Choices for LNG FPSOs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 19884, ’Choices for LNG FPSOs,’ by L.J.F.M. Festen and J.B.P.M. Leo, CB&I Lummus B.V., originally prepared for the 2009 Offshore Technology Conference, Houston, 4-7 May. An estimated quarter of world gas reserves currently is stranded in offshore gas fields. The energy markets and dwindling oil reserves highlight the necessity to find new ways for offshore gas production. Liquefied-natural-gas (LNG) floating production, storage, and offloading (FPSO) vessels will be required to develop the fields that are too far from market for pipelines or are in extreme water depths. Despite many design studies, technical and economic constraints have prevented LNG FPSOs from being developed as fast as oil FPSOs. Introduction More than 20 years of development work has been performed on LNG FPSOs, but apparently there are challenges that have prevented the realization of such a project even with rising gas prices. Both the organization of the project and the actual design of an LNG FPSO are considerably more complicated than those of an oil FPSO. The full-length paper discusses the choices to be made during selection and design of a project on the basis of many years of development work. Table 1 in the full-length paper lists the main parts of an LNG FPSO where choices have to be made. For an offshore facility, the interdependence of the choices is much stronger than onshore because of the limited plot area on an FPSO. Selection of Gas Field and Pretreatment The design of the LNG FPSO begins with the selection of the gas field to be developed and the intended market for the LNG. The higher-heating-value (HHV) specification for the LNG together with the hydrocarbon part of the feed-gas composition determines the natural-gas-liquids (NGLs) separation. A very rich gas (i.e., high C2, C3, C4, C5+ content) and a market that requires lean LNG will require a pretreatment process that also produces liquefied petroleum gas (LPG) and condensate. The leaner the LNG specification, the more NGLs that have to be separated. When the mol ratio of methane to ethane in the feed gas is lower than 14 to 11, it becomes very difficult to produce a lean LNG specification with HHV less than 1,070 Btu/scf unless ethane is produced as a separate product. Producing a fourth product, liquid ethane, is rather improbable for an FPSO.