Why heat tracing would have been the most viable way of inhibiting wax in a Uganda crude pipeline

There are basically five ways to deal with waxy crudes such that there is assurance of flow of oil within the pipeline.

 

The Foster Wheeler Report discusses the five ways that Uganda would have oted to take on if it had chosen to build a crude pipeline for its oil and the five methods include heating the crude such that the temperature will not fall below the pour point during the transportation, Diluting crude with lighter oils, transporting the crude as an aqueous emulsion, using chemical additives to depress the pour point and the method of heat tracing to maintain the bulk crude temperature.

 

When using the method of heating the crude, let us say it is assumed that the crude is heated to eighty degrees at each pumping station prior to entering the pipeline; it will arrive at the next pumping station above sixty degrees subject to soil temperature. For the longer sections, 100 degrees may required, which is not recommended. Therefore an additional heating station becomes a necessity.

 

To enable this option, a small fired heater would be required using some of the product crude from the pipeline as a means of energy. This seems acceptable until there is no flow in the pipeline. In such instances, the fluid would continue to cool and there is the very real risk that if the fluid flow is not resumed within a defined period of time (which can be established by a more detailed wax deposition study), the temperature will cool below the wax deposition temperature forming wax and potentially blocking the line. Even if the temperature were to fall a few degrees below 60, viscosity issues with the crude will make restarting the line problematic. This option on its own would not provide assurance of flow.

 

Diluting the crude with lighter oils is the other option. It is impossible to say what the effect would be without performing pilot tests. Even if a lighter oil did indeed aid the flow (by reducing viscosity and lowering the pour point, there still remains the issue of where the lighter oil will be obtained. In all probability, a second pipeline would be required from Mombasa or Dar es Salam to bring light oil from a port to the start of the pipeline where it would be mixed with the waxy crude. This would add significant costs to the venture and at that stage of production it was not considered a viable option.

 

Or Uganda would have gone for the option of an Aqueous Emulsion. In order to transport the crude as an aqueous emulsion in the pipeline, water and the waxy crude are mixed before entry to the pipeline in such a way that an emulsion is formed, (the idea is that the low viscosity fluid is in contact with the walls of the pipeline. Typically, the required water, oil ratio is large and the unstable nature of the emulsions would bring all sorts of problems. There is also the suggestion that for some waxes, the presence of water assists wax deposition.

 

A pilot test would be required to see whether water would assist in this  instance, however even if it did there would be logistical issues to overcome, not least where would the water come from and how would the water be separated from the crude at the end of the pipeline. There is the possibility for stable emulsions to form during transportation that may prove difficult to separate. For the purposes of this study this option will not be considered as there are too many doubts as to the technical feasibility of the study.

 

Chemical Additives is yet the other option. Using Chemical additives to depress the pour point can be successful and indeed are used following rigorous pilot plant and field testing. The chemicals used include combinations of ethylene, vinyl acetate, acrylate and fumarate components such as ethylene vinyl acetate copolymer. The lack of availability of specific additives may not be appropriate. One article written on the subject states that “the complex nature of the interaction between these various precipitating high molecular weight species (asphalt, malt, waxes, resins) has resulted in specific additives being effective only with very specific types of crudes.”

 

The author goes on to say that following empirical testing to match specific crude with an additive that “some crudes continue to elude effective treatment with commercially available additives.” In this instance, because the crude is coming from different reservoirs with properties in varying quantities, the appropriate additive for start of life may be different from that required for middle or end of life.

 

This method is not guaranteed to work with the available combination of crudes and even if it does, the amount of suppression may not be sufficient to eliminate wax formation should the crude cool to the soil temperatures. The availability of the additives may also be a limiting factor.

 

Additives may be used as a risk mitigating measure in conjunction with heat tracing for occasions when there is a known shutdown of the upstream facilities that will lead to static fluid in the pipeline. However due to the uncertainties surrounding the effectiveness of this method and the nature of waxy crudes in Uganda, it is not possible to put this method forward as a viable solution for solving the flow assurance problem, even though with sufficient work an additive may be found.

 

And lastly we discuss the heat tracing method. Maintaining the crude temperature above its pour point represents the single most effective solution for ensuring that wax formation is minimized and that the crude can be pumped but this is not to say that wax wont deposit as “it is now generally recognized that that the pour point represents only a qualitative and at that a very mediocre measure of the rheological properties of any waxy crude. Consequently pigging of the line is still required. The best scenario is to maintain the crude temperature above that of wax appearance temperature or what is called the cloud point.

 

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