The selection criteria for a liquefaction process for an offshore floating LNG (FLNG) process are different from that for onshore liquefaction plants due to offshore specific considerations like safety requirements, space and weight constraints, and ship motion effects. For a small and medium scale FLNG project suitable for the development of strained gas production and extended oil production, both the Single Mixed Refrigerant (SMR) cycle and the Nitrogen expansion cycle are most widely considered feasible liquefaction process, as they have considerable advantages in terms of space and weight. However compared to SMR, nitrogen as refrigerant has strong additional advantages, even though it has lower thermal efficiency. Nitrogen is nonflammable gas and therefore inherently safer, as the need for flammable refrigerant inventory is eliminated. Also the nitrogen expansion cycle is a reverse Brayton cycle, which has no phase change throughout the refrigeration cycle. This results in a single phase process, which is less prone to reduction in production performance caused by adverse ship motions. Based on the above the nitrogen expansion cycle seems more appropriate for a small and medium scale FLNG.

This paper presents a LNG liquefaction cycle configuration using three stages of nitrogen expansion to improve the efficiency of the conventional nitrogen double expansion cycle, based on a LNG production rate in the range of 1 MTPA. The chosen configuration further optimizes the composite cooling and heating curve of the liquefaction cycle, resulting in higher thermodynamics efficiency. The efficiency of the liquefaction cycle will be improved in order to reduce the interval between the cooling curve of the natural gas and the warming curve of the refrigerant: the closer both curves are, the better the efficiency of the cycle. This optimization is achieved by adjusting the refrigerant operating temperatures and pressures.

The three nitrogen expander liquefaction cycle includes three levels of expansion, each having different temperature and pressure levels: warm, intermediate and cold. This configuration allows the nitrogen warming curve to closely match the cooling curve of the natural gas cooling curve by changing the nitrogen warming curve from two straight lines into multiple intersecting straight lines of different gradient. That is to say, the additional new nitrogen expander generates an additional inflection point within the cold composite curve. As a result, thermodynamic inefficiencies are minimized and power requirements are reduced when compared to the double expansion cycle.

A case study is presented for an open sea associated gas FLNG concept showing a comparison using a liquefaction process based on a two-expander cycle and a three-expander cycle. A Life Cycle Cost (LCC) analysis based on Net Positive Value (NPV) shows an improvement on the project NPV with minor incremental CAPEX.

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