When oil is pumped over considerable distances, usually between the subsea wellhead and installation, large pockets of gas or congealed oil will build up at the highest points in the pipeline. As the velocity in the pipeline is increased, the gas pockets or congealed oil will become dislodged and make their way towards the platform or processing unit.
A slug catcher is normally used to vent off gas and remove the congealed oil for processing. Accurate flow measurement of the oil from the outlet of the slug catcher is vital for allocation and accounting purposes. The process conditions downstream of the slug catcher are far from ideal, with varying large levels of aeration and solids.
In addition, coatings and pressure surges rule out insertion meters and conventional non-intrusive devices.
These difficult process conditions are why Able has developed the SlugMaster™ system, which is a unique dual-technology flow metering solution for accurate, dependable flow measurement on slugging applications.
The Able SlugMaster system deals effectively with the problematic process conditions by the use of dual ultrasonic technologies. The system uses time of flight metering during low aeration periods and switches to reflexor metering during heavy aeration and slugging, slurry or solid particle entrainment.
Able’s ultrasonic flow meters using time of flight technology have exceptional measurement accuracy for a clamp-on meters, even with small levels of aeration. In the SlugMaster system, this accuracy is used to calibrate a tandom reflexor meter during the window when both meters are able to operate due to low aeration in the fluid. Hence by calibrating the reflexor to the time of flight at the point where time of flight is just failing to measure, the reflexor becomes vastly more accurate than a standard reflexor out of the box.
Ordinarily, the change over point between these two technologies would be difficult to achieve, yet with the SlugMaster, an accurate and seamless changeover between the two metering technologies is achieved with unique mathematical software that enables accuracies that exceed the raw change over of both meters.
SlugMaster software cross calibrates the reflexor to the time of flight so that a calibrated reflexor flow is developed and an accurate transition is achieved between both metering devices. Able’s unique mathematical calculations are used to enable a smooth transition. The calibrated accuracy that is achieved with the reflexor meter is similar to that of the time of flight metering even during the heavy aeration and particle entrainment, so considerable accuracy is achieved in the most difficult metering conditions.
This accuracy may not be instantly apparent due to the swelling in the pipe during heavier aeration, as fluid transfer becomes erratic and staggered. When tested against a Coriolis Mass Flow meter at a point in the flow where there is no aeration, the ultrasonic meters would see large variations in the flow during slugging as the liquid is pushed and pulled by the slugs and cavitations of gas. The following graphs demonstrate this principle and explain this effect on Able’s flow test rig in Aberdeen, where the actual fluid flow was measured against a Coriolis meter at the pump outlet after degassing with an overall totalisation experiment conducted.
If accurate metering of the liquid has been achieved, the overall totalisation from both systems will agree within a very small percentage.
During factory testing and customer demonstrations, accuracies of well within 3% were achieved under all test conditions and most tests were within 1% accuracy against the Coriolis Mass Flow metering device. With such accuracies, the SlugMaster is a very accurate solution to challenging metering applications where only clamp on metering transducers are required and achieves exceptional accuracy compared with other metering technologies.