Generically, establishing a reassessment interval to deal with a time-dependent threat to pipeline integrity requires calculating the failure pressure of the worst-case anomaly remaining in the segment after an initial assessment and determining the time it will take for the anomaly to reach a size that will cause failure at the MOP. Calculating failure pressures requires the use of a failure-pressure-versus-anomaly-size model as discussed in Annex D. The time for the failure pressure of a growing anomaly to decay from the benchmark value established by the last assessment to the MOP depends on the rate of growth. Since it is not prudent to allow this entire calculated time period to expire before carrying out a reassessment, a safety factor is embodied in the calculation.
The pipeline operator should establish the lengths and depths of the anomalies that remain after an integrity assessment and the amount of growth that would cause their failure pressures to decay to 1.1 × 72 % of SMYS as described above. The operator should also establish the rate of growth appropriate to the time-dependent growth mechanism as described in 9.2.2 and 9.2.3. For anomalies that are believed to grow at linear rates with time (external and internal corrosion, selective seam corrosion, and SCC), the operator may then use Figure 5 to establish a reassessment interval, to calculate a reduced MOP to schedule mitigation or reassessment, or to schedule remediation of individual anomalies as their failure pressure approaches 1.1 times MOP.
ΔK C1ΔS πa ---Q
=
To use Figure 5 correctly, the operator should consider the nominal wall thickness of the pipe, the MOP, and the established corrosion or crack-growth rate. For example, consider the case for dealing with the effect of external corrosion on the 20-in. OD, 0.250-in., X52 pipeline that has a MOP of 72 % of SMYS represented in Figure D.2.
Assume that it was assessed by ILI and that the operator repaired all anomalies having failure pressures below 100 % of SMYS. From Figure D.2, it can be ascertained that 50 mils of growth for a 14-in.-long anomaly is about the least amount of growth required to erase the margin of safety established by the integrity assessment, that is, to cause the ratio of 100 % of SMYS to 72 % of SMYS (1.39) to decline to a ratio of 1.1 × 72 % of SMYS. If the operator determines that the corrosion rate is 5 mils/year, reassessment should be carried out within 10 years. This situation is represented by the line labeled “Above 50 % of SMYS” in Figure 5. The line slopes from a Y axis failure pressure/
MOP ratio of 1.39 at a time of 10 years on the X-axis (remaining safe life) to a failure-pressure-to-MOP ratio of 1.1 at a time of zero years (meaning that time has run out). An anomaly that had a rupture pressure exceeding 100 % of SMYS that is growing at a rate of 5 mils/year can be placed farther to the right of 10 years on the time axis by extrapolation of the line to the appropriate failure-pressure-to-MOP ratio for the anomaly, and then it can be remediated within the indicated time. Additionally, an anomaly indicated by ILI to have a failure-pressure-to-MOP ratio of 1.55 (end of the dashed line extension) has a remaining safe life of 15 years. Conversely, if the operator had left unremediated, an anomaly with a failure-pressure-to-MOP ratio less than 1.39, the time to address the anomaly would be less than 10 years (e.g. 5 years for an anomaly with a failure-pressure-to-MOP ratio of 1.25). It is very important to note that this example applies only to a pipe with wall thickness of 0.250-in. with a MOP of 72 % of SMYS corroding at a rate not exceeding 5 mils/year. For other conditions, the operator should adjust Figure 5 in an appropriate manner as outlined below.
The effect of a MOP other than 72 % of SMYS can be seen in Figure 5. Suppose that the 20-in. OD, 0.25-in. wall, X52 pipeline that was corroding at a rate of 5 mils/year had a MOP of 50 % of SMYS. The “Above 30 % of SMYS but not exceeding 50 % SMYS” line on Figure 5 indicates that if reassessment results in repair or removal of all anomalies
Figure 5—Timing for Scheduled Responses 1
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6
0 5 10 15 20 25
Failure Pressure/MOP
Response Time, years
Above 30 % of SMYS but not exceeding 50 % SMYS
Above 50 % of SMYS
Not exceeding 30 % SMYS
with failure pressures of less than 100 % of SMYS, then the remaining safe life is 15 years. That is because a minimum failure-pressure-to-MOP ratio of 2 has been established. If the MOP of the pipeline were 30 % of SMYS, by the same reasoning (a minimum failure-pressure-to-MOP ratio of 3.3), Figure 5 indicates a remaining safe life of 20 years. These principles also would apply to an operator using a pressure reduction to delay remediation or reassessment. Users are cautioned, however, that these gains in time are absolutely tied to the minimum failure- pressure-to-MOP ratio being validated by the current integrity assessment. For example, if the current assessment consisted of a hydrostatic test of to a level of 1.5 times MOP for a pipeline with a MOP of 50 % of SMYS, the remaining safe life assured would be only seven years based on the point where the “Above 30 % of SMYS but not exceeding 50 % SMYS” slope intersects the failure-pressure-to-MOP ratio of 1.5.
The effects of wall thickness and anomaly growth rate on the remaining safe life are illustrated in Figure 6.
Figure 6 is based on a pipe wall thickness of 0.312-in. and a MOP of 72 % of SMYS. The more steeply sloping line represents a 0.312-in.-wall pipe with an anomaly growth rate of 7 mils/year. Note that the remaining life following an assessment to 100 % or SMYS (a failure-pressure-to-MOP ratio of 1.39) is 10 years. Recall that the 10-year remaining life for the 0.25-in. wall pipe corresponded to a corrosion rate of only 5 mils/year. The difference arises from the facts that the thinner pipe will be penetrated more deeply in relation to its wall thickness in a fixed amount of time than the thicker pipe and that for a fixed value of remaining life, the ratio of growth rates has to be the inverse of the ratio of the thicknesses. Now consider the effect of a lower growth rate on the 0.312-in. wall pipe (see the line with the least slope in Figure 6). If the growth rate is cut in half from 7 mils/year to 3.5 mils/year, the remaining safe life is doubled.
Figure 6—Effects of Wall Thickness and Defect Growth Rate
1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45
0 2 4 6 8 10 12 14 16 18 20
Failure Pressure/MOP
Time, years Response time relationship based
on Figure 1 (assumed rate of growth = 7 mpy)
Response time for maintaining an equivalent margin of safety if the rate of growth is 3.5 mpy
The examples described herein indicate that a pipeline operator should establish a Figure 5 for the specific circumstances of wall thickness, anomaly growth rate, MOP, and minimum failure-pressure-to-MOP ratio achieved by the current assessment in order to determine either when reassessment is needed or when it is necessary to remediate a particular anomaly. The process can be applied to corrosion-caused metal loss, SCC, and selective seam corrosion (i.e. to any time-dependent anomaly growth mechanism where it is safe to assume a constant anomaly growth rate). For each particular type of threat other than corrosion-caused metal loss in the body of the pipe, however, the user should account for the effect of material toughness on the sizes of defects that will fail at particular benchmark pressure levels.