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, 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.