Any decision to repair an area of corrosion must include an appropriate safety factor. The current published guidance on safety factors in pipeline codes1-5 is inconsistent and in some codes arbitrary. In assessing the significance of corrosion in a pipeline and selecting an appropriate factor of safety the following should be considered:
- the location of the pipeline in terms of density of the surrounding population
- the hazardous nature of the product being transported
- the maximum operating stress level of the pipeline
- the uncertainty associated with the pipe material properties and pipe geometry
- the corrosion sizing accuracy associated with the inspection tool used
- the assumed rate of future corrosion growth
Modern reliability methods allow the above parameters to be modelled and the resulting factors of safety to be quantified. This paper uses reliability methods to investigate the actual level of safety inherent in the published assessment methods. A simple methodology is proposed which ensures the selection of consistent, safe and cost effective factors of safety for use in the assessment of corroding pipelines. Examples illustrating this approach are provided.
Corrosion assessed on the basis of modern ?fitness-for-purpose? methods avoids unnecessary repairs associated with codified, prescriptive methods, which are generally accepted as being over conservative (Table 1, located after the References).