Page 126 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Riskvariables and scoring 5/103
sures. A 100-psi pressure cycle will have a potentially greater Example 5.6: Scoring fatigue potential
effect on a system rated for 150 psi MOP than on one rated for
1500 psi. Most research points to the requirement oflarge num- The evaluator has identified two types of cyclic loadings in a
bers of cycles at all but the highest stress levels, before serious specific pipeline section: (1) a pressure cycle of about 200 psig
fatigue damage occurs. caused by the start of a compressor about twice a week and (2)
In many pipeline instances, the cycles will be due to changes vehicle traffic causing a 5-psi external stress at a frequency of
in internal pressure. Pumps, compressors, control valves, and about 100 vehicles per day. The section is approximately 4
pigging operations are possible causes of internal pressure years old and has an MOP of 1000 psig. The traffic loadings
cycles. The following example schedule is therefore based on and the compressor cycles have both been occurring since the
internal pressures as percentages of MOP. If another type of line was installed.
loading is more severe, a similar schedule can be developed. For the first case, the evaluator enters the table at (2
Stresses caused by vehicle traffic over a buried pipeline would startdweek x 52 weekdyear x 4 years) = 416 cycles across the
be an example of a cyclic loading that may be more severe than horizontal axis, and (200 psig/lOOO psig) = 20% of MAOP on
the internal pressure cycles. the vertical axis. This combination yields a point score of about
This is admittedly an oversimplification of this complex 13 points.
issue. Fatigue depends on many variables as noted previously. For the second case, the lifetime cycles are equal to (100
At certain stress levels, even the frequency of cycles-how fast vehicles/day x 365 daysiyear x 4 years) = 146,000. The magni-
they are occurring-is found to affect the failure point. For tude is equal to (5 psig/lOOO psig) = 5%. Using these two
purposes of this assessment, however, the fatigue failure risk values, the schedule assigns a point score of 7 points.
is being reduced to the two variables of stress magnitude The worst case, 7 points, is assigned to the section.
and number. The following schedule is offered as a pos-
sible simple way to evaluate fatigue’s contribution to the risk Cracking: a deeper look
picture.
One cycle is defined as going from the starting pressure to a All materials have flaws and defects, if only at the microscopic
peak pressure and back down to the starting pressure. The cycle level. Given enough stress, any crack will enlarge, growing in
is measured as a percentage of MOP. depth and width. Crack growth is not predictable under real-
In this example of assessing fatigue potential, the evaluator world conditions. It may occur gradually or literally at the
uses the scoring protocol illustrated in Table 5.2 to analyze speed of sound through the material. (See also discussions on
various combinations of pressure magnitudes and cycles. The possible failure hole sizes in Chapters 7 and 14.)
point value is obtained by finding the worst case combination As contributors to fatigue failures, several common crack-
of pressures and cycles. This worst case is the situation with enhancing mechanisms have been identified. Hydrogen-
the lowest point value. Note the “equivalents” in this table; induced cracking (HIC), stress corrosion cracking (SCC), and
9000 cycles at 90% of MOP is thought to be the equivalent of sulfide stress corrosion cracking (SSCC) are recognized flaw-
9 million cycles at 5% of MOP; 5000 cycles at 50% MOP is creating or flaw-propagating phenomena (see Chapter 4). The
equal to 50,000 cycles at 10% of MOP, etc. In moving around susceptibility of a material to these mechanisms depends on
in this table, the upper right corner is the condition with the several variables. The material composition is one of the more
greatest risk, and the lower left is the least risky condition. important variables. Alloys, added in small quantities to iron-
The upper left corner and the lower right corner are roughly carbon mixtures, create steels with differing properties.
equal. Toughness is the material property that resists fatigue failure. A
Note also that Table 5.2 is not linear. The designer ofthe table trade-off often occurs as material toughness is increased but
did not change point values proportionately with changes in other important properties such as corrosion resistance, weld-
either the magnitude or frequency of cycles. This indicates a ability, and brittleductile transitions may be adversely
belief that changes within certain ranges have a greater impact affected. The fracture toughness of a material is a measure of
on the risk picture. The following example illustrates further the degree of plastic deformation that can occur before full fail-
the use of this table. ure. This plays a significant role in fatigue failures. Much more
energy is required to fail a material that has a lot of fracture
toughness, because the material can absorb some of the energy
that may otherwise be contributing directly to a failure. A larger
Table 5.2 Fatigue scores based on various combinations of
pressure magnitudes and cycles defect is required to fail a material having greater fracture
toughness. Compare glass (low fracture toughness) with cop-
Lifetime cycles per (high fracture toughness). In general, as yield strength goes
up, fracture toughness goes down. Therefore. flaw tolerance
often decreases in higher strength materials.
Another contributor to fatigue failures is the presence of
IO0 7 5 3 I 0 stress concentrators. Any geometric discontinuity such as a
90 9 6 4 2 1 hole, a crack, or a notch, can amplify the stress level in the
7s 10 1 5 3 2 material. Coupled with the presence of fatigue loadings, the sit-
50 11 8 6 4 3 uation can be further aggravated and make the material even
25 I2 9 1 5 4 more susceptible to this type of failure.
10 I3 IO 8 6 5
5 14 II 9 7 6 The process of heating and cooling of steel during initial
formation and also during subsequent heating (welding) plays