Fiscal assurance, Part 2

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

New technologies bring higher accuracy, reliability and assurance

New technologies and solutions are now available to help optimise measurement systems. These include 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. These technologies can also be incorporated to optimise integration and turn-key systems along with lifecycle services.

Flowmeters

Although there are a wide range of technologies that can be employed for fiscal measurement applications, ultrasonic and Coriolis are emerging as flowmeter technologies of choice for fiscal measurement systems, as demonstrated by their rapid adoption in the oil and gas industry. These devices are approved by the majority of industry bodies, such as GPA, AGA and API, and national metrology standards such as OIML (International), NIST (USA), PTB (Germany), CMC (China) and GOST (Russia).

Ultrasonic meters

These provide volumetric flow rate. The meters measure flow rate via the transit-time method, where sound waves transmitted in the direction of fluid flow travel faster than those travelling upstream. The transit time difference is proportional to fluid velocity. The average axial velocity multiplied by the area of the pipe then gives the uncorrected volumetric flow rate through the ultrasonic flowmeter transmitter. Mass flow rates can also be determined in conjunction with densitometers. Ultrasonic meters (Figure 3) have 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.

Figure 3. Daniel ultrasonic meter.

Coriolis flowmeters

These provide direct mass flow measurement in both gas and liquid streams with high accuracy and repeatability over wide turndown ratios. The sensors maintain those qualities even when fluid conditions such as density, viscosity and composition frequently change. In a Coriolis meter (Figure 4), the material to be measured passes through one or more oscillating tubes; the rate of mass flow affects the oscillation of the tubes, and from this both mass flow and density can be determined.

Figure 4. Micro Motion Coriolis meter.

Coriolis flowmeters do not require 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.

As such, these technologies can provide more sustained, accurate flow measurement under a wider range of application parameters than other flow metering technologies.

Data and diagnostics

The increased availability of diagnostic data for fiscal measurement systems and its ease of access and utilisation improve flow assurance by identifying potential accuracy deviations between proving or calibration. Outputs such as the real time speed of sound calculation and velocity profiles for the ultrasonic meter or drive gain/density curves for the Coriolis meter can be effectively used to detect pipe blockage or entrained gas in liquids.

Advanced in-situ diagnostics are becoming more prevalent in newer technologies. Ultrasonic meters utilise multiple path transducer velocity ratios to characterise meter performance under different flow rates and automatically alert operators of upstream piping conditions that degrade overall measurement accuracy. Some Coriolis meters have the ability to perform self-checks on the integrity of the flow tubes, sensor components and electronics without the need to remove the meter or interrupt flow measurement.

WirelessHART

The ability to access the added data, diagnostics, and information from measurement system components is enhanced through the use of wireless technology. All the system parameters and flowmeter data can communicate wirelessly with the control system, with countless advantages in terms of flexibility, ease of installation and start-up.

Flow computers

Centralised control stations collect, assimilate, and manage data received from all the instruments and devices present in the system, including flow computers and RTUs (Figure 5). The ability to access, assess and transmit flow rate, operational status, pressure, temperature and diagnostic information to central monitoring facilities is used to monitor the status of the measurement system at any time and provide early detection of any unusual behaviour. They can also incorporate the same information to provide actionable alerts indicating the possible root cause of degradation in measurement accuracy.

Figure 5. ROC800 RTU.

Systems and lifecycle assurance

New technologies can be very helpful, but they need to be well understood, in order to be properly used and integrated in a complete system (Figure 6). Experienced integrators and fabricators will have the skills and knowledge to properly design and engineer best practices associated with new technologies to enhance flow assurance, minimise maintenance intervention and enable remote information management.

Figure 6. Turnkey metering skid.

Remote access to the system information enables collaboration with system fabricators, suppliers, subject matter experts and metrology services to improve lifecycle operability and reliability. Leveraging newer technology and expertise to augment or fill gaps in the lifecycle care programme will take this burden off the operator, who can then be confident in its system health throughout its lifetime.

Conclusion

Fiscal measurement systems are complex and, with regulatory requirements getting tighter, operators face more and more challenges on a daily basis. 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 throughout the entire oil and gas value chain.

Part 1 of this article can be reached here.

Adapted by David Bizley

Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/17022014/fiscal_assurance_part_2/