LNG and steam turbines

Hello.

I could not find a more appropriated section!

I've reading about LNG Carries switching from steam turbines to dual-fuel reciprocating engines since a while due to the economical advantages of adding HFO as a fuel etc.

This is all quite clear but I couldn't find if in a 'Pure LNG' (or ever LNG + diesel) setup that advantage would hold, specially when using these more advanced/efficient steam turbines like MHI UST series.

That said, in a context of a 'Pure LNG' large vessel using the most efficient available equipment, how the "steam turbine vs reciprocating engine" efficiency would be?

PS. This is more a matter of curiosity.

Thank you.

 

Even with the most advanced steam cycles for marine propulsion, they are slightly lower overall efficiency than dual fuel reciprocating engines. Modern dual fuel recips, with very high pressure common rail fuel injection injecting a very small amount of fuel oil, are more efficient as well as avoiding the costs and complications of a marine steam plant.

 

Actually, I don't think it has anything at all to do with fuel efficiency of the main plant. Steam was generally selected in 1970s-1990s LNG construction because it could use low-cost boil-off while the cost of fuel oil was high. As dual fuel engines improved and the cost ratio between LNG and fuel oil changed, the economics of manning, lack of marine steam support in the industry, and the maintenance lay-days has forced steam into its little niche market. In terms of overall oil usage (which includes LO), steam still excels over reciprocating engines at the higher horsepowers, as well as having volumetric, weight, and arrangement advantages. However, the reduced manning, ability to do at sea maintenance, and less costly lay-days drive the over-all economics.