Wärtsilä BOG Reliquefaction

Wärtsilä is a leading designer, developer and supplier of energy efficient LNG Boil-Off Gas (BOG) reliquefaction plants. These systems are designed to be efficient, reliable, safe, robust and flexible.
With more than 50 systems in operation, Wärtsilä has the highest number of BOG reliquefaction plants installed in the global LNG carrier fleet.
We work closely with our customers to develop technological solutions that meet their needs for increased fuel flexibility, energy efficiency, and environmental performance in today's fast changing LNG market.

    Technical data

    The BOG reliquefaction process is based on reversed nitrogen Brayton cycle refrigeration technology. This means that the process is a closed nitrogen cycle for extracting heat from the boil-off gas. Typically, the reliquefaction system is used to control the cargo tank pressure by liquefying boil-off gas. They have the capability to handle all boil-off gas (100% capacity) or only excessive boil-off gas not burned in the engines (partly liquefaction).

    Our compact reliq module is our updated reversed nitrogen Brayton offering, our experience and proven track record is now translated in to a compact and reliable module. Key benefits are low maintenance and safe operation. The integrated magnetic bearing compressor only needs minimal maintenance every 5th year and with commercial grade nitrogen as refrigerant we ensure a safe operation.

    For smaller BOG capacities we do also have the Mixed Refrigerant (MR) system who offers supreme efficiency.

    The BOG reliquefaction system could be installed for vessels, offshore installations, or onshore facilities such as LNG terminals.

    Figure 1: Nitrogen loop
    Nitrogen loop
    Figure 2: Compact Reliq
    Deckhouse arrangement
    Figure 3: Mixed refrigerant (MR) loop (for onshore applications only)
    Mixed refrigerant MR loop
    Wärtsilä has commercialised and improved reliquefaction technology since it was originally patented by Kværner and licensed by Hamworthy in the late 1990s.

    Our boil-off gas (BOG) reliquefaction plants have been installed on numerous vessels, including the entire Q-Flex fleet of 216.000 m3 LNG carriers. The 31 Wärtsilä LNG BOG reliquefaction plants delivered to the Q-Flex fleet are installed on ships driven by slow speed diesel propulsion engines. We have also delivered the same type of systems to several LNG carriers with DFDE, XDF and MEGI propulsion systems.

    Wärtsilä are supplying complete and integrated systems that include BOG reliquefaction and fuel gas supply systems as compressor solutions, and LNG pump solutions required for feeding engines with low and/or high pressure gas.

    The installation of a BOG reliquefaction system on dual-fuel LNG carriers, allows ship operators the added flexibility to switch between fuels to take advantage of price differentials between LNG and heavy fuel oil. State-of-the-art propulsion systems are efficient and not all the boil-off gas can be utilized in the engine. Furthermore, slow speed sailing and ship holding operations often result in an excess of boil-off gas. Instead of burning this gas in a gas combustion unit, it can be reliquefied and returned to the cargo tanks.

    The liquefaction of BOG on LNG carriers results in increased cargo deliveries and allows owners and operators to choose the optimal propulsion system and operating profile.  The advantages are extremely attractive; a flexible fuel system, optimized operating costs, and increased delivered cargo capacity.
    Key benefits

    Reliq in demand for retrofit and newbuilds due to environmental focus from charters around the globe – GCU only allowed for emergency situations.

    • Chartering models / contracts have changed the latest years – short term and spot market driven
    • Reliq is interesting to ship owners - will make the vessels more attractive and flexible towards charters.

    Reliable and proven plants, with the most extensive BOG reliquefaction reference list.

    • More than 50 BOG reliquefaction plants delivered.

    The most comprehensive system solutions throughout entire gas value chain

    • Competitive capex in all capacity ranges, two technologies (MR and Brayton)
    • Optimized process for low opex cost
    • Could supply larger and complex systems

    Wärtsilä is a life cycle oriented company 

    • World wide service network
    • Smart digital solutions for optimal performance, 24/7 service available.  

    Compact Reliq™

    Newly developed Wärtsilä Compact re-liquefaction unit is a based on the reversed nitrogen technology. The re-liquefaction plant will condensate boil of gas and return the condensate to the cargo tank. The system utilises an integrated magnetic bearing compressor, with a single-sealed compressor/expander. The re-liquefaction unit is smart marine ready with OPERIM, instrumented for remote operations, health monitoring and online operational supports.


    • Compact Reliq for Knutsen LNG carriers
      The easy to install solution is designed to reliquefy the boil-off gas (BOG) from gas carriers and LNG bunker vessels, and to keep the cargo cool under all operating conditions.

      Compact Reliq for Knutsen LNG carriers

    • LNG vessel for Knutsen OAS Shipping
      The latest development of the LNG infrastructure provides clear evidence of the increasing importance of LNG fuel to the marine industry. Wärtsilä has a lot of experience and know-how throughout the LNG value chain, and we are confident that their solutions are what we need for this latest expansion to our fleet.

      LNG vessel for Knutsen OAS Shipping

    Press releases

    Related products


    BOG reliquefaction takes another step...

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    21 April 2021
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    Completing the LNG value chain

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    17 May 2013
    • In Detail article

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