Wax Deposition in Crude Oil Pipelines

Abstract

ABSTRACT The presence of wax in crude oil can lead to the formation of wax deposits on the walls of pipelines and as a consequence significant increases in pumping pressure caused mainly by an increase in the pipewall roughness. This can be particularly pronounced with the high cooling rates typical of submarine pipelines and is not restricted just to very waxy crudes. Using an extension of standard molecular diffusion theory the parameters influencing wax deposition rates are identified, and from simple laboratory tests on stabilized crude the rate of deposition in full scale pipelines can be predicted. Further, these principles and test methods can be applied to evaluate the effectiveness of the many available deposit-inhibiting additives under conditions appropriate to pipeline operations. INTRODUCTION Pipelines are widely used to transport crude oil. To ensure efficient use it is important to maintain their integrity and to minimise any flow restrictions. Water drop out in pipelines carrying wet oil can cause hydraulic hold-up; wax deposition leading to an increase in roughness and loss of effective diameter, will increase pressure drop or reduce throughput; turbulence also leads to nonproductive dissipation of energy. In this paper, we are going to focus on wax deposition. Wax deposition in a pipeline, particularly if it is left untreated, may have severe consequences on the operational efficiency of a pipeline system. This is certainly the case with North Sea submarine lines. The first, and by far the most important effect, is that the deposit increases the surface roughness and so leads to increased pressure drops in turbulent flow. The second effect is to reduce the effective cross sectional area of the pipe. The overall result is a loss in throughput for systems which are pressure limited. The normal method of controlling wax build-up for subsea pipelines is by regular pigging. The improved flow due to the passing pig is probably caused by a combination of smoothing of the rough wax layer and removal of the wax back into the flowing oil. Evidence that the smoothing of pipeline deposits can significantly reduce the friction factor in turbulent flow has come from work performed on bitumen-lined water pipes.