In different inspection standards, corrosion pits in drill pipe tubes are treated in different ways. Sometimes they are considered a stress concentrator and subjected to relative depth limitations (like a maximum of 10% of the adjacent wall); sometimes they are only considered as a location of wall loss, so the steel thickness underneath them is the only acceptance parameter. This work clarifies the difference in severity between these two types of imperfections, and enables the definition of a reasonable inspection process.
Slip cuts and corrosion pits with varying depths were modeled using Finite Element Analysis (FEA) techniques for multiple drill pipe sizes. The resulting stresses and strains under tension and bending loads were used to calculate the fatigue lives of these components. The resulting number of cycles to failure does not correspond to the real lifetime of real pipe—there are too many variables and neglected parameters—but the results can be compared within the data set to determine which options are better and worse and by how much.
Significantly, this research validates the approach taken by some standards to neglect the stress concentration effects of corrosion pitting, only considering the wall thickness remaining for the purposes of inspection. The commercial benefit to this is huge: it allows the inspector to not have to reject or repair a drill string’s worth of light corrosion pitting. This research indicates this is valid and safe in many cases, but in the more-stringent inspections used for offshore drill pipe the relative depth of the pitting also needs to be considered.
Number of Pages
API RP 7G-2, Recommended Practice for Inspection and Clasification of Used Drill stem Elements, First Edition. 2009. Washington, DC: API
Ellis, S., Reynolds, N., Lee, K. 2004. Use NC56 Connections on 8" Drill Collars and Cut 1" or 3/4" Pin Stress Relief Grooves on Rotated BHA Connections NC38 and Larger. Paper presented at the IADC/SPE Drilling Conference, Dallas, TX, USA, 2-4 March. IADC/SPE 87191. https://doi.org/10.2118/87191-MS.
Standard DS-1®, Volume 2: Drill Stem Design and Operation, Fourth Edition. 2012. Houston, TX: Bureau Veritas North America, Inc
Standard DS-1®, Volume 3: Drill Stem Inspection, Fourth Edition. 2012. Houston, TX: Bureau Veritas North America, Inc
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