Conventional crude and dilbit are subject to quality control measures and regulation. Pipeline operators employ further measures during transportation to manage and control the quality of delivered crude. Transportation of Canadian crude from the Alberta oil sands using transmission pipelines to distant refineries has been met with public concern that the transported dilbit is more corrosive than conventional crude products and is more likely to result in pipeline failure. This report has reviewed the current status of information and concerns regarding the corrosivity of dilbit in pipeline transportation as compared to conventional crude oil, thereby assessing the validity of the concerns. Naphthenic acid, organic sulfur, and chloride salt concentrations in dilbit crude oils were comparable to conventional Alberta heavy crudes. These compounds are too stable at pipeline operating temperatures to be corrosive. The sediment levels of most dilbit crudes were comparable to or lower than the conventional crudes, although the solids size distribution is unknown as is the role of larger size solids in the formation of pipeline deposits. Erosion corrosion was found to be improbable and erosion, if present, will be gradual and observed by regular mitigation practices. The dilbit viscosities are comparable to those of heavy crudes, supporting acceptable operating temperatures that are monitored along the pipeline. Pipeline steel wet by oil does not corrode. The regulated water content of crude oil pipelines is less than 0.5 volume percent and this water is primarily present as a stable emulsion, maintaining an oil wet pipe. A water phase may form underneath sediment deposits, and localized pitting corrosion has been observed. Similar underdeposit corrosion is not unique to dilbit lines and also has been observed in pipelines transporting conventional crudes. Cleaning pigs and chemical inhibition have proven to be effective mitigation strategies. Sufficiently turbulent flow rates also promote the suspension of sediments. Further understanding of the sludge deposition and corrosion mechanis will aid in optimizing the mitigation frequency and, where possible, operating conditions. Adjustment of the Alberta and U.S. pipeline failure statistics to compare similar crude oil pipeline systems on an equivalent basis indicated that the Alberta systems (with a large percentage of dilbit lines) experienced comparable internal corrosion failure rates than the U.S. systems (predominantly conventional crude lines). This review has indicated that the characteristics of dilbit are not unique and are comparable to conventional crude oils.
Corrosivity of Dilbit and Conventional Crude Oil in Transmission Pipelines
2012
16 Seiten, 3 Bilder, 2 Tabellen, 45 Quellen
Aufsatz (Konferenz)
Englisch
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