Design of a Floating-Liquefied-Natural-Gas System for Severe Metocean Conditions

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27074, “Design Case Study for a 4-Mt/a FLNG System for Severe Metocean Conditions,” by Robert M. Shivers III, SPE, and Richard P. Michel, LoneStar FLNG; and Norman P. Kolb, Strategy Engineering, prepared for the 2016 Offshore Technology Conference, Houston, 2–5 May. The paper has not been peer reviewed. Copyright 2016 Offshore Technology Conference. Reproduced by permission. In the past few years, several floating-liquefied-natural-gas (FLNG) projects have become commercial. However, most FLNG systems designed to date remain complex, costly, and operationally limited to areas with benign-to-moderate metocean conditions that allow side-by-side offloading of the liquefied natural gas (LNG). This paper describes a split-process FLNG design where primary production and gas-treatment functions are provided on a host platform while liquefaction occurs on a separate vessel. The arrangement works in moderate-to-severe metocean conditions. Introduction Today, FLNG is generally accepted in the industry as feasible. The focus is now on technology selection, liquefaction and storage capacity, and safe operational considerations, particularly with regard to LNG offloading. Concerns typically raised with pro-posed FLNG schemes include Size, weight, and complexity of topside Topside hydrocarbon inventory and effect on safety design LNG-offloading method and sea-state limitations Overall project cost, schedule, and uncertainty Design for generic application and residual value FLNG-System Configuration—Integrated Arrangement. Most FLNG concepts on the market involve an integrated FLNG vessel that incorporates all functions into a single vessel: primary processing, gas treatment, natural-gas-liquids (NGL) fractionation, liquefaction, storage of condensate/NGL/LNG, and offloading of each product from storage. This approach leads to very large and complex purpose-built vessels. Processing requirements for treating wellhead gas to LNG inlet quality vary considerably from field to field. It is not practical to design a single generic topside-process scheme to deal with the range of conditions that may be encountered if the FLNG vessel has to produce two, three, or more fields over its useful life. However, once the gas is rendered suitable for liquefaction, with water, acid gas, contaminants, and heavier hydrocarbons removed, the facility requirements for liquefaction, storage, and offloading of the LNG are the same from one field to another, varying only according to capacity.