The value of ILI evaluation and mid-cycle excavations

Without the ILI evaluation made by Lamontagne Pipeline Assessment Corporation for the following section of pipeline, four mid-cycle excavations would not have been made and leaks of hazardous liquids would possibly have occurred prior to the next in-line inspection.

An engineering evaluation was made on the NPS 12, 140 mile Eagle River to Bear Springs section based on the comparison of 2003 and 2006 MFL inspections. The initial run for this analysis was conducted on April 29, 2003 and identified 13667 anomalies down to an estimated depth of 10% wall loss. The comparative run was undertaken in May 2006 and identified 30170 anomalies down to a depth of 10% wall loss. Of the anomalies identified in 2003 and 2006, 10254 anomalies were matched between the inspections. The majority of the unmatched features from the 2003 inspection were within the areas of high corrosion and therefore has merged into groups in 2006. There were 365 external corrosion anomalies denoted in 2006.

Figure 1. Growth from 2003 to 2006 inspections

It is evident that the internal corrosion growth is more rapid where product is being injected at Darby and Thesalon Stations as illustrated in Figure 1. The run-to-run comparison as referred to above provided over 10000 data points from which to extract growth information of the internal anomalies. Figure 2 graphically illustrates that the rate of growth of the internal anomalies was dependent upon the orientation of the anomaly. A varying growth rate with a confidence level of 95% is obtained when using the solid black line while the dashed line contains all of the most rapid values of growth.

Figure 2. Internal anomaly growth rate per orientation

Applying the appropriate growth rates based on orientation to the anomalies delineated in the 2006 inspection an excavation timeline based on pressure (1.0 ERF) and anomaly depth limits (70%) was established. This was done on a six-month interval over a ten-year period. As this evaluation was done in 2007 it may be seen in Table 1 that by including the growth rate eight more excavations had to be made to cover 2006 and another 9 to cover the 2007 year period. It was in the initial eight excavations that the 4 locations were sand blasted through as the pipe was being cleaned for examination. 

Table 1. Timeline for excavation of anomalies

Dig Date	Failing Anomalies	Excavations
Nov-06	9	8
May-07	12	3
Nov-07	15	6
May-08	54	13
Nov-08	87	12
May-09	95	58
Nov-09	110	36

Based on the excavation estimates presented in Table 1 and further over the ten-year period an economic assessment using net present value (NPV) may be made. A net present value assessment is generally used to optimize the mitigation plan by taking into account the future repair cost versus the cost of re-inspection. In this case the underlying assumptions were an 8% discount rate, $22,000 per excavation and $125,000 per in-line inspection. The corrosion growth rate employed is aggressive therefore a more reasonable set of limits (1.0 ERF, 70% Depth) was used. The NPV in Figure 3 demonstrates that some sweeping measures must be taken to address not only the corrosion anomalies within the pipeline but the root cause of the corrosive medium to lessen the aggressive attack on the pipe. In particular the product input at both Darby and Thesalon stations must be treated appropriately to prevent further aggressive attack.

The proposed re-inspection date would be on or about May 2008 as long as the excavations are made as outlined for the 2006 and 2007 time period.

Figure 3. Net present value assessment to provide a re-inspection date

In-line inspection evaluations pull together all the information gathered from previous ILI and excavations to provide a more definitive answer to dig scheduling and re-inspection intervals. The cost to an operator to have such evaluations completed for their whole system is negligible relative to the cost of one in-service failure. As a result of the case study examined here, four anomalies in different locations were actually highlighted and remediated just prior to failing in-service.