A new report by Penspen Integrity, commissioned by the Canadian Energy Pipeline Association (CEPA), finds that diluted bitumen (dilbit, or synbit—bitumen extracted from oil sands and diluted with lighter crudes or synthetic crudes for transport through pipelines) is not more corrosive than comparable heavy sour crudes in pipelines—and in cases may be less corrosive.
|Transmitting oil sands production|
|Pipelines require a product with a density of 940 kg/m3 and/or a viscosity of 350 cST at the pipeline reference temperature. Bitumen has a density of 960 - 1020 kg/m3 and a viscosity of 760,000 cSt at 15°C—i.e., it won’t flow through the pipeline with modification.|
|Generally, bitumen is diluted either with naphtha-based diluents, NGL liquids, ultra light sweet crudes, condensates, and so on with a density of 650 - 750 kg/m3 to produce dilbit, typically with a 30:70 diluent to bitumen ratio.|
|Alternately, the bitumen is diluted with synthetic crude (density 840 - 870 kg/m3) at a typical ratio of 50:50 synthetic crude to bitumen for synbit. Dilbit has a typical viscosity of 350 cSt @15°C and synbit has a viscosity of 128 cSt at 15°C. The actual diluents to bitumen ratio may change from winter to summer as the temperature changes.|
|(Synthetic crude is produced by upgrading bitumen and is a blend of naphtha, distillate and gas oil range materials produced by hydrotreating the naphtha, distillate and gas oil generated in a delayed coking unit.)|
|Dilbit and synbit are classified as heavy sour crudes with an API gravity around 20—similar to other conventional heavy crude.|
As a result, the report authors concluded, there are no significant additional implications for corrosion control in a pipeline carrying dilbit and synbit as part of pipeline integrity management over and above what is already standard practice.
The oil and gas industry have viewed Dilbit and Synbit as much the same as any other heavy sour crude and there has been little published information on its potential corrosivity under transmission pipelines conditions. This void of information has led to speculation about the corrosive nature of these products. This absence of information has resulted in some literature that concludes these products are highly corrosive.—“Dibit Corrosivity”
The Penspen report examined 40 studies addressing the behavior of diluted bitumen and conventional crude. In these studies, which spanned more than 40 years, the research concluded that diluted bitumen is no more corrosive when compared to conventional heavy crude oil.
The report also showed that tests were carried out using internationally recognized standards, which found that small differences in some components of the product did not increase the chances of corrosion developing in oil transmission pipelines. In addition, monitoring and preventative maintenance programs used by transmission pipeline operators on conventional crude oil pipelines were equally as effective on pipelines containing diluted bitumen.
Other conclusions of the review were:
Dilbit and Synbit have similar characteristics as conventional heavy sour crudes in terms of density, TAN and sulfur content. While the TAN number, sulfur and salt content are important parameters for refineries, they cannot be used to assess the corrosion threat to an oil transmission pipeline, and these parameters are not used by pipeline corrosion engineers.
The presence of an electrolyte, essentially water, is necessary and that water must wet the internal surface of the pipeline for corrosion to occur in crude oil pipelines.
The corrosion mechanisms that oil transmission pipelines suffer from are well understood and transmission pipelines carrying dilbit and synbit have similar corrosion threats as those carrying conventional heavy sour crudes. The highest risks are associated with under deposit corrosion and sulphate-reducing bacteria. The key parameter used to reduce the risk of internal corrosion is the basic sediment and water (BS&W) value which is 0.5% for conventional crude as well as dilbit or synbit. This reduces the risk of water wetting the surface of the steel and of settlement of sediment which may lead to under deposit corrosion.
Crude oil transmission pipelines including those that carry dilbit and synbit are operated at flow velocities above that at which water and sediment drop out tend to occur but below the velocities where erosion corrosion can occur.
ASTM G205 has been used to assess the corrosivity of a number of conventional crude oils and dilbit and synbit. The results show that the dilbit and synbit are no more corrosive than comparable conventional crude oils.
Crude oil transmission pipelines have carried dilbit and synbit for more than 20 years with no discernible increase in corrosion failure incidents.
All pipelines have a corrosion control strategy as part of an integrity management plan. For a crude oil transmission pipeline this will include the running of cleaning pigs as required to remove any deposits and stationary moisture. It may also include batch treatment of inhibitor and or biocide. It will also include inline inspection using an MFL or UT inspection tool.