Transforming LNG custody transfer

LNG has decided to stay.

Despite uncertain financing and regulatory conditions, LNG trade has grown by 1% from 401 million t of LNG in 2023 to 406 million t in 2024 and is forecasted to continue to grow to up to 700 million t by 2040. While overall market conditions remain uncertain, the technology behind global LNG trade continues to evolve, driven by multiple innovations that improve the efficiency, reliability, and safety behind the scenes of liquefaction, distribution, and regasification facilities along the LNG value chain.

The custody transfer quantity measurement of LNG is becoming increasingly important as the growing use of LNG as a fuel for energy supply and mobility simultaneously increases the number of LNG transfer points and the need for accurate and reliable measurements at each transfer point.

What are the challenges when it comes to accurate and reliable measurement?

LNG changes hands several times along the value chain, whether in internal company sales, between two companies, or even between countries. Considering the latest Q-Max class LNG carriers, with a capacity of up to 266 000 m³ of LNG, the financial value of an LNG load is approximately €50 million per carrier (based on average values for density, calorific value, and average future prices for LNG traded at EEX European Energy Exchange in 2026). This LNG needs to be measured in matters of energy being transferred from seller to buyer. An uncertainty of 0.1% in this measurement corresponds to roughly €50 000 worth of LNG per carrier during loading or unloading. These uncertainties cannot be fully eliminated, but they can be minimised.

The following points (amongst others) need to be specially considered and corrected to achieve an accurate quantity or volume reading on the LNG carrier:

• Ship individual tank geometries (tank tables), which transfer level to volume readings and correct for tank internals and temperature induced geometry changes.
• LNG tank movement due to ship movement (list/trim) or due to convection current inside the tank.
• Boiling LNG inside the tank, blurring the phase border between liquid and gas.
• Dead volumes between tanks on the LNG carrier and the tanks in the terminal.
• Proper calibration and sealing of all involved instruments and the check by a surveyor that all of these are valid and in place.
• Sufficient tank settling time before and after loading to allow stable readings, while on the other hand there is a need to reduce berth occupancy charges with fast LNG-transfer.

For quality measurement in the liquefaction or regasification terminal:

• Representative LNG vaporisation and sampling with minimum time lag.

Typically, the instrumentation for quantity measurement belongs to the shipping company or vessel owner, while the instrumentation for quality measurement belongs to the plant (liquefaction/regasification), which may introduce additional complexity in case of disputes.

When it comes to measurement of tank level, temperature, pressure, flow/LNG-bunkering, and fluid composition, Endress+Hauser has helped to reduce uncertainties in measurement and control of process parameters for decades. With the strategic partnership between Endress+Hauser and SICK, the offering for LNG facilities has been enriched by measurement solutions for ultrasonic flowmeters (UFM) for flare, feed, sent-out, and boil-off gas (BOG), continuous emission monitoring systems (CEMS), and meters for liquid LNG. The latest development from this partnership is the FLOWSIC900 – a UFM for custody transfer and process LNG metering.

How does ultrasonic technology solve these challenges?

Ultrasonic flowmeters (UFMs) and Coriolis mass flow meters (MFM) both belong to dynamic in-line measurement methods in comparison with static measurement methods like tank gauging or weighing (via weigh bridges).

With changing from a static to a dynamic measurement method, the following challenges are resolved:

• Individual tank geometries: Movement of the ship or fluid movement inside the tank does not add application uncertainty anymore.
• No dead volumes or fluid flows (LNG/BOG) inside the LNG-carrier (e.g. fuel gas) or plant (e.g. compressors) need to be considered any more. Upstream of the custody transfer point belongs to the seller, downstream belongs to the buyer.
• The number of instruments which may need to be checked by a surveyor for proper calibration and sealing is reduced dramatically and the instruments are located close to each other.
• Instrumentation (quantity and quality) can be owned by one party completely, theoretically a master/duty configuration of whole setup possible (one skid on the ship, one skid on the jetty of the plant).

In addition to this, UFM offer the following advantages specifically for LNG metering of large scale quantities:

• Available in large line sizes up to 36 in. or larger.
• No pressure loss (which could lead to BOG/cavitation).
• Additional process diagnostics (e.g. speed of sound) for LNG quality monitoring.
• Being nearly maintenance and drift-free.
• Custody transfer approved UFM available (e.g. OIML R117).

Outlook

Challenges and concerns which have hindered the common use of UFM in LNG custody transfers have largely been overcome – the technology is ready. In the near future, UFMs are expected to be seen more and more in LNG plants. Firstly, they will be used as process meters on (un)loading lines for LNG pump monitoring or for LNG rundown measurement, and secondly as check meters as reference to level metering before they finally may get industry standard for LNG custody transfer. Global standards will continue to develop and ease usage of UFM or Coriolis-based LNG metering systems for LNG transactions from small to large scale. Ultimately, measurement uncertainties will further decrease, allowing LNG operators to focus on the economic and political uncertainties which will probably remain.

Read the rest of this abridged article in the full issue here!