Letton-Hall Ultra Deepwater Measurement Workshop

Research Partnership to Secure Energy for America-backed initiative to fill gaps in ultradeepwater measurement. We report on Eric Kelner’s evaluation of uncertainty in subsea metering.

Last month, the Houston-based Letton-Hall Group was home to an industry workshop targeting ‘Improvements to Deepwater Measurement.’ Attendees heard results from a joint industry partnership backed by the Research Partnership to Secure Energy for America (RPSEA).

The project goal is to ‘fill gaps’ in ultra-deepwater measurement in several areas. Here we focus on Task 6, ‘Measurement and Meter System Uncertainty,’ a subject area with high IT content, and indeed of some topicality in view of this month’s lead.

Letton Hall’s Eric Kelner noted the growing importance of subsea metering in the face of multi-billion dollar capital costs, complex, deepwater projects and ever lengthening tie-backs. These can approach 80 kilometers and are further complicated by ‘second wave’ tie backs to the ends of other tie backs. All of which amounts to more subsea commingling, greater ownership disparity and complex allocation and reservoir management. Equal uncertainty of meters is not a valid assumption. All measurement points carry different uncertainties, the problem is how to account for them. Measurement methodologies include single and multiphase flow meters and well tests. Separator performance is another factor influencing measurement. Flow line performance (especially slugging), instrument uncertainties and operating equipment outside of recommended ranges (considered a ‘necessary evil’) also impact measurement. Building on earlier work by API and the DeepStar project, Letton Hall is now looking for a way to calculate uncertainty based on first principles, one that accounts for the entire metering system including subsea meter, flow lines and riser, separator and single-phase meters downstream of the separator.

Multiphase Systems Integration (MSI) was subcontracted to extend an earlier model it built for Chevron to include the complete metering system as above. The result is ‘UBProdAlloc’ a standalone program with an Excel interface that predicts uncertainties and allocates flow rates, taking meter operating conditions, in-situ PVT properties and other system variables into account. The model determines the overall system imbalance, distinguishing between ‘normal’ and ‘abnormal’ by comparing flow meter data and assessing the overall uncertainty of reference separator measurements. A suggested use case for the tool when finalized could be to detect when a production system is operating outside of its normal range and then use the tool ‘in reverse’ to identify possible bad measurements. The tool can then be applied to categorize well test uncertainties and determine whether the SMPFM needs servicing. Industry partners in the JIP are BP, BHP Billiton, Chevron, ConocoPhillips, Shell, Statoil, and Total.

Presentations from the deepwater workshop are available on www.oilit.com/links/1107_26.

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