Assessment of Long Axial Corrosion Defects

Abstract

Severe Microbiologically Induced Corrosion (MIC) has been discovered in several offshore pipelines, and in particular in water injection pipelines. The corrosion is characterised by long axial channels around the 6 o’clock position. The channelling corrosion generally shows an irregular form which influences the quality of the ultrasonic inspection results; both due to the irregular form itself, and because it affects the ability to clean the pipeline prior to the inspection. The common practice is to produce a feature list reporting the deepest point of a metal loss defect and associate this with a long defect length, typical equal to the pipe joint length for channelling corrosion. Calculating the pressure capacity based on the reported defect depth and length will give a too conservative result for each defect. At the same time; a pipeline with severe channelling corrosion will have many pipe sections subject to an invariant loading condition, and potential structural failure may occur in connection with the lowest structural resistance among the pipe sections. This system effect is a statistical effect that will result in a lower pressure capacity. A procedure to establish the remaining wall thickness profile, a river bottom profile (RBP), based on detailed wall thickness and stand off data from inspections has been developed. A total probability of failure (PoF) for the pipeline given a design pressure is estimated. In case this total PoF exceeds the allowable PoF level, a procedure to establish a pressure adjustment factor, γs, has been developed. This paper outlines the procedure to establish RBPs along the pipeline based on detailed inspection results together with the methodology to calculate the total PoF, including system effects, and the pressure adjustment factor, thus coming up with a pressure capacity at the time of the inspection. The paper also reflects on how consecutive inspections can be used to estimate the yearly corrosion rate between the inspections.