Examining the importance

of starting at the drawing board in terms

of long-term measurement reliability. A discussion

between Wim Volmer, Senior Product Manager Oil & Gas at NMi, the Netherlands and Eveline Janse,

Comorama, the Netherlands.

to theStraight

A s oil and gas are valuable commodities, accurate measurement gets increasingly important in the oil and gas industry. Oil and gas flowmeters with lower measurement uncertainties are being

developed, but are we aware which additional effects also play a role in the overall uncertainty and performance of the installation as a whole? What will happen with the measurement uncertainty in five years? What expenses are involved?

The performance of a flow measurement installation is complex and involves a number of factors. Constructing or upgrading such an installation therefore requires expertise. Flow profiles, possibility for calibration and temperature stability, are only a few of the factors that can affect the accuracy of the measurement results.

Maintenance: return on investmentMeasurement uncertainty cumulates over time. In addition, measurement uncertainty directly corresponds with revenue. The measurement uncertainty x price/litre x number of

point

litres/year will be visible on the balance as ‘contingency’.

A good analysis of the installation enables the various effects to be quantified and improved. Performance over time can be calculated and compared to the legal requirements or the customer’s criteria. This information

can be used to optimise installation and draw up a maintenance plan.

Replacing components during the installation phase should be well considered; other metering techniques are not necessarily suitable for a specific installation. In addition, replacing components provides an opportunity to check whether the performance of the installation can be improved. For example, is it possible to improve the flow profile when you replace certain valves?

A number of measurement principles are sensitive to asymmetrical and swirling flows. Measurement differences can be multiples of the initial uncertainty of a well-installed flowmeter. With the effects being so large and the products valuable, the benefits of one valve over another is easy to calculate, provided the attention for metrology is there.

Important decisions regarding the installation, which will affect overall performance and uncertainty, are consequently being made at a much earlier stage. Putting together the tendering document, calculations in the design phase and decisions in the construction phase, all accumulate to the accuracy of flow measurement.

CapeX versus opeXWhen making decisions regarding metering principles, various types of expenditure, both short-term and long-term, should be taken into account.

Constructing a flow measurement installation involves serious investment and capital expenditure (CAPEX) is an important factor. As a result, when designing and constructing the installation, the main focus on CAPEX is to put together an installation that is as cost-effective as possible. Tendering puts more pressure on bidding companies to search for the cheapest solution. However,

Figure 1. Flow measurement computer.

Table 1. Calculation example: investment versus accuracy

Instrument Scenario 1 Scenario 2

% Investment % Investment

Uncertainty quantity 0.10 100 0.25 65

Uncertainty quality 0.50 30 0.20 65

Uncertainty total 0.51 130 0.32 130

Figure 3. Construction of the gas flow calibration facility of EuroLoop. Figure 4. Pressure meters in a flow measurement installation.

Figure 2. Based on a throughput of 500 000 m3 (6 in. batching), a price of 20 cents/litre and an increase in the measurement uncertainty of 0.01%/yr, the costs increase substantially.

World pipelines / REPRINTED FROM APRIl 2014

one should not neglect the operational expenditures (OPEX), being the costs involved in operating the system, which will return on the balance year after year.

A good measurement protocol can help reduce OPEX. For a producer of plastics that generates its own energy, NMi performed an uncertainty analysis proving their energy generation and consumption was so efficient the company was eligible for subsidies. Most people perceive uncertainty analysis as being limited to single points of measurement, but in this case it involved a large number of measurements of various quantities, spread over two separate plants.

At a later stage, NMi effectively devised a way to keep the measurement accuracy such that it met with the subsidy related requirements. The measurement protocol in combination with a “metrological maintenance programme” (when to re-calibrate what) formed an efficient tool for the maintenance department.

Short-term versus long-termCAPEX and OPEX are strongly related to short-term investments and long-term decisions. The costs of modifications required as a result of ill-considered and short-term solutions soon mount up (Figure 2). Accuracy always pays for itself if it improves process or invoice management.

It is more costly to add proving connections to an onsite situation than to a measuring skid still in a workshop. Once onsite modifications are needed, the related expenses easily mount to several times the initial cost-cuts made in the procurement stage.

For example, natural gas is usually traded by its energy content. For the determination of that so-called calorific value, it is needed to measure both the quantity and the composition of the gas. The uncertainty in energy content is equally determined by the uncertainty in the flowmeter and in the gas chromatograph. It pays, therefore, to balance the investments in these components over the two, as illustrated in Table 1 (in imaginary monetary units).

tenderingTendering is the magic word when it comes to the construction or upgrade of a cost-efficient installation. The tender call typically states specifications and requirements, such as the measurement uncertainty. In practice however, CAPEX often beats quality when it comes to awarding the contract. This forces companies to save on all possible expenses, which has a detrimental effect on the quality and long-term stability and reliability.

For example, certain flowmetering techniques are more expensive to purchase, but are more stable over time and require less maintenance due to the self-check options. When making a bid, these long-term operational costs are of little importance and cheaper, less suitable options will replace the best long-term option. This type of ‘cost saving’ results in higher measurement uncertainty and sub-optimal measurement characteristics.

It is evident that mistakes can be costly. It would not be the first time that the meters are impossible to calibrate because of their size. In addition, a lack of test connections and spare parts is often encountered. Clear specifications for the tender prevent many problems. For instance, having multiple flowmeters in parallel allows for consecutive calibration of each and provides an inherent degree of redundancy.

In the past 10 years, NMi has been asked for its expertise in various big projects. NMi encountered different situations in installations that were the result of lack of knowledge in the tendering phase. What you should or should not do in the tendering call regarding the metrological aspects:

Do: ) Apply a helicopter view, taking the complete

installation into account.

) Include relevant (legal) requirements.

) Include design specifications from international norms and regulations.

) Include certification of the installation.

) Hire expertise when putting together specifications.

) Consider OPEX when assessing the bids.

) Translate metrological effects into financial terms the DMU is sensitive to; highly technical terminology is prone to be ignored.

) Invest in a proper system design; mistakes are cheaper to correct in the drawing board stage than later in the lifecycle.

Do not: ) Only determine the uncertainty of the flowmeter.

) Accept meters exceeding available calibration capabilities, for example, meters too large to be calibrated on existing facilities.

) Underestimate the consequence of incorrect software (configuration).

) Assume everybody understands what the consequence of measurement uncertainty is.

Smart, future proof design In the design phase, different options to meet the requirements and criteria for measurement uncertainty can be calculated. Investing is this early stage of the development results in an optimised installation, reducing both CAPEX and OPEX. Calculations of both impact on expenditures, measurement uncertainty and operational costs can be surprising. When designing the gas meter calibration facility of EuroLoop, specific uncertainty reducing investments were loosened to enable higher

RepRinted fRom ApRil 2014 / World pipelines

investment in other factors, which gave a better overall performance. Calculations showed, for example, that investment in temperature stability instead of pipeline uncertainty resulted in a lower measurement uncertainty.

Tuning the various components is much easier and cheaper on the drawing board than in the construction phase. Even though flow measurement installations are in most cases ‘one of a kind’ and become proven only in the field, asking experts to review the design can reduce costly modifications at a later stage. There may be good arguments that make decisions plausible, such as placing an oil meter in the top of a flow circuit because it is easier to reach. Still this is not a good idea

since gas bubbles, for some physical reason, also prefer the top of the installation, where they will influence the measurement accuracy. Or the other way around: placing a gas meter at the bottom of a flow circuit is not the best thing to do, as all disposal that enters the circuit in the gas flow will settle down and cumulate in this lower part of the system.

What uncertainties do you find acceptable?

Oil and gas products are charged based on measurement data. As a result, precise

measurements are essential. The measurement uncertainty of the flowmeter is often considered the main factor in the precision of measurement results. However, in flowmeter installations, the measurement uncertainty is much more complex. For example, the accuracy of the measurement can be influenced by the flow profile as a result of pipelines, pressure, pulsations and – last but not least – human error.

Differences in measurement results between supplier and buyer are often a point of discussion, since small differences in big pipelines correspond with millions of Euros per year. Certification by an independent metrology institute, such as NMi, is often needed for acceptance of measurement results.

What uncertainties does your CFo find acceptable?The oil and gas industry is one where multiple disciplines work together, one of which usually being finance. Without some extent of understanding between the various disciplines, it is more challenging to include metrological specifications in, for instance, an invitation to tender or a maintenance contract. Therefore, taking the time and making the effort to translate and/or explain the link between metrology and economy is key. In other words, that extra step should be taken and the short and long-term financial effects of any purchase should be shown; it is highly likely to be an attractive return on investment.

Figure 5. Wireframe of a flow measurement installation.

Figure 6. Flow measurement installation in operation.

World pipelines / REPRINTED FROM APRIl 2014