Assuring Accuracy in Fiscal Measurement and Custody Transfer

by | Nov 22, 2013 | Industry, Measurement Instrumentation, Oil & Gas | 0 comments

As hydrocarbons and other fluid-based products change ownership through the value chain, accurate measurement and accounting is critical. We have discussed this custody transfer process in several posts.

Emerson's Rossella MimmiOilfield-Technology-Fiscal-AssuranceIn an Oilfield Technology article, Fiscal Assurance, Emerson’s Rossella Mimmi describes how advancing flow measurement, diagnostic, and communications technologies assist in providing more accurate and reliable custody transfer operations.

Rossella opens explaining that the flow measurement required for custody transfer is not only used for accounting purposes, but also:

…is required for production management, leak detection systems, batch operations and loss/gain balance. These applications constitute key elements for operators who want to maximize efficiency in production, transportation and storage facilities.

She introduces the term fiscal measurement and how it differs from custody transfer. Fiscal measurement includes allocation, typically not bound by a buy/sell contract, which involves:

…assigning aggregated product quantities back to owners, leases, or individual source streams.

Custody transfer agreements are typically governed by contractual obligations and involve the transfer of ownership between buyer and seller.

Given the high stakes and financial impact of the flow measurements involved, accuracy is paramount. Unfortunately, many factors can impact accuracy of these measurements:

…product quality, fluid properties and composition, operating parameters, maintenance practices and technology type.

Rossella notes that the concepts of accuracy and uncertainty often get confused. Accuracy compares the flowmeter reading to a known reference standard. 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.

All the components within the measurement system must combine to be within the maximum allowable uncertainty level specified in the agreement established. Accuracy must be maintained over the flow range of the application.

Components in these flow measurement systems include fluid composition capture, pressure control, instrumentation, and flowmeter proving/calibration operations. Fluid composition is measured through manual sampling or on-stream continuous analyzers. Constant pressure levels help the accuracy of flow measurement.

Turnkey metering skid

Turnkey metering skid

Depending on what is being measured and the stability of its composition and fluid properties, instrumentation can include pressure, temperature, density, and viscosity instruments. The performance of the flow meter must periodically be compared against an external reference to prove compliance to the stipulated accuracy and repeatability requirements.

FloBoss S600+ Flow Computer

Rossella highlights some of the new technologies that bring higher accuracy, reliability and assurance including:

…recent developments in flowmeter technology and advanced diagnostics, flow computers with the ability to pass through critical sensor information and diagnostics, and WirelessHART to capture stranded device diagnostics.

Ultrasonic flow meters measure volumetric flow rates. I shared the theory of operation in an earlier post. Advantages for this application include having:

…negligible pressure drop, high turndown capability and can handle a wide range of applications, including natural gas and crude oil production, transportation, and processing. The latest versions extend the temperature and viscosity range to address applications such as shale oil, LNG and heavy crudes found in oilsands.

Coriolis flow meters measure direct mass flow for both gases and liquids with high accuracy and repeatability over wide turndown ratios. Advantages include not requiring:

…flow conditioning and mass calibration is valid on any fluid type, therefore they are a very good fit in both liquids and gas custody transfer lines. Coriolis meters also work well in applications where fluid density is not stable, like critical phase ethylene, as well as slurry and high-viscosity products.

You’ll want to read the article to see the role that diagnostics, wireless technology, flow computers, and remote access capabilities play in the accurate and reliable measurement. Some examples of the advanced diagnostics are to detect pipe blockage and entrained gas in liquids.

Rossella concludes:

New technologies play an essential role in increasing overall accuracy, achieving consistent and reliable measurement, and providing better insight into system conditions. The result is improved overall fiscal flow assurance through the entire oil and gas value chain.

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