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.
This study presents a methodology based on the Life Cycle Cost (LCC) to define the optimum yearly Liquefied Natural Gas (LNG) production of a Floating LNG (FLNG) based on the proven gas reserve of an Associated Gas (AG) Field. This methodology developed specifically for these specific FLNG Concepts can be extrapolated to other type of FLNGs such as full-fledged FLNGs. For this purpose, several FLNG concepts of Associated Gas (AG) FLNGs are compared. A previous study presented to the OMAE under OMAE2016 – 55152 is used to develop variations based on the same concept with one liquefaction train and two liquefaction trains for both nitrogen refrigerant and Single Mixed Refrigerant (SMR) concepts. These concepts are defined as follows: Case A-1: 1 liquefaction train - nitrogen used as refrigerantCase A-2: 2 liquefaction trains - nitrogen used as refrigerantCase B-1: 1 liquefaction train - SMR used as refrigerantCase B-2: 2 liquefaction trains - SMR used as refrigerant These FLNG concepts are designed to monetize associated gas from existing offshore fields located within convenient distance of an existing LNG plant or port with LNG storage facility. The economic model assesses the LCC using Net Present Value (NPV) for both cases. The LCC and Internal Rate of Return (IRR) show that each concept has its own merits depending on gas reserve and production profile. The study provides details on areas of applications for each FLNG concept and how it is influenced by key parameters such as reserve size, production profile, discount rate, estimation accuracy, and development phase. Based on this, the study provides guidance for the selection of the optimum LNG production capacity for an associated gas production profile with reserves from 0.5 TCF to 1.5 TCF.
This study presents a comparative evaluation of a Floating Gas to Liquid Facility (FGTL) and a Floating Liquid Natural gas Facility (FLNG) as a way to monetize Associated Gas (AG) from existing oil producing offshore fields or from remote offshore gas fields. The study aims at showing that the FGTL is an option to be considered as a way to monetize AG. For this purpose, a previous study presented to the OMAE under OMAE2016 – 55152 is used for the FLNG with one liquefaction train for nitrogen refrigerant. This FLNG concept serves as a benchmark to evaluate the technical and economic relevance of the FGTL through the Net Present Value (NPV) of the Life Cycle Cost (LCC). The LCC compares the difference between the FGTL and the FLNG NPVs on the basis of a selling price for the gasoline of 1.4, 2.0 and 2.4 USD/Gal with a price for the LNG varying from 3.5 to 7.5 USD/MMBTU. The AG FGTL is presented in detail with a capacity of 6,000 bpd of gasoline (methanol production being an option to Gasoline). Natural gas is converted into gasoline by a proprietary natural gas to Liquids technology awarded with AIP (Approval In Principal) by ABS for off-shore GTL, which includes five principal steps in one continuous gas-phase process loop using only standard components: (1) Steam Methane Reforming; (2) Methanol Synthesis; (3) Gasoline Synthesis; (4) Gasoline Treatment; (5) Separation. This proprietary technology has several advantages over other GTL technologies including single product, long catalyst lifetime and higher allowable levels of CO2 in AG. Gasoline produced using a proprietary natural gas to Liquids technology can be blended with refinery gasoline or sold directly into the wholesale market.
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