By DNV GL
Fiscal metering of gas needs to ensure traceability to international standards. It must also reduce errors to levels that meet contractual or legal obligations, and which can be justified on technical and economic grounds.
The huge volumes of gas flowing through transmission pipelines internationally means that puzzling differences between what goes into a network and what goes out to customers can create an accounting headache and disagreement between supplier and customer.
This situation is compounded by the increasing variation in international trade and when there are multiple offtakers of gas from a network.
The potential reasons for these gas losses or unaccounted-for-gas (UFG) can include:
• Administrative errors
• Unmetered gas such as fuel consumed by compressor drivers, heaters in pressure reduction stations, and ‘vented’ gas that leaks from equipment
• Systematic metering errors, either initial installation or technical failures or related to the longer-term behaviour of metering systems (drift).
Fiscal consequences can be substantial. ”For example, if 5% of transported gas is unaccounted for because of bad measurement, you are talking tens of millions of dollars a year that the system operator somehow has to make up for,” said Bert Kiewiet, head of section, gas system management in DNV GL.
This threat is one reason that DNV GL advocates a fiscal metering policy that ensures traceability of gas to international standards, elimination of systematic errors, and the pursuit of the lowest measurement of uncertainty that is affordable. This view is based on experience supporting major transmission system operators (TSOs) worldwide, and operating facilities such as DNV GL’s Multiphase Flow Laboratory in Groningen, Netherlands, and the Flow Centre in the United Kingdom where flow meters and instruments for gas measurement and trading are tested, validated and calibrated.
”Measurements can never be 100% perfect, but best-in-class TSOs worldwide are achieving a UFG of ±0.1% of total traded energy volume in their grids, and our experts work with customers to reach that goal,” Kiewiet added. Assisting operators to achieve fiscal metering excellence according to ISO 10012 involves DNV GL performing a gap analysis on all aspects of a metering organization to establish what can be a reason for large UFG.
Questions about metering
There is continual debate in the gas industry about which meters to use and how often they should be calibrated.
Calibration intervals vary greatly. ”We generally advise recalibrating high-risk flow systems every three years,” said Dr Henk Riezebos, senior principal consultant – flow and flow acoustics, gas system management, DNV GL. ”High-risk refers to the high financial exposure of systems transporting annual volumes valued at more than EUR100 million (more than USD100m). The recalibration period for lower-risk systems could be up to 10 years.”
Frequently, transmission system operators’ policies on metering gas volume, capacity and quality (heating power) are governed by national regulations. Such a case is Gasunie, the high-pressure gas TSO in the Netherlands and in parts of Germany, whose systems link up to Denmark and the Siberian gas fields via the Nord Stream pipeline in the Baltic Sea.
When it comes to the company’s ‘city gate’ gas regulating and metering stations, Gasunie is regulated in the Netherlands by the TSO Dutch Metering Code. These buildings are where gas pressure is stepped down to transfer it to local gas distribution companies, large industrial customers and power plants. Temperature and gas volume flow are precisely metered at delivery pressure to the distributor so that invoicing is accurate.
Gasunie also has larger stations where its pipelines connect at national borders with those of other transmission system operators. Agreements between Gasunie and the other operators cover metering standards and practices at these cross-border stations. In addition, the European Network Interoperability Code (INT NC) started providing guidance in 2016 on how such facilities should be operated.
”For the city gate stations, we have some room to manoeuvre on metering policy,” said Peter van Wesenbeeck, manager metering and allocation, Gasunie. ” We mostly use turbine meters and positive displacement meters. We could also use ultrasonic meters, but turbine meters are currently a cheaper option while being fit-for-purpose.”
Having two meters in series at each border station allows an online comparison between the two to assess how well they are running.
Cost considerations are important
The Dutch Metering Code prescribes a statistically-based method in which a representative sample of gas meters is checked to obtain a valid snapshot of how all meters are performing. While considered sufficient to check on metering integrity, this also means that Gasunie is saving the cost of having to recalibrate all meters at the same time.
”Compared with the value of gas in our system, the cost of metering and calibration for a TSO is comparatively low,” van Wesenbeeck observed. ”However, metering quality is important for our customers, and still represents a significant amount of money as we have nearly 1,100 offtake stations connected to the grid.”
In a low gas price climate, operating expenditure is a key consideration for operators while remaining focused on safety and meeting supply requirements.
”Like all transmission and distribution system operators in Europe, we want to cut costs further,” said Robert Kruithof, metering specialist in Gasunie’s metering and allocation department, which takes care of the information exchange and IT system that centralizes and analyses metering and other data from the network and connected parties. ”So we are looking at smarter ways to perform calibrations and to make use of information from ultrasonic flow meters and diagnostic tools for turbine meters, for example. We are following closely what is being developed by meter manufacturers.”
Innovation and technical qualification
Innovation in metering tends to be incremental because of the necessarily conservative approach of network operators supplying large quantities of valuable gas. For the same reason, it makes sense for operators and manufacturers to pre-qualify equipment by using services such as DNV GL’s technical qualification (TQ) scheme for high accuracy gas flow meters. The new technology qualification scheme provides hard, scientific evidence in field-simulated conditions on whether recalibration intervals can be extended and is the industry’s first qualification scheme that provides assurance of the meter’s field robustness.
The TQ process is a methodology developed with specific acceptance criteria for technology under review and can be applied during any stage of the development lifecycle of systems or technology.
”Meters are specified to operate optimally within particular ranges of operating parameters and/or locations. In real-field conditions, efficiency and accuracy can be affected by factors such as dirt or deposits from the gas, as well as vibrations from the surroundings. We see many examples. In a specific recent case, we observed a huge (more than 40%) over-measurement in a flow system under extreme dynamic flow conditions.”
Kruithof stated: ”For external connections to our network, TQ can help manufacturers to understand our requirements.”
Summarizing the key issue facing operators, Riezebos said: ”There is a simple calculation. Metering in such a large grid represents a value of more than five billion dollars annually. So anyone can calculate the millions of dollars that would be represented by a reduction in unaccounted for gas, or financial exposure, from a level of, say, 1% down to below 0.1%.”
This can be achieved by using cost-effective, best industry metering and calibration practices and improving equipment quality, he advised. “For individual flow solutions, this is assured by a TQ assessment which determines the field robustness and cost savings potential of the technology.”