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 35 systems in operation and an additional 8 plants under construction, Wärtsilä has the highest number of BOG reliquefaction plants installed in the global LNG carrier fleet. Similar solutions could also be installed with onshore LNG terminals.

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, combined with a process solution for separating nitrogen from the BOG.  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).

    For smaller BOG capacities the Mixed Refrigerant (MR) system offers a number of advantages, such as a standard component with shorter delivery time, reduced CAPEX, and easier maintenance.  The screw compressor in the MR loop is well proven in refrigerant loops.

    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: Mixed refrigerant (MR) loop
    Mixed refrigerant MR loop
    Figure 3: Deckhouse arrangement
    Deckhouse arrangement
    Applications

    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 a number of LNG carriers with dual-fuel diesel electric (DFDE) propulsion.

    Wärtsilä has a number of new contracts where our reliquefaction systems are combined with different propulsion systems.  This means that we 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 so as 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
    • The reliquefaction and gas supply systems have full flexibility in order to condensate the BOG at different capacities according to the specific voyage.
    • The gas supply system has full flexibility to feed the engine with a different capacity of BOG and/or LNG from cargo tanks.
    • The gas supply system is designed to handle different fuel combinations.
    • The reliquefaction and gas systems have full flexibility to handle changes in the LNG composition.
    • The reliquefaction system uses advanced technology with reduced power consumption.
    • The system is controlled fully automatically using a proven control system.
    • The system has a short response time for capacity adjustments.
    • The reliquefaction system is based on a well established and tested technology.Wärtsilä has several onshore and marine references for this application.
    • The system use only proven components that are first class, high quality with extensive references.
    • Engine operation does not affect the reliquefaction unit.

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