Page 391 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

366 Pipe Strength Determination
In all pipe materials, special allowances must be made for Areas where the calculated remaining strength of the pipe
‘‘stress risers.” Notches, cracks, or any abrupt changes in wall results in a safe operating pressure that is less than the cur-
thickness or shape can amplify the stress level in the pipe rent established MOP at the location
wall. See further discussions about fracture toughness in Areas of general corrosion with a predicted metal loss of
Chapter 5. >50% of nominal wall
Areas where the predicted metal loss of >50% of nominal
wall at crossings of another pipeline exists
Anomalies and defects Weld anomalies with a predicted metal loss >50% of nomi-
nal wall
A defect in a pipe wall is considered to be any undesirable pipe Potential crack indications that when excavated are deter-
anomaly, such as a crack, gouge, dent, or metal loss, that mined to be cracks
reduces pipe strength or could later lead to a leak or spill. Note Corrosion of or along seam welds
that not all anomalies are defects. Some dents, gouges, metal Gouges or grooves greater than 12.5% ofnominal wall.
loss, and even cracks will not affect the service life of a
pipeline. Possible defects include seam weaknesses associated Additional anomalies that might warrant attention in the risk
with low-frequency ERW and electric flash welded pipe, dents evaluation include
or gouges from past excavation damage or other external
forces, external corrosion wall losses, internal corrosion wall 0 Any area where the data reflect a change since the prior
losses, laminations, pipe body cracks, and circumferential weld assessment
defects and hard spots. Any area where the data indicate mechanical damage that is
Damages can be detected by visual inspection or through located on the top half of the pipe
integrity verification techniques. Until an evaluation has shown Any area where the data indicate anomalies that are abrupt in
that an indication detected on a pipe wall is potentially serious, nature
it is normally called an anomaly. It is only a defect if it reduces Any area where the data indicate anomalies that are longitu-
pipe strength significantly-impairing its ability to be used as dinal in orientation
intended. Any area where the data indicate anomalies that are occur-
Many anomalies will be of a size that do not require repair ring over a large area
because they have not reduced the pipe strength from Any area with anomalies located in or near casings, cross-
required levels. However, a risk assessment that examines ings ofanother pipeline, and areas with suspect cathodic pro-
available pipe strength should probably treat anomalies as tection.
evidence of reduced strength and possible active failure
mechanisms. There are several industry-accepted methods for determin-
A complete assessment of remaining pipe strength in con- ing corrosion-flaw severity and for evaluating the remaining
sideration of an anomaly requires accurate characterization of strength in corroded pipe. ASME B31G, ASME B31G
the anomaly-its dimensions and shape. In the absence of Modified, and RSTRENG are examples of available method-
detailed remaining strength calculations, the evaluator can ologies. Several proprietary calculation methodologies are also
reduce pipe strength by a percentage based on the severity of used by pipeline companies. These calculation routines require
the anomaly. measurements of the depth, geometry, and configuration of
Higher priority anomalies-those with a very high chance of corroded areas. Depending on the depths and proximity to one
being defects-include: another, some areas will have sufficient remaining strength
despite the corrosion damage. The calculation determines
Areas of metal loss greater than 80% of nominal wall regard- whether the area must be repaired.
less of dimensions For cracklike defects, fracture mechanics and estimates of
Areas where predicted burst pressure is less than the maxi- stress cycles (frequency and magnitude) are required to deter-
mum operating pressure at the location of the anomaly mine this. For metal loss from corrosion, the failure size for
Any dent on the top of the pipeline (above the 4 and 8 o’clock purposes of probability calculations can be determined by two
positions) with or without any indicated metal loss. criteria: (1) the depth of the anomaly and (2) a calculated
remaining pressure-containing capacity of the defect configu-
Important anomalies-probable defects-might include ration. Two criteria are advisable since the accepted calcula-
tions for remaining strength are not considered as reliable when
Any dents with metal loss or dents that affect pipe curvature anomaly depths exceed 80% of the wall thickness. Likewise,
at a girth or seam weld depth alone is not a good indicator of failure potential because
Any dents with reported depths greater than 6% of the pipe stress level and defect configuration are also important vari-
diameter ables [86].