Page 130 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 511 07
(H- l)x30=pointscore(uptoarnaxirnumof 15points) detection. In-line inspection is also relative costly. Precleaning
of the pipeline, possible service interruptions, risks of unneces-
and where the minimum = 0 points. sary repairs, and possible blockages caused by the instrument
(2) Time since last test: Points = 10 - (“ears since test) are all possible additional costs to the operation. The ILI
(minimum = Opoints) process often involves trade-offs between more sensitive tools
(and the accompanying more expensive analyses) requiring
A test 4 years ago 6 pts fewer excavation verifications and less expensive tools that
A test 1 1 years ago 0 pts generate less accurate results and hence require more excava-
Add points from (1) and (2) above to obtain the total hydro- tion verifications. Because this technique discovers existing
static test score. In this schedule, maximum points are given to defects only. it is a lagging indicator of active failure mecha-
a test that occurred within the last year and that was conducted nisms. ILI must be performed at sufficient intervals to detect
to a pressure greater than 40% above the maximum operating serious defect formations before they become critical.
pressure. General types of anomalies that can be detected to varying
degrees by ILI include
Example 5.8: Scoring hydrostatic pressure tests Geometric anomalies (dents, wrinkles, out-of-round pipe)
Metal loss (gouging andgeneral, pitting, and channeling cor-
The evaluator is studying a natural gas line whose MAOP is rosion)
1000 psig. This section of line was hydrostatically tested 6 Laminations, cracks, or cracklike features.
years ago to a pressure of 1400 psig. Documentation on hand
indicates that the test was properly performed and analyzed. Some examples of available ILI devices are caliper tools,
Points are awarded as follows: magnetic flux leakage low- and high-resolution tools. ultra-
sonic wall thickness tools, ultrasonic crack detection tools, and
H= 1400/1000= 1.4
elastic wave crack detection tools. Each of these tools has spe-
(1) (1.4- 1) x 30 12 pts cific applications. Most tools can detect previous third-party
(2) 10-6years 4pts damage or impacts from other outside forces. Caliper tools are
Thus, used to locate pipe deformations such as dents or out-of-round
12+4 16pts areas. Magnetic flux leakage tools identify areas of metal loss
with the size of the detectable area dependent on the degree of
resolution of the tool. Ultrasonic wall thickness tools detect
In-line inspection general wall thinning and laminations. So-called “crack tools”
are specifically designed to detect cracks, especially those
The use of instrumented pigs to inspect a pipeline from the whose orientation is difficult to detect by other means.
inside is a rapidly maturing technology. In-line inspection. also Currently, no single tool is superior in detecting all types of
called smartpigging or intelligentpigging. refers to the use of anomalies and not all ILI technologies are available for smaller
an electronically instrumented device traveling inside the pipeline sizes.
pipeline that measures characteristics of the pipe wall. Any Depending on vendor specifications and ILI tool type, detec-
change in pipe wall can theoretically be detected. These devices tion thresholds can vary. The degree of resolution (the ability to
can also detect pipe wall cracks, laminations, and other mate- characterize an anomaly) also depends on anomaly size, shape,
rial defects. Coating defects may someday also be detected in and orientation in the pipe. The probability of detecting an
this fashion. The pipe conditions found that require further anomaly using ILI increases with increasing anomaly size.
evaluation are referred to as anonialies. Smaller anomalies as well as certain anomaly shapes and orien-
The industry began to use these tools in the 1980s, but ILI tations have lower detection thresholds than others.
presently benefits from advancements in electronics and com- The most common tools employ either an ultrasonic or a
puting technology that make it much more useful to the magnetic flux technology to perform the inspection. The ultra-
pipeline industry. State-of-the-art ILI has advanced to the point sonic devices use sound waves to continuously measure the
that many pipeline companies are basing extensive integrity wall thickness around the entire circumference of the pipe as
management programs around such inspection. A wealth of the pig travels down the line. The thickness measurement is
information is expected from such inspections when a high- obtained by measuring the difference in travel time between
quality, in-line device is used and supported by knowledgeable sound pulses reflected from the inner pipe wall and the outer
data analysis. It is widely believed that pipe anomalies that are pipe wall. A liquid couplant is often required to transmit the
of a size not detected through failure under a normal pressure ultrasonic waves from the transducer to the pipe wall.
test can be detected through ILI. The magnetic flux pig sets up a magnetic field in the pipe
While increasingly valuable, the technology is arguably wall and then measures this field. Changes in the pipe wall will
inexact, requiring experienced personnel to obtain most mean- change the magnetic field. This device emphasizes the detec-
ingful results. The ILI tools cannot accommodate all pipeline tion of anomalies rather than measurement of wall thickness,
system designs-there are currently restrictions on minimum although experienced personnel can closely estimate defect
pipe diameter, pipe shape, and radius of bends. All current ILI sizes and wall thicknesses.
tools have difficulties in detecting certain types of problems- In either case, all data are recorded. Both types of pigs are
sometimes a combination of tools is needed for full defect composed of several sections to accommodate the measuring