Hydrocarbons transported in transmission pipelines contain solid particles with micro-attached water. Subjected to flow conditions, these particles may have sufficient density to reach the pipe floor and enable bacteria growth and local Under-Deposit Corrosion (UDC), with this form of corrosion being one of the principal threats to the integrity of oil and gas transmission pipelines. NACE International has published a variety of UDC related standard practices to manage corrosion in the oil industry such NACE 61114, but few of them are representative of, or applicable to, low water cut hydrocarbon transmission pipelines. Further, there are presently no industry recognized key performance indicators (KPIs) suitable for managing UDC in low water cut hydrocarbon transmission pipelines.
Enbridge (the “Company”) operates North America’s largest interconnected liquid hydrocarbon transmission pipeline network. For the purposes of this paper, when the word ‘transmission’ is used to modify ‘crude oil’, ‘hydrocarbons’, or ‘pipelines’, it implies medium to long distance transport (100’s to 1000’s of km) as well as clean, “refinery-ready” crude oil (oil containing less than 0.5% sediment and water). This quality of oil renders it generally non-corrosive at pipeline operating conditions. However, if water wet particulates accumulate on the pipe floor, it can lead to UDC. The Company collects sludge samples produced during pigging operations on a regular basis to establish the composition of these materials and quantify bacterial population/activity. These solids represent an amalgamation of material removed from the pipe floor, and thus can be used as an indicator of the UDC threat in the pipeline. This paper builds upon previous work of the Company  by considering a larger data set in order to generate a more meaningful assessment of bacteria population/activity and provide better correlations with crystalline compounds, water content and elements found in the sludge.
This paper presents these data and associated statistical analysis, and proposes KPIs for evaluating the UDC threat based on numerous variables, including in-line inspection Magnetic Flux Leak (MFL) data (through signal to signal corrosion growth rates), sludge analysis, flow conditions and pipeline operation; this paper also suggests mitigation activities and intervals relative to these KPIs.
Personnel involved in pipeline integrity management (e.g. field operations, technical and management staff) may find the concepts, strategies and correlations presented herein to be useful in developing their own UDC management programs.