As LNG production capacity continues to grow and the value of natural gas remains high, the impetus to monetise non-traditional gas resources also grows. Offshore floating LNG production has generated interest because it offers the potential to avoid flaring or reinjection of associated gas and to monetise smaller or remote fields of non-associated gas. With the realisation of large floating production, storage, and off-loading (FPSO) facilities for oil production and more recently LPG production, an LNG FPSO project appears to be increasingly more likely in the future.

By : Michael Barclay and Noel Denton, Foster Wheeler Energy Limited, UK

As LNG production capacity continues to grow and the value of natural gas remains high, the impetus to monetise non-traditional gas resources also grows. Offshore floating LNG production has generated interest because it offers the potential to avoid flaring or reinjection of associated gas and to monetise smaller or remote fields of non-associated gas. With the realisation of large floating production, storage, and off-loading (FPSO) facilities for oil production and more recently LPG production, an LNG FPSO project appears to be increasingly more likely in the future.

Offshore natural gas liquefaction has different process requirements to the traditional on-land baseload plants. While thermodynamic efficiency is arguably the most important process selection criteria for large onshore natural gas liquefiers, other factors become more important for offshore projects. Thermodynamic efficiency is likely to remain critically important. However, for offshore applications criteria such as compactness and process safety become more significant considerations. The high efficiency pre-cooled mixed refrigerant cycles that dominate onshore LNG installations will likely not meet the needs of the future mobile and offshore liquefaction projects. The authors review natural gas liquefaction cycles in the context of compactness, ease of operation, process safety, and efficiency for offshore liquefaction. Particular attention is paid to the lower-efficiency single and dual expander processes.

These cycles offer several advantages over traditional cascade and mixed refrigerant (MR) liquefiers for offshore, onboard and mobile applications. The authors share their insights gained through their design and operating experience on small-scale N2 cycle LNG plants and marginal field developments.

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