When thinking about maintenance, a shipping channel is probably not the first thing that comes to mind. However, maritime professionals know that these waterways require periodic attention. As tides, currents and river flow cause sand, gravel and silt to build up over time, the depth of a channel is reduced. The sediment must occasionally be removed from the bottom to maintain safe water depths for ship navigation and to ensure that traffic flow to ports continues uninterrupted.

In addition, as the amount of trade and cargo moving through ports has increased, the size of commercial vessels also has increased. These larger ships require deeper channels to navigate.

The U.S. Army Corps of Engineers constantly surveys the depth of waterways to determine when sediment buildup may become a navigation concern and require dredging to clear the channels. In fact, more than 300 million cubic yards of dredged materials are removed from U.S. navigation channels each year to accommodate the shipping industry's larger vessels.

MAINTENANCE DREDGING

Donjon Marine Co. recognizes maintenance dredging as an opportunity to increase its business. Founded in 1964, the company focuses primarily on marine salvage, transportation and ocean engineering. However, Donjon Marine recently capitalized on the changing shipping industry to expand its services by entering the dredging arena.

Based in Hillside, N.J., a suburb of Newark, Donjon Marine takes advantage of its proximity to the Atlantic Ocean by working on dredging projects for nearby river channels and straits that feed into the ocean. The company recently began working on a Corps project to deepen the Arthur Kill River channel, the waterway that separates New Jersey from the New York City borough of Staten Island. Corps engineers determined that the channel needed to be taken from a depth of 38 feet down to 43 feet in order to accommodate the new, larger ships that navigate the waterway.

According to the Corps' survey of the project area, the dredging would be so deep that Donjon Marine's crew would not only have to remove requisite silt, but 60,000 cubic yards of rock as well. This survey result is not uncommon; as channels become deeper, layers of sand and silt often give way to rock.

SEEKING THE RIGHT TOOL

The Donjon Marine project crew knew it would load the company's Caterpillar 385C hydraulic excavator onto the dredge, Newark Bay, but needed a tool that attached to the excavator to break up the rock. Finding a recommended attachment that was right for the job was key. A tool that was too small would slow down the project; a tool that was too heavy would place too much weight on the front of the dredging barge and cause it to tip. Donjon Marine turned to its long-time equipment supplier, Hoffman Equipment of Piscataway, N.J., for a suggestion on equipment that would fit the excavator and be capable of handling the rock removal task.

After reviewing the requirements of the job, Hoffman Equipment determined that the Atlas Copco HB 7000 hydraulic breaker was suitable, not only to the excavator and the demanding job, but also to underwater rock breaking. Considering the challenging task ahead, the Donjon Marine crew decided to use this machine.

Weighing 15,400 pounds, the breaker fit the weight specifications for the excavator and dredging barge while offering an impact force that could effectively break up the rock underwater. Often used in mining and rock quarry applications, the HB 7000 also is available with an underwater kit that makes it suitable for underwater breaking applications.

Atlas Copco's underwater system applies a compressed air source to the percussion chamber of the breaker to keep a positive displacement of air constantly coming out of the bottom of the breaker. Without this compressed air, the piston would draw water into the percussion chamber on its upstroke when the breaker is submerged. If the piston comes down on water, it can pressurize the water enough to break internal components and damage the breaker. Since the breaker would be working more than 40 feet under water, this kit was essential to ensure that the tool operated properly.

Jim Wright, Donjon Marine project superintendent, knew that using the right breaker would be paramount to removing the rock quickly and efficiently. “We had a large amount of rock to get through, so we wanted the largest and best breaker available,” Wright noted.

After the silt was cleared away, the HB 7000 started breaking up the rock. Next, the large pieces of rock were gathered from the bottom of the channel with a dredging bucket and deposited onto a dump scow. The scow carried the load of rock 22 miles out to sea for disposal.

The HB 7000 offered features to help the dredging crew keep the project running smoothly. The machine's StartSelect system helped facilitate the rock removal process. With the system set to Auto Stop, the HB 7000 will not function until the breaker comes up against a solid object, which helps an operator avoid hitting areas where rock is not present and eliminates the damaging effects of blank firing. This feature reduces the possibility of internal damage and increases the life of the hammer.

GPS VISION ASSIST

The HB 7000 also worked in conjunction with a GPS unit and excavator-cab-mounted screen to help operators monitor the dredging area. The GPS unit creates a map of the area to be dredged and allows operators to position the breaker with precision.

“These new systems help operators work more precisely in the dredging area because they prevent extra passes that cost time and money,” Wright said. “The systems also prevent us from missing any materials that need to be dredged, which means we do a better and more thorough job.”

Even with state-of-the-art technology and ideal working conditions, the harsh environment of working underwater can cause equipment to require service. When the breaker developed a hydraulic oil leak during the Arthur Kill River channel project, Atlas Copco dispatched service technicians to the site to fix the leak. This got Donjon Marine back on the project quickly.

Although extremely large ships that needed extra room to navigate or choppy seas (caused by inclement weather) sometimes caused the project to slow down, the HB 7000 helped Wright and his crew keep things on schedule by removing an average of 700 cubic yards of rock each day. When the water was calm and channel traffic was low, the breaker enabled the crew to remove up to 1,000 cubic yards of rock in one day. Donjon Marine finished the channel deepening project in two years.

DEEPER CHANNELS

Wright anticipates that the HB 7000 will become part of Donjon Marine's equipment fleet. In fact, the company recently purchased a second breaker to prepare for upcoming rock-breaking projects. It likely will see a quick return on investment, as the Corps plans to bid future contracts for dredging the Arthur Kill River channel down to 50 feet.

This continued deepening of the channel reflects the Corps' dedication to provide larger ships access to the river's docks. Moreover, this trend extends beyond the United States. According to a Corps report, container ports around the world are deepening navigation channels (down to 49 and 53 feet) to accommodate larger ships that will travel international waterways.

The shipping industry's need for deeper channels means more dredging contract opportunities for companies such as Donjon Marine. With more than 40 years of experience and tools such as the HB 7000, the company expects to dredge up more business while creating safer navigation channels to facilitate the flow of maritime traffic in and out of the nation's ports.

Information provided by Atlas Copco Construction Tools LLC, Westfield, Mass.