TRACERCO Diagnostics™ - Flow Assurance
Tracerco had been requested by a North Sea major to determine the total amount of deposit within a 22km long, 16” diameter subsea pipeline running between two platforms, by the use of the TRACERCO Diagnostics™ Flow Assurance Study. Measurement of the accurate velocity of a pulse of tracer between the platforms would be compared to the flow rate through the line and used to determine the effective average internal diameter of the pipe and therefore highlight the extent of deposit within the line.
The 16” oil line had not been pigged for a number of years, therefore the amount of deposit in the line needed to be measured to enable a cleaning program to be devised, and confirm the amount of expected deposits that would have to be disposed.
A sharp pulse of suitable radiotracer was injected into the pipeline at the sending platform, and a Tracerco data logger mounted externally on the pipework measured the exact time that it passed the known position on the riser. A second Tracerco data logger measured the exact timing of the eventual arrival of the pulse of tracer on the receiving platform, and a simple measurement of flow velocity obtained by dividing the time of flight of the pulse by the total distance travelled. The accuracy of the transit time was +/- 0.05%, derived from the error produced in the measuring of the centroid of the tracer pulse at the receiving platform.
The fiscally metered flow rate during the tracer test was then compared to the measured velocity, allowing the effective average internal diameter of the pipe to be calculated. Any reduction from the internal diameter schedule of the pipe will be due to deposit within the pipe, as long as the flow is full bore and turbulent during the measurement period.
An average deposit throughout the full 22km of 10mm was detected within the line, which equated to a total measured deposit volume of approximately 140 cubic metres. The accuracy of this measurement depends on the accuracy of the fiscal flow meter.
Unfortunately, it was not possible to arrange for the deployment of subsea detectors, which would have provided further intermediate velocity measurements, which would give further information on the location of the deposit within the line.
Detector responses on each platform: