Contractual and Economical Consequences of LNG Boil-Off Quality
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- Institutt for marin teknikk 
The gas evaporating from the Liquefied Natural Gas (LNG), usually called Boil-off Gas (BOG), has previously been out of focus regarding its composition and corresponding heating value. A concentration of the BOG has been assumed to be identical to the loaded LNG, or has been given by another constant value, as the gas has been something to the carrier has to get rid off . However, these assumptions have seemed to give deviating results in reported fuel consumption for the vessel in some cases. This master thesis is therefore investigating the difference in concentration of the BOG evaporating from the transported LNG during voyage, and corresponding deviations in reported fuel consumption by assumed characteristics of the BOG. To do this, a dynamic model is developed to see the change in concentration of the BOG per day of the voyage by a given loaded LNG quality. By knowing the change in concentration of the BOG, the model calculates the corresponding change in Higher Heating Value (HHV), since the BOG is used as fuel for the vessel. Any changes in concentration of the LNG during the loading process is taken into account as well, as it will influence the initial concentration of the product at the first day of the voyage. To report the total fuel consumption for an LNG Carrier (LNG/C), a Fuel Oil Equivalent (FOE) is used to convert amount LNG to tons of Heavy Fuel Oil (HFO). This equivalent depends on the HHVs of both the BOG and the HFO, which is also calculated by the dynamic model. Since it has been a normal procedure for a ship owner to assume a value for this FOE, the last part of the master thesis looks into deviations in reported fuel consumption by different assumptions, and corresponding economical consequences. The results from the model showed a significant increase in fraction of nitrogen in the BOG by only a small increase of nitrogen in the transported LNG. Since nitrogen is highly volatile at the given cargo temperature of -162°C, the LNG tended to contain a lower fraction of nitrogen when finally loaded. The difference in concentration of the BOG during voyage caused a lower HHV compared to the LNG, as the fraction of nitrogen in the BOG was higher. This affected the calculated FOE of the BOG, and caused significant deviations in reported fuel consumption when utilizing common ways to assume the FOE. By having a HFO price of 600 USD/ton, this led to an unnecessary cost of 366,000 USD for one voyage of 28 days, when transporting LNG with approximately 1 Mole% nitrogen.