The article, Reducing Measurement Uncertainty, puts the current liquefied natural gas (LNG) trade in perspective as we look at dynamic (flow metering) and static (tank level) measurement methods of LNG and the technologies used for LNG bulk transfers. We discuss the reasons behind the demand for dynamic and more accurate measurement of delivered LNG flow.
The current practice for measurement of the quantity of LNG delivered to or received from a ship’s tanks is made in the form of energy transferred based on measurements of volume. To date there is no standard governing this method of measurement but the industry seems to be satisfied with the guidelines and sales agreements covered by the G.I.I.G.N.L. Handbook. This handbook reports that the accuracy in the measurement of LNG energy transferred is +/- 0.5 % which is acceptable within the context of long term sales agreements. There is general consensus that dynamic LNG flow metering will provide for much higher accuracy in the range of 0.15% – 0.20% provided there is a means to calibrate the LNG flow meters.
The demand for LNG flow measurements will increase in the future due to:
- The preference for custody transfer to be based on more accurate measurements by all stakeholders in the LNG chain, especially with the emerging LNG spot market where volumes are typically smaller.
- The trend towards floating offshore production, storage/bunkering and re-gasification of LNG. Sloshing of LNG due to the constant movement of offshore tanks hinders tank level measurements.
- Recent developments with a common LNG production facility where operators share storage capacity and flow meters are used for the allocation of ownership.
- The need for accurate book keeping (mass balance) of both the incoming and re-gasified LNG for operational and governmental regulation (emission) purposes.
These considerations have led to the main focus in developments in LNG flow measurement technology being in Coriolis and ultrasonic meters. We review the use of these two technologies and how some of them have been designed to address specific LNG flow measurement challenges including:
- The effects of the cryogenic temperature of LNG on the performance of flow metering equipment
- Unsteady nature of LNG—it is stored and transported at cryogenic temperatures close to its boiling point. As a result, LNG can easily become a two-phase liquid if there are hot spots in the pipeline or if there is an excessive pressure drop anywhere in the system.
- Lack of large-scale cryogenic flow laboratories to calibrate meters at conditions similar to operating conditions.
- Lack of traceable calibration system for inline flow meters at large LNG flow rates
- Potential sources for lost and unaccounted for (LAUF) gas in LNG terminals as a result of discrepancies between calculation standards for LNG and natural gas.
We also make recommendations on how some of these technologies can be used to supplement the current tank gauging approach by adding flow metering points to LNG transferred into, within, and out of the terminal. This would improve operational efficiency and reduce lost and unaccounted for (LAUF) LNG.
Read the article to see ways to reduce measurement uncertainty in your LNG measurements and transfers.