(215e) Optimal Operation for LNG Ship Tank Commissioning and Loading Via Rigorous Dynamic Simulations

Natural gas is fastest growing energy economy. Its production and demands are increasing rapidly. Natural gas is transported as liquid over long distances. In 2017, liquefied natural gas (LNG) trade reached 293 million tonnes per annum (MTPA), as per Shell LNG Outlook 2018 report. LNG imports grew by 29 MTPA which is 30% more than what was expected. Spot cargo deliveries increased by 17% in 2017 over 2016. Natural gas and LNG production rates are expected to increase over next couple of decades.

In this work, dynamic process simulation for both LNG ship-tank commissioning and LNG loading has been performed to study the optimal procedures when new LNG ship-tanks are put to service or old LNG ship-tanks come back to service after maintenance. At the beginning, the tanks contain humid air at ambient temperature. Before LNG can be loaded, the tanks require drying, inerting, gassing up, and cooling. Drying is necessary to avoid moisture from air to condense and solidify during cooling of the tank. Air is purged using inert gases to remove oxygen from the tanks. Then methane vapors are used to remove inert gases because some inert gases like CO₂ can solidify after adding LNG and can chock valves. Cold methane is used for initial cooling of the tank. LNG liquid is sprayed for further cooling to desired temperature. Finally, LNG is loaded into the ship tanks. The LNG ship-loading is constrained by maximum allowable cooling-rate of ship-tanks, jetty boil-off gas (BOG) compressor capacity, LNG loading line capacity, and LNG level in ship-tank. Overall, LNG loading is complicated process. Dynamic simulation is helpful to study the commissioning and loading operations. The changes in the composition of material inside the tank are reported for each step in the commissioning and loading procedures. Optimization of the process to minimize time of commissioning and loading operations, emissions, and energy requirements is carried out using the Aspen Plus Dynamics simulation tool. This study would help LNG industry to operate LNG exporting terminal smoothly and in an optimal way.