9 minute read
A dynamic metering philosophy
Mike Shepherd, Alderley, UK, explains the advantages of dynamic measurement systems for LNG automation and control, and how they can open up new opportunities in today’s digital age.
LNG has a critical role to play in today’s energy landscape and the transition to a lower carbon future.
This highly versatile and tried-and-tested fuel is relatively easy to transport on an international scale, is an energy dense hydrocarbon and, when compared with other hydrocarbon fuel sources, is clean-burning and low carbon emitting.
With these credentials LNG will remain a cornerstone fuel for decades to come, making it a very attractive transition fuel whilst even lower carbon-intensive energy solutions are developed.
Indeed, a recent Bloomberg report forecast that global LNG supply could reach 449 million t by 2025 – up 23% from 2020.1 The analysis references LNG project backfills or expansions that are sustaining output in Australia, the Middle East, and Africa, with the US and Russia seen as key drivers for the growth. The US in particular could see its liquefaction capacity rise by more than half, overtaking Australia as the world’s largest exporter by 2024.
The need for effective measurement and control
In value terms, the Bloomberg report predicts that the global LNG market is expected to reach US$61.85 billion by 2026, at a compound annual growth rate of 6.92% during 2021 - 2026, with LNG predicted to be the fastest growing source of traded natural gas.
Evidently, with the volumes and values being traded, it is essential that the measurement of LNG is efficient, traceable, and accurate.
Figure 1. An Alderley dynamic metering system for LNG operations in Qatar.
This is where the dynamic measurement of LNG, i.e. measurement of flowing LNG, comes to the fore.
A dynamic solution
Dynamic measurement of LNG utilises a fiscally accurate measurement system at a point where there is a transfer of custody of hydrocarbons between two parties, or where the potential for hydrocarbon losses is of a significantly impactful value. High precision flowmeters and instrumentation is needed to provide the mechanism for direct measurement of the products.
Two meter types are favoured for gas and LNG fiscal measurement: ultrasonic meters (USM) and Coriolis meters. They both have their merits, with the line size USM essentially providing a volume flowrate whilst the Coriolis meter provides a mass flowrate. Both use sophisticated electronics and have extensive diagnostic capabilities. Precision is inherent in a fiscal metering system: complex calculations are performed on the meters and flow computers to determine the correct production and transfer figures – essential for the management of any high-value media.
Each flow computer is configured to automatically perform metering calculations, monitoring, and control functions for each meter on its individual pipe stream. The flow computers retrieve data direct from field instruments, primarily the meter, pressure, and temperature instruments, and perform all the primary flow calculations to internationally agreed standardised parameters. Based upon Alderley’s lengthy experience in LNG measurement and the recent improvements in fiscal LNG meters and sampling systems, dynamic measurement is the preferred method to optimise measurement performance and control.
Dynamic measurement also enables the opportunity for integrated automation and control, and forms the basis for digitally integrated solutions.
Focusing on energy value
As the primary measurement unit for LNG is its energy value, confident analysis of the chemical properties of LNG is essential and is best undertaken using online automated gas chromatography techniques.
The primary purpose of the gas chromatograph is to automatically calculate the calorific/energy value of the LNG as it is loaded onto the tanker.
Online gas chromatography has been a challenge in the LNG industry as the super cooled liquefied gas needs to be rapidly warmed to convert it into a representative gas sample to determine the quantities of methane, propane, butane, etc., so that an accurate energy or calorific value can be calculated.
Recent improvements on the design of specialised LNG sampling systems have overcome many of these issues. However, the pursuit for LNG measurement performance encompasses more than the utilisation of high-precision instrumentation.
Taking into consideration the significant temperature challenges of transporting cryogenic LNG at approximately -162˚C in sometimes 50˚C+ environments, the key components for an effective dynamic measurement system are:
z Careful mechanical design. z Appropriate isolation philosophies. z Suitable instrumentation selection.
z Robust automation and control systems.
The value of automation
Figure 2. Representation of a dynamic LNG measurement system. With the increasing emphasis on maximising value – achieved through enhancing efficiency/reducing costs – industries, including oil and gas, are increasingly turning to the automation of as many processes and procedures as possible.
Automation provides the opportunity for real-time performance and process monitoring for operators and technicians. This supports effective and efficient control through live monitoring of measurement and process events, better utilisation of resource through informed decision-making, and facilitates condition- and performance-based maintenance routines to ensure the optimum ongoing integrity and performance of their assets.
Much of the automation capability is managed by the supervisory computers. This provides a platform for integrating the communication interfaces; hosting the HMI, remote operated valves and controls interface via graphical representations of the metering package; various reporting systems; analytical packages such as condition-based management, instrument validation and calibration, and maintenance management systems.
A typical dynamic LNG measurement system is shown in Figure 2. It shows the fiscally measured raw feed gas coming into the LNG processing train, where the refrigeration and processing is undertaken to chill the gas into LNG, before progressing through to the storage tanks and ultimately loading onto the tanker for international sale.
During this LNG process, there can be unquantified hydrocarbon losses through flaring, the unmeasured consumption of gas for power generation, and general fugitive emission losses, etc. – losses that a proper integrated measurement philosophy, with its associated automation and control systems, can help to quantify and minimise. Once the LNG production process is complete, the super chilled LNG is measured and stored in large, heavily insulated tanks. At this point there is inevitable warming of the LNG in the tanks and the consequential release of boil-off gas (BOG) which needs to be controlled and measured whilst it is recirculated into the process train for re-refrigeration. Normally, the point of fiscal measurement is on the pipelines from the tanks to the tanker loading system as
Figure 3. Condition-based and measurement system performance monitoring of an ultrasonic metering system.
this is where the effective custody transfer and ownership change takes place. During the loading process the tanker is chilled and filled, and the consequential BOG is recirculated back into the LNG train for refrigeration once again.
Throughout the loading process, automated sampling of the LNG will be conducted by online gas chromatography to measure the caloric value of the loaded LNG – calorific value being the primary method of selling a cargo.
Dynamic fiscal measurement of the loading LNG and BOG returns is automatically calculated to ensure that the final stored volumes are correct once loading is complete.
To optimise the control of measurement processes, Alderley recommends the use of a condition-based monitoring system incorporating dynamic uncertainty for all gas, liquid, and LNG measurement systems.
Through continuous assessment of the metering systems, a bespoke condition-based monitoring (CBM) solution can provide the early detection of potential problems with these assets allowing appropriate control decisions to be made.
The availability of live information can also allow for the scheduling of maintenance exactly when it is needed, rather than using the industry norm of time-based maintenance, saving considerable costs in time and scheduling.
Alderley recommends a measurement CBM with clear ‘traffic light’ status indicators against each piece of equipment to support the early detection and prevention of potential faults, with built-in troubleshooting and deep-dive capabilities, including trend analysis and historian capabilities.
This helps to deliver assured performance of the metering system, ease of operation, and reduced operating costs throughout the life of the measurement asset, as well as the ability to provide prompt and effective remedial activity in the event of a fault.
For an LNG system, where pressure drop issues preclude the use of a master meter, the live CBM can also be used to automatically identify if a meter is drifting from its original calibration by comparing the meter in operation with its original calibration footprint. This can be used to provide the validation of accuracy of the loaded product. By collecting historical information from the meter CBM and meter calibrations, an argument can be presented to the local authorities to allow extended periods between calibration – saving cost and time.
It is also possible to connect the LNG metering system to a dynamic, or ‘live’, uncertainty calculation which would automate the production of a system uncertainty for every load or unload.
Global access and control
With the recent travel difficulties, especially internationally, there has been the need to remove barriers that keep critical plant data domiciled on industrial sites. This is because technical experts who would normally travel to these locations to check the quality of operations, diagnose problems, and provide solutions can no longer do so with the same frequency and ease.
To overcome these obstacles, and moreover to optimise operational performance and value, data must now be transmitted to remotely located stakeholders for review and action, and suppliers must now provide the technological services which facilitate these data transfers.
Alderley recommends solutions that harness industrial IoT technologies for securely transmitting business critical data from disparate industrial sites into corporate or Cloud-hosted databases. From these databases, stakeholders can be presented with live dashboards, charts, and reports showing the industrial system status and equipment performance.
Increasingly automated analytical tools are also being deployed to process this data to remotely manage and predict potential problems that might impact high-value commercial transactions such as automated and remote controlled routine system audits and calibrations.
A smart solution
Smart Asset Management from Alderley is one such solution. It is an advanced, fully integrated, and fully supported IoT connectivity solution to ensure the ongoing performance of the most critical assets: measurement systems.
Combining the company’s industry knowledge of custody metering systems with the latest connected and Cloud platform technologies, Smart Asset Management transforms measurement data into remotely accessible insight with the flexibility, scalability, and control to manage all measurement assets.
Summary
Automation and control is a critical requirement to ensure safe operation and cost-effective systems.
In this case, a well-designed and plant-wide dynamic metering philosophy including appropriate information-based automation and control systems, as described for a modern LNG facility, can provide considerable cost-saving opportunities and easier operator interaction whilst also providing the backbone for the digitalisation of the information.
This is crucial for the modern operation of large facilities and will allow operators to benefit from the new age of digital integration and IoT technologies.
