S07 ORME 4 2021 Flow measurement_Layout 1 29/06/2021 12:06 Page 24
Technology
How to be certain of the
uncertainty of an MFM Dr Bruno Pinguet, multiphase domain senior advisor at TÜV SÜD National Engineering Laboratory, discusses the importance of measurement uncertainty analysis for multiphase flow meters. ULTIPHASE FLOW METERS (MFMs) have been employed as complex measurement systems for the oil and gas sector for many years. As they can eliminate the need for a test separator, which are big and difficult to maintain, smaller platforms are possible. Existing facilities can also be upgraded to take subsea tie-backs without having to add an extra test separator. MFMs also give continuous measurements, allowing better reservoir management, well optimisation, and a quick response to water break-through and similar events. As MFMs are becoming drastically cheaper, most oil and gas operators could now take advantage of this capability. However, while they reduce CAPEX, fundamental questions remain over the reliability and capability of such equipment to perform accurately when compared to previously used systems. This is because the claimed theoretical uncertainty of meter manufacturers does not always match that of a more conventional test separator setup, making some oil and gas companies question their accuracy in service. The buying process therefore very often involves some comparison tests being made either before delivery, or later in the field on a regular basis, or as a spot-check against standard equipment, such as separators. While flow meters are calibrated under ideal laboratory conditions, the environments into which they are installed vary greatly. It is therefore incredible to see today in this billiondollar oil and gas industry that most flow measurement systems are reporting flow rates without any consideration of measurement uncertainty. Uncertainty analysis is therefore essential to determine whether measurement systems are capable of meeting required performance targets.
What is uncertainty? It is a popular misconception that measurement is an exact science. In fact, all measurements are merely estimates of the
24
oilreview.me
Issue 4 2021
Image Credit: TÜV SÜD National Engineering Laboratory
M
Dr Bruno Pinguet, Multiphase Domain senior advisor, TÜV SÜD National Engineering Laboratory.
interval defined around the average, which is based on the data collected over a given period that is considered a stable flow condition. The true value can be expected to be within the confidence level inside the interval defined during the measurement. The size of the interval is described as a confidence interval or in terms of sigma (i.e. standard deviation from a statistical point of view). To make MFMs easily comparable and effectively report their performance, the multiphase flow metering community has decided to follow the 95% confidence level or 1.960 multiplied by one sigma. From a practical point of view, this means when a measurement is made repetitively (under continuous stable flow), then a standard deviation can be calculated using the numerous collected data. This is defined as one sigma (σ) which means that there is a 68% chance that the true value is within this interval. This standard deviation “σ” is multiplied by 1.960 and provides the performance or uncertainty interval with a 95% confidence interval. This is shown in Figure 1.
Uncertainty elements true value being measured, and the true value can never be known. The terms ‘accuracy’ and ‘uncertainty’ are also misused in the oil and gas industry. Accuracy refers to the agreement between a measurement and an expected true value. Therefore, accuracy requires two measurements with two different meters. Accuracy cannot be discussed meaningfully unless the true value or most probable value is known or can be recognisable. On the other hand, uncertainty is an
It is a popular misconception that measurement is an exact science.”
So, how can operators effectively allow for this uncertainty? The ideal approach would be to calibrate each individual device for the specific conditions it will encounter. However, this is not financially realistic, and to establish the performance of an MFM, an uncertainty budget must therefore be constructed, taking account of additional uncertainties arising from interpolation and extrapolation from calibration conditions. Firstly, calibration against a single-phase meter at least three to four times better than the device in question is needed. This is done in a third-party flowloop facility, the best being a primary calibration facility. Secondly, the repeatability performance of the MFM must be established. The repeatability of the deviceunder-test (effectively the closeness of agreement between successive measurements made under the same conditions) is its estimation of the overall
