InSitu Analysis of Pipeline Metallurgy - dnV

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pipe related to the pipeline segment of interest is not always available. Further, while a single tappedcoupon or pipe cylinder may not be representative of the range of metallurgical characteristics orproperties in a long pipeline segment, obtaining multiple pipe samples for destructive testing can beimpractical. Fortunately, visual examination and interpretation of specific surface featuressupplemented by nondestructive metallurgical analyses can provide a wealth of information tosupplement and/or validate pipeline records.THE VALUE OF VISUAL EXAMINATIONTo a trained inspector, visible features on a pipe surface can reveal details about manufacturingmethods, installation and inspection - details easily overlooked if the focus is merely on characterizingin-service degradation. For example, fillet welded plugs or small patches at the 12:00 position within afew inches of girth welds made in the late 1940s through early 1950s typically indicate that aradiographic isotope was lowered into the pipe to enable single wall exposure radiographic inspectionof the girth weld. 1 The plug or patch was used to seal hole after the inspection was complete.Inspection by radiography has been identified as the leading indicator of girth weld quality.Flush rectangular patches welded across girth welds are indicative of mobile tensile testing units thatwere used on the right of way to spot check girth weld quality before reliable nondestructive testing(NDT) methods were developed. If the test result failed the acceptance criteria the weld would havebeen removed and replaced with a new weld. “Good” welds were patched. While the patch indicatesthat the weld met the specifications for mechanical properties the practice of inserting the welded patchto replace the test specimen was abandoned after it was discovered that the pipelines often leaked atthe patch. 2As another example, specific types of surface textures (“spellerizing”) are uniquely associated with lapseam pipe. The surface pattern was embossed onto the pipe surface by the patterns engraved on therollers that gripped the pipe during processing. The patterns typically occur in two bands with the lapseam located in one of the two bands. Furthermore, since the specific pattern of the spellerizing wasunique to each lap seam pipe manufacturer characterizing the spellerizing pattern can help confirmrecords indicating the pipe manufacturer. 3Surface features are also useful in differentiating furnace butt weld seams from ERW seams. The abilityto differentiate these two types of seams from each other is important mainly because of the lowerseam efficiency factor assigned to butt welded seams by United States federal pipeline safetyregulations (see for example, Reference 4). In addition, butt weld seams are less likely than earlyvintage ERW seams to have very high hardness microstructures in the seam heat affected zone. Thehigh hardness microstructures increase susceptibility to sulfide stress cracking and to brittle fractureinitiation.A few characteristics enable butt seams to be differentiated from ERW seams. First, butt weld seamswere and are only manufactured in pipe no larger than 4.5 inch outside diameter (OD). Second, duringmanufacturing the ERW seam is characterized by flash being expelled to the inside diameter(ID) andoutside diameter of the pipe. It is subsequently machined off nearly flush with the pipe surface. Incomparison, furnace butt welding, or the more modern variant of continuous butt welding, often leavesa perceptible groove along the OD where the seam fusion line is located. Third, the butt weld processoften leaves a characteristic band of scratch marks alongside the seam (Figure 1).Other important surface features are even more subtle than the features on butt weld seam pipe. From1940 to 1951 manufacturers of API Specification 5L pipe were required to stamp the pipe with a markidentifying the manufacturer within 305 mm (12 in.) of the end of the pipe, After the twelfth edition ofAPI 5L painted stencil marks were an acceptable alternative to stamped marks. The stamp marks areshallow and each character is normally only about 6-10 mm (1/4-3/8 in.) across (Figure 2). However,
careful removal of coating and close visual examination of the area near girth welds will sometimesreveal the stamp marks.Girth weld joint designs and weld metal appearance are not frequently characterized during directexamination but they can be significant from the standpoint of assessing likely resistance to large axialstrains such as those associated with ground deformation. Reference 5 describes the evolution ofseveral different types of girth weld joint designs. For example, fillet welded bell+spigot joint designswere largely abandoned in the early 1930s in part because of the relatively low axial strain capacity ofthe fillet welds. In comparison, the bell-bell-chill ring (BBCR) joint design that followed was found tohave much better strain capacity, even when workmanship was imperfect.Oxyacetylene weld deposits are notorious for having workmanship flaws in the weld root and oftenhave a uniquely high and wide cap on the OD surface. While the dimensions of the weld cap areseldom measured in the course of routine bellhole inspection, it has been demonstrated that very widecaps (width at least five time the wall thickness) having a height greater than 50% of the wall thicknessreduce the applied stress intensity factor by a minimum of 25%. Smaller caps have smaller influencewith the effect being related to cap height to width ratio and dimensions relative to the pipe wallthickness. As a result, the strain capacity of the welds can be somewhat larger than what is expectedbased on conventional fracture mechanics evaluations that disregard the cap effects. 6 Therefore,recording the dimensions of weld caps can support fitness for service assessments of welds.One typically overlooked feature of early girth welds is the consistency of the weld metal ripple patternthat indicates the direction of weld progression. Early pipeline construction techniques included thepractice of making several girth welds above ground while pipe was rolled underneath the welding arcor torch. Segments of welded pipe were then placed into the ditch and welded together in the morechallenging fixed position. Sometimes different welding processes were used for the two different typesof welds. The rolled position and fixed position welds result in different weld ripple patterns that areeasily distinguishable from each other. In more modern pipe similar weld to weld variations inworkmanship and properties exist when pipe segments are made using pipe that is “double jointed”,typically using submerged arc welding, at the mill prior to being delivered to the construction site. As aresult of the two different welding positions and/or use of different welding processes for different girthwelds the long pipeline segments can have two (or more) distinctly different populations of girth welds.Each weld population can have different workmanship quality and mechanical properties that occurrepeated sequences. Identifying the presence of multiple populations of girth welds can influenceintegrity assessment sampling plans and assessment methodologies.Sometimes visual examination must be supplemented with NDT to differentiate among seam types.For example, single side submerged arc welded (SSAW) and double sided submerged arc welded(DSAW or SAW-L) seams look the same from the OD surface. Only the ID appearance is different.Normally, the differences are apparent in radiographic inspection.In another example, two separate types of seams are commonly mischaracterized as a product of anearly submerged arc welding process when in fact they are both unique types of seams manufacturedfrom about 1928 to 1932 by A.O. Smith Corporation. (1) The external appearance of both types ofseams is the same as a result of both types using a relatively broad, flat cap pass of metal deposited byshielded metal arc welding (Figure 3). In both types of seams weld metal solidification cracks are oftenfound in the cap pass. Until 1930 the seam consisted only of multiple passes of weld metal depositedby shielded metal arc welding process. Incomplete penetration and lack of fusion in the root region iscommonly observed. By 1930 the root portion of the seam was first formed by the flash weld processand the cap pass was used only for additional reinforcement. As a result, the OD of the pipe isindistinguishable from the earlier vintage pipe while the ID looks like a conventional flash welded seam.1 Trade name.
Page 1: In-situ Analyses to Characterize th
Page 5 and 6: from a pipe having a nominal origin
Page 7 and 8: In-situ MetallographyIn-situ metall
Page 9 and 10: ScratchesFigure 3:Typical appearanc
Page 11 and 12: Figure 10: Magnified view of a norm
Page 13 and 14: 11. B.L. Bramfitt, “Structure/Pro

InSitu Analysis of Pipeline Metallurgy - dnV

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