Lng Fpso

Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference Rhodes, Greece, June 17–22, 2012 Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set)

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Optimal Synthesis of LNG FPSO Liquefaction Cycles
Jihyun, Hwang1, Namkuk-Ku2, Joon-Chae Lee2, Myung-Il Roh3, andKyu-yeul Lee2
1 SBM Atlantia, 2 Seoul National University, 3 University of Ulsan 1 Houston, TX, USA, 2 Seoul, Korea, 3 Seoul, Korea

ABSTRACT
The liquefaction process is regarded as primary among all topside systems in liquefied natural gas floating production storage and offloading (LNG FPSO) applications. The liquefaction process, which typically accounts for 70% of the capital cost oftopside process systems and 30 to 40% of the overall cost of LNG FPSO plants, condenses the separated and pre-treated natural gas into LNG. The volume of liquid occupies about 1/600th the volume of the natural gas. The cycles in the liquefaction process consist of seven main pieces of equipment: compressor, sea water cooler, expansion valve, heat exchanger, phase separator, tee, and common header.Many different types of liquefaction cycles are determined according to their respective synthesis and optimized operating conditions. This study first proposes the generic liquefaction model to represent various types of liquefaction cycles. Twenty-seven feasible liquefaction configurations derived from the generic model are selected to perform the most effective synthesis for the optimization ofthe liquefaction cycle. With the minimum amount of power required for the compressors, the optimized liquefaction cycle then is proposed by obtaining optimal operating conditions in 27 cases. The optimized liquefaction cycle is compared with the dual mixed refrigerant (DMR) cycle, which is considered for the application of LNG FPSO. Finally, the results show that the amount of power required for theoperating conditions is decreased by 1.2% compared with that of the DMR cycle.

KEY WORDS: liquefaction cycle, generic liquefaction model, optimal operating conditions, optimal synthesis, LNG FPSO INTRODUCTION
Because of the increasing demands for LNG consumption worldwide, liquefied natural gas floating, production, storage and offloading (LNG FPSO) is considered for offshore applications(Lee et al., 2010). LNG FPSO units consist of a hull, turret, and topsides. The latter is divided into two parts: the process system and the utility system. The process system consists of the separation, pretreatment, and liquefaction processes (Jihyun Hwang et al., 2009). In the liquefaction process, the separated and pre-treated natural gas is condensed into liquefied natural

gas (LNG), thevolume of which occupies about 1/600th the volume of the natural gas. The resulting LNG is stored in atmospheric tanks ready for export by ships. Therefore, the liquefaction process is important in the LNG FPSO topside process system and typically accounts for 70% of the capital cost of this system and 30 to 40% of the overall plant cost (Cha et al., 2010; Shukri, 2004). Considering the limited spacesin offshore applications, the main considerations of potential liquefaction cycles for LNG FPSO are compactness, safety, ship motion, and energy efficiency (Hwang et al., 2009). In particular, compactness is the most important factor in achieving offshore applications, and it is effective in reducing ship motion as well. The change from one state to another state through the equipment is referredto as the “process.” The cycle is the means by which the process returns its initial state. In the liquefaction process, the cycle consists of major pieces of equipment to configure the system, which include the compressor, condenser, expansion valve, evaporator, phase separator, common header, and tee. Various kinds of liquefaction cycles consist of a combination of this equipment. This paper...