Fiscal assurance, Part 1

Rossella Mimmi, Emerson Process Management, USA, explains how the latest flow measurement technologies can help to provide more accurate and reliable custody transfer solutions.

Accurate flow measurement has grown in importance due to its widespread use for accounting purposes and in other upstream and pipeline operations. In addition to fiscal and custody transfer applications, correct flow measurement is required for production management, leak detection systems, batch operations and loss/gain balance. These applications constitute key elements for operators who want to maximise efficiency in production, transportation and storage facilities.

What is fiscal measurement?

Fiscal measurement is a term that commonly includes both allocation and custody transfer flow measurement. Allocation involves the assigning of aggregated product quantities back to owners, leases or individual source streams and is typically not governed by a buy/sell contract. Custody transfer is based on a change of ownership between a buyer and seller and is managed against a contractual obligation. Both require adherence to specified accuracy, repeatability, linearity or uncertainty values as defined by the appropriate owner policies, regulatory agencies and industry standards.

Fiscal measurement applications are everywhere in the oil and gas value chain and play an essential role in solving various industry challenges that customers face in each area. For onshore and offshore, fiscal measurement supports achieving real time product flow measurement data to optimise production and ensure regulatory compliance. In terminals and storage, use of the right flow measurement technology helps increase loading/offloading efficiencies. Finally, to ensure efficient product movement and maximise utilisation for pipelines and transportation, accurate flow measurement is key in leak detection systems, batch operations, loss/gain balance and throughput optimisation.

Oil and gas flow measurement accuracy and sustainability depends on many factors such as product quality, fluid properties and composition, operating parameters, maintenance practices and technology type. The selection and effective utilisation of newer measurement technologies are proven to minimise many of these factors, resulting in improved accuracy and reliability of fiscal measurement systems.

Flow measurement accuracy and uncertainty

All meters and metering systems are subject to uncertainty. It is a common mistake to mix accuracy and uncertainty; however they are subtly different. ‘Accuracy’ refers to matching the meter output to a known standard or reference (using terms like bias, readability and precision), and this can be considered the best estimate according to the scale of the measurement. ‘Uncertainty’ is related to repeatability and is an estimate of the limits where the true value is expected to lie for a given confidence level. Figure 1 gives an indication of flow measurement uncertainty levels that are generally required throughout the oil and gas supply chain.

Figure 1. Typical flow measurement uncertainty levels.

The overall design objective of a measurement system is to continuously provide precise, reliable, and traceable volume measurement of the quantity of fluid or gas being sold. Given the maximum allowable uncertainty of the system, for example ± 1.0% of the actual reading, the other system components that combine to achieve this accuracy must provide measurement accuracy less than ± 1.0%. Selecting the right flowmeter technology with the best accuracy, in combination with other high accuracy components, helps ensure the overall measurement accuracy is consistently achieved. Inaccurate measurements can lead to increased fiscal risk, litigation and possible internal/external audits.

The measurement system is required to maintain the specified accuracy over the flow range, compliant with industry standards and regulations, provide the necessary fluid composition and ensure sustainable measurement performance between proving/calibration cycles. Overall system reliability is also a critical design element to minimise complex and frequent maintenance procedures. Other systems associated with gas measurement may include gas conditioning equipment for particulate and liquids removal, odorant injection and pressure control to stabilise the meter working pressure. The main components of a natural gas fiscal measurement system are shown in Figure 2.

Figure 2. Schematic of typical NG measurement system.

Components of flow measurement systems

Referring to system components, it can be seen how each one contributes to the overall uncertainty of the total system.

Fluid composition

Manual samples and analysis or continuous analysers are used to provide information on fluid composition. For natural gas, this is associated with the fluid energy content, which determines the product value and price. For NGLs, it is required to determine component volumes at reference conditions. Inaccurate measurement of fluid composition and analysis can result in additional maintenance where contaminants are not detected.

Pressure control

A constant operating pressure can greatly help in the stable operation of some types of metering technologies and increase the measurement accuracy. Maintaining constant pressure is a prime reason why metering lines are usually installed downstream of pressure reducing stations.

Instrumentation

In measurement systems, a variety of pressure, temperature, density and viscosity instruments are used to help compensate for changing fluid properties that impact meter factors and correct fluid volumes to reference conditions. Inaccurate measurement, associated with any one of these components, will contribute to overall inaccuracy of converting flow measurement to reference conditions.

Flowmeter proving/calibration

One of the challenges faced in fiscal measurement is ensuring sustainable measurement performance over time. Meter performance can be affected by a change in fluid physical properties, mechanical wear in the meter, obstructions in the pipe and coating of the pipe wall. For these reasons, meter performance must be regularly verified against an external reference to ensure appropriate compliance to accuracy and repeatability requirements over time.

In addition to these challenges, undetected parameters associated with facility operations can occur on an intermittent or continuous basis and jeopardise measurement performance. For example, operators can encounter low volumes of entrained gas in a liquid stream, entrained liquids in a gas stream, or coating/obstruction of a flow conditioner. All of these conditions can impact the uncertainty of the flow measurement and may go unnoticed until the next verification event. It is clear that one of the main contributors to maintaining flow assurance is to detect factors that can potentially increase uncertainty before the next calibration.

Part 2 of this article can be reached here.

Adapted by David Bizley

Read the article online at: https://www.oilfieldtechnology.com/exploration/14022014/fiscal_assurance_part_1/