Accurate modelling of future corrosion growth is critical to long term pipeline integrity. A lot has been published within the industry regarding how to accurately determine corrosion growth rates (CGRs) from repeat in-line inspection data but there is limited guidance to support pipeline operators in deciding how to apply CGRs in order to support the development of repair plans and investigate pipeline remnant life. The consequences of significantly under- or overestimating CGRs are high and getting the balance right between the two is not straightforward.
Future integrity decisions can be made using maximum, upper bound, or average corrosion rates, and these rates may be calculated for and applied to individual features, pipe joints, or segments. All of the commonly used methods have advantages and disadvantages.
In this paper 10 CGR application methods have been applied to 3 onshore pipelines in which 3 inspections were conducted. The methods were compared against each other by reviewing the corrosion growth predictions from the 1st and 2nd inspections with the actual results of the 3rd inspection.
The aims of the study were:
i. to identify the methods that limit underestimation of corrosion growth but also do not severely overestimate corrosion growth, and
ii. to identify which methods are most appropriate for the different future integrity decisions, e.g. repair & re-inspection planning and remnant life estimation.
A scoring methodology was developed in order to compare and rank the CGR application methods. The methodology was based on an underestimation score (to identify which methods minimize underestimation) and an overestimation score (to identify which methods limit unnecessary repair work). The scoring method allowed for weightings to be applied to the under- and overestimation scores to take account of how the priorities vary when making different future integrity decisions.