Flow Assurance: Wax Deposition and Gelling in Subsea Oil Pipelines
Authors: Hilbert Jessica 
Published: February 2011

Process surveillance of the Offshore Snapper Platform in Bass Strait identified an exponential increase in differential pressure in the 17.5km subsea oil pipeline after the 2008 Rig 175 Snapper-Moonfish infill drilling campaign. The increase was attributed to a build-up of wax which resulted when the crude (with pour points between 25 and 45°C) was cooled to the 15°C operating temperature of the pipeline. Process upsets and an increase in platform backpressure resulted. Further, the crude gel strength as determined by laboratory tests was such that the pressure required to reinstate flow in the pipeline exceeded that available from the existing platform facilities, resulting in a threat to continued production if the platform was to shutdown.

An understanding of the wax characteristics specific to the Snapper & Moonfish crude was obtained by leveraging expertise from ExxonMobil's global organization including ExxonMobil Upstream Research Company, local technical experts, external consultants and research groups. To reduce the risk of a blocked pipeline the frequency of pigging was increased and a minimum water cut of 40% was imposed as a short term risk reduction measure while facilities to continuously inject a wax inhibitor were pursued.

Differing crude mixes, water cuts, emulsions, and pipeline cooling properties are all factors which impact wax behaviors in subsea oil pipelines. Rheological tests showed that the shear yield stress of the pipeline fluid decreased with increasing water cut, with the Moonfish crude having a yield stress approximately double that of the Snapper crude. By comparing these results to proven operating practices, operating boundaries under which the pipeline could be reliably operated were established. A 40% water cut minimum was implemented to ensure the pipeline could be restarted.

Further tests determined the selection of a suitable wax inhibitor and the required dosage rate. The application of a wax inhibitor chemical resulted in the formation of weaker wax matrices which improved restart performance and hence operational confidence in the event of a shutdown. The design of the retrofit facilities required consideration of operational, safety and financial implications to ensure a reliable system was installed.

Monitoring of changing platform conditions allowed the early identification of a financial risk which had not previously been present. Understanding theoretical and practical wax characteristics enabled this risk to be mitigated and the crude and condensate platform production rate to be sustained. The continuous injection facilities removed the need for the 40% water cut minimum, reduced corrosion inhibitor costs, and relieved pressure on downstream water handling facilities.

The discovery of a wax deposition and gelling issue in the SNA oil pipeline highlighted the importance of technical monitoring programs. Further, the studies conducted have led to an enhanced knowledge of wax behavior in subsea oil pipelines and specifically an understanding of how changes in water cut impact pipeline restart which has application beyond this pipeline and Bass Strait.