style tanks with a stainless-steel membrane. Membrane tankers are more common because they are smaller than moss ships for the same amount of LNG fuel carried, but they create more boil-off gas than Moss style ships. A study by MEC Intelligence found said that LNG will be the primary fuel source for all merchant ships within 40 years. Many companies have already begun looking at the process of switching their fleets over to LNG propulsion systems. LNG vessel propulsion systems typically come equipped with WHRs, because they can lead to reduced emissions, reduced fuel consumption, and better efficiency. Switching to LNG powered vessels is a complicated task for companies, but combined with modern Waste Heat Reduction systems (WHRs), LNG vessels can be more efficient than diesel or steam propelled vessels. Propulsion systems Most propulsion systems in LNG carriers use the BOG and liquid fuels. In a steam plant, the BOG is used to fire the boilers and produce steam. The steam drives the turbines and propels the ship. The advantage to this type is that when the LNG cargo tank pressure is elevated the excessive BOG is burned simultaneously with liquid fuel. If there isn't enough BOG, liquid fuel (heavy fuel oil or HFO) is used to keep the plant operating. An alternative to the steam turbine engine is the dual-fuel marine diesel engine. Commercial ship propulsion system manufacturers such as Finland's Wärtsilä and Germany's MAN Diesel are producing large bore dual-fuel diesel engines. The MAN B&W ME-GI Engines have extremely flexible fuel modes that range from 95% natural gas to 100% HFO and anywhere in between. A minimum of 5% HFO for pilot oil is required as these are compression ignition engines and natural gas is not self-combustible. Steam turbines are exclusively the primary moving source for LNG ships, even though 2-stroke diesel engines are more efficient. This is because the boil-off gas from LNG needs to be utilized. Cost benefits Recent research has been focused on using LNG for fuel on ships other than LNG tankers. These studies show that LNG stands out in terms of emissions reduction and reduced operational costs. Some economic incentives have been shown to be advantageous to running an LNG propulsion system. When certain systems such as waste heat recovery (using waste heat to do work rather than dissipate) are added to the power plant, significant savings can be observed. One study shows