Return To Day 1Return To Day 27Day 28. Dredging

By Edwin Schiele

October 23, 1998. Our voyage has entered its third phase. No more sitting in the control van staring at pillow lavas on a video screen. Now we have a chance to hold the pillow lavas in our hands.

ARGO II recovery (L. Dolby)
ARGO II rises for the last time (at least on this cruise). (L. Dolby)

Yesterday, we all gathered on the back deck and watched the water streaming off of ARGO II as the crane lifted it out of the water for the final time. There was time for a quick wax core, but that was just a small preview of what was coming up. Wax cores yield enough chips of rocks and glass to cover the bottom of a beaker. Dredging often brings up as much as several hundred pounds of rocks at a time—enough to fill several large buckets.

Frank Trusdell and Gary Drewry retrieve dredge (L. Dolby)
Frank Trusdell and Gray Drewry retrieve the dredge. (L. Dolby)

The dredges are steel rectangular frames with chain nets. The ship drags the dredge along the seafloor and breaks the rocks off the bottom. The rocks then get funneled back into the chain net. Dragging a heavy dredge over rocks and rough topography has its risks. If the dredge becomes snagged, it will anchor the ship in one place. Since we neither want to stay in the boat for the rest of our lives, nor do we want to cut the cable, which is far too valuable to lose, we have introduced safeguards. Just below the spot where the cable attaches to the dredge, we have added two weak links that are designed to break if the dredge becomes lodged and we cannot extract it. The two links do not have the same strength. If the dredge does get stuck, the weaker link breaks first. When the link breaks, the cable yanks the side of the dredge and almost always dislodges it. If the dredge remains lodged, the second link, which is set 500 pounds stronger, will break. We would lose the dredge, but nothing else.

Replacing weak links (L. Dolby)
Wenlu Zhu, Tim Dulaney, Eric Bergmanis and Kevin Johnson replace the weak links. (L. Dolby)

The nature of the watches has changed dramatically. Most people now must work two four-hour watches per day. Kevin Johnson and Jennifer Reynolds, who are coordinating the dredging operation, are on duty for 12 hours each day. Unlike the previous watches, our tasks are no longer well-defined. We help raise and lower the dredges, monitor the dredges as they pick up rocks, and process the rocks that come on board. It actually feels good to work out on the deck and do some physical activity rather than sit in the darkened control van in front of a monitor for four hours.

I was on the first watch that lowered the dredge into the water. The mood was tense. Before, we relied on the DSL group and crew to raise and lower the DSL 120 and ARGO II and make sure everything was working properly. We just sat back and collected the data. Now operating the dredge was largely up to us. The most complicated and critical task was setting the weak links. First we had to calculate the strength we wanted. The deeper we dredge, the weaker the weak links must be. The links themselves are composed of two metal plates held together by a series of pins. The pins vary in strength based on their thickness and the metal from which they are made. For example, a steel pin is stronger than an aluminum pin. So we must choose the right combination of pins to match the strength we want.

Weak Links (L. Dolby)
An example of a weak link. (L. Dolby)

It took a while, but when the crane finally hoisted the dredge over the side, we retreated into the computer room (not to be confused with the control van) to watch its progress. We had attached a pinger (a tube that makes a pinging sound) and a transponder to the cable so we could determine the altitude and position (longitude and latitude) of the dredge. Digital readouts on the wall showed the amount of cable that had been let out and the tension on the cable. If the tension suddenly increases, we know that we have a bite—the dredge has grabbed some rocks.

Mike Avgerinos logs dredge position (L. Dolby)
Mike Avgerinos logs the position of the dredge. (L. Dolby)

Again the mood was tense. Jennifer Reynolds closely monitored the length and tension of the cable, when the dredge touched the bottom, when the ship started pulling the dredge, when we got the first nibble, and when the dredge left the bottom. My job was to write down the time and position of the dredge every step of the way.

Finally, at 1:00 in the morning, the dredge broke the surface with its first payload of rocks.

Ship Tracks October 21 through October 23

Ship Tracks October 21-23
Blue = Days 21 and 22
Red = Day 23
Go back to day 27. On to day 29