Density measurements

Density is a measurement of mass per unit of volume. Because

LNG is not a pure substance, the density varies between the

different gas fields due to different composition.

The traditional way of obtaining the density of LNG as input to

the CTS report, is by taking liquid samples that are vapourized

and sent to a gas chromatograph to analyze the composition.

The density is then calculated by using the revised Klosek

McKinley equation, where the composition of the LNG found by

the gas chromatograph forms the input.

The chemical composition of LNG is a function of the source

and type of processing/aging. it is a mixture of mainly methane,

ethane, propane and butane with small amounts of heavier

hydrocarbons. Besides the hydrocarbons, mainly Nitrogen exist,

which is an important component to watch due to its altered

properties compared to the hydrocarbons. Methane is by far the

major component, usually over 85 % by volume.

With KONGSBERG radar based densimeter, the density of the

LNG is monitored continuously as long as the sensing part of

the pipe is immersed in LNG. This is done at each immersed

segment in the tank, and can be presented for each tank or

averaged over several tanks. By knowing only the N2

content to

certain accuracy, the uncertainty of the density measurement is

reduced to a level well below the uncertainty of today’s accept

ed standards.

Accurate density measurement is possible regardless of the

tank atmospheric conditions. High instrument accuracy and

flexible software ensure continuously monitoring of the density

and its change both in cases with natural and forced boil-off and

re-liquefaction.