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Skipper Describes workings of Trailing Suction Hopper Dredge, the “McFarland”

Source: Professional Mariner Magazine

I am one of two alternating skippers of the 319-foot trailing suction hopper dredge “McFarland”, which is operated by the U.S. Army Corps of Engineers. We are underway about 180 days a year, and I mean literally underway all the time: we spend our watches running back and forth within a channel sucking up bottom material like a giant vacuum cleaner and then running off-shore to dump it in designated depositories. Other times we extend our 222-foot “side caster” out to one side of the channel as we go. And, in yet another operating mode, we sometimes pull up to an anchored barge and pump the contents of our hopper through the barge onto a shoreside landfill.

The “McFarland”‘s hull was built by the Bethlehem Steel Shipbuilding Corporation with all the design drawings and dredging machinery supplied by Ellicott® International, of Baltimore, Maryland. She was built in 1967 for the Corps of Engineers hopper dredge fleet, which then consisted of 15 trailing suction hopper dredges. The vessel was delivered to the Galveston District Corps of Engineers, and primarily maintained the main ship channels in Texas and the Gulf of Mexico. Nowadays, the “McFarland” is one of only four such hopper dredges operated by the Corps, and we tend to work on the East Coast or in the Gulf of Mexico as far west as the Mississippi.

Right now our ship is working the outer reach area of Port Canaveral, Florida. We’re sucking up sand from the main channel just outside the jetties and then hauling it out to a dumping ground about six miles away. Although the clean Florida sand from the bottom is a pleasure to work with–smells sort of like a seafood restaurant–the job is made somewhat complex there because we have these protrusions like old-fashioned cowcatchers mounted on the drag heads along with underwater cameras which let environmental observers see if we can dredge without harming sea turtles which live in the area.

When I mention to friends or acquaintances that I work on a dredge, you can almost see their noses turn up as if they are thinking….Yuck, all that muck.

In many areas, surprisingly, bottom material has virtually no smell at all: places like the coast of Maine, the lower Delaware, and here in Florida. Even the lower Mississippi is normally not too bad since it’s mostly silt and sand washed down from America’s heartland.

Material from the bottom is sucked up by two large trailing suction drag heads and dumped into the midship hopper which can hold 3,140 cubic yards of material–it can be as much as 6,000 tons. The ship displaces 10,000 tons when loaded and draws about 23 feet. To get rid of the dredged material we have several choices — at an authorized disposal site, or we can pump it overboard through the side-casting system involving 222 feet of 36-inch pipe, or pump it ashore through the alongside mooring barge. Once we are on site and begin dredging, it can take anywhere from 30 minutes to two hours to fill up the hopper.

For a deck officer, handling a dredge is pure ecstasy. The “McFarland” is a twin screw vessel with 3,000 horsepower (2237kw) on each shaft, and twin rudders. Also included is a 500-horsepower (373kw) electric bow thruster, which helps turn the ship around easily in her own water. That’s something we are constantly doing, since we conduct our dredging by running back and forth over a certain patch of water, and at the end of each sweep, we’ve got to turn around and head back.

The Mississippi River is one of the busiest channels in the world, and it seems as though there are vessels arriving there every five minutes, 24 hours a day. And the spring of the year, with maximum river runoff, is when most shoaling occurs. This past spring, we had two dredges from the Corps there, plus a large contingent of private commercial dredges. Silting was so bad, and there were so many ship groundings, that it was called a dredging emergency. Most of us were assembled at Head of Passes, and as I said before, it was a shiphandler’s dream–or nightmare, depending on the perspective.

In the middle of the night with a fast current running and not much water under a big ship’s keel, it can get confusing very fast. But we have this saying that there’s no need to get alarmed if you are working on a dredge until you actually see the sparks begin to fly when ships make contact. Last spring we had one 900-foot tanker come as close as 15 feet to our dredge while we were both underway. That was a close one!

The “McFarland” is actually quite a maneuverable vessel. Her maneuverability comes from twin screws with controllable-pitch propellers and the bow thruster. For propulsion power we have a pair of ALCO V-12 diesels clutched to each shaft. These produce about 1,500 horsepower (1119 kw) each. We can run with just one engine on line for each shaft in light-load situation, and underway while loaded we can crank her up to 12 knots. In addition we have three V-16 ALCO diesels putting out a total of 2,800 horsepower (2088 kw), all linked to AC generators to power most of the onboard electric pumping equipment, including the main suction pumps. All seven of these engines are arranged athwartships in the engine room.

Overall, the ship is packed to the bulwarks with machinery of all kinds, making her an engineer’s paradise. Indeed, we routinely have a staff of three engineers on each tour plus a chief electrician. Most of us on the crew work a steady rotation of 16 days on and 12 days off.

The bridge of a hopper dredge today is right up-to-date. We are loaded with electronics and banks of computers to give accurate positioning both on top of the water and below the surface for drag depth indication. Most dredges today use differential global positioning systems. Differential GPS is a positioning technique which uses two receivers, the differential station at an unknown location, and the reference station at a known fixed location. The reference receiver computes corrections based on the differences between its actual and observed ranges to the satellites being tracked. The coordinates of the unknown location can be computed with great precision applying these corrections to the satellite data received at the differential station.

The “McFarland” is really quite a modern ship in terms of her machinery and bridge equipment. The Corps spends whatever it takes to keep its ships properly maintained. Ellicott® still provides after-sales parts and service to the “McFarland” despite her age; last year, for example, Ellicott® delivered spare drag heads.

The Corps of Engineers’ fleet of seagoing dredges today consists of four trailing suction hopper dredge ships. These are the “McFarland”, operating from Maine to Texas; the “Wheeler”, operating in the Mississippi River, and the ports in the Gulf of Mexico and Puerto Rico; and “Essayons II” and “Yaquina”, operating on the West Coast and off Alaska and Hawaii. Over a 40 year period Ellicott® International built 18 of the over 30 hopper dredges ever owned by the Corps.

Years ago, The Corps operated many more dredges, but increasing pressure from commercial industry has obliged the government to “privatize” much of its dredging work. By congressional mandate the fleet has shrunk to its present size, with each vessel working only 180 days a year. In addition, the Corps still maintains three small side-casting dredges for coastal work.

Excerpted from Professional Mariner Magazine

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