## 2 Description of Target System  
In this study, two systems in LNG-FPSO is selected as a target for the availability estimation. These are air compression and nitrogen generation systems in LNG-FPSO. LNG-FPSO is a huge facility for LNG production in offshore, and its concern has been increased because of the growing demand for LNG. LNG-FPSO is a floating unit for production, processing, storage, and offloading of LNG in remote offshore gas fields. Conventionally, the natural gas in an offshore field is transported by pipeline to onshore for processing. LNG-FPSO does not require the pipeline because it processes the natural gas itself in offshore. It is specialized for small scale gas field. Topside modules of the LNG-FPSO can be categorized into two: a processing module and a utility module. The processing module handles the primary hydrocarbon, whereas the utility module deals with utilities including energy, water, air, and diesel oil. The utility module provides utilities to the processing system for safe and stable operation. Some failure of the utility module can be critical because safety systems for preventing an accident are operated by the utility module. The topside of LNG-FPSO can be divided into ten modules as shown in Figure 1. A produced feed gas come up through a turret, and it is transported to an inlet facility module. Slug in the feed gas is removed by a slug catcher, and liquid is separated by a separator. CO\({}_{2}\), Hg, and H\({}_{2}\)O in the feed gas is removed in a pre-treatment module. The treated natural gas is liquefied by a liquefaction module, and a refrigeration module supplies the refrigerant to the liquefaction module. The heavier components than methane like ethane, butane, and propane are separated by a fractionation module. Some amounts of natural gas are transferred to a fuel gas compression system, and it is utilized for power generation. The liquefied natural gas is stored in storage tanks with LPG and condensate. A condensate stabilizer module separates the relatively light components for safe operation. Condensate is mainly composed of propane, butane, pentane, and heavier hydrocarbon. When condensate contains light components like methane and ethane, it can be vaporized and increase the pressure of a storage tank during storage. These light components should be separated before storage. A blowdown module treats combustion fluids in emergency situations. The utility module supplies various utilities to other modules for the operation. In this study, air compression and nitrogen generation systems are analyzed in the utility module because those are important systems for stable and safe operation. A general utility module contains  
Figure 1: Topside modules of liquefied natural gas—floating production storage and offloading unit (LNG-FPSO). an instrument and service air system, a nitrogen generation system, a cooling water system, a seawater system, a hot oil system, a portable water system, a produced and wastewater system, and a diesel oil system. The instrument and service air system compresses the air up to approximately 10 bar for the usage of the instrument and others. The nitrogen generation system supplies nitrogen to the customers. The cooling water system is used to provide the cooling medium for all of the topside modules. The sea water system provides the seawater to various systems. The hot oil system increases the temperature of the oil within a specified range. It utilizes waste heat from flue gas using waste heat recovery units installed in a power generation system. The portable water system distributes water to topside eyewash and safety shower, and hot and cold water for personal usage. The produced and wastewater system removes the oil in the produced water from topside separators. The diesel oil system distributes the diesel oil to customers by transferring and purifying it. Figure 2 indicates the air compression and nitrogen generation systems.