Measurements of gas and liquid natural gas (LNG) quality, along with flow measurements, ultimately determine the commercial value of the gas and LNG, which is vital for the long-term successful operation of a custody transfer / fiscal measurement system. The linearity of these instruments can lead to errors which contribute to the measurement uncertainty.
All elements of a measurement system can contribute to the overall uncertainty budget:
- Sampling
- Sample conditioning
- Calibration
- Analysis
- Data handling
- Staff training
- Quality control
- Maintenance
Although measurement uncertainties cannot be removed entirely, they can be reduced to acceptable levels by ensuring the appropriate ISO standards, technical procedures and maintenance regimes are used.
To demonstrate the level of risk at stake, here are the typical relative errors for a shipment of 260,000 tonnes of LNG at $5 / MMBtu, total cargo value $30,000,000*
- A well-performing system (good GC, good calibration gas, no issues with the sampling)
- A poorly-performing GC
- insufficient sub-cooling when the sample is vaporised
- a poor calibration gas
|
Uncertainty contribution |
% Rel. uncertainty |
Cargo value at risk of mismeasurement* (USD$) |
|
A well-performing system |
~0.002 |
~ 500 |
|
A poorly-performing GC |
~0.2 |
~ 60,100 |
|
Insufficient sub-cooling when sampling |
~0.4 |
~116,000 |
|
A poor-quality calibration gas |
~1.6 |
~443,500 |
As can be seen from the table above, the most important factor is to have a good-quality, accurate calibration gas. Failure to do this could incur up to half a million dollars’ worth of mismeasurement.
Because of the large volumes, even very small errors correspond to large financial discrepancies. Even a relatively small error for the GC (approximately 0.3%) gives rise to a potential risk of $60,000 per cargo.
Even if the GC and calibration gas are good, problems when sampling LNG can increase the financial mismeasurement risk. If vaporisation of the LNG is not done correctly, fractionation and enrichment can occur. We estimate this could contribute as much as 0.4%, equating to approximately $116,000.
However, when a good quality calibration gas is used in conjunction with a good GC and a well-performing sampling system, the financial risk can be significantly reduced.
To see how important it is to reduce uncertainties in these measurements, here is the cost of each of these errors:
A well performing system:
A poorly performing GC using good calibration gas:
Insufficient sub-cooling when sampling:
Poor quality calibration gas with a well-performing GC:
(*the data shown in this blog are for illustrative purposes only, based on a single cargo, QMax LNG vessel. To find out more information about the risks of mismeasurement for your system, contact cac@cacgas.com.au).
