Retrofit to X-DF dual-fuel propulsion for Moss-type LNG carriers

In October 2023, NYK was granted Approval in Principle (AiP) from the Japanese classification society Class NK for converting the steam turbine of an LNG carrier to a dual-fuel diesel engine. This is the first AiP granted by Class NK for main engine conversion on LNG carriers. 

As part of the project, low pressure Otto cycle dual-fuel engines will replace the LNG carriers’ existing steam-turbines. The ship owner and operator plans to install WinGD’s X-DF engines. 

The project is expected to see the vessels utilise boil-off gas generated from the vessels’ Moss-type LNG tanks to power the X-DF engines, which is expected to reduce fuel consumption. The ability to combust BOG gas, along with significant advances in the efficiency of low-speed low-pressure dual-fuel engines has contributed to the eclipse of steam turbine propulsion solutions for LNG carriers in recent years. NYK noted that the conversion project was expected to extend the operational life of steam turbine–driven LNG carriers, which would otherwise be scrapped due to their inferior fuel economic performance compared with dual-fuel LNG carriers.

The Motorship notes that the retrofit programme is understood to relate to Sayaendo-class LNG carriers, which were first introduced in 2014. The design was superseded by the Sayaringo-class of steam-turbine powered LNG carriers, which were first introduced into NYK’s fleet in 2018. The twin skeg Sayaringo vessels feature an ultra-steam turbine (UST) plant on the port side, as well as a dual-fuel diesel engine on the starboard side, unlike the Sayaendo vessels.