What do you do when you need a flowmeter for a high-pressure system where reliability and accuracy are of great importance, the system is under high pressure and the meter needs to be installed in a Zone 1 hazardous area? This is the exact situation that was faced by BG Group on its new North Sea Minerva platform.

While examining the design of its new platform, the company realised that it was going to encounter problems with the measurement of liquid produced by a number of selectable gas lines.
A flowmeter had not initially been included in the design of this system, although it was later decided that it was necessary to be able to monitor the quantity of water being produced in order to better understand how efficiently the gas wells were operating.
The main challenge faced by BG Group engineers was the process conditions that the pipeline was to be subjected to.
As Gordon Robinson, Juno commissioning manager, explains: "The platform process system is designed for normal operating pressure of 90 bar g, but this may reach
280 bar g in certain shutdown conditions."

Clamp-on flowmeter

It was therefore decided that the best option available was to install a clamp-on ultrasonic flowmeter, as this ensured that system pressure was not a factor in the reliability of the flow readings.
The engineers also discussed the possibility of using a turbine flowmeter on the system but as produced water system can be subject to gas blow-by there was a risk of compromised results and damage to the turbine flowmeter. It was therefore decided that this was not a viable option.
"A non-intrusive system removes the need for process tapping points and hence improves the overall safety by reducing the number of possible leak paths," added Robinson.
To further complicate matters, the area in which the flowmeter was installed was classified Zone 1. It was therefore essential that any equipment purchased was not only certified for hazardous area use, but also preferably to the new EU ATEX directive standards.
It was at this point that BG Group turned to Katronic Technologies to supply a solution in the form of the new KATflow 160 clamp-on ultrasonic flowmeter.
The meter and sensors are not only fully ATEX certified but also satisfied the project requirement for a non-invasive system ensuring that process conditions were not compromised.

Non-invasive

As the clamp-on sensors for the KATflow 160 are fully non-invasive there was no need to cut into the pipeline. This ensured that installation time and costs were kept to a minimum and that no system
re-engineering was required in any way.
Furthermore, as the sensors are mounted on the outside of the pipeline there was no risk that they could be damaged by any of the adverse conditions that are known to exist within this produced liquid system.
When making the choice of which flowmeter to use, BG Group had to consider other factors beyond simply the pressure and the hazardous area legislation requirements. There were also serious concerns about the accuracy of the meters under consideration, owing to the non-ideal installation conditions for the flow sensors.

Robinson again explains: "The Katronic system was the only system that would provide a 10 per cent accuracy without considerable pipe-work modifications. Most instruments will require suitable straight lengths to provide the specified accuracy. The Katronic system obtained the required accuracy with existing pipe work that seemed unachievable by others".
The advanced sensor and digital processor technology employed within the Katronic flowmeters allows all of their meters to produce reliable flow readings even when subjected to difficult conditions such as high levels of flow disturbance, or solid and gas content.
Once offshore the flow transmitter and sensors were mounted adjacent to each other directly on the pipeline, this allowed for easy commissioning and verification of the flow readings through the local digital display. All programming of the flowmeter was done using the magnetic pen provided with the flowmeter ensuring the adjustments can be made to the settings without needing to expose the flowmeter electronics to the hazardous environment.
Following commissioning the measured data was then fed back to the DCS in the control room via the digital relay and 4 - 20 mA outputs included in the meter as standard.
Robinson was impressed with how easy the flowmeter was to install and commission and described the KATflow 160 as 'simple to maintain and operate' and also that the unit was 'easy to interface with other instrumentation and the DCS'.

As the project was under considerable time pressure an engineer from Katronic Technologies was made available during the commissioning phase. Although the flowmeter is straightforward to operate it was felt that this was necessary in order to ensure that everything went as smoothly as possible during the complex procedure of bringing the platform on-line.
Since the KATflow 160 was put into operation in March this year the unit has been subject to frequent performance checks. This is done in order to ensure that the meter continues to maintain the performance that it showed at the time of commissioning and that it remains within the project accuracy requirements.

Tests

TIn reality what these tests have shown is that the flowmeter is in fact far surpassing expectations, as Robinson explains. "Our initial requirement was to install a meter capable of 10 per cent accuracy. The accuracy of the Katronic meter has been checked against batch measurements and is currently providing an accuracy of <5 per cent".
As well as the ease of installation, one of the big advantages of this type of ultrasonic flowmeter is that as there are no moving parts and no contact with the fluid, there is no reason why this level of performance could not be maintained.
Providing that the robust electronic housing or clamp-on sensors are not physically damaged in some way there should be no need for recalibration, thereby ensuring the long term cost of ownership of such a meter is effectively zero.