Session: 07-04-02 Crack Management

Paper Number: 97332

97332 - Probabilistic Surviving Population Remaining Life and Inspection Timing Guidance 

The estimation of the Post Hydrostatic Test (PHT) baseline crack Inline Inspection (ILI) timeline for oil pipelines, and their ILI re-inspection intervals afterwards, can be a technically challenging task for some operators due to the complexity of the analysis and the computational resources required to perform the analysis. In addition, a balance between the proper level of conservatism in the analysis assumptions needs to be considered to ensure that the targeted level of safety is achieved while maintaining optimized inspection intervals to reduce cost.

While detailed guidance for re-assessment timing is provided in STP-PT-011 [1] for gas pipelines susceptible to stress corrosion cracking, no analogous guidance is offered for pipelines susceptible to longitudinal weld seam imperfection crack growth due to pressure fluctuation. This paper provides guidance to pipeline operators on recommended crack ILI timelines for baseline inspection and future re-inspections, thereby closing this gap. These recommended timelines are based on validated probabilistic assessments which consider a wide range of pipe properties, defect populations and morphologies, and were found to be primarily sensitive to operating pressure cycling severities.

To estimate recommended timelines for baseline crack ILI PHT, assessments were completed using the probabilistic surviving flaw approach described in [2]. This approach provides a realistic probabilistic assessment for remaining life PHT in comparison to traditional deterministic analysis which assumed worst case scenarios for distance from pump station and defect severities. In this paper, the probabilistic approach is baselined and generalized. A baseline mean time to failure (MTTF) was probabilistically estimated over a wide range of pressure cycling severities using typical random variable inputs. The baseline MTTF results were found to be scalable to any pipeline with known condition and operational parameters. In addition, a user-friendly life reduction factor approach is recommended and baselined with three validation points to account for uncertainties in possible growth accelerators and pressure monitoring provisions.    

To estimate recommended future ILI re-inspections (re-assessment) intervals, Monte Carlo simulations were completed using probabilistic crack growth analysis. In addition to applying growth analysis on reported ILI features, the study also accounted for the list of undetected ILI features by implementing a methodology developed in a Pipeline Research Council International (PRCI) project and which is presented in IPC2022-XXXX. Multiple scenarios have been studied covering the same abovementioned pressure cycling severity range and groups of pipeline grades. Like the PHT analysis, reduction of the projected MTTF to a conservative deterministic re-assessment timeframe is recommended through a life-reduction factor methodology which considers asset-specific uncertainties.

Refinement and validation testing of this re-assessment timing methodology is recommended for future work. 

Presenting Author: Lyndon Lamborn Enbridge LP

Presenting Author Biography: Employed at Enbridge since 2016, Lyndon is currently the Cracking Threat Subject Matter Expert and a Professional Engineer in the province of Alberta, Canada. Lyndon holds a bachelor of science degree in Mechanical Engineering and a Masters degree in Aeronautical Engineering and comes to the pipeline industry with 30 years of experience in the aerospace industry.