Determination of the Failure Mode and the Rupture Pressure in a Mechanically Damaged Pipeline
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CASE HISTORY—PEER-REVIEWED
Determination of the Failure Mode and the Rupture Pressure in a Mechanically Damaged Pipeline B. A. James Æ R. A. Sire Æ R. D. Caligiuri
Submitted: 18 April 2008 / Published online: 29 May 2008 Ó ASM International 2008
Abstract The following case study outlines metallurgical, finite element, and fracture mechanics analyses conducted to determine the cause of a pipeline rupture that resulted from prior external damage. Metallurgical and fractographic analyses indicated that the rupture initiated from a shear crack caused by mechanical damage, and that the fracture mode was overload; no indication of progressive growth was observed. Stress and fracture mechanics analysis indicated that the pipe ruptured at a pressure below the maximum allowable operating pressure. Keywords Failure analysis Fracture mechanics Fractography Pipe burst
Background The subject pipe was manufactured and installed in the 1960s. It was made from American Petroleum Institute (API) Standard 5LX Grade X52 steel, had a 16-inch diameter, a 0.312-inch nominal wall thickness, and was electric resistance seam-welded. Excavation for another pipeline project was known to have occurred some thirty years after installation in the vicinity of the rupture. The subject pipe ruptured several years after the excavation.
dents on the upper outside surface of the pipe at and near the rupture site. The size, shape, and spacing of the gouges were consistent with the metal teeth typically found on excavating equipment. The rupture occurred along one of the longitudinally oriented gouges, located at roughly the 11:00-o’clock position on the circumference when looking downstream. The ERW seam was identified at the 8:00-o’clock position in the ruptured section, well away from the rupture location. Figure 1 shows a photographic montage of the ruptured pipe section. Gouges were characterized from the inside of the pipe using laser profilometry. An image of some of the gouges developed from the profilometry data is shown in Fig. 2. The laser profilometry data was used in the fracture mechanics analysis, and is discussed later. The fracture surface exhibited a region of flat fracture near the center of the fish-mouthed rupture area shown in Fig. 3. The flat fracture area extended approximately 3.5 inches along the length of the pipe. Ductile tearing was observed on either side of the flat area, evidenced by 45° fracture surfaces (‘‘shear lips’’) and gross plastic deformation. These fracture surface features helped identify the ‘‘flat’’ region as the fracture origin.
Fractography Visual Examination Visual inspection clearly indicated that the pipe had been severely damaged, as evidenced by numerous gouges and B. A. James (&) R. A. Sire R. D. Caligiuri Exponent Failure Analysis Associates, Menlo Park, CA, USA e-mail: [email protected]
Fractographic analysis of the subject pipe fracture surfaces was conducted to further confirm the origin location and help determine the failure mode. Portions of this analysis were necessarily dest
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