Ageing of Liquified Natural Gas during Marine Transportation and Assessment of the Boil-Off Thermodynamic Properties

During LNG storage and transportation by ship, a fraction of the LNG in the cryogenic tanks evaporates due to heat ingress through the insulation, resulting in boil-off gas (BOG) production and a change in LNG composition, a phenomenon known as LNG ageing. Common practice is to assume that BOG composition and related density are identical to the initial LNG or pure methane, resulting in inaccuracy in BOG mass flow measurements. This is particularly important regarding LNG shipping economics and the utilization of BOG as a fuel for ship propulsion. This work investigates the influence of LNG ageing on the produced BOG thermodynamic properties relevant to the mentioned inaccuracies' estimation and correction. An established, simplified, dynamic boil-off model is utilized for the simulation of LNG and BOG properties' changes during the voyage. Four cases represented by limiting the minimum and maximum values of methane and nitrogen content are used to estimate the general influence of the compositional variability over the whole range of practically possible LNG source mixtures. Research results provide an insight into the relevant BOG properties' variability and confirm that BOG flow measurements should be corrected with dynamic model simulations results due to significant differences between the total BOG mixture and forced BOG mixture corresponding to the LNG composition.