A case of continual product refinement stimulated by product failures was described. Brittle fracture of gas transmission line pipe steels occurred demonstrating a poor combination of materials, environment, manufacturing and installation problems, and loads. Initial efforts were concentrated towards decreasing the Charpy ductile-to-brittle transition temperature to avoid brittle fracture. It was subsequently revealed that the absorbed energy on the upper shelf of the Charpy energy-temperature curve was critical for arresting a moving crack. Both fracture initiation and fracture propagation were needed be controlled. It was concluded that improved steel processing procedures, chiefly hot-working temperature and deformation control, were also required to optimize microstructure and properties.

Schedule 80 low-carbon steel pipes used to transfer kraft liquor in a Kamyr continuous pulp digester failed within 18 months after installation. Visual and metallographic examinations established that the cracking initiated on the internal surfaces of the equalizer pipes in the welds and heat-affected zones (HAZs). Fracture/crack morphology was brittle and primarily intergranular and deposits at crack tips were primarily iron oxides with significant amounts of sodium compounds. On these bases, the cracking was characterized as intergranular stress-corrosion cracking (IGSCC). Corrosion-related deterioration was not found, indicating that the material was generally suitable for the intended service. High residual tensile stresses in the welds and HAZS, resulting from field welding under highly constrained conditions using inadequate weld procedures, were the most probable cause of the failures. Minimizing residual stresses through use of welding procedures that include appropriate preweld and interpass temperatures and postweld stress relief heat treatment at 650 deg C (1200 deg F) was recommended to prevent further failures.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

During the construction of a large-diam pipeline, several girth welds had to be cut out as a result of radiographic interpretation. The pipeline was constructed of 910 mm (36 in.) diam x 13 mm (0.5 in.) wall thickness grade X448 (x65) line pipe. The girth welds were fabricated using standard vertical down stove pipe-welding procedures with E7010 cellulosic electrodes. The crack started partially as a result of incomplete fusion on the pipe side wall, which in turn was a result of misalignment of the two pipes. The crack was typical of hydrogen cracking. Girth welds can be made using cellulosic electrodes. For high-risk girth welds, an increase in preheat and/or a reduction in the local stress by controlling lift height or depositing the hot pass locally before lifting may be required.

A section of pipe in a hydrocarbon pipeline was found to be leaking. The pipeline was installed several decades earlier and was protected by an external coating of extruded polyethylene and a cathodic protection system. The failed pipe section was made from API 5L X46 line pipe steel, approximately 22 cm (8.7 in.) OD x 0.5 cm (0.2 in.) wall thickness, which was electric resistance welded along the longitudinal seam. The pressure at the time and location of the failure was 2760 kPa, which corresponds to 20% of the specified minimum yield strength. The cause of failure (based on visual inspection, magnetic particle inspection, stereoscopic analysis, scanning electron microscopy, tensile and hardness testing, and chemical analysis) was attributed to damage resulting from a lightning strike.

A 75 cm OD x 33 mm thick pipe in a horizontal section of a hot steam reheat line ruptured after 15 years in service. The failed section was manufactured from rolled plate of material specification SA387, grade C. The longitudinal seam weld was a double butt-weld that was V-welded from both sides and failure was found to propagate along the longitudinal seam and its HAZ. The fracture surface near the inner wall of the pipe was found to have a bluish gray appearance, while the fracture surface near the outer wall was rust colored (oxides). The transverse-to-the-weld specimen from the longitudinal seam weld was revealed to have lower elongation and a shear type failure rather than the cup-cone failures. It was concluded that the welded longitudinal seam exhibited embrittlement. A low-ductility intergranular fracture that progressed through the weld metal was revealed by scanning electron microscopy. The cracks were revealed to be in existence for some time before the final failure which was indicated by the extent and amount of corrosion products. It was concluded that low ductility was responsible for the original initiation of cracks in the pipe.

A newly installed pipeline leaked during cleaning prior to hydrotest at a pressure of approximately 400 psig. The intended hydrotest pressure was 750 psig. The pipeline was constructed from spiral-welded API 5L-X65 HSLA steel and was intended for seawater injection. Analysis included nondestructive testing, metallography, and scanning electron microscopy. Based on the results, the failure was attributed to transit fatigue, caused during highway transportation. Cracks along the toes of the weld from both the outside and inside surfaces, the transgranular nature of cracking, and the presence of fatigue striations all supported transit fatigue as the damage mechanism.

This article describes the failure characteristics of high-pressure long-distance pipelines. It discusses the causes of pipeline failures and the procedures used to investigate them. The use of fracture mechanics in failure investigations and in developing remedial measures is also reviewed.

This article discusses the failure analysis of several steel transmission pipeline failures, describes the causes and characteristics of specific pipeline failure modes, and introduces pipeline failure prevention and integrity management practices and methodologies. In addition, it covers the use of transmission pipeline in North America, discusses the procedures in pipeline failure analysis investigation, and provides a brief background on the most commonly observed pipeline flaws and degradation mechanisms. A case study related to hydrogen cracking and a hard spot is also presented.

A flanged 100 mm (4 in.) diam low-carbon steel spool piece lined with Teflon was removed from a sulfuric acid denitrification system after cracks were observed in the painted coating. Visual and microstructural examination along with SEM fractography revealed scaled iron oxides on all opened crack surfaces. The surfaces had a faceted morphology, indicating intergranular fracture. Cracks originated at the interface between the tube and the Teflon liner Corrosion products were found caked into the intergranular region between the liner and the spool. The portion of the liner that had been exposed to the process stream was discolored. Failure of the spool was attributed to stress-corrosion cracking promoted by the presence of nitrates. Nitric acid contaminant in the sulfuric acid stream had diffused through the liner and accumulated in the annular space. Use of a liner that is more impermeable to the diffusion of ionic species was recommended.