PRCI Report 194

L51665e

Battelle Memorial Institute

Need: Historically safe operation of thousands of miles of gas- transmission pipeline underscores the merits of hydrotesting. The hydrotest is applied to remove defects, which might exist in the line and could be critical in subsequent service. The hydrotest involves a proof pressurization that is designed to remove defects that could otherwise become critical in service by failing these defects during the time the line is held at the over-pressure. Water is used as the pressurization media to avoid the potential for running fracture, which is possible in cases where defects become critical during the test when using more compressible media (e.g., gases). Hydrotesting is not the only means to ensure defects that could become critical in subsequent service are identified and removed from a line. For example, inline inspection (ILI) procedures are available that permit the identification of specific types of defects, such as corrosion. But because not all lines can be inspected with ILI tools and the need to find types of defects that are not currently detected by inline procedures, hydrotesting remains the method of choice in the USA for natural gas transmission lines.

Benefit: The objective of this study was to determine the growth of various flaw sizes as a result of having been subjected to combinations of test pressures and hold times and to assess the subsequent serviceability of pipelines containing those flaws in gas-transmission service. The purpose of the flaw-growth calculations is to define hydrotest procedures for gas-transmission pipelines that remove flaws that could become critical in service while minimizing the growth of any remaining flaws. This report is the fourth in a series of topical reports dealing with quasi-static ductile flaw growth in pipelines, such as can occur in a hydrotest. Prior reports presented relevant material properties, the theoretical analysis, and the model development and validation. This report presents the results and analysis of an extensive parametric study of ductile flaw growth as a function of maximum test pressure, hold time, flaw geometry, and strength and toughness of the pipe steel representative of submerged-arc welded pipe in grades from X52 through X70 with a yield-to-ultimate ratio less than 0.90.

Result: Flaws of near-critical size can be removed by hydrotesting a pipeline without adversely advancing the flaw population or damaging the line. The results show that a specific sequence of pressures and hold times can be identified that enhance the ability of a hydrotest to fulfill its purpose. This study has shown this sequence involves a peak pressure and hold to advance the near-critical defects to a through-wall state followed by a hold at a reduced pressure to locate these now leaking defects.

Product Details

Published:

07/31/1992

Number of Pages:

57

File Size:

1 file , 920 KB