110
Improving Global Quality of Life
Through Optimum Use and Innovation of Welding and Joining Technologies
technology based on the existing pipeline materials, welding technology and standards on transmission of
hydrocarbons.
9.4.4
Testing of pipes
Although a very small number of pipeline failures is caused each year by stress-corrosion cracking (SCC), it
remains a concern to pipeline operators and regulators. The primary methods for managing the problem
include periodic hydrostatic testing, in-line inspection (ILI), or SCC direct assessment (DA). The choice of
methods depends on many factors unique to each pipeline. No single method is appropriate for every case.
Since all three methods are very costly, especially compared to the relatively low risk of failures, it would be
of considerable benefit to the industry if more-cost-effective ways to manage SCC were available.
Hydrostatic Testing:
Hydrostatic testing is the most common method for locating SCC. It is well accepted by
the industry and by regulatory agencies, and it is certain to find every crack that is larger than a critical size,
which depends upon the test pressure. There are a number of limitations to hydrostatic testing, however. It
requires that flow of the product through the pipeline be interrupted, which may not be practical if the line
is not looped. In areas with large elevation differences, it might not be possible to achieve sufficient pressure
at the high points without excessive segmentation of the line. Furthermore, water might not be available,
or, if it is, the cost of disposing of it, due to environmental concerns, might be prohibitive. In situations such
as those, ILI or SCC DA may be preferable.
In-Line Inspection:
ILI has certain advantages over hydrostatic testing in that it can find cracks that are
smaller than those that would fail at the hydrostatic-test pressure, thus potentially providing greater margins
of safety, and, in many cases, it can be conducted in the fluid that the pipeline is meant to carry, thus not
requiring interruption of service. Ultrasonic techniques have been most successful at locating and sizing
stress-corrosion cracks in liquid pipelines where the liquid petroleum product serves as the couplant for
energy flow between the transducers and the pipe. Achieving satisfactory coupling in gas pipelines has been
a serious problem. A recently developed technology involving electromagnetic acoustic transducers shows
promise for overcoming this problem, but it has not yet been validated through field trials. Another serious
limitation of current ultrasonic ILI tools is their inability to find cracks in dents. This is because the geometry
of the dent throws off the angles of transmitted and reflected waves.
SCC Direct Assessment:
SCC DA is an important method of integrity assurance, especially for portions of the
pipeline where it is impractical or even impossible to do ILI or hydrostatic testing. The SCC DA process was
formalised with the publication of an NACE Recommended Practice. Essentially, SCC DA involves making
intelligent choices about where to excavate the pipeline and then directly examining the pipe to determine
the severity of cracking as a basis for deciding what actions should be taken and how soon. The foundation
to SCC DA is proper site selection, which remains a major challenge.
Internal SCC in Ethanol Pipelines:
With the increasing use of ethanol as an additive to gasoline, consideration
is being given to transport of denatured ethanol by pipeline. Currently, denatured fuel grade ethanol is
transported primarily by railroad tanker cars and tanker trucks. The discovery of SCC in user terminals,
storage tanks, and loading/unloading racks in contact with denatured fuel-grade ethanol has raised concern
about potential effects in pipelines. Research is needed to develop a better understanding of the causes of
SCC in ethanol and, based upon that understanding, to suggest and evaluate possible ways to prevent it.
Crack Arrest:
When a crack in a welded structure propagates under the action of a load, it can extend in
principally two different manners, stably by ductile tearing, or unstably by either brittle fracture or disruptive
ductile fracture. Crack growing stably will arrest unless the stress intensity applied to the crack tip continues
to increase at a greater rate than the crack resistance curve. Analysis and prediction of an arrest event of a
dynamically running unstable crack, however, is a major challenge for higher grade pipeline steels (X 80 and
beyond) and welds.
