The presence of wax in crude oil can lead to the formation of wax depositson the walls of pipelines and as a consequence significant increases in pumpingpressure caused mainly by an increase in the pipewall roughness. This can beparticularly pronounced with the high cooling rates typical of submarinepipelines and is not restricted just to very waxy crudes.
Using an extension of standard molecular diffusion theory the parametersinfluencing wax deposition rates are identified, and from simple laboratorytests on stabilized crude the rate of deposition in full scale pipelines can bepredicted. Further, these principles and test methods can be applied toevaluate the effectiveness of the many available deposit-inhibiting additivesunder conditions appropriate to pipeline operations.
Pipelines are widely used to transport crude oil. To ensure efficient use itis important to maintain their integrity and to minimise any flow restrictions.Water drop out in pipelines carrying wet oil can cause hydraulic hold-up; waxdeposition leading to an increase in roughness and loss of effective diameter,will increase pressure drop or reduce throughput; turbulence also leads tononproductive dissipation of energy. In this paper, we are going to focus onwax deposition.
Wax deposition in a pipeline, particularly if it is left untreated, may havesevere consequences on the operational efficiency of a pipeline system. This iscertainly the case with North Sea submarine lines. The first, and by far themost important effect, is that the deposit increases the surface roughness andso leads to increased pressure drops in turbulent flow. The second effect is toreduce the effective cross sectional area of the pipe. The overall result is aloss in throughput for systems which are pressure limited.
The normal method of controlling wax build-up for subsea pipelines is byregular pigging. The improved flow due to the passing pig is probably caused bya combination of smoothing of the rough wax layer and removal of the wax backinto the flowing oil. Evidence that the smoothing of pipeline deposits cansignificantly reduce the friction factor in turbulent flow has come from workperformed on bitumen-lined water pipes.
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