Accurate Custody Measurement for the Oil and Gas Industry

In the oil and gas sector, the quality of operations largely relies on the measurements. While moving the product from one place to another, it is highly crucial to perform accurate custody transfer measurement.

Without an exact report of what has been delivered, it would not be possible for operations to accurately charge for their services. Moreover, this can have significant financial implications.

This article reviews the difficulties involved in accurate custody transfer measurement, and the possible financial loss caused by mismeasurements. It also reviews the choices available for accurate real-time evaluations, and the way certain operations make it to the court due to minor custody transfer errors.

What is the Use of Flow Measurement Systems in the Custody Transfer of Oil and Gas?

In general, flow measurement systems comprise temperature and pressure transmitters, flow meters, and other optional field devices fitted in a pipeline adjacent to the delivery point and linked to a flow computer.

All field signals are measured by the flow computer, which then accurately computes the amount of oil or gas delivered (based on internationally recognized standards), stores all essential data, and provides a number of reports. The custody-transfer flow measurement system is regarded as the cash register.

Why is Accurate Measurement so Crucial at the Time of Custody Transfer?

Accurate measurement is highly crucial since enormous quantities of oil or gas are transferred in several custody transfer measurements, so even a minor error could have a huge financial impact. Even the most insignificant measurement calculation or other error can end in a high figure with very large quantities.

For instance, if an oil tanker is loaded with 700,000 barrels of crude oil, and the custody transfer system performs measurement with a combined uncertainty of a realistic 0.3%, an error in absolute value of $105,000 (at $50 per barrel) may then be introduced. Hence, it is vital to employ the most accurate measurements available since the cost of the equipment used can be gained back within a very short time period.

What are Examples of Things that Generally Go Wrong with Measurement at the Time of Custody Transfer? How do Flow Measurement Systems Account for Them?

Several different measurement problems have been observed, such as incorrectly calibrated equipment, flow computers with incorrect algorithms, human errors, drifting analog inputs, and low-cost and inaccurate measurement instruments. The statement “Penny wise, pound foolish” is probably applicable in this case.

Typically, these errors must be back-calculated upon detection. However, this is a time-consuming and laborious manual process. At times, companies even approach the court when they are unable to agree on the amount of financial effect of the measurement error and the final compensation between the buyer and the seller.

Why Do Several Operations Presume They are Receiving Accurate Measurements at the Time of Custody Transfer?

This is because they might assume that other things are less important once they have bought a high-cost meter, which is not always true. Since huge quantities are involved, every aspect of the cash register is important, where even the most insignificant error introduces a huge financial impact.

Are There Less Evident Problems that can Cause Inaccurate Measurement? If so, What Generally Leads to These, and How do They Slip Through the Cracks?

Obviously, several different problems could lead to inaccurate measurements. Hundreds of different mismeasurement causes have been observed so far. At times, the measurement system maintenance is not done properly, thereby causing problems; or it can be due to human error—for instance, a pressure transmitter that has been unintentionally left to a keypad value following a calibration.

Due to this mistake, the entire custody transfer measurement turns nearly worthless. Sometimes, operating staff at a refinery or on a platform focus more on handling the chemical process, for example, than monitoring the custody transfer systems.

In a gas plant, one-tenth of 1% of pressure variation may not have a huge impact on the chemical process itself. However, it largely affects the volume correction and hence has a huge impact on the custody transfer measurement, with a substantial financial impact.

What are some of the consequences that follow these mismeasurements?

A lot of effort has to be done to manually correct the error. This is a tedious and cumbersome process. In the example above, if there was only one pressure transmitter which has been kept to a keypad value, rather than a real-time measured value, how can you convince your customer what the right value has been during the entire mismeasurement period? Who pays for this, and where is the proof?

What are Certain Methods for Automatic Detection of These Mismeasurements in Real Time?

Several things can be done to make the measurement system smarter. For instance, flow computers and a supervisory computer system—including not just sophisticated diagnostics but also smart mismeasurement management software—can be installed at a custody transfer metering system.

This novel software will help eliminate mismeasurements since these computers can now smartly and continuously perform validations of field signals in real time, raise an error condition, and take suitable measures in the case of measurement failure.

Based on the actual underlying cause, in several cases, it not only finds an error and warns the operator, but more significantly, it can perform corrections for the mismeasurement incident semi-automatically or even fully automatically, as well as reproduce the flow calculation results, in near real time, without including any more uncertainty.

This turns out to be a huge advantage in terms of cost for all involved parties since the mismeasurement is now precisely resolved in a matter of seconds, as opposed to several weeks. This saves a lot of sweat, blood, and tears when compared to resolving it manually.

What is the Role of API’s Manual of Petroleum Measurement Standards (MPMS) in Alleviating Mismanagement Problems?

The MPMS could establish an improved standard for resolving mismeasurement issues, which is more in-depth. Since there are several different mismeasurement problems, the process will not be simple, as all parties involved must agree and be in line with the methodology. However, it is definitely worth trying.

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.


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