Liquefied natural gas (LNG) custody transfer has traditionally involved fiscal risk given the uncertainties in traditional tank measurement technologies. Quantities are invoiced in units of transferred LNG energy based on a calculation of volume, density, and gross caloric value, which in turn are a combination of direct and derived measurements. The end result is measurement uncertainty of a half of percent up to one percent.
In an LNG world shipping article, Dynamic measurement solutions in LNG custody transfer, Emerson’s Rossella Mimmi shares an alternative approach via dynamic metering to improve measurement accuracy and reliability. Rossella highlights the complexities in current measurement methods:
The complex scheme used to calculate the energy value involves LNG sampling, the direct measurement of quantities and the calculation of derived quantities. The direct measurement of quantities, in turn, relies on the LNG tank level, composition and temperature while the calculation of derived quantities depends on the LNG volume inferred through the method known as ‘tank strapping’ as well as on LNG density and gross calorific value.
The impact of measurement uncertainties at the 1% range:
…on the total value of the global LNG trade in 2010 (approximately 200 million tonnes) represents US$607 million.
Ultrasonic flow measurement provides an alternative form of fiscal measurement:
Dynamic measurement of LNG with ultrasonic meters has already been accepted by the industry as a reliable solution that can provide improved accuracy.
Rossella explains reasons why ultrasonic flow measurement is a good fit for this application:
Ultrasonic meters allow mitigation of the sources of pressure drop in a metering system, as they are full-bore devices and do not generate any incremental pressure drop beyond normal pipe friction. They are also generally sized to operate at relatively low velocities to keep the meter size the same as the pipe size. The electronics are remotely mounted to avoid a heat source close to the pipe and, consequently, reduce the hot spots.
Many LNG custody transfer arrangements are based on 20-year contracts which rely on used and accepted measurement practices. Rossella notes:
…traditional measurement techniques can be supplemented by adding flowmetering points, which will improve operational efficiency and reduce lost-and-unaccounted-for quantities. The addition of flowmetering points also means having more data inputs available to better control the plant systems.
Dynamic measurement methods have also been hampered due to the lack of an in-situ proving system for meter calibration per industry guidelines such as API Chapter 4. She highlights a solution [hyperlink added]:
With Emerson’s LNG prover, it is now possible to provide an effective solutions for a complete LNG measurement system in compliance with the current standards. It is a concept that has evolved from field-proven, bi-directional piston provers.
Future LNG project contracts can consider a change to dynamic methods per the API custody transfer standard for liquid transfer. Dynamic metering allows measuring LNG with increased accuracy and reliability with reduced fiscal risk.