An FLNG (floating liquefied natural gas) or LNG FPSO (floating production, storage and offloading) unit is a notable offshore unit with the increasing demand for LNG. The liquefaction process on an FLNG unit is the most important process because it determines the economic feasibility, but would be a hazard source because of the large quantity of hydrocarbons. While a high efficiency process such as C3MR has been preferred for onshore liquefaction processes, a relatively simple process such as the SMR (single mixed refrigerant) or DMR (dual mixed refrigerant) liquefaction process has been selected for offshore units because they require a more compact size, lighter weight, and higher safety due to their space limitation for facilities and long distance from shore. It is known that an SMR has the advantages of a simple configuration, small footprint, and lower risk. However, with an increased production rate, the inherent safety of SMR needs to be evaluated because of its small train capacity. In this study, the potential explosion risks of the SMR and DMR liquefaction processes were evaluated at the conceptual design stage. The results showed that an SMR has a lower overpressure than a DMR at the same frequency, only with a small production capacity of 0.9 MTPA. With increased capacity, the overpressure of the SMR was higher than that of the DMR. The increased number of trains increased the frequency in spite of the small amount of equipment per train. This showed that the inherent risk of an SMR is not always lower than that of a DMR, and an additional risk management strategy is recommended when an SMR is selected as the concept for an FLNG liquefaction process compared to the DMR liquefaction process.
Liquefied natural gas (LNG) floating production storage offloading, or floating liquefied natural gas (FLNG), is an offshore unit used to produce LNG from offshore gas reservoirs. The liquefaction is critical process for liquefying natural gas (NG) into LNG. Among NG liquefaction technologies used in the industry, single mixed refrigerant, dual mixed refrigerant (DMR), and the nitrogen expansion liquefaction process have been considered for FLNG on account of its space limitations and higher safety standards. In particular, the DMR liquefaction process is preferred for a large FLNG because of its high efficiency. Many studies have been suggested about an efficiency of DMR, but a few studies have been conducted on their process safety although different configurations in process concepts can cause meaningful differences in operating conditions and safety. In this study, two DMR process configurations were optimized to maximize the efficiency and conceptual explosion risk was analyzed to compare their risk at the conceptual design stage. The results showed a difference between the explosion risks by the differences in the optimal mixed refrigerant compositions and number of devices, with similar efficiencies. These results can provide insight for a risk management strategy at the conceptual design stage, to minimize the unexpected cost generation.Additional Supporting Information may be found in the online version of this article.
The potential risk of an offshore processing facility is the major important part in the oil and gas industry due to its limited space causing difficulties in evacuation. An offshore processing facility is normally exposed to flammable oil and gas in the operating phase. Especially, uncontrolled hydrocarbon leaks or ruptures of the equipment present main threats. These failures can lead to fire and explosion disaster. Some studies have proposed fire and explosion assessment methodologies and made fire and explosion assessment tools. These tools can provide risk assessments result using physical effect modelling software and following the related standards or engineering practices according to accident scenarios. Nevertheless, existing fire and explosion assessment procedures are still not comprehensive enough to applicate a specific process due to its complexity and are not clear which stage in a project is appropriate for applying it. This paper focuses only on explosion accidents and discusses the development of an explosion risk analysis procedure possible to apply at process flow diagram (PFD) level. The explosion risk analysis procedure using PFD has 6 steps; modelling of a process, scenario selection, inventory calculation, frequency calculation, consequence modelling and risk estimation. It starts at modelling of a specific process using process simulation software, HYSYS. The process modelling can be optimized by the existing methods and finally provide the PFD for the specific process. In the scenario selection step, the information required to perform a risk analysis is identified. The inventory calculation conducts to calculate the inventory of a defined segment after sizing of the equipment in the PFD. The frequency calculation consists of leak frequency and ignition probability. The leak frequency can be calculated with historical database and the ignition probability can be calculated with a specific ignition probability model. The consequence modelling is conducted by using physical effect modelling software, PHAST. It can provide the distance to specified overpressure. Finally, at the risk estimation step, the risk results are evaluated. This procedure can help to applicate a specific process easily and provide explosion risk assessment tool at PFD level. This paper conducts the case study for a liquefied natural gas floating production storage offloading (LNG-FPSO) which is one of the representative offshore processing facilities. Especially, a natural liquefaction process in a LNG-FPSO, which liquefies the processed natural gas to store in a storage tank of a LNG-FPSO, is the most important process in terms of cost and risk. In the situation the most of ongoing or prospective projects for LNG-FPSO adopt dual mixed refrigerants (DMR) liquefaction process, the representative configurations of the DMR liquefaction processes are evaluated and compared. It can help decision making through providing which configuration has an advantage in terms of explosion accidents.
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