Measured success

Dr Michael Reader-Harris takes a look at measuring the changing flows in mature fields

Many offshore platforms in operation today were designed and built in the 1960s and 70s to extract oil and gas from large fields. Since that time, many fields have matured and declined in production levels, and are no longer operating at the same volumetric flowrate. This results in meters operating close to or below their minimum operating limits, which leads to an increased uncertainty of measurement. So, what are the options for using meters at flow rates lower than those for which they were originally specified?

Economic considerations
Most metering situations are unique. However, generally, meters operating at or below the lower end of their turndown will result in greater operating costs as a result of the increased uncertainty of measurement. For example:

• Where flow measurement data are SS used to optimise production conditions, operational efficiency can be compromised by the increased uncertainty.
• Where measurement data are used for allocation or fiscal purposes, exposure (the financial value associated with the uncertainty of measurement) increases.

There are, broadly, three approaches to mitigating these issues, though not all are applicable in every situation. In roughly increasing order of cost, these are:

• Use of the same meter below the original range of flow rates, through re-calibration or use of established equations.
• Physical modification of the meter to facilitate range extension.
• Replacement (downsizing) of the complete meter.

The cost will also be influenced substantially by the meter specification and its location/ease of access. Often, the overall cost of meter or part replacement can be many times that of the item itself. There may also be resulting effects in ongoing operational costs, for example if meter downsizing results in a greater pressure drop.

The options for a given situation may be compared and ranked economically using established calculation methods, such as Return-on-Investment, Net Present Value or Payback. Most organisations will have their preferred methods in this regard.

Technical considerations
These depend strongly on the meter types under consideration, which are discussed below.

Orifice meters
Orifice meters offer the greatest scope for extending meter range as they are very well standardised and characterised using well-established equations. They are almost always used without calibration in a flowing fluid; the Reader- Harris/Gallagher (1998) equation is widely used.

Existing orifice plates can sometimes be used as is at reduced flow rates, with only a modest trade-off in terms of uncertainty of measurement, which may be readily quantified. However, a minimum differential pressure of around 25 mbar is probably required at high static pressure (c. 50 bar), owing to limitations in the calibration of differential-pressure transmitters.

It is usually preferable to exchange the orifice plate for one of a smaller value (the ratio of plate hole diameter to pipe diameter) to extend the measurement range. As regards the minimum value of that can be used, traditional wisdom (ISO 5167-1:1991) suggests 0.2. However, in ISO 5167- 2:2003 the minimum value is 0.1.

Provided that certain limitations are observed, in theory there is no minimum value of . However, two possible physical consequences of reducing to a lower value are:

1. Increased pressure drop across the meter – for which there may be a consequential engineering requirement, such as an increase in pumping capacity.
2. An increased likelihood of liquid hold-up in gas-lines. If this is considered a possibility then drain-holes can be introduced to mitigate the issue. These are covered in ISO/DTR 15377 (Guidelines for the specification of orifice plates, nozzles and Venturi tubes beyond the scope of ISO 5167).

If turbine, positive-displacement and Coriolis meters are used, extending their range requires that:
a) The K-factor (which describes the relationship between pulse outputs and flow rate) curve shape is known for the range of flow rates of interest. This can be established if:

• the meter in question is itself re-calibrated over the range of interest or,
• a number of similar meters have been previously calibrated at lower flow rates and the K-factor curve shape shown to be sufficiently similar.

b) The K-factor does not become significantly non-linear with decreasing flowrate.

Turbine meters
Independent research evidence based on eight inch turbine meters suggests that condition (b) is particularly difficult to achieve. This is because uncertainty of measurement has been shown to increase rapidly as flow rate reduces below the design range of this class of meter.

While turbine meters give good performance over their normal operating range, it would be challenging to extend their performance very far towards lower flow rates, even with re-calibration. Turndown ratios for many turbine meters are limited to about 5:1.

Positive displacement meters
Independent research evidence for an eight inch positive displacement meter suggests this type of meter is capable of good performance over a turnover ratio of at least 15:1. The actual figure may be higher, though the limited scope of the independent data suggests caution as regards how widely this observation may apply. The need for re-calibration is therefore emphasised as part of any assessment.

Coriolis meters
Coriolis meters are relatively new technology as regards deployment, compared with turbine and positivedisplacement meters. There are also a very wide number of variants being deployed. Consequently, there is relatively little independent research data on which generalisations can be made regarding their range-extension capability.

Thus, by default, the same guiding principles apply as for turbine and positive displacement meters. That is, that range extension requires K-factor knowledge, including confirmation that it does not become significantly non-linear with decreasing flowrate.

Reducing the economic impact of maturing fields
Meters operating close to or below their intended range of flow rates impart economic penalties in terms of operational efficiency and financial exposure. For any meter type, it is only possible to extend the measurement range when the K-factor or discharge coefficient (which corrects for ‘nonideality’ relative to the theoretical relationship between differential pressure measurement and flowrate) curve shape is known and it does not become significantly non-linear with a decreasing flow rate.

Orifice plates offer by far the greatest flexibility in terms of range extension as they are well standardised with a comprehensive set of equations governing their use over a very wide range of flow rates and values. Turbine meters, by contrast, do not lend themselves well to range extension. Positive displacement meters have some scope for range extension if certain conditions are met, but they are inflexible compared with orifice meters, while Coriolis meters remain a relative unknown.

With flow rates typically reducing as fields mature, it is being increasingly found that meters are not fit for purpose, as the conditions for which they were designed are no longer applicable. With flow rates reducing, meters are often operating at the lower end of their turndown, which is characterised by poorer repeatability and accuracy, resulting in greater financial exposure for the entire measurement system. The solution is to either replace the metering skid with a more appropriately sized system or to amend the existing system to suit. As we have seen, each has their advantages and disadvantages, with cost being the primary decision criteria.

As most metering situations are unique, all relevant technical, economic and human factors should be taken into account in any decision. Manufacturers and regulators should be consulted, appropriate standards observed and a risk assessment should always be carried out.

NEL
Dr Michael Reader-Harris is Principal Consultant at NEL. NEL is a world-class provider of technical consultancy, research, testing and programme management services. Part of the TÜV SÜD Group, NEL is also a global centre of excellence for flow measurement and fluid flow systems and is the UK’s National Measurement Institute for Flow Measurement.

For further information please visit: tuvnel.com