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Title: Determination Of Measurement Uncertainty For The Purpose Of Wet Gas Hydrocarbon Allocation
Author: Robert A. Webb, Winsor Letton, Martin Basil
Source: 2002 North Sea Flow Measurement Workshop
Year Published: 2002
Abstract: The use of measurement uncertainty in the allocation of oil and gas is gaining interest in the industry. This so-called Uncertainty-Based-Allocation (UBA) utilizes the relative uncertainties of reference meters and allocation meters. The uncertainty of the allocation meters may differ significantly from one another and from the reference meters in both design and performance. The UBA approach attempts to use knowledge of meter uncertainty to equitably assign imbalance in the system. It has found immediate application in assigning the imbalance for wet gas systems in which one or all of the allocation meters is located subsea 1. While the mathematics used to perform Uncertainty-Based-Allocation is straightforward in its derivation and application, determination of the meter uncertainties that will be input to the calculations can be a formidable task. Not only are the systems that would use the approach often complex, but there are other problems which must be addressed as well. One is the determination of the uncertainty of each wet-gas meter at subsea flowing conditions. Another is the transformation of those uncertainties to the environmental conditions of the reference meter in order that the appropriate equations can be applied. Perhaps the most significant is the detection and allowance for systematic, or bias, errors in different parts of the system. Bias errors are particularly worrisome. After being effectively eliminated by calibration prior to system startup, they may gradually become significant in one or more meters during system operation over time, and be virtually undetectable without removal and test of the offending meter. Their effect is discussed further in Section 5.3. The effective combination of the Uncertainty-Based Allocation technique with the ability to determine meter and overall system uncertainty is of greatest value to the industry. The process has the potential to deal with difficult allocation situations and to open the door for incremental and marginal production, such as subsea tiebacks. The challenges which must be overcome in this process are not simple, however. Some of the most important will be discussed in what follows.

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