T C R

A notable COGAG plant powers the Stena HSS 1500-class high speed passenger/vehicle ferries, whose service speed of 40 knots is secured by twin General Electric LM2500 and twin LM1600 gas turbines arranged in father-and-son configurations with a total output of 68 000 kW. All four turbines are deployed for the maximum speed mode, with the larger or smaller pairs engaged alone for intermediate speeds; this enables the turbines to operate close to their optimum efficiency at different vessel speeds, with consequent benefits in fuel economy (Figure 31.2).

Figure 31.2 Final preparation of a General Electric LM2500 propulsion module for a large high speed ferry

An example of a CODEG plant is provided by QQQueen Mary 2, the world's largest passenger ship, whose 117200 kW power station combines twin General Electric LM2500+ sets with four Wartsila 16V46 medium speed diesel engines, all driving generators.

Combined-cycle gas turbine and steam turbine electric (COGES) plants embrace gensets driven by gas and steam turbines. Waste heat recovery boilers exploit the gas turbine exhaust and produce superheated steam (at around 30 bar) for the steam turbine genset. Such an arrangement completely changes the properties of the simple-

cycle gas turbine: while gas turbine efficiency decreases at low load the steam turbine recovers the lost power and feeds it back into the system. The result is a fairly constant fuel consumption over a wide operating range. Heat for ship services is taken directly from steam turbine extraction (condensing-type turbine) or from the steam turbine exhaust (back pressure turbine), and there is thus normally no need to fire auxiliary boilers.

Installations supplied by General Electric Marine Engines for large Royal Caribbean Cruises' liners pioneered the COGES plant at sea, the first entering service in mid-2000 (Figures 31.3 and 31.4). Each 59 000 kWe outfit comprises a pair of GE LM2500+ gas turbinegenerator sets, rated at 25 000 kWe apiece, and a 9000 kWe non-condensing steam turbine-generator. Heat recovery steam generators located in the exhaust ducts of the gas turbines produce the steam to drive the steam turbine and feed auxiliaries such as evaporators and heating systems. Since no additional fuel is consumed to drive the steam turbine, the additional power it generates represents a 1518 per cent increase in efficiency with the gas turbines operating at rated power. The auxiliary steam represents a further efficiency improvement.

Service condensate

Service condensate

turbine(s)

Figure 31.3 Schematic layout of COGES plant for a large cruise ship (GE Marine Engines)

turbine(s)

Figure 31.3 Schematic layout of COGES plant for a large cruise ship (GE Marine Engines)

Lightweight gas turbine-generator sets (weighing approximately 100 tons) have allowed naval architects to locate them in the base of a cruise ship's funnel. Such an arrangement replaces the gas turbine inlet and exhaust ducts normally running to the enginerooms with a smaller service trunk housing power lines, fuel and water supplies to the gas turbine package. A significant area on every deck between

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