Dual Fuel Engines for LNG Vessels

The American Bureau of Shipping published a Guide for Design and Installation of Dual Fuel Engines in response to an increased interest in applications of dual fuel engines. Every large LNG vessel constructed so far has been fitted with a steam turbine powered propulsion system. Recently, many owners and shipyards, in view of the considered inefficiency of the steam plant and lack of availability of experienced crews for steam vessels, are seeking alternatives to steam. It appears that the most favored arrangement by the shipyards and the owners consists of four sets of dual fuel diesel generators supplying power to the propulsion motors.

There are various proposed machinery arrangements being considered by the industry which include:

• Two direct driven main diesel engines and a reliquefaction plant
• Diesel engine or gas turbine driven generators with one propulsion shafting system and a reliquefaction plant
• Diesel engine or gas turbine driven generators with two propulsion shafting systems and a reliquefaction plant
• Diesel engine or gas turbine driven generators with two azimuthing thrusters and a reliquefaction plant

The dual fuel engines have been installed for offshore and for onshore power plant applications for several years. The dual fuel engines proposed for LNG vessels have been developed on the sameprinciples. A double wall piping system with the gas fuel contained in the inner pipe or the gas fuel piping installed within a ventilated pipe or duct is an ABS Steel Vessel Rules requirement based on the IGC Code for the installation of gas fuel supply piping. Both concepts seek to encase the gas fuel pipes in double wall piping or ducting, as well as to provide a hood over the engine with ventilation, such that in the event of gas leakage, the exhaust fans will remove any leaked gas immediately. For diesel engine installations, however, it would be very difficult to carry out maintenance on the engines if a hood was installed over the engines or double wall piping was fitted in the engine compartment containing the dual fuel engines. Further, the risk of improper installation will be increased each time the double wall pipes, hood, etc. are dismantled and reinstalled. Therefore, industry is proposing to designate the entire engine compartment containing just the dual fuel diesel engines with single walled gas fuel supply piping as being equivalent to the hood required by the present Steel Vessel Rules and IGC Code. The subject Dual Fuel Engine Guide is built around this concept.