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API PUBL 1156 Document Information:
Title
Effects of Smooth and Rock Dents on Liquid Petroleum Pipelines (Phase II)
American Petroleum Institute
Publication Date:
Nov 1, 1997
Scope:
EXECUTIVE SUMMARY
This report represents an addendum to API Publication 1156 which has
the same title as this report, "Effects of Smooth and Rock Dents on
Liquid Petroleum Pipelines". It presents a description of work which
was done after the completion of APT Publication 1156. The aim of the
second phase of the work remained the same, namely to provide the
liquid pipeline industry with information on how to deal with smooth
and rock dents in pipelines. On the one hand if these dents pose a
threat to pipeline serviceability, they need to be found and removed or
repaired. On the other hand, to the extent that they do not pose a
threat to pipeline serviceability, the industry can more effectively
and justifiably spend maintenance dollars to address other
safety-related issues.
As was the case in the first phase, the work of the second phase
was largely experimental. Dents were created in actual line
pipe materials. The specimens were then subjected to various types
of service-simulating stresses. The results were assessed in terms
of the effects of dent parameters on the modes of pipe failure and
the stress levels or numbers of stress repetitions required to
produce failures. The findings of the experimental work were used as
the basis for a "field guide" to assist pipeline operators in
evaluating the severities of smooth and rock dents in pipelines. The
field guide is incorporated into this document as Appendix C.
As a separate issue, the effects of buckles in pipelines were
also addressed briefly by means of three pressure cycle experiments.
The objective of this work was to determine what degree of buckling
if any, can be tolerated without significantly affecting
pipeline integrity.
The essential findings of this work (based on both the Phase I
and Phase II experiments) are that
• Dents (single unconstrained smooth dents) created by an external
smooth object that was pressed into an unpressurized pipe rebounded due
to elastic spring back upon release of the load. A substantial amount
(up to two-thirds) of the initial indentation was recovered in this
process. As internal pressure was applied the unconstrained smooth
dents continued to reround and virtually disappeared as the pressure
level was taken high enough to significantly exceed the yield strength
of the pipe. Such dents had no deleterious effect on the burst pressure
of the pipe in a one-time pressurization-to-failure. Similar results
were achieved in prior investigations; the study described herein
merely reconfirmed what others had established.
• Single unconstrained smooth dents subjected to
simulated operational pressure cycles of a range one-half the
maximum operating pressure of most pipelines had fatigue lives greater
than the expected life of most pipelines. Stress concentrating
features within such dents such as girth welds, seam welds, and
manufacturing score marks tended to reduce the fatigue lives of the
affected dents, but the lives observed were still quite long, longer
than the expected cyclic life of many pipelines.
• The most important parameters controlling fatigue lives were the
radii of curvature at the location of the fatigue crack.
An enlarged-radius transverse curvature (i.e., a flattened area)
was found at all crack locations. The role of curvature in the
axial plane was not as clear cut, but relative flattening in
that direction appears to contribute to a shorter fatigue life.
The relatively flat area between two "overlapping" dents has been
the source of fatigue crack leaks on a few occasions in
operating pipelines.
• Constrained dents designed to simulate rock dents exhibited
a unique mode of failure (a transversely oriented fatigue
crack propagating from the ID surface to OD surface) in response to
large numbers of pressure cycles. The equivalent fatigue lives in
the experiments were much greater than what would be expected for
an unconstrained dent. No example of this mode of high cycle
fatigue failure in an actual pipeline has been brought to our
attention. However, it is entirely possible that rock dents in which
the local shear stress is nearly high enough to cause shear failure of
the wall thickness will be caused to fail with the application
of relatively few cycles of stress.
• Puncture tests in which objects of two different shapes
were pressed into a pressurized pipe produced failures in the form
of transversely oriented shear cracks, the mode of failure most
often associated with leaks at rock dents in actual pipelines.
Fully-constrained dents containing axially-oriented external notches to
simulate rock-contact damage or internal notches to simulate damage
from the interference with pig passage exhibited fatigue failures in
response to pressure cycles just like those of the constrained dents
with no notches. In other words the leaks that developed were at
transverse cracks that propagated from the ID surface to the OD
surface. The fatigue cracks did not initiate from the notches. In
contrast, when the constraint was partly or fully relaxed (i.e. the
dent was allowed to flex) fatigue cracks developed quickly at the
external notches. No internal notch was tested under these conditions.
• Pressure tests to failure of dents containing actual
and simulated external corrosion-caused metal loss showed that
corrosion in a shallow to moderate unconstrained dent has no more
deleterious effect than the same amount of corrosion in an undented
pipe.
• Some types of experiments conducted in this project were similar
to experiments conducted in a project on dents sponsored by the Office
of Pipeline Safety, U.S. Department of Transportation. It is
significant that similar results were obtained in both projects in the
cases where the experiments were of the same type.
The conclusions reached as a result of this work are as follows.
1. Because of the potential for rerounding, it is highly unlikely that
unconstrained dents with depths exceeding 5 percent of the pipe's
diameter will exist in areas of a pipeline which have been pressurized
to levels of 72 percent of SMYS or more. The results were obtained in
tests of pipes with diameter/thickness ratios of 68or more. It is
likely that pipe materials with lower diameter/thickness ratios would
behave differently exhibiting less rerounding.
2. Pipeline operators need not be concerned about truly
smooth, unconstrained dents. Concern arises, if and only if, the dent
will be subjected to aggressive service pressure cycles over a
long period of time, or if it also contains some type of stress
riser such as a score mark, a weld crown, a distinct crease or a crack.
3. Flattening of the pipe in conjunction with the area between
two smooth dents or two rock dents may facilitate the development of
a leak from fatigue crack growth within the useful life of the pipeline
that is subjected to significant pressure cycles.
4. It is prudent to repair dents (or replace the pipe) involving a seam
weld or a girth weld or score or trim mark if the depth of the dent
exceeds 2 percent of the pipe's diameter.
5. Rock dents where the rock remains in place are of concern only
to the extent that, if the load is great enough, the pipeline may
be punctured. The mode of failure of such a puncture will be a leak
nota rupture.
6. The results of this project suggest that aside from issues
of corrosion control and monitoring, rock dents will at worst
cause punctures and that such punctures will occur only if the rock
is sharp enough to produce a concentrated load that exceeds the
shear strength of the wall thickness. The results further show that
if deep dents exist, they are most likely constrained, and
therefore, are most likely rock dents. Most people assume that dents
found on the bottom half of the pipe are caused by rocks. In view of
these findings, it is prudent for pipeline operators to focus their
first responses to excavating dents that appear on the top or sides of
the pipe.
7. Leaving "rock" dents in place undisturbed would be preferable
to digging them up and removing the rocks, from a fatigue standpoint. It
must also be pointed out that leaving rocks in place
potentially creates increased risk of corrosion problems for the life
of the facility.
8. For pipelines buried in stable soil areas, buckles of 2 percent or
less of the pipe's diameter can be considered non-injurious.
It is important to note that before pipeline operators act on
these conclusions, they should consider the possibly that some of
them, particularly Number 4 and Number 8, may conflict with
restrictions in the ASME B31.4 and B31.8 Code and certain parts of the
U.S. Code of Federal Regulations, Title 49, Parts 192 and 195. To the
extent that field experience continues to bear out the correctness of
the data generated in this study and a similar study funded by the
U.S. Department of Transportation a rationale may become available
for changing the codes and regulations.
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