Criteria for Reinspection Intervals for Low-stress
Criteria for Reinspection Intervals for Low-stress Steel Pipelines by
This document presents criteria that could be used to establish appropriate reinspection intervals for “low-stress” natural gas pipelines comprised of steel pipe. Low-stress pipelines for this purpose are defined as those with maximum operating stress levels of 30 percent or less of the specified minimum yield strength (SMYS) of the pipe. To put these pipelines in perspective, it is noted that the vast majority of cross-country natural gas transmission pipelines typically are operated at stress levels ranging from 50 percent of SMYS to 72 percent of SMYS.
Low-stress pipelines typically consist of short segments (50 miles or less in length) that serve as connectors between the major cross-country transmission pipelines and local distribution piping networks. The low-stress systems are inherently more difficult to test or inspect than the long-distance transmission pipelines for a variety of reasons. For example, their configurations and operating conditions may not be readily conducive to the in- line tools typically used to inspect long distance, high-stress transmission pipelines. Further, they typically consist of single- line systems that cannot be easily taken out of service to be hydrostatically tested. These difficulties, coupled with the fact that their lower operating stress levels present less risk in many cases than long distance, high-stress pipelines provide a rationale for longer reinspection intervals than those that are required for the long-distance, high-stress systems. The bases for appropriate reinspection intervals are provided herein and specific intervals are suggested.