Split-hopper dredge keeps ship traffic moving through QuebecJan 27, 2016 03:58 PM
The split-hopper dredge Ocean Traverse Nord operates off La Petite-Floride, Quebec, with its drag arm lowered.
Mate Robert Martin kept a keen focus on the screens and controls around him, all while scanning for traffic in the dredge’s vicinity on the St. Lawrence River. That focus was interrupted only by terse exchanges between Martin and operator Yannick Moreau, adjusting the depth of the trailing suction arm in coordination with the ship’s movements. The dredge Ocean Traverse Nord is basically a 215-foot-long vacuum cleaner.
Martin and Moreau both studied the same image on their respective DredgePack screens. DredgePack is a bathymetric software package. Martin maneuvered the vessel to sweep the drag head over the high spots of riverbed identified for removal by the Canadian Coast Guard.
Mate Robert Martin pilots the Groupe Ocean vessel while operator Yannick Moreau, in background, controls the dredge equipment.
In the St. Lawrence River below Quebec City, a 16-nm channel called Traverse du Nord is a bottleneck of sorts for shipping between ports in the upper midwestern United States, as well as the Canadian provinces of Ontario and Quebec and the rest of the world. Traverse du Nord, little known outside the people who work there, collects silt so predictably that without a yearly period of dredging many of the 6,000 ships that use the river would not be able to pass safely. This annual dredge work is called maintenance dredging, as opposed to capital dredging, which is defined as creating new channels or modifying the depth or breadth of existing ones. Since 2012, this stretch and others in the lower St. Lawrence have been kept clear by a new dredge called Ocean Traverse Nord, its name a melding of its owner — Groupe Ocean — and the job that spawned it. Among crew and friends, it’s simply OTN.
For several decades, maintenance dredging in the Traverse du Nord had been provided by an antique trailing suction hopper dredge called Port Mechins. In 2009 when Transport Canada refused to renew the seaworthiness certificate of Port Mechins, which was built in 1949, a solution was urgently needed. Bringing in a foreign-flagged dredge would conflict with Canadian cabotage law, and building a replacement for Port Mechins was prohibitively expensive until the Canadian government decided to seek bids for a seven-year contract. It was the guarantee of this long-term contract that convinced Groupe Ocean to bid. Groupe Ocean tendered the winning bid; however, at that time it possessed no trailing suction hopper dredge. But what the company lacked in equipment it possessed in determination. Only 12 months elapsed from signing a dredging contract to keel-laying to christening in August 2012. To make this happen, according to Capt. Paul Drouin, “Ocean used off-the-shelf plans for a Spanish split-hopper vessel combined with a dredging package from Damen and built the biggest possible vessel they could in their own yard in right here in Quebec’s Isle-aux-Coudres.”
Dredge spoils are pumped in through hopper loading grate valves.
Split hopper means that essentially the dredge has two hulls. In dredge mode, the two are held together, enclosing 1,308 cubic yards of “cargo” hopper. In dump mode, huge hydraulic rams open up the underside of that hopper so that the dredge spoils are released, a process that takes about two minutes. When the hulls open, the superstructure maintains its dimensions, rising as needed and staying level.
Since its launch, the dredge has been very busy either working or transiting from job to job. Clearing its namesake waterway takes about six weeks per year. Working with Canadian Coast Guard sounding vessels such as FCG Smith, OTN systematically removes excessive silt and then discharges it to a designated dumping area.
It’s maintenance — a never-ending job. “Sand and silt are constantly moving across the riverbed,” said Drouin, “so we try to get the Smith to verify that we have the channel to the contractual depth as soon as we have dredged it to the required specifications.”
The hopper dredge loaded almost to full capacity.
OTN also has its own soundings vessel, crew boat Korok, equipped with a multi-beam echo sounder (MBES) that covers a 66-foot swath of riverbed and processes the results. “Korok’s probe directs 256 beams toward the riverbed and allows the production of a bathymetric image that looks like a photograph,” said project manager Sylvain Babineau. “With the R2Sonic 2022 MBES, if there’s a shipwreck there, we can image it so well you’ll think we took a picture of it.” Variations in bottom contours show up as precise images in depth-significant colors.
Besides the Traverse du Nord, OTN works other locations in the St. Lawrence River and the Gulf of St. Lawrence, such as the Magdalen Islands, or Isles de la Madeleine — the source of most of the winter road salt used in eastern Canada. Maintenance dredging keeps the channel to the salt facility there open to Salarium, the self-discharging bulk carrier that often visits the facility to load salt.
In dump mode, Ocean Traverse Nord’s hull opens along centerline hinges while the superstructure rises.
But as cold weather approaches, the vessel moves to jobs in warmer waters. “Ice and trailing hopper dredging are just not compatible,” Drouin said. For the past two winters, OTN has worked on dredging projects in Mexico, the Dominican Republic and the British Virgin Islands. Louis-Pierre Dorval, director of dredging operations at Groupe Ocean, said these contracts present not only new market opportunities, but help develop and expand dredging expertise while keeping precious “manpower’’ in-house and fully employed during what would normally be a down period at their home base of Quebec or eastern Canada. Drouin added, “Working ‘down south’ where the weather is great in the winter is a welcome change of pace for the crew. While dredging is dredging, each contract has its own special challenges — in the Caribbean, our logistics are sometimes a little more complicated than back in Canada."
OTN carries a crew of nine: a captain, two mates, two engineers, three operators/seamen and a cook. The day is divided into three watches, with dredging and discharge going on around the clock for six weeks before crew change. Once about every two weeks, the boat ties up at the nearest port for fuel and fresh water. The whole crew goes ashore for a few hours of rest and relaxation. In addition to the OTN crew, Korok has a crew of two or sometimes three who live ashore and do survey work and supply runs as needed.
Moreau’s work station offers an ideal view of the activity and electronic display.
An onboard cradle for Korok in the dredge’s spoils hopper is one of the modifications made to OTN; while the dredge travels to its next job, Korok is loaded there. In fact, when OTN has traveled to the Caribbean, sections of floating pipe are carried in the hopper. Another modification is the catwalk over the starboard side of the hopper for safer access to the bow.
OTN operates on two Caterpillar C32 main engines driving two Schottel SRP 550 FPs. In addition, a Caterpillar C18 in the bow operates a 300-hp bow thruster made by Hidraulica Vigo. When worked in combination with the z-drives aft, it allows OTN to rotate in its own length. A third Caterpillar C32 drives the Damen BP5045 pump, which has the capacity of 7,193 cubic yards per hour.
While dredging, the captain (or mate) and operator monitor identical screens running DredgePack. The captain can maneuver the vessel over an area identified for removal as the dredge operator, seated nearby, controls the depth of the dredge arm. “It’s like erasing pixels from a screen,” explained Drouin. The two omnidirectional z-drives and bow thruster position the vessel — and thereby the drag head — as needed.
Once the spoils have been dumped and the vessel returns to the designated dredge area, the drag arm is lowered and the process of filling the hopper begins anew.
The time it takes to fill the hopper depends on factors such as depth, composition of the bottom material and weather. “Our record is 52 minutes,” said Drouin, “but that was at the beginning of the job at Becancour.” As an area gets dredged closer to the required depth, filling the hopper can take as long as six hours. Also, debris or garbage along the bottom can slow the fill times.
Dumping, on the other hand, can take as little as two minutes. It is accomplished by huge hydraulic rams, about 16 feet in length when closed and over 23 feet when extended, which force the two “hulls” apart 45 degrees. Alternate means of dumping include using a floating pipe and rainbowing — using a gooseneck attachment to propel spoils in an arc to a target location like the beach.
In June 2015, OTN was nearing completion of dredge operations in an area called La Petite-Floride where the Becancour River flows into the St. Lawrence. When the dredge hopper had reached capacity, the drag arm was raised and the vessel headed downriver about an hour to the dump area designated by the Canadian Coast Guard and Environment Canada — a grid with about two dozen points among which dumps were to be alternated. When the mate had the vessel precisely in location, he removed the cover from the touch-screen hydraulic controls and initiated the hopper opening sequence. In less than two minutes, the smooth drying sand piles in the hopper slid and tumbled downward, darkening as they mixed with water below. As soon as the bottom of the hopper was filled with just water, the vessel returned to La Petite-Floride.