Explosion Risk Analysis on the Liquefaction Process of LNG-FPSO at the PFD Level

Abstract

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.