The client was looking into importing LNG from multiple sources. The aim of the study was to develop a predictive model to manage the composition and heating value variability due to LNG ageing during storage. The model focused on controlling the maximum heating value and gas composition through nitrogen spiking and control of the minimum send-out rate. The model also identified potential LNG cargo compositions outside the third-party pipeline specification that could not be managed through nitrogen spiking.
When the LNG is received with different compositions, the issue of LNG ageing, pressure build-up in the storage tank and change in the output gas heating value is quite common. There are several factors influencing BOG: the LNG delivery temperature and pressure, tank temperature and pressure, ambient conditions, loading conditions, insulation, and sloshing. The boil off could be managed by supplying boil off gas to auxiliary equipment, controlling minimum send-out, and/or re-liquefaction as required. Predictive modeling to manage the above challenges was required to provide indicative results and guidance to the client.
BE&R developed and supplied a user friendly excel model to integrate with client process systems which calculated the amount of BOG and change in export gas specifications over a range of LNG compositions and send-out rates. The model can be used to determine the rate of increase in stored LNG heating value as the volatile components (methane and nitrogen) boil-off and provides the client with a predictive strategy to manage change in composition with time.