The elevator then releases a “drop weight” that is attached to the lifting line. Now positively buoyant, the elevator begins to rise toward the surface, spooling out line as it goes.
- After the elevator reaches the surface, the ship’s crew lifts the elevator and the drop weight out of the water and then attaches the lifting line to a winch.
- Using the lifting line, the ship’s crew lifts the heavy instrument off the seafloor and on to the ship.
In April 2011, Conway finally got a chance to test his new system in the open ocean. The line elevator worked just as he had hoped, allowing MBARI researchers to lift two heavy instrument stands (“benthic instrument nodes”) from the bottom of Monterey Canyon.
In addition to providing a new tool to help keep the floor of Monterey Bay free of unused equipment, the line elevator provides another advantage for marine operations crews. When it reaches the sea surface, the line elevator will not drift with the currents because its drop weight acts as an anchor.
Conway hopes that the advantage of not having to chase instruments around the bay, combined with the environmental benefits of keeping the seafloor clean, will more than make up for the extra gear involved in using the line elevator.
Conway is optimistic that the new line elevator will be a useful tool for MBARI’s marine operations crews, helping them recover heavy instruments from the deep sea. “The ROV pilots have assured me that we’ll have a lot of opportunities to use this,” he says. “The gear we’re putting out there keeps getting bigger.”
For additional information or images relating to this article, please contact: Kim Fulton-Bennett