LNG has been used as an alternative fuel to heavy oil and diesel oil on board ships for decades. Primarily this has been in LNG carriers, where, since the early 1960s, it has been used as a way to mitigate the natural boil-off gas generated within the atmospheric cargo tanks, exploiting it for efficiently covering the ship’s propulsion and electrical powering demands, and for reducing ship emissions.

Global shipping emissions regulations have become progressively tighter and more widely applicable since 2008, with reduction targets introduced for SOX and NOX. 1,2 More recently, CO₂ reduction schemes imposed by the EU (including the EU Emissions Trading System and FuelEU, which entered into force in January 2025³) as well as the International Maritime Organization’s forthcoming Net Zero Framework (scheduled for adoption between 2026 and 2030⁴ ) are both designed to achieve net zero greenhouse gas emissions by 2050. As a result, LNG has emerged as the most attractive fuel option for almost all ship types and sizes, driving the need for efficient and robust fuel gas supply systems (FGSS) to be installed on ships.

The main internal combustion propulsion engines for ships which can burn LNG can be either a 2-stroke diesel cycle where the delivery pressure is elevated at up to 380 bar high pressure (HP), 2-stroke otto cycle at delivery pressure of 13 bar, and similar low pressure (LP) for 4-stroke generating sets.

A typical FGSS is designed to transfer the LNG stored in a cryogenic containment tank (pressurizer of atmospheric) at the required temperature and pressure conditions demanded by the main consumers, such as the main propulsion engine, the generating sets and the boilers which can be either 2-stroke LP, 2-stroke HP, and/or 4-stroke LP.

It comprises of a low pressure transfer (booster) pump which sends the LNG to two major fuel supply sources: the high pressure pumping unit and the low pressure vaporizer, while natural boil-off gas can also be transferred through a BOG compressor directly to consumers.

In such FGSS configurations, the most critical and demanding component of a HP fuel gas system is the pumping unit, since it provides for the main propulsion power of the ship over a high delivery pressure (up to 380 bar). The most efficient method for a pumping unit is through cryogenic reciprocating (piston) pumps.

Each reciprocating piston pump is direct-driven by an electric motor through a gearbox or a belt drive, a lubrication system which provides lubricating oil to the warm ends (crankcases) of the pump and all the necessary controls, safety devices, and instrumentation, all of which is mounted on a common structural steel base or skid.

The cryogenic reciprocating pump consists of a ‘warm end’ or drive assembly containing the drive mechanism and a ‘cold end’ in which the cryogenic liquid is actually pumped from low pressure to high pressure. The warm end housing consists of modular ductile iron castings encompassing the drive components for all three cylinders. An intermediate housing connects the warm end housing to each cold end cylinder housing. This intermediate housing thermally isolates the cold end from the warm end. The warm end contains a crankshaft supported on main bearings with end play controlled by a thrust bearing. A connecting rod, wrist pin, and crosshead that convert the rotary motion into reciprocating motion complete the drive components for each cylinder. The warm end drive components (bearings, crosshead, and wrist pin) are pressure lubricated with oil from an external lubrication system.


Special intermediate housings are used, completely sealed from the atmosphere, and are provided with purge gas inlet and discharge ports. The purge gas maintains safe conditions if a cold end packing should leak and allow LNG to penetrate into the intermediate; in addition, the dry nitrogen purge gas prevents formation of ice on the cold piston surface exposed in the intermediate. The purge gas and any entrained natural gas are vented overboard. Piping systems for the inlet and outlet purge gas are provided on the pump skid.

Depending on the pump size (piston diameter and number of cylinders) and drive speed discharge, pressures can reach up to 6600 psig (450 bar) and flows from 1.2 to 52 gal./min. (4.5 to 200 l/min.)

Cold ends are available in different diameters for flexible matchup to the desired flow rate and pressure. Each cold end consists of a piston rod driven from the warm end crosshead, a cylinder sleeve, and suction and discharge valves all contained within a stainless steel housing. The suction valve is a flat plate type located at the end of the piston chamber. The discharge valve, also located at the end of the piston chamber, is a flat-faced poppet type that opens automatically when the pressure in the cylinder is above the downstream system pressure and resets during the suction stroke. The piston is provided with several sets of piston rings to seal the pumping chamber from the remainder of the housing. The stainless steel piston is of sufficient length to provide adequate thermal isolation, thus limiting heat leak into the liquid being pumped while allowing the piston packing, located near the warm end, to remain relatively warm.

In general, the life expectancy of a cold end can reach up to 4000 hrs or more, depending on LNG quality and operating conditions.

The cold ends include packing seals and piston rings which have to be replaced each time the cold end is overhauled to ensure tightness and the ongoing good condition of the packing seals.

During shipboard operations, certain challenges are faced which require a robust, reliable, and redundant HP fuel gas pumping system to be considered for uninterrupted operations.

For instance, LNG quality is impactful to the system functionality and risk for cavitations exist as it can vary from bunkering spot to bunkering spot, while all such propulsion fuel systems on ships are dual-fuel, hence operators can either switch frequently from diesel oil to LNG with repeated cycles of warming up/cooling-down that stresses the cold ends. The adaptability of the HP pumping system to main engine load variations is also very important, especially at heavy seas, while electric variable frequency drive-controlled reciprocating pumps present an ideal solution to this challenge. Turn-down ratio should also be as small as possible to avoid excess BOG generation in the tank.

A well designed and robust high pressure fuel gas pumping unit for a ship requires elevated redundancy (i.e. one pump operating and one stand-by at each 100% capacity), smart and integrated control functions aligned with main engine load demand and profile, high quality soft elements, precision engineering, low net positive suction head (NPSH) design features, and the ability for augmented functionality, such as the incorporation of heat exchangers based partial BOG management during voyage.

[1] IMO Resolution MEPC. 320 (74) - implementation of 0.5% sulphur limit in marine fuels.

[2] IMO Nox Technical Code MEPC.177(58).

[3] Regulation EU 2023/1805.

[4] IMO Net Zero Framework, MEPC 83.