Design and Operability Analysis of Multi-Tank Boil Off Gas Compressor System Using Dynamic Simulation

Multi-train LNG facilities are economically attractive option for LNG producers and several of these facilities are in operation all over the world. These facilities use one or more LNG storage tanks and common boil off gas (BOG) compression system that process the BOG generated in the tanks and during ship-loading. In some cases, asymmetrical plant, tank farm and jetty layouts introduce an intrinsic imbalance in operation of the BOG system. In addition, these asymmetrical layouts introduce an operating challenge for a steady and balanced distribution of ship vapor return and BOG generated in LNG tanks. A detailed analysis using dynamic simulation helps in verifying the hydraulic system and control scheme adequacy for various operating scenarios comprising varying number of tanks and number of BOG compressors in operation.

Bechtel has successfully used simulation technology for BOG compressor network analysis over the years. For design studies, a fundamental dynamic simulation model is used. This is developed based on the actual equipment sizes, piping isometrics, heat leak calculations and other relevant information. If more rigorous representation of the control scheme is desired, it is possible to utilize an emulator to give an exact replica of the control system. For the case study presented here, an actual emulation of the compressor control system is connected to the dynamic plant model of the entire BOG system. This is useful to check the anti-surge and master performance controller performance and identify any potentially undesirable interactions.

This paper will present the design and operability studies of BOG compressor network done on a greenfield facility with multiple tanks and compressors. Various operating scenarios including startup and shutdown of BOG compressors, emergency shutdowns, ship loading, and suction and discharge valve failures were studied. The results from these simulations were valuable in verifying engineering design, providing operational advice and in enhancing the process control schemes. The various insights gained from these analyses will be presented in this paper.