Posted on June 19, 2017
The U.S. Army Corps of Engineers operates two hopper dredges on the west coast of the U.S.A. to maintain shipping channels. The largest and most modern of them is the “Essayons,” built by Bath Iron works in Bath, Maine in 1982. Based in Portland, Oregon, the 106.7 meter Essayons works in harbors between Alaska and California, as well as in Hawaii. Routine work occurs on the edge of shipping channels while commercial ships pass nearby, and also close to jetties, reefs and wrecks, even in marginal weather, so both vessel and crew must be prepared for every eventuality.
Essayons was originally powered by 4x EMD 645 main engines (two for propulsion and two for dredge pumping) and 3x Cat D399 generator sets. In addition to its normal navigation equipment and hotel needs for a crew of 24, the ship needs 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 in service, the main engines were showing their age, and did not meet the latest air-quality standards for California harbours.
Consequently, the U.S. Congress approved funds to re-power the ship and install a new power-distribution system. The new line-up consists of eight EPA Tier 2 compliant Cat marine engines: 4x Cat C280-12 main engines, 3x Cat 3512C generator sets and 1x Cat C18 emergency generator set. The Halton Company, the local Cat® dealer, provided consulting services for the installation, with Cascade General providing project management at the Portland Shipyard.
Unfortunately, engine rooms seldom allow for easy engine replacement. All the piping and wire runs in the forward engine room bulkhead had to be dismantled, and an opening was cut toward the hopper to prepare for the re-power. After the old engines were removed, the new Cat C280-12 engines, weighing a total 40 tons with generators, were craned into the hold and skidded into position.
The Cat C280-12 is a 222 L, vee-type, 12 cylinder, medium-speed marine engine with electronic ADEM™ A3 control. It produces 3,460 kW at 900 rpm for continuous service and meets EPA Tier 2 emission standards. The dredge’s two outer C280-12 units are fitted with reduction gears turning controllablepitch propellers that enable the engines to run at an efficient 750-950 rpm while the ship is dredging at only 1-2 knots. The two inner units are connected to Kato 600 V generators each producing 3,250 kW of electrical power.
The smaller Cat 3512C generator sets are located in a separate engine room and are also connected to Kato generators, each rated 1,030 kW at 1,800 rpm. All three generator sets are set on flex mounts to reduce vibration and noise.
An automated power management system monitors engine functions, temperatures, and pressures, which are displayed on ten computer work stations in various locations on the bridge, engine room, and fire-fighting station.
The two Cat C280 generator sets supply power to the 600 V bus, whereas the three Cat 3512C generator sets serve the 480 V bus. The dredge pumps and the bow thruster run off the 600 V bus, and the dredging hydraulics, as well as the rest of the ship’s electrical load, run off the 480 V bus. Both busses are cross-connected via circuit breakers and a transformer, guaranteeing maximum flexibility in load sharing.
Located high above the waterline is the 6 cylinder Cat C18 emergency generator set, developing 425 ekW at 1,800 rpm, which is sufficient to keep the lights running should the ship be damaged by some hazard.
“The new engines have greatly improved our operational efficiency,” said Captain James Holcroft, who has been in command of the Essayons for six years. “With the old engines, when dredging upstream and going against a strong current, we barely had enough power to maintain forward motion. With the new Cat engines, we have an extra 2,000 hp enabling us to get the job done even under difficult conditions.”
Source: CAT
