Determining the Yield Strength of In-Service Pipe Using Hardness Testing

Abstract

Enbridge Energy Partners (EEP) (“Enbridge”) purchased a Tank Storage Facility in Cushing, Oklahoma in 2004. After the acquisition, it was discovered that pipe records were incomplete and hydrotest documents were unavailable for some of the pipe segments in the tank facility. In order to comply with the Code of Federal Regulations (CFR) 49 Part 195, it was determined that piping at the Cushing Facility should be operating under “low stress” conditions. To determine if this condition could be met, the internal design pressure and yield strength (“YS”) were required for each pipe segment. Without pipe records, neither the internal design pressure, nor the YS was known. In 2007, a project was undertaken by Enbridge to inspect and test all pipe segments identified to have missing pipe records. The project’s objectives were: (1) To establish procedure and process for nondestructive evaluation of tensile properties of in-service pipe; (2) To collect pipe characteristic information (i.e. diameter, wall thickness); (3) To determine if the piping could be considered low stress piping; and (4) If the piping could not be considered low stress, to select a course of action from the following options: a. Lower the Maximum Operating Pressure (MOP), b. Hydrotest the piping, c. Remove/Abandon all unnecessary lines. Two existing reports justified the use of hardness as a means of determining the YS of in-situ piping. Based on these reports, Enbridge developed the following project scope: (1) For all piping with missing records: a. to collect hardness data of in-situ pipe using two portable hardness testers (any hardness measurements taken with these portable hardness testers are hereinafter referred to as “field hardness”), b. to measure wall thickness of in-situ pipe; (2) To collect field hardness, lab hardness, YS and tensile strength data of pipe coupons of various diameters in order to establish a field hardness to YS correlation; (3) To determine the YS of the in-situ pipe using field hardness measurements and the established field hardness to YS correlation determined above; (4) To determine if the pipe could be considered low stress at existing operating pressures; (5) For pipe segments not deemed low stress at existing operating pressures, to calculate an internal design pressure (Allowable Working Pressure or “AWP”) required to maintain low stress conditions. This paper will describe the method used to determine YS and AWP of in-service piping using field hardness and compare the results obtained using this method to the YS and AWP determined using CFR Title 49 Part 195.106.