Computer Fluid Dynamics analysis of flare lines aboard a major offshore oil and gas facility in the North Sea, has provided its operator with a cost-effective and fast mechanism to improve the accuracy of its metering.
“NEL was contracted by the client to investigate potential metering errors due to installation effects in an ultrasonic flare gas metering system,” says CFD Team Manager, Marc Laing, who led the research. “We have supplied the client with correction factors that they can program into their flow computers to improve the accuracy of their measurements. This removes the need to make any costly and time-consuming engineering changes such as removing and re-positioning meters.”
According to Marc, the challenge facing the operator is a common one for many offshore installations where there are space constraints. The solution provided could therefore be of relevance to many operators.
“It is often the case that meter installations cannot be optimally positioned and this leads to problems such as asymmetry and swirl being present at the measurement point,” he explains. “This is a particular problem for ultrasonic meters, which are known to be sensitive to distortions in the velocity flow profile. Potentially, this can result in a reduction in measurement performance and accuracy.”
The CFD analysis involved evaluating the mean axial ultrasonic transducer path velocity in the installed system. This was then compared with the mean path velocity in an ideal setup where the meter has been given the required upstream length to allow the flow profile to fully develop.
The analysis was performed for both high and low pressure flare lines. A range of scenarios were modelled that represented the range of flow conditions that were experienced in the system. These were based on flowrate data and fluid properties, supplied by the client.