DE FACTO HYDROSTATIC TEST PRESSURES - A STUDY IN DOUBLE STROKING

摘要

The pipeline industry has traditionally utilized the double stroke method to limit pipe yielding during pressure testing. Yet few case studies are available which indicate the actual state of stress in a pipeline segment where the pressure -volume plot indicates that the double stroke point has been reached. Examination of a typical stress-strain curve of pipeline steel indicates that the double stroke point typically occurs midway between the elastic limit and the defined yield strength and that the elastic limit can occur at 80% of the defined yield stress. The purpose of this paper is to develop a model to better understand the actual stresses a pipeline experiences during a hydrostatic test. For analysis, the double stroke point is used as the test case, which results in insights about the actual stresses seen on the pipeline and an understanding about what double stroking actually means. The double stroke point was chosen for analysis since it is commonly used as a ceiling on hydrostatic test pressure and, although literature provides clarity on its definition, few experiments and tests have been performed to gain a practical understanding of its affect on a pipeline. Analysis was done on a real world example and the results were compared to a Monte Carlo simulation to model the sensitivity of the various parameters. The results of the real world test case of known pipe attributes indicated that the strains in all pipe joints would stay below 0.5% at the double stroke point. When a Monte Carlo simulation was applied to the pipe variables of the same test section, 0.5% strain was exceeded on several outlying joints, but in those cases the hoop stress utilizing Barlow's equation was in-excess of 125% SMYS. The analysis in this paper provides a tool for predicting the range of actual stresses the pipeline is experiencing when hydrostatically tested, and in doing so, also provides a firmer grasp on what double stroking actually means for a pipeline.