Offshore/subsea systems

New Improvements to Deepwater Subsea Measurement

A RPSEA project identified the gaps in technology that are the most pressing for multiphase-flow measurement.

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Fig. 1—Conceptual drawing of an ROV-conveyed apparatus for sampling at a deepwater measurement point.
Source: OTC 25686

In an effort to develop new measurement technologies for ultradeepwater oil and gas production, the US Department of Energy’s National Energy Technology Laboratory under the Research Partnership To Secure Energy for America (RPSEA) has initiated a new project. This project, titled More Improvements to Deepwater Subsea Measurement, has addressed those gaps that were identified by an earlier RPSEA project as the most pressing for multiphase-flow measurement.

Introduction

Approximately 15 years ago, as offshore production in the Gulf of Mexico and elsewhere moved off the continental shelf and into truly deep water, it became apparent that methods of measuring the flow of oil and gas would have to change in a significant manner. Not only did formidable technical challenges exist in performing production tests on wells that were many miles away in 1000 m or more of water, but the economics of installing test lines to perform such tests simply was not acceptable. Production from many wells would be commingled, and one of two alternatives would have to be chosen: Either a clever way of testing well rates from the combined flow would be developed, or each well’s rate of production would be measured before commingling.

Discussion

History. Past efforts of RPSEA and others to research improving deep­water exploration-and-production operations are detailed in the complete paper. At the conclusion of one such RPSEA project, running from 2008 to 2011, several leaders in a joint-industry project (JIP) that had supported that effort expressed an interest in a followup RPSEA project to refine certain results from the first project, and to investigate those areas where it was felt there were still questions needing answers. Consequently, a new proposal was submitted for RPSEA’s consideration, resulting in the new RPSEA project: 10121-4304-01, More Improvements to Deepwater Subsea Measurement. The following topics were identified:

  • Deepwater fluid sampling
  • Deepwater meter-verification technology
  • Early kick detection
  • Downhole differential-pressure-sensor development
  • Virtual-flowmeter evaluation
  • Detection of meter fouling

Each of these topics could have qualified as a project on its own, but the fact that the common thread of measurement ran through all, requiring the same kind of expertise in investigators, suggested that grouping the various parts under a single RPSEA project, managed by those experienced in measurement, made good sense. Therefore, this was the approach taken once again.
This JIP was organized somewhat differently from that formed in support of Project 1301. Chevron, ConocoPhillips, Statoil, and Total were retained from the 1301 JIP, and General Electric was added as a fifth member. Each agreed to contribute a proportionate share of the cost-sharing portion required, totaling 20% of USD 4.057 million, or approximately USD 810,000. Project 10121-4304-01 will conclude on 2 July 2015.

Overview of Results. Subsea Sampling and Sensor-Insertion Systems.Two parts of the new RPSEA project addressed the issue of fluid-properties monitoring during the life of a subsea well. In the first part, the RPSEA Task 5 effort, a system was developed to collect a fluid sample from a subsea flow point, typically on the production tree, for the purpose of capturing the fluid-properties information required by the multiphase meter. In earlier days, these data could be collected topside from the separator, but they are generally unavailable in deepwater systems because of the extensive commingling of wells that takes place. The new sampling system is conveyed by a remotely operated vehicle (ROV) to the receiving point.

In the sample-system development of RPSEA Project 4304, considerable attention was paid to lessons learned from testing of the 1301 equipment, especially in addressing ROV operator-handling issues. This resulted in a smaller and considerably more maneuverable unit. Tests for both sampling capability in the field and ROV-operator handling in a large-volume tank were made to assess the unit’s operability and its ability to perform the sampling functions. In Fig. 1 (above), a conceptual drawing of an ROV-conveyed apparatus for sampling at a deepwater measurement point is shown.

In the second part of the fluid-monitoring effort, a methodology for safely inserting or removing a probe sensor in a live production flowline was developed and tested. With few constraints other than dimensional considerations, individual sensors of one’s choosing can be inserted through a sophisticated series of valves by ROV, ­either horizontally or vertically.

Clamp-on Subsea Multiphase Meter. In RPSEA Project 1301, a first attempt was made at using an ROV to clamp another meter to a section of the subsea pipework. Although the mechanical aspects of the clamping and sensor orientation were demonstrated to work well, the measurement technique selected was not sufficiently robust to be considered seriously for meter verification.

To find a multiphase measurement robust enough to work in a clamp-on mode, an unusual approach was taken. Because the approach taken was to clamp on to normal steel pipe, and because the only known clamp-on metering sensors were those able to measure through the steel, certain kinds of sensors were ruled out, in particular those using electromagnetic (EM) methods for sensing the flowing fluid in the pipe. However, it was known that high-strength nonmetallic tubulars were being developed and were in test, and thus consideration was given to EM methods. A method of multiphase measurement based on electrical-capacitance tomography (ECT) was chosen for development here, with the goal of a laboratory demonstration of a clamp-on ECT meter operating around an EM-transparent pipe in a saltwater environment.

New Measurement Techniques for Kick Detection at the Mudline. One goal of RPSEA Project 4304 from the outset was to develop new methods for detecting a kick during drilling operations. To this end, two different methods were prototyped and evaluated experimentally. Incorporating a mud-density sensor at the mudline allows the monitoring of changes that could be caused by oil and gas inflow, and allows the detection of such an event long before it can be sensed topside—perhaps several hours in advance of its arrival at the surface for ultradeepwater wells.

The two techniques selected were (1) observation of various ultrasonic propagation properties and (2) measurement of the density of the mud using precision differential-pressure (DP) sensing.

True Downhole Measurement of DP. Measurement of downhole DP, in both drilling and production applications, has been an elusive goal for many years. The primary problem is the difficulty one encounters in measuring a very small difference in pressure in the presence of a very large background pressure. Bottomhole temperatures may be greater than 200°C, which presents a problem for most DP gauges. Consequently, attempts to use the difference between two absolute-pressure sensors have been completely unsuccessful.

In this Project 4304 task, the micromachined silicon sensor developed for RPSEA Project 1301 has been repackaged to meet the requirements for use in a downhole gauge. It is designed to operate with a precision of 0.1% of full scale at 15,000 psi and 250°C, and there is not another comparable gauge commercially available. The overall measured uncertainty at 10,000 psi was well within the specification of 0.1% of full-scale DP.

The packaged unit measures 0.95 in. in diameter and is 1.13 in. in length. Subsequent to assessment at room temperature, calibrations were performed first at 250°C and then at 10,000 psi; separate calibrations for high temperature and high pressure were necessary, because there is no known facility that concurrently is capable of providing both.

In-Situ Detection of Subsea Meter Fouling. When production meters are subjected to contamination or fouling as they are used, the effects on the quality of measurement can be disastrous. These kinds of inaccuracies are possible as scale is deposited on the interior meter surfaces. Similar effects in creating systematic errors in discharge coefficient, but in the opposite direction, are observed when the fouling is attributable to erosion rather than scaling.

Systematic errors from deepwater meters as significant as those described here can be harmful in reservoir-management applications but can be catastrophic in those cases where meter outputs are used for fiscal purposes (e.g., in production allocation). Given the possible consequences of making such errors in measurement, an obvious question is whether one can detect the fouling conditions that are the root cause. This was the goal of another dedicated RPSEA Project 4304 task.

Seven possible directions to address the problem were considered initially. From this list, three were chosen as most promising, one of which was eliminated because it required access to diagnostic parameters generated by today’s commercial multiphase meters, a need that was not likely to be fulfilled for these proprietary devices. The two remaining techniques were selected for in-depth studies.

The first was a look at innovative ways to use DP to indicate the presence of fouling. The second implemented the method known as data validation and reconciliation, wherein certain systems that are overspecified—which have more measurements than independent measurands—can be used to suggest which of the measurement devices is most likely to be the source of any errors in the measurement.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25686, “More Improvements to Deepwater Subsea Measurement: Overview,” by W. Letton, Letton Hall Group; J.M. Pappas, Research Partnership To Secure Energy for America; and J. Shen, Chevron, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2015 Offshore Technology Conference. Reproduced by permission.