- £80,000 /month error was found due to valve timing error
- £300,000 /month error was found due to undetected wrong calibration composition entered
- £150,000 /month error was found due to heavy end drop out in sample let down system
Most of oil and gas companies in the UK and North Sea need to have their GC audited once a year to ensure that the GC performance is acceptable. There are various methods presently exist which are used to obtain the GC repeatability test such as ISO 10723, ASTM D1945, ISO 6974, ISO 6976. ISO 10723 with multilevel calibration method to determine the GC linearity and measurement bias, is one of most popular approaches adopted by the majority of operators to evaluate their GCs.
ISO 10723 performance test is a very important check to ensure that GC operates within acceptable linear measurement range of its detector. It is particularly important where the calibration gas cannot be made similar to the process gas being measured, for instance in the case where various wells are in operation which results in high variation of composition being measured by GC. However, when gas composition is stable, the risk of not performing yearly ISO 10723 performance test can be quantified. GCAS® provide feature to determine the risk by comparing multi level calibration mole percent against single point calibration mole percent.
The following are some of the limitations of ISO 10723 that can be covered by GCAS®:
- The performance test does not include setting up the GC to ensure it is performing at its optimum settings - GCAS® identifies and help correct this issue
- The performance test is only performed once a year - GCAS® can monitor the rest of the period
- The constants from performance test is only appropriate for the operating condition and settings of the GC at the time of the test - GCAS helps monitor the changes in operating condition
- The constants provided on the certificate issued are not always implemented in the GC to optimise its performance for the conditions during the verification process - GCAS® provides both results
The following is comparison table between GCAS method and ISO 10723 method:
Combining GCAS® with ISO 10723 will provide operators with the highest confidence of GC measurement accuracy. GCAS® will utilise constants from ISO 10723 performance test to calculate uncertainty from linearity and repeatability, and also calculate actual bias from each measurement that will help determine the risk of not performing ISO 10723 test.