In the United Kingdom sector of the North Sea, many assets are approaching or have already
passed their design life. Ensuring the integrity of ageing subsea carbon steel pipelines is an
increasingly important requirement, especially where an extension of field life may be required.
The Authors and their company have worked with a number of operators, performing pipeline
internal corrosion assessments, fitness for purpose assessments and future life prediction.
This type of assessment is often used to determine the requirement for in line inspection where
inspection data is absent, or to estimate the condition of pipelines that cannot be intelligently
pigged. This publication describes the steps and methodologies involved in this process. This
begins with corrosion modeling, and assessment of mechanisms such as erosion, CO2
corrosion, top of the line corrosion, oxygen corrosion and microbial corrosion. The utilization
and effectiveness of mitigation measures is then assessed. This is performed for both present
conditions and historical conditions. The corrosion modeling and assessment results are then
compared and tuned to available inspection results, corrosion monitoring and chemical and
microbial analyses, where available. Predicted metal loss is then used alongside
methodologies such as PD8010, ASME B31-G and DNV-RP-F101 to assess fitness for
purpose. The next step is often verification by inspection of selected worst case pipeline
sections, which may involve PEC and subsea UT measurements where intelligent pigging is
not practicable. The final step will then involve future life prediction. Additionally, this
publication examines the types of issues that have been identified in practice, including
examples, and the ways forward where problems have been identified.