The U.S. Army Corps of Engineers operates two hopper dredges on the United States West Coast and also contractswith private companies to keep shipping channels open. The largest and most modern vessel of the two is the recently repowered 107 m Essayons, built by Bath Iron Works in Bath, Maine, U.S.A., in 1982. It is based in Portland, Oregon, U.S.A. and its annual schedule includes work in U.S. harbors between Alaska and California, plus Hawaii.
The Essayons was launched with four 2685 kW engines, two for propulsion and two for dredge pumping, and equipped with three 850 kW, 1200r/min generator sets to support its high service power demand when dredging. In addition to its normal navigation equipment and hotel needs for a crew of 24, the ship requires 60 Hz power to run numerous valves, including those used for flushing and jetting in the hoppers and filling the ballast tanks. After 25 years of service, the main engines were showing their age and also did not meet the latest air-quality standards for California harbors. The U.S. Congress approved the funds to repower the ship and install a new power-distribution system.
The new power lineup consists of eight new Caterpillar engines that meet Tier 2 standards.
The repowering contract was awarded to Cascade General, and the project commenced in December2007 at the Portland Shipyard. The Halton Company, the local Caterpillar dealer, provided consulting services for the installation. M. Rosenblatt and Sons Group, a division of AMSEC LLC, was the design and engineering contractor.
Ships’ engine rooms are rarely laid out to allow easy removal of an entire engine, and a great deal of planning and effort was required to clear an exit path for the original main engines. All the piping and wire that runs in the center of the forward engine room bulkhead had to be dismantled, and an opening cut to give access to the hopper.
The old engines were then removed and the new Cat C280s, were installed. Each, with generators attached, were lowered into the hold by crane endfirst, turned 90°, maneuvered through the opening and skidded into position.
The 222 L, Vee-configuration, 12-cylinder, medium-speed engines produce 3460 kW at a continuous service rating. Fuel for all four engines passes through an Alfa Laval fuel purifier.
The two outer units are fitted with Haley 5.8:1 reduction gears turning controllable-pitch propellers that enable the engines to run at an efficient 750 to 950 r/min while the ship is dredging at only 1 to 2 knots. The two inner units are for electrical generation and each produces 3250 KW. The 600 V generators are manufactured by Kato.
After the three old service generators were disconnected, they were lifted off their beds and skidded to the side of the ship where a patch was opened in the topsides. They were replaced with Cat’s 3512C 12-cylinder, Vee-configuration engines fitted with Kato generators. Each is rated 1030 kW for continuous service at 1800 r/min, and set on VMC flex mounts to reduce vibration and noise.
The generators are controlled using touchscreen interface panels through an automated power management system designed and supplied by Ockerman and Associates. The 600 V and 480 V switchboard was supplied by Lloyd Controls. All engine functions, temperatures and pressures are monitored and displayed on ten computer workstations mounted in various locations on the bridge, the engine room and the firefighting station. This integrated control and monitoring system was designed and supplied by Siemens.
The two C280-powered generators supply power to the 600 V bus. The three 3512C engines supply power to the 480 V bus. The 480 V and the 600V buses are cross-connected via circuit breakers and a transformer. Therefore, a 600 V engine can drive all the 480 V equipment through the transformer or the 480 V generators can drive the 600 V equipment through the transformer. The dredge pumps’ electric motors and the 746 kW HRP bow thruster run off the 600 V bus. The dredging hydraulics and the rest of the ship’s electrical load run off of the 480 V bus. Since the 480 V and the 600 V buses are cross-connected, the crew can operate the dredge with a variety of generator configurations.
For example, when dredging, they can use two C280s to supply power for all electrical loads on the ship at 480 V and 600 V, if they are transiting and do not need the dredge pumps, they can run just two of the 3512 engines for the bow thruster (600 V) and all the rest of the load (480 V). The Ockerman power management system automatically controls the load and the generator load sharing across both the 600 V and 480 V buses.
“These new engines will greatly improve our operational efficiency,” said Captain James Holcroft, who has been in command of the Essayons for five years. “With the old engines, when we were dredging upstream in a river and going against a strong current of 6 to 7 knots, we barely had enough power to maintain forward motion. With these modern engines, we will have an extra 2000 hp [1491 kW] to enable us to get the job done in difficult conditions. We will also benefit by being able to spot potential problems by checking the engines’ performance on the screens, and by only having one brand of engine parts to stock.”
The last of the eight engines is the emergency generator, located high above the waterline on top of the deck house. This is a six-cylinder Cat C18, developing 425 kWe at 1800 r/min.