gas carriers
Gas turbines renew challenge for power
A strong environment-led case for dual-fuel gas turbine-based propulsion plant can now be made
by Doug Woodyard
iesel propulsion in LNG carriers was pioneered in 1972 by the 29,000m3 Venator with an installation based on a dual-fuel burning Sulzer 7RNMD 90 low speed two-stroke engine. Steam turbines shrugged off this short-lived challenge to their traditional dominance of the sector, however, and retained total supremacy in the LNGC propulsion market for another 30 years.
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The introduction of significantly larger carriers than before – with cargo capacities up to double the classic 125,000m3 size – stimulated interest in other propulsion systems and eventually stifled steam’s hegemony. Dual- fuel medium speed engines arranged in electric power stations and low speed heavy fuel oil- burning diesel engines largely ousted the turbine from the newbuilding lists. A more recent challenge was presented by dual-fuel low speed engines designed to burn the cargo boil-off gas. A new generation of larger LNG carriers
in the past decade also attracted the interest of the major gas turbine suppliers GE Marine and Rolls-Royce, whose power-dense
derived designs promised significant benefits, particularly in conjunction with electric drives. Compactness and machinery layout flexibility could be exploited to enhance cargo carrying capacity within given ship dimensions. Excellent emissions characteristics were also cited, along with low weight and volume; high reliability; reduced installation
costs; freely-
located plant; and low noise and vibration. Rolls-Royce vigorously marketed its proposals over several years after joint studies with Shell Shipping Technology on various propulsion options for LNGCs. A dual-fuel gas turbine- electric system for larger ships, particularly those over 200,000m3 capacity, featured two gas turbine gensets in a father-and-son configuration. The larger genset was based on the (then)
new Rolls-Royce MT30 turbine, flat rated at 36MW, and the smaller set on the group’s 501 turbine, rated at 5,000kW. The MT30 set would provide all power for seagoing service, while the 501 supplied power for cargo pumping and port duties; in addition, the smaller set provided a get-home facility in the event of non-availability of the main genset. A diesel-driven harbour genset was rated at around 1,500kW.
The gas turbine gensets would be supplied aero-
as packaged units and located in a housing at main deck level aft of the accommodation; the housing would be similar in dimensions to the boiler room casing of contemporary steam- powered LNGCs.
Such an arrangement eased access for removing gas turbines for maintenance and facilitated short and efficient intake and exhaust trunkings. It also contributed to a short engineroom which, together with the low weight of the gas turbine plant, allowed the aft hull form to be optimised and the cargo volume maximised within given hull dimensions. A reduced engineroom length was one of the merits of an electric rather than a direct-drive system. A short machinery room was further fostered by installing the electrical gear and controls for the main propulsion motors in the top of the engineroom, with the geared motors arranged at the bottom. Primary fuel for the turbines would be cargo
boil-off gas supplemented by vaporised LNG as required. Liquid fuel of marine gas oil quality would be carried as a secondary emergency fuel and for transits to and from drydock when gas was not available. Tapping years of experience with Rolls-Royce dual-fuelled gas turbines in the oil and gas
Compact LNGC propulsion configurations proposed by Rolls-Royce were based on its MT30 aero-derived gas turbine
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