Storage and shipping in the oil & gas industry

Typically LNG is sold from the liquefaction plant and transported by sea using a specialized LNG tanker vessel. Custody transfer meters are used to measure the volume of the liquefied gas delivered, and this involves an analytical measurement of the LNG calorific value. The LNG tanker voyage takes some days, and the LNG tanks, while insulated, are not cooled, so some of the lightest fraction, the methane, boils off. Normally this is used to fuel the ship’s engines.

Upon arrival at the receiving terminal the volume of LNG available is slightly less than it was during the loading of the ship, so once again a custody transfer measurement of the delivered volume of LNG is essential, with the associated calorific value measurement. Traditionally, the volume of the LNG in the ship’s tanks was determined by level measurement, however more and more companies are installing flow measurement based systems to overcome the limitations of such cryogenic tank level measurement.

At the receiving terminal, the gas in the tanks that is displaced by the delivered LNG is frequently compressed and returned to the tanker, to equalize the system pressures. This returned volume is measured by another (gas phase) custody transfer flowmeter, to give accurate accounting.

KROHNE supplies flow metering instrumentation and systems for the custody transfer measurement of cryogenic LNG. For high volume applications - such as tanker loading and delivery flows - these are based on ultrasonic flowmeters. For lower flowrate applications these often use Coriolis mass flowmeters. Currently the calibration of these systems is achieved using water flow rigs, with an NMi certified correction method applied for the declared meter use on cryogenic LNG. For the future, the industry plans to establish a large scale LNG flow calibration facility. KROHNE cryogenic level switches are also used for LNG tank level measurement and control.