When the first LNG carriers were built nearly 40 years ago, the easiest way of handling the ever-present boil-off gas (BOG) was to use it for fuel in a simple steam boiler. In fact, the steam system design seen in modern LNGCs is very similar to the layout on the early ships. The plant usually comprises two boilers supplying steam to high and low pressure turbines, which, in turn, drive a single screw via a gearbox. Two steam turbo-alternators and one diesel genset typically provide electrical power. This arrangement is a proven means of propulsion for gas carriers, with the advantages of low maintenance and practically non-existent lubricating costs, plus the ability to burn HFO and BOG in any proportion. Furthermore, the steam plant offers an easy method to dispose of excess BOG by burning it to raise steam, which is then condensed. The major drawback, however, is the low efficiency of the turbine plant with the inevitable high fuel costs and consequent excessive CO_2 emissions.
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