Crude oil is a complex mixture of hydrocarbons which consists of aromatics, paraffins, naphthenics, resins, asphaltenes, diamondoids, mercaptans, etc. When the temperature of crude oil is reduced, the heavy components of oil, like paraffin/wax (C18–C60), will precipitate and deposit on the pipe wall. The pipe internal diameter will be reduced with wax deposition, resulting in higher pressure drop.Wax deposition problems can become so severe that the whole pipeline can be completely blocked. It would cost millions of dollars to remediate an offshore pipeline that is blocked by wax.

Wax varies in consistency from that of petroleum jelly to hard wax with melting points from near room temperature to over 100°C. Wax has a density of around 0.8 g/cm^ and a heat capacity of around 0.140 W/(mK).

In subsea system, following issues caused by waxes should be addressed by flow assurance analysis:

Several methods of wax control and management are practiced by production operations, but the transportation of crude oil over a long distance in subsea system demands significant planning and forethought. The wax management strategy generally is based on one or more of the following methods,

The most common method of wax control is flowline pigging if the wax has formed. The solid deposit is removed with regularly removing the wax layer by the scouring action of the pig. Chemical inhibitors can also help control the wax deposition, although these chemicals are not always effective and tend to be expensive. In case that the flowline pigging option for the production lines is not practical, particularly for subsea completions, wax deposition is controlled by maintaining fluid temperatures above the cloud point for the whole flowline.

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Source:

Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007.

Guo, Boyun, Shanhong Song, Jacob Chacko, Ali Ghalambor. USA. 2005.

Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.