The selection criteria for a liquefaction process for an offshore floating LNG (FLNG) process are different from that for onshore liquefaction plants due to offshore specific considerations like safety requirements, space and weight constraints, and ship motion effects. For a small and medium scale FLNG project suitable for the development of strained gas production and extended oil production, both the Single Mixed Refrigerant (SMR) cycle and the Nitrogen expansion cycle are most widely considered feasible liquefaction process, as they have considerable advantages in terms of space and weight. However compared to SMR, nitrogen as refrigerant has strong additional advantages, even though it has lower thermal efficiency. Nitrogen is nonflammable gas and therefore inherently safer, as the need for flammable refrigerant inventory is eliminated. Also the nitrogen expansion cycle is a reverse Brayton cycle, which has no phase change throughout the refrigeration cycle. This results in a single phase process, which is less prone to reduction in production performance caused by adverse ship motions. Based on the above the nitrogen expansion cycle seems more appropriate for a small and medium scale FLNG. This paper presents a LNG liquefaction cycle configuration using three stages of nitrogen expansion to improve the efficiency of the conventional nitrogen double expansion cycle, based on a LNG production rate in the range of 1 MTPA. The chosen configuration further optimizes the composite cooling and heating curve of the liquefaction cycle, resulting in higher thermodynamics efficiency. The efficiency of the liquefaction cycle will be improved in order to reduce the interval between the cooling curve of the natural gas and the warming curve of the refrigerant: the closer both curves are, the better the efficiency of the cycle. This optimization is achieved by adjusting the refrigerant operating temperatures and pressures. The three nitrogen expander liquefaction cycle includes three levels of expansion, each having different temperature and pressure levels: warm, intermediate and cold. This configuration allows the nitrogen warming curve to closely match the cooling curve of the natural gas cooling curve by changing the nitrogen warming curve from two straight lines into multiple intersecting straight lines of different gradient. That is to say, the additional new nitrogen expander generates an additional inflection point within the cold composite curve. As a result, thermodynamic inefficiencies are minimized and power requirements are reduced when compared to the double expansion cycle. A case study is presented for an open sea associated gas FLNG concept showing a comparison using a liquefaction process based on a two-expander cycle and a three-expander cycle. A Life Cycle Cost (LCC) analysis based on Net Positive Value (NPV) shows an improvement on the project NPV with minor incremental CAPEX.
The main purpose of the study is to ensure whether the safety systems of an offshore drilling platform maintain their integrity and perform their duty under probable accident scenarios. This assurance is completed via reliability and availability analysis of the systems, verifying the design outcomes and recommending design and operational changes. The scope is limited to determining the reliability and availability for continuous systems, and only the reliability, often represented probability of failure on demand (PFD), for stand-by systems. Qualitative analysis is to be carried out to identify each failure mode and the sequence of events associated with it for the systems within the scope. Based on the result of qualitative analysis, failure scenarios are logically constructed to comprise basic events and failure effects. The failure data is taken from generic references such as the offshore reliability data handbook (OREDA). In the final analysis, the values of availability for the all systems are quantified to be above 99%. It is considered that the analysis results are broadly acceptable for general safety requirements.
This study presented the dynamic simulation of a gas compression system, proving the viability of operational philosophy and emergency shutdown logic with quantitative process responses in various situations. To avoid unnecessarily high peak in an initial stage of blowdown, this study employed the controlled blowdown and investigated its safety level. Introduction This study concentrated on the dynamic simulation of a gas compression system of a topside module on offshore facilities. The gas produced from the wells was separated in the Inlet Separator and routed to the Gas Compression Module. Such various process specifications and operation logics as dew-point control, turbine speed, and compressor surge were taken into account in the dynamic simulation. Revealing the time-varying behavior provided the viability of operational philosophy and emergency shutdown logic, illustrating the detail and quantitative process responses to various disturbances. For example, it showed the transient behavior of the compressor control system consisting of the recycle valve opening at 15% above the surge flow and the surge control valve opening at 10%. Event scenarios included the possible cases of emergency and relief operation. The history of such process variables as pressure, temperature, liquid level, and flow rate were demonstrated to the virtual event scenarios. Flare system should be designed to the peak flow rate. The conventional blowdown systems is easy to install and simple to operate. But, it suffers an unnecessarily high peak flow rate in an initial stage of blowdown. In offshore processes, this high peak rate means costly flare facilities. Especially, the flare stack location and weight distribution demand careful consideration for floating units like FPU, FPSO, and LNG FPSO. Therefore, it is important to mitigate the peak flow rate for environmental protection and optimization of the flare system design. In addition to rigorous dynamic simulation, this study also examined the feasibility of the controlled blowdown application to the floating units, to decrease the peak flow rate. The blowdown system was compared with the normal practice in terms of safety level as well as of flare load. Process description The gas compression and treatment facilities are rated for gas export capacity of 60 MMscfd and fuel gas requirement of 4 MMscfd at 108 F ambient temperature and 100 % relative humidity. The overall compression requirement is satisfied by two process stages. The Low Pressure (LP) and the Intermediate Pressure (IP) stages are in the form of a complete 100% train, comprising a single driver with two compressors in a tandem arrangement. The gas turbine for LP and IP Compressors is capable of running both on fuel gas and diesel. Diesel is to be used only for field 'black start' when fuel gas is not available. The discharge pressure of the IP compressor is fixed at 638.5 psia (43 barg) to fulfill the required hydrocarbon dew point specification. The compression module receives the gas from the Inlet Separator, the gas produced from the wells is separated into two phases of gas and liquid. Compression is performed in two stages increasing the pressure from the Inlet Separator operating pressure to the required export discharge pressure.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.