Accurate measurement in field conditions is difficult, says guest contributor Dr Bruno Pinguet, yet in-situ flowmeter calibration is achievable and benefits the effectiveness of control and instrumentation services.
For the oil and gas industry, accurately measuring a mixture of oil, water and gas in field conditions is a major challenge. Multiphase flowmeters (MFMs) have opened the door to the development of marginal assets and enabled more efficient exploitation of larger fields.
The ability to provide real-time flow measurement will lead to a greater uptake in MFMs worldwide but, because of the costs required to send them back to facilities for calibration, the challenges of validating them in-situ throughout their time in service must be addressed. However, the use of MFMs in oil and gas applications, particularly when installed for remote or subsea applications, is fraught with challenges.
It is important to have a proper mapping of the performance and uncertainty of an MFM based on the expected profile of oil and gas production. However, end users face a substantial challenge when trying to fully understand meter performance: the information that is readily available will most likely be generic, and not relevant to the actual field conditions that the meter will experience.
Research carried out at TÜV SÜD National Engineering Laboratory (NEL) has shown that generic meter performance figures are unsuitable for use in actual field service.
When the end user has selected an MFM, the fluid behaviour should also be considered, as this establishes the true performance at standard conditions. This step is especially important as it will highlight the level of uncertainty that should be achieved for the different fluid properties faced in field conditions.
It should also be remembered that it is the combination of the performance of the MFMs under well-established flow and process conditions, and the estimation of the uncertainty of the relevant PVT (pressure-volume-temperature) package, from line to standard conditions, that will provide the overall uncertainty of the system in field conditions.
Two methods can be used to address in-situ flowmeter performance. The first is to take the manufacturer’s statement, literature and the laboratory’s knowledge to establish the performance of the water, oil, and gas at line conditions. This performance analysis should then be coupled with the PVT uncertainty performance.
The next step is then to combine both uncertainties from the MFM performance and the equations of state (EOS) and propagate this to the standard conditions. This allows the end user to establish, without ambiguity, the performance and how the meter will behave in field conditions.
The second MFM performance review method is at the well site, either by remote or physical witnessing. This is usually required if there is some doubt about the performance that requires secondary equipment for verification, or when advice is required on the best metering solution to be defined as a reference. In field conditions, the uncertainty will be substantially higher than what can be delivered in well-controlled conditions such as in third-party facilities.
Real-time flow measurement will lead to a greater uptake in MFMs worldwide but, because of the costs required to send them back to facilities for calibration, the challenges of validating them in-situ throughout their time in service must be addressed
After meter selection, a test programme is established and a specific procedure is defined to validate the response of the MFM, employing engineering expertise and some statistical evaluation. It is then possible to understand the typical response of the MFM in the specific field conditions, identify the sweet spot and what should be avoided. This could result in a new manufacturer maintenance programme, the MFM’s replacement, or the installation of a complementary device following the end user’s expectation.
Overall, the work to be done to state the uncertainty accurately, and therefore fully understand the performance of MFMs, requires expertise and precise calculations. Thorough mapping of MFM performance against its in-situ application should be established by either oil and gas operators, or third-party MFM experts – and be validated where possible at a calibration facility.
- Dr Bruno Pinguet [pictured above] is multiphase domain senior advisor at TÜV SÜD National Engineering Laboratory
One size fits all is rarely the right approach when you want to ensure accurate flow. Ensure the assets you have are right for the job and remain that way...
Sensitive dosing operations rely on flowmeters and control valves working in an integrated system.
Pipework is vital to process optimi- sation, says industry account manager for water applications at Bürkert, Greg Wainhouse.
“Process application engineers commonly oversize pipework with the goal of reducing pressure drops across the system, as well as offering the flexibility of upscaling capacity at a later date. However, oversized pipe diameters and inappropriate configurations can cause inaccuracies that impact the quality of the process as a whole,” he emphasises.
An oversized pipe will often lead to low velocity flow, resulting in an uneven or transitional flow profile characteristic across the sensor, adds Wainhouse. Likewise, fluctuating flow readings cause variable output, with consequences for connected pumps and valves.
Oversizing also increases wear on valves and seals. Fluctuating input causes the control valve to open and close frequently and at uncontrolled rates in an effort to identify the correct position and maintain target flow. Likewise, bends in pipework close to sensors can also cause turbulence, rendering flow slow or uneven.
A factor to bear in mind, he states, is that in liquid flow applications 1m/sec is the normal flow velocity for most flow measuring technologies, because this gives stable flow profile for reliable measurement. Reduce the pipe diameter and you increase velocity, thus improving the accuracy of your flow sensor measurement. And the addition of a modulating control valve can increase dosing accuracy.
Just as optimising the supply of the chemical being dosed reduces process costs, so too can the action of refurbishing a key asset reap appreciable benefits.
Optimisation experts Riventa identified annual savings of almost £20,000 for a food processor’s cooling system. They did so by refurbishing the five pumps sending chilled water from heat exchangers. Operating these had previously accumulated annual energy costs of £181,400.
The firm’s specialist thermodynamic measure- ment technology evalu- ated the horizontal end-suction pumps that were driven by a motor with an inverter drive to control the flow rate.
This provided data enabling calculation of differential head, hydraulic efficiency and volumetric flow.
Pumps were altered gradually, with each asset throttled to reduce flow rate, to allow the other four to increase in speed to compensate. Pump curve shape indicated severe internal recirculation due to high wear ring clearances, says Riventa MD Steve Barrett:
“Measurements provided a strong case for robust savings: refurbishment with internal coating and replacement of wear-rings, bearings and seals – or comprehensive refurbishment.”