ExxonMobil SENT test method and application to Strain-Based Design

Abstract

Strain-Based Design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Large ground movements may impose significant longitudinal strains to the pipelines. One of the key elements of the SBD approach is a Tensile Strain Capacity (TSC) prediction model for pipelines, which is usually provided in the form of an equation based on Finite Element Analysis (FEA). ExxonMobil's TSC prediction model has been validated against more than 100 full-scale tests. A key input to the prediction model is the tearing resistance of pipe and weld materials in terms of a Crack Tip Opening Displacement Resistance (CTOD-R) curve. Therefore, a test methodology is needed to appropriately characterize the ductile tearing resistance of pipes and girth welds. To this purpose, in recent years, ExxonMobil has developed a Single Edge Notched Tensile (SENT) test method, which has been shown to provide a level of crack tip constraint close to what is observed for defects in pipeline girth welds. In addition to providing input to TSC prediction, SENT testing is an integral part of Strain-Based Engineering Critical Assessment (SBECA) for pipelines.This paper describes the details of the ExxonMobil SENT test procedure and its application to carry out TSC predictions for SBD. Particularly, the paper covers the following aspects: (1) test procedure including analysis and interpretation of results; (2) validation of selected specimen geometry through comparison with data from pipe Full-Scale Tests (FST); (3) use of the CTOD-R curve as input to the ExxonMobil TSC prediction model; (4) finally, recent challenges related to crack path deviation are discussed with potential ways to address this issue.