Geared for success

ARCO Marine's eclectic approach to the development of a mold-breaking new generation of crude carriers to handle coastwise shipments out of Alaska has provided a broad platform for marine engineering technology. Just as experience culled from North Sea shuttle tanker operations helped build the technical basis for the Millennium tanker project, distinguished by its emphasis on service dependability, system redundancy and environmental compatibility, proven engineering plant design has formed the foundation for an innovative concept of oil carrier.

Reliability, durability and a tried and tested design derivation have been common threads in the selection of machinery, albeit applied in an unusual configuration comprising two, low-speed engines, twin propellers and shaftlines, each with a high-capacity shaft alternator driven off a power take-off (PTO) gear, and twin rudders. Each main engine and allied equipment is separated from the other by a longitudinal bulkhead, and each 'side' of the power train can be operated independently of the other.

While 'proven' engineering design and practice is a watchword for the project, the nature of the ship operating criteria laid down by ARCO has resulted in the adoption of the latest or newly-developed version of each individual element in the overall package. German marine transmissions specialist Renk, for example, has provided PTO gears which individually cover a generator power of 8,600 kW, and has developed a new disconnection clutch to enable the shaft alternator to be driven by the respective main engine with the propeller shaft stopped. For Vulkan Kupplungs, another German expert in a particular discipline, the design requirement for a PTO coupling rated for 8,600 kW necessitated an extension of its range to 800 kNm nominal torque from the previous maximum of 630 kNm. It was able to draw on 10 years' operating experience with its 630 kNm couplings fitted in shuttle tankers powered by low-speed diesels. Definition and basic design of the propulsion plant for the 125,000 dwt Millennium-class tankers was entrusted to MAN B&W licensee Manises Diesel, a Spanish enterprise owned by Astilleros Espanoles (AESA) and MAN B&W Diesel. The selected prime mover, the S50MC-C in its seven-cylinder version, is lighter and shorter than the preceding engine type, producing some 10 percent more power than the equivalent MC model. For each of the newbuilds at Avondale, two such engines producing 15,015-bhp (11,060-kW) apiece will be delivered from Europe.

Circle 243 on Reader Service Card The Renk friction clutch connecting the engine to the propeller and incorporating an integrated thrust bearing is perhaps the most innovative element of the propulsion system. The clutch allows the screw to be engaged and disengaged with the engine running at low speed, and its admissible torque is twice the nominal torque of the engine.

Thus, with the propeller disconnected, the PTO can be employed up to the full 8,600 kW rating of the ABB shaft generator for cargo handling purposes. The arrangements mean that the plant may be operated in a total of three modes, namely sea mode, maneuvering mode and cargo mode.