FEATURE
OFFSHORE
Diesel Electric Offshore
A proven success; a promising future
By Alan Haig-Brown
The Seattle-based naval architect firm of Guido Perla and Associates (GPA) has become well known for their platform supply vessels (PSV) utilizing diesel electric propulsion. The vessels have enjoyed a wide welcome in the industry with all of them going to work as soon as they have been delivered from shipyards in the U.S. and China. GPA had long promoted the technology but it was not until Larry Rigdon launched his Rigdon Marine Inc. in 2002 that they found a client to match their enthusiasm. The first of the new Rigdon vessels went to work in June of 2004 and the demand for more has not stopped since. When Rigdon Marine was sold to Gulf Mark Offshore in 2008 the company had 20 diesel electric PSVs. Bourbon Offshore is in the second year of a 76-vessel order of GPA OSV and anchor handling tugs (AHT). These are being delivered at the rate of one about every eleven days from Zhejiang and Dayang shipyards in China assuring that the technology will gain a lot more exposure in the global market. Guido Perla and Associates designer Dan Koch recently explained some of the advantages that diesel electric systems can bring to the marine environment. "A vessel's power systems should be designed to meet the particular operating modes of that vessel," he said. "GPA started considering this when designing the large factory trawlers for the Alaskan fishery where there were large electrical demands for freezing as well as propulsion. However, the industry wasn't yet ready. Later we designed casino boats where the electrical needs of the slot machines and other hotel services were primary and propulsion was secondary." When they became involved in the off shore oil industry they saw a ready-made role for diesel electric in the vessels that spend a good part of their time standingby off oil platforms. When customer demand for Dynamic Positioning II was added with full redundancies, the diesel electric advantages became even more pronounced. The components of a dieselelectric system as designed and recommended by GPA are: a set of three or more diesel-powered electric generators, propulsion switchboard with power man44
agement software, a variable speed drive and electric motors with azimuthing thrusters. The diesel generators on boats such as the Rigdon/Gulfmark GPA654 vessels (where "6" designates PSV and 54 is the length in meters) are a pair of Cummins
KTA-50(D)M1-powered 1,235 kW and one Cummins KTA-19(D)M1-powered 425 kW set. Koch calls this the father-son arrangement and it can work well for the varied operational profiles of the vessels. With this system a PSV holding position under an oil rig in DP2 mode would have
Capt. Bill Barlow at the helm of the well equipped wheelhouse on the PSV Double Eagle. Below: Oiler Terrence Wagner in the engine room of the PSV Double Eagle with two Cummins KTA50 and one KTA19-powered gen sets.
the two larger generators running. They would feed current to the bus which would send it on to the driver. GPA favors the use of variable speed drivers which are the heart of the system. This in turn allows a variable speed motor to allow full control of the azimuthing drive propulsive power without adding the complexity of controllable pitch propellers. Each variable speed drive provides the power requirements of the two Z-drives and the two bow-thrusters to turn the propellers in such a manner as to keep the vessel on position. Also in the event that a problem causes one of the generators to go off-line, the second generator would be able to handle the load to maintain the dynamic positioning function, as the redundancy demanded by DP2 requires. Working in the DP mode with these redundancy requirements, GPA calculates that there is about a 30 percent load factor with the three-generator arrangement. "Our goal is to optimize the machinery with the requirements of the ship," Koch said, "which may require additional generators to provide smaller increments of power, or other energy storage devices." In another role, where the PSV is serving as a survey vessel and is towing sonar-type devices at slow speeds for hours on end, redundancy is no longer required. In this mode the load percentage has been seen to increase from 35 to 40 percent. If the boat is holding position in calm waters, the smaller genset alone may be able to provide the required power without any redundancy or spinning reserves required. The idea with diesel electric, in applications with variable power needs, is to have multiple increments of available power. "We try to balance initial capital costs with fuel savings over time. The GPA654 vessels are showing a 13 to 19 percent fuel saving over all for time at sea." Diesel electric is not always the best choice. If a vessel is designed to go only from point A to point B a mechanical drive may be better. Mechanical drives like a gearbox, shaft and propeller will lose about three percent of their power in the transmission of that power from engine to propeller. An electrical drive may lose seven to nine percent between engine and propeller. However, there are
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