IPC PRESSURE CYCLING MONITORING HELPS ENSURE THE INTEGRITY OF ENERGY PIPELINES. Peter Song Enbridge Pipelines Inc. Edmonton, Alberta, Canada

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1 Proceedings of the ASME 8 th International Pipeline Conference IPC2010 September 27 October 1, 2010, Calgary, Canada IPC PRESSURE CYCLING MONITORING HELPS ENSURE THE INTEGRITY OF ENERGY PIPELINES Peter Song Doug Lawrence Sean Keane Scott Ironside Aaron Sutton ABSTRACT Liquids pipelines undergo pressure cycling as part of normal operations. The source of these fluctuations can be complex, but can include line start-stop during normal pipeline operations, batch pigs by-passing pump stations, product injection or delivery, and unexpected line shut-down events. One of the factors that govern potential growth of flaws by pressure cycle induced fatigue is operational pressure cycles. The severity of these pressure cycles can affect both the need and timing for an integrity assessment. A Pressure Cycling Monitoring (PCM) program was initiated at Enbridge Pipelines Inc. (Enbridge) to monitor the Pressure Cycling Severity (PCS) change with time during line operations. The PCM program has many purposes, but primary focus is to ensure the continued validity of the integrity assessment interval and for early identification of notable changes in operations resulting in fatigue damage. In conducting the PCM program, an estimated fatigue life based on one month or one quarter period of operations is plotted on the PCM graph. The estimated fatigue life is obtained by conducting fatigue analysis using Paris Law equation, a flaw with dimensions proportional to the pipe wall thickness and the outer diameter, and the operating pressure data queried from Enbridge SCADA system. This standardized estimated fatigue life calculation is a measure of the PCS. Trends in PCS overtime can potentially indicate the crack threat susceptibility the integrity assessment interval should be updated. Two examples observed on pipeline segments within Enbridge pipeline system are provided that show the PCS change over time. Conclusions are drawn for the PCM program thereafter. INTRODUCTION Enbridge operates, in Canada and the U.S., the world s longest crude oil and liquids pipeline system. The system is comprised of approximately km (8500 miles) of pipeline with vintage ranging from 1940 s to current, and delivers more than 2 million barrels per day of crude oil and liquids. Modern and vintage manufactured pipe can contain flaws in the long seam weld as well as in the pipe body. One of the factors that govern potential growth of flaws by pressure cycle induced fatigue is operational pressure cycles. Two commonly used methods in the pipeline industry to mitigate critical flaws are hydrostatic pressure testing and running inline inspection tools followed with field excavation programs. For either method, the pipeline company must determine the integrity re-assessment interval and monitor pipeline conditions to ensure the continued validity of the scheduled interval. This paper describes the Pressure Cycling Monitoring (PCM) program that was initiated at Enbridge to monitor the (PCS) during pipeline operation. In determining the PCS for the pipeline, fatigue analysis was carried out using the operating pressure data from Enbridge SCADA system, and a flaw with dimensions proportional to the pipe wall thickness and the outer diameter. The estimated fatigue life is then plotted on the PCM chart and the trend of the PCS change with time is monitored. 1 Copyright 2010 by ASME

2 The PCM program can allow users to recognize notable fluctuations of the PCS. These fluctuations potentially identify a change to crack threat susceptibility. As well, the PCS used within re-assessment interval determination can be compared to most recent PCS to ensure the continued validity of established integrity re-assessment interval. In addition, the PCM program can be used to identify notable changes to PCS that may be attributed to specific operational practices. Two case histories demonstrating the use of the PCM program are presented. The PCM program identified specific time periods that were associated with the increased PCS. These time periods were examined and operational practices that contributed to the pressure cycling were identified, thus facilitating a review of potential changes to operational practices and/or changes to integrity programs. NOMENCLATURE a Depth of the surface crack da/dn Crack growth per cycle D Pipe external diameter D i Pipe internal diameter K max Stress intensity factor corresponding to the max. stress level within one pressure cycle K min Stress intensity factor corresponding to the min. stress level within one pressure cycle t Thickness of the pipe wall Y Dimensional constant that depends on the geometry and the mode of loading (fracture mechanics) K (K max K min ) p i Pipe internal pressure differential Hoop stress differential in the pipeline due to the internal pressure differential, p i D i /(2t), PRESSURE CYCLING IN LIQUID PIPELINES Liquids pipelines undergo pressure cycling that can be induced by line start-stop, batch pigs by-passing pump stations, product injection or delivery as well as other unexpected line shut-down events. Figure 1 shows an example of the pressure history plot for a 12 month period at one pump station discharge from the Enbridge pipeline system. The pressure data plotted in Figure 1 was acquired from the Enbridge Supervisory Control and Data Acquisition (SCADA) system, which collects the pressure data along the pipeline system in real time. The pressure plot in Figure 1 does not overtly indicate the PCS over this one year period. This same pressure data is represented on the PCM chart shown on Figure 5 and clearly indicates the change in PCS.. Discharge Pressure (psi) Jun-2007 Jul-2007 Sep-2007 Nov-2007 Jan-2008 Date Figure 1 Discharge Pressure for 6 Month Period at One Station Discharge In order to conduct the PCM program, the pressure data must be separated into equally spaced time spans. The time span selected should be sufficiently long so that the normal line operation characteristics, such as line start-stop, batch pigs by-passing pump stations, will not show up as noise on the PCM chart. On the other hand, the time span used in the PCM program should not be so long that the important changes to the line operations become normalized and unidentifiable. Typical time spans used by Enbridge range between monthly and quarterly. Once a time span has been selected, the pressure data is analyzed using rain flow counting techniques (ASTM E ) and representative pressure spectrums are created for each time span. At this stage, the pressure data is ready for fatigue analysis required for the PCM program. SELECTION OF THE PARAMETERS USED IN THE FATIGUE ANALYSIS OF THE PCM PROGRAM For fatigue analysis, the parameters affecting the fatigue results are the geometry of the flaw, the representative pressure spectrum and the fatigue growth constants. This section will describe how the relevant parameters used in the fatigue analysis of the PCM program were determined. 2 Copyright 2010 by ASME

3 The depth of the flaw used within the PCM program should be selected such that the spectrum of the fatigue lives that comprise the PCS graph provide a measurable change associated with change in operating pressure cycles. Depth wise fatigue crack growth with time follows a logarithmic curve, i.e., Paris Law. As such, if the selected initial flaw depth is too deep, the fatigue life change associated with change in operating pressures will be minimal as the fatigue lives will all be very short. Conversely, excessively shallow initial flaw depth will result in all fatigue lives being excessively long and also result in minimal change in fatigue life. It should be noted that the flaw dimensions used in the PCM program are not necessarily those used in determining the re-inspection intervals, but rather are determined such that the resulting spectrum of fatigue lives provided a measureable change associated with change in operating pressure cycles. An initial flaw depth of 20% wt is used as the starting depth in the fatigue analysis of the Enbridge PCM program. The length of the flaw used within the PCM program is 2 Dt. This length was selected as it aligns with fatigue cracks observed in failures (OPS TTO5, Michael Baker Jr. Inc et al. (2003)). The wall thickness used within the PCM program corresponds to the pipe subjected to the measured pressure cycling. PRESSURE CYCLING MONITROING (PCM) PROGRAM In conducting the PCM program, fatigue analysis was carried out using the selected parameters as discussed in the previous section. The obtained fatigue life corresponding to one month or one quarter of pressure data is plotted on the PCS chart as a single point and continued fatigue analysis will display the PCS change over time for the line segments monitored. In conducting the fatigue analysis, Enbridge used Paris Law equation and stress intensity solutions per BS 7910, which are expressed as below: da dn K = = n C K (1) σy πa (2) pid 2t i (3) where C and n are material constants that are determined experimentally and have recommended values for use in industry practice to conduct fatigue analysis, such as those by API 579 (2007) as mentioned in the previous section. The Paris Law equation and the stress intensity solutions are also well described in industry literature such as OPS TTO5 (Michael Baker Jr. Inc et al. (2003)). Enbridge uses the API 579 fatigue crack growth constants of C = 8.61E-19 (unit in psi and inch) and n = 3. Within the PCM program, the goal of the fatigue analysis is to determine the time for the initial flaw to grow to a defined depth. Since the PCM program is intended to monitor for changes to pressure cycling, this final depth does not need to align with critical flaw dimensions at the Maximum Operating Pressure (MOP). In order to develop a consistent approach to a diverse pipeline system with various design and operating conditions, Enbridge has selected a final flaw depth of 95% through wall within the PCM program. It is noted that, because of the logarithmic relationship between the depth and the time, the time for a flaw to grow from critical dimension to 95% through wall is very short in comparison to the time for a flaw to grow between starting flaw size and the critical dimensions. For each time span, fatigue analysis is used to determine the time for a flaw to grow from the initial flaw dimensions to final depth. The results are graphed with the time span identifier (i.e. month or quarter) on the horizontal axis and the time to grow the initial flaw to final depth on the vertical axis. The resulting graph is called (PCS) chart, an example of which is shown in Figure 2. The following three regions are identified on a PCS chart: High pressure cycling severity region, which corresponding to a fatigue life less than 17 years. Medium pressure cycling severity region, which corresponding to a fatigue life between 17 years and 34 years. Low pressure cycling severity region, which corresponding to a fatigue life over 34 years. OPS TTO5 defined categories for the relative aggressiveness of the operational pressure cycles. These categories are then related to the calculated fatigue lives and the crack threat susceptibility. It is important that an understanding of the alignment between the PCM program and the categories defined by OPS TTO5 be made. For an initial flaw with 30% depth to thickness ratio, a fatigue life less than 22 years is defined as aggressive pressure cycling. This fatigue life would be longer if a shallower initial flaw was considered. Additionally, the fatigue life would be longer if the final flaw depth was deeper than that defined by the critical dimensions. Within Enbridge PCM program, a 3 Copyright 2010 by ASME

4 shallower initial flaw depth (20% depth to thickness ratio) and a deeper final flaw depth (95% depth to thickness ratio) were used, a fatigue life of 17 years was used to define aggressive operational pressure cycling. Thus, the Enbridge definition of aggressive pressure cycling is conservative in comparison with that defined by OPS TTO5. Similarly, OPS TTO5 associates moderate cycling to a fatigue life of 56 years for a 30% initial flaw. This compares to the Enbridge values of 34 years. It should be noted that, for alignment with the pressure cycling severity categories described within OPS TTO5, the C and n values used within the PCM program must be the same as those used within the fatigue assessment used to determine the re-inspection interval. APPLICATION OF THE PCM PROGRAM - CASE HISTORY Presented below are two case histories demonstrating the use of the PCM program. Figures 2 and 3 show the PCS charts for portions of two pipelines within Enbridge pipeline system that exhibit variability of pressure cycling severity over time. Figure 2 shows the variability of the PCS along the length of a single segment of a pipeline. Prior to mid 2007, the PCS of the two pump stations follow similar trends. Change in operations during the latter part of 2007 resulted in start/stop operations at pump station B which introduced a change in PCS. Station A Station B The PCS change observed on Figure 3 identified the need to update the crack threat susceptibility assessment and integrity assessment interval on this line segment. In addition, time periods during which PCS changes required an associated shift in integrity assessment planning were identified, thus facilitating detailed examination of the possible factors contributing to the notable changes in pressure cycling Figure 3 PCS Chart for Case History Two Figure 4 shows the detailed analysis of the pressure cycles at this station discharge for a 10 day period. Operational review identified several factors associated with injection and deliveries and pigging practices that contributed to the change to PCS. The PCM program and associated focused review of operations facilitated a review of potential changes to operational practices and/or change to integrity programs Discharge Pressure (kpa) Pig bypass station stations No flow t ti Low flow rate Unscheduled shut down t ti Figure 2 PCS Chart for Case History One 1000 Figure 3 shows a trend of increasing PCS aggressiveness, settling into the high pressure cycling severity region, over a five (5) year period for a single pump station. A case history study regarding the mitigation of the PCS at this line station is presented below. 0 11/10/08 11/11/08 11/12/08 11/13/08 11/14/08 11/15/08 11/16/08 11/17/08 11/18/08 11/19/08 11/20/08 Date Figure 4 Pressure Profile for 10-Day Period 4 Copyright 2010 by ASME

5 Identification of the key factors contributing to pressure cycling and subsequent modification to operations contributed to the change in the PCS trend as shown in Figure 5. REFERENCES American Petroleum Institute, 2007, Fitness-For-Service, API Recommended Practice 579, Washington, DC, USA Figure 5 Updated PCS Chart for Case History Two American Society of Testing and Materials, 1985, ASTM E (Reapproved 2005), Standard Practices for Cycle Counting in Fatigue Analysis, ASTM International, West Conshohocken, PA, USA British Standard Institute, 2005, BS 7910 Guide on Methods for Assessing the Acceptability of Flaws in Metallic Structures, London, UK Michael Baker Inc. et al., 2003, OPS TTO5 Low Frequency ERW and Lap Welded Longitudinal Seam Evaluation, Integrity Management Program Delivery Order DTRS D CONCLUSIONS Enbridge PCM program is a convenient tool for the pipeline operating company to understand the pressure cycling severity on a pipeline. When effectively applied within an integrity management program, PCM can help ensure the integrity of the pipelines by identifying pipeline segments that have notably changed operations. Based on the descriptions of the PCM program and the case history study of the PCM program on two pipeline segments within the Enbridge pipeline system, the following conclusions are obtained: 1. The PCM program allows users to recognize notable fluctuations to the PCS. 2. When the PCS is aligned with crack threat susceptibility indicators, such as described in OPS TTO5, these fluctuations potentially identify a change to crack threat susceptibility. 3. The PCS used within re-assessment interval determination can be compared to current PCS to ensure the continued validity of established integrity re-assessment interval. 4. The PCM program can be used to identify notable changes to PCS that may be attributed to specific operational practices. 5. periods during which PCS changes required a shift in integrity assessment planning can be identified, thus making possible a focused examination of the possible factors contributing to the changes in pressure cycling. The PCM program and associated focused review of operations can facilitate a cost analysis to optimize between revisions to operational practices to reduce PCS and changes to integrity programs. 5 Copyright 2010 by ASME