PRCI NG - 18 Report 162

L51518e

Battelle Memorial Institute

Need: Damage that occurs in post-hydrotest operations, such as third party contractors hitting the pipeline with mechanical equipment at a later date, clearly cannot be removed by the hydrotest and may cause failure at the time the damage occurs or at some later time depending on the defect severity and the service conditions. Damage induced while a line is in service, such as during pipeline repair or from miscellaneous nearby construction, presents the most common type of service defect and potentially the most hazardous. Therefore, it is desirable to determine the severity of such defects in order to assess the serviceability of the damaged area (and thus avoid costly downtime). Understanding the basic mechanisms of mechanical damage failure also will assist in specifying fracture toughness requirements for future line pipe steels.

Result: The specific objective of this research is to develop a means of assessing the severity of mechanical damage defects and the effect of fracture toughness in resisting the failure of these defect types. Prior research studies provide in-depth consideration of the separate effects of plain dents and simulated gouges (sharp machine surface flaws with negligible indentation) on gas line pipe integrity. The failure characteristics of such defects are well understood. Recent studies on mechanical damage have concentrated on the complex behavior of a localized gouge within a dent, the most typical form of damage that occurs in the industry. However, these experimental studies were hampered in that the defect combinations had to be introduced into the pipe test section at ambient pressure in order to produce adequate repeatability. The past research has been included for completeness. The most recent research differs from past work in that the effects of dynamically produced dent-and-gouge defects in line pipe under pressure are studied. This provides a more realistic representation of the type of outside force damage that occurs in service. Nearly all research on this subject has been conducted at two laboratories, at the Battelle Columbus Division and at British Gas Corporation research facilities at Newcastle-upon-Tyne, Great Britain. British Gas conducted most of their research using rings cut from pipe, damaged, and then tested on a ring yield-test-machine, using the same type of ring tester used by pipe mills to determine pipe yield strength. British Gas also conducted tests on pipe that was damaged while not pressurized and later on pressurized and damaged pipe. Battelle's efforts involved pipe testing both on pipes that were damaged and then pressured to failure and on pipes damaged while pressurized. It is believed that gouge length is a strong influencing parameter, and it is not possible to use gouge length as a variable in the ring test method.

Benefit: Benefits of this experimentation suggest the following: if mechanical damage defects are found that are gouges within an indented area, pipeline pressure should be lowered to 2/3 of pressure level in the pipeline at the time the defect was discovered before conducting a detailed inspection. The gouge should be examined for cracking by grinding smooth a small area of the gouge making it suitable for crack inspection. If a crack is found, a small circumferentially oriented grind should be made until the bottom of the crack is found. This crack depth should be added to the gouge depth to arrive at "d" in the analysis. If the defect has a failure pressure above 1.25 times the operating pressure according to the criteria herein, it is probably safe to operate for a short time period at the operating pressure, but it is recommended that it be replaced or repaired. An encircling sleeved and/or epoxy resin wraps constitute the most appropriate repair, with the spaces between the dent and sleeve filled with polyester or epoxy molding compound.

Product Details

Published:

08/01/1986

Number of Pages:

58

File Size:

1 file , 990 KB