As LNG is adopted as a key transition fuel for meeting environmental targets, LNG reliquefaction systems are set to perform an important role

Gavin Forward CEng, Senior Project Manager

With the imminent IMO requirements for reducing global fleet emissions by 2030, LNG is set to be adopted by many operators as a key transition fuel for achieving these emission targets. Increased demand for LNG as a bunker fuel will consequently lead to an upsurge in the global ship-to-ship bunker market, owing to its ability for high capacity and quick transfer. This increased demand for new bunker tonnage, coupled with rising world fuel prices and an emphasis on more economical operations with less waste will see LNG reliquefaction (subcooler) systems playing an increasingly important role. New subcooler technologies for the management of boil gas have allowed for economical and compact retrofit solutions on these smaller vessels.

LNG Market

LNG is becoming of increased importance within the maritime market. Current world events in the first quarter of 2022 have led to further demand for LNG transportation and the resultant upsurge in the LNG carrier market, with over 200 new LNG carriers on order¹. With increasing pressure to reduce maritime emissions, LNG is now seen a recognised and preferable alternative fuel for ships, offering a simple and low risk transition to meet operators clean energy demands. LNG as a bunker fuel can substantially reduce a vessels carbon emissions, allowing compliance with the new IMO guidelines for decarbonisation and reduction in Green House Gas’s (GHG) of 40% by 2030 and 50% by 2050. Over 30% of gross tonnage currently on order is LNG fuelled, increasing to over 50% for some segments¹, equating to more than 400 vessels. With the increased use of LNG as a marine fuel, LNG bunkering vessel demand and associated port infrastructures are also expanding rapidly, with over 30 new build LNG bunker vessels on order². It has been forecasted that LNG will account for over 10% of global bunkering by the end of this decade¹, with ship-to-ship bunkering predicted to see growth of over 60% by 2024⁴.

Boil off Gas

LNG is cryogenically stored onboard vessels at -163⁰C (-261⁰F) within specially designed and insulated cargo tanks. Although these tanks are well insulated, cargo warming is unavoidable, this warming causes the LNG to evaporate upon reaching its boiling point, known as boil-off gas (BOG). This heating is caused by a number of external sources, including the motion of the cargo within the tank (Sloshing) and during cargo discharge operations where the largest losses can be seen, the latter being of particular significance for LNG bunker vessels. BOG rates are typically around 0.1% for larger LNG carriers and up to 0.15% for smaller carriers and bunker vessel³ where there is a larger surface to volume ratio within the tanks. With average LNG bunker prices exceeding $1300 / mt as of April 2022, losses through boil off can result in considerable cost for operators. This equates to over $100,000 / day on a 200,000 CBM LNG carrier.

BOG creates increased vapour pressure within cargo tanks and must either be removed or managed to maintain tank integrity. Some boil of gas can be fed to gas engines, however for larger qualities of BOG and when there is low demand for engine power, alternative solutions are required. With the rising price of LNG fuel, loss of cargo through boil-off or burning within a gas combustion unit is uneconomical. In addition, boil off has a detrimental effect on cargo quality, known as ‘LNG ageing’ where the composition of the LNG is altered as evaporation occurs.

LNG bunker vessels can see the biggest advantages for maintaining BOG, this is due to increased tank warming through regular cargo operations, high tank volume ratio and the necessity to maintain LNG composition when delivering to customers.

Reliquefaction

BOG can be manged by the implementation of a Reliquefaction plant. LNG cargo is removed from the tank, passed through a cooling skid where it is cooled by approximately 10⁰ C -15⁰ C and returned to tank via a bottom feed line or via top spray rail. Reintroducing this cooled LNG maintains the tank temperature, resulting in reduced boil off gases and subsequent tank pressure.

Reliquefication systems now available on the market operate on the reverse Turbo-Bryton refrigeration principal, allowing for standalone compact units with minimal space claim, permitting application onboard all vessel sizes, including smaller LNG tankers and LNG bunker vessels. Systems range from 0.2 mt/h to over 3 mt/h of LNG boil off reliquefaction.

In summary, installation of an Reliquefication system gives several advantages for vessel operators and owners: –

• Avoiding cargo losses
• Preventing LNG ‘ageing’ or loss of cargo quality
• Maintaining LNG at cold temperature, delivering ‘cold LNG’ during bunkering operations
• Allowing for recovery of vapour from receiver after bunkering operations

Houlder Ltd

Houlder has a proven track record on the Design and Engineering for LNG boil-off reliquefaction plant retrofits, specifically the integration of Air Liquid’s Turbo-Brayton subcooler systems on a range of LNG vessels types and sizes for various clients. Houlder has completed the detailed design and engineering for the integration on six vessels from 20,000CBM to 174,000CBM.

Houlder can offer a flexible design and engineering package to suit our clients’ requirements, drawing on a wealth of in house knowledge and expertise’s, including but not limited to piping design, electrical system & automation design, marine system and machinery design, structural engineering, 3D modelling, naval architecture, site survey, laser scanning and marine operational experience across all maritime sectors.

Read about Houlder’s Capabilities within LNG here:

Houlder LTD- LNG Capabilites

References  ¹Clarkson’s Research data – April 2022, ²Sea-LNG, ³Wartsila, ⁴Insight Energy,