FAILURE MARGIN ANALYSIS USING RISK CURVES TECHNOLOGY Description
Definition
Direct failure margin analysis using the risk curves technology evaluates the effect of broken prestressing wires on the performance of the pipe and its margin to failure using a calibrated and verified model. Failure margin may be evaluated using risk curves corresponding to limit states related to serviceability, damage, and ultimate strength of the pipe with broken prestressing wires (Figure 6.1). Repair priorities are assigned to pipes with broken prestressing wires in order to identify pipes with unacceptable risk of failure when subjected to the maximum internal pressure and gravity loads.
History
Failure margin analysis is often performed using a model that was developed by Zarghamee in 2001 as a part of a research program on failure of PCCP with broken wires sponsored by the PCCP Users Group. An experimentally verified model was developed that relates the number of broken wires and maximum pressure in the pipe to limit states of serviceability, damage, and strength (rupture). The program included, field investigations, nonlinear finite element analyses (Figure 6.2), and hydrostatic pressure testing (Figure 6.3) from which a practical failure margin analysis procedure was developed. The results of this research program were published in Zarghamee and Ojdrovic (2001), Zarghamee et al. (2002), and Zarghamee et al. (2003). Experimental verification was published by Zarghamee (2003). The accuracy of the risk curves technology was verified by comparing the predicted number of broken wires and maximum pressure corresponding to failure of the pipe with the actual value for highly distressed pipes prior to failure and failed pipes (Ojdrovic et al. 2011). The application of the procedure to LCP was published by Erbay et al. (2007).
Other failure margin analysis methods using limit states curves have been developed, but no experimental verification of these methods have been published or made available.
Physics of Technology
The failure scenario of PCCP with broken prestressing wires progresses as follows: (1) microcracking of the concrete core (serviceability limit state) occurs within the prestress loss zone as tensile stresses increase, (2) visible longitudinal cracking of the concrete core (damage limit state) occurs within the prestress loss zone as the core expands radially, (3) circumferential cracking of the concrete core occurs at the center and the edges of the broken wire zone, and (4) the ultimate strength of the distressed pipe is reached, when the steel cylinder yields, cracked concrete core reach its strength, and soil resistance is overcome (ultimate strength limit state).
Analysis Process
The first step in failure margin analysis of a pipeline is collection of pipeline data including pipe design, maximum pressures expected in the pipeline, soil cover heights, and any available information regarding past performance, inspections, and aggressiveness of the environment toward PCCP.
Risk curves are then constructed for each pipe design class using the earth load corresponding to the actual soil cover height (usually within about ±2 feet). The serviceability limit state is based on the onset of concrete core cracking, the damage limit state is based on structural cracking of the core and on increase in wire stress adjacent to the BWZ, and the strength limit state is based on the ultimate strength of the pipe (Zarghamee et al. 2003).
Thinning of the steel cylinder due to corrosion is accounted for in analysis of LCP (Erbay et al.
2007), but is generally not included in analysis of ECP unless identified as a concern based on the pipe design or external inspection.
The risk curves divide the plots of pressure versus number of broken wires into repair priority zones, as shown in Figure 6.1 for ECP. Repair priorities are calculated for each pipe containing broken wires using the maximum expected pressure in the pipe and an effective number of broken wires that accounts for the estimated number of broken wires from NDT data, uncertainties in estimation of the number of broken wires, and progression of broken wires with time. The expected rate of wire breaks can be either calculated using historical results of condition assessment on the same pipeline or obtained from documented rates observed on other similar pipelines. Each pipe containing broken wires is assigned a repair priority, depending on the margin of pipe failure and the need for repair.
Due to the uncertainties in the numbers of wires predicted with the nondestructive condition assessment technologies, the results of failure margin analysis are considered to be preliminary until verification of the predicted number of broken wires is performed on selected pipes through external inspection. After verification is complete, the failure margin analysis and repair prioritization are adjusted as needed and final recommendations are made regarding the future management of the pipeline.
Benefits and Issues
The benefits of the technology can be summarized as follows:
Technology is based on structural analysis, hydrostatic pressure testing to failure, and field verification.
Accounts for uncertainties in electromagnetic inspection results and condition of wires adjacent to the broken wire zone.
Accounts for progression of wire breaks using rates calculated from historical inspection data or from similar pipelines in similar environments.
Provides a means to evaluate the structural significance of inspection results.
Allows prioritization of repair of distressed pipe.
Figure 6.1. Example risk curves for a specific ECP design subjected to a specific height of cover and bedding and backfill condition
Strength with contribution of soil
Strength without contribution of soil
Serviceability
Damage
Source: Zarghamee et al. 2003, with permission from ASCE
Figure 6.2. Strains in outer core at failure of cracked outer core
Figure 6.3. Hydrostatic testing of PCCP with broken prestressing wires Application
Failure margin analysis has been performed on PCCP pipelines since 2001 and is used as part of condition assessment and asset management program that includes periodic re-inspection and repairs/replacement of pipes at high likelihood of failure. Some sample applications are as follows:
1. Central Arizona Project, Arizona: Nonlinear finite-element analysis was used to predict the performance of 252-inch-diameter non-cylinder prestressed concrete pipe (NCP) and 72-inch-diameter PCCP, both subjected to combined effects of internal pressure, pipe and fluid weights, and earth load as it loses prestress due to gradually increasing number of broken wires. The model incorporates a nonlinear stress-strain relationship for concrete that includes compressive crushing, tensile softening and cracking. The results of analysis for 252-inch NCP show that the final failure mode of the pipe is in fact in form of progression of wire breaks due to the increase in the stresses in the wires, rather than concrete crushing or major leakage. The results of analysis for the 72-inch-diameter PCCP subjected to a prestress loss and increasing internal pressure show that interlocking strength of cracked outer core and steel cylinder ultimate strength, rather than progression of wire breakage, governs the strength of pipe. For pipe with large number of wire breaks, structural cracking can occur exposing the steel cylinder to corrosive soil environments. Corrosion of the steel cylinder can result in leakage and premature failure of the steel cylinder. The results of the nonlinear finite element analyses are used to validate engineering criteria and procedures used for risk analysis and repair priorities determination of pipes with broken wires and for comparison with confirmation testing of the pipe with broken wires (Zarghamee et al. 2002).
2. Cedar Creek Pipelines and Richland Chambers Pipelines, Tarrant Regional Water District, Texas: Beginning in 1989, Tarrant Regional Water District (TRWD) recognized that 2 of their 75-mile-long, large diameter PCCP lines were deteriorating. A high percentage of pipes in the Cedar Creek pipeline that were under impressed cathodic protection were found to have wire breaks due to hydrogen embrittlement. TRWD participated in the Prestressed Concrete Pipe User’s Group and, along with a number of other agencies, funded a study by SGH to develop a simplified finite element analysis to determine the residual strength of damaged pipes. TRWD employed SGH to develop specific models and risk curves for their 2 pipelines and to calibrate the model for wire failure due to hydrogen embrittlement. As a part of calibration, several pipes with embrittlement were pressure tested to failure. Results of the study showed that pipes with embrittlement had high residual strength. Using the risk curves technology with and without embrittlement, SGH prioritized pipes for repair and TRWD was able to focus repairs on the pipes at highest risk of failure. TRWD’s approach of predictive maintenance and replacement of pipes at high risk of failure allowed repair costs to be spread over decades to minimize the impact on budget and operation (Marshall et al.
2005).
3. Central Pipeline, Santa Clara Valley, California: PPIC inspected 4.2 miles of PCCP without shorting straps using RFTC technology in 2001 and inspected 6.6 miles in 2005.
The 2001 inspection identified 24 distressed pipes (about 2.2%) and the 2005 inspection identified 59 distressed pipes (about 3.4%). SGH performed failure margin analysis of the distressed pipes using results of RFTC inspection and the maximum pressures in the pipes. Preliminary results of failure margin analysis were used to select 5 distressed pipe pieces for external inspection to verify the results of RFEC/TC and to reduce the uncertainties in the failure margin analysis. External inspection consisted of wire continuity measurements, visual inspection and sounding of the exposed pipe surface, and measurement of coating thickness, soil cover height, and prestressing wire diameter
and spacing. External inspection revealed absence of active corrosion and lower numbers of broken wires than predicted by RFTC. Failure margin analysis was revised based on the results of field inspection, and SCVWD determined that repairs were unnecessary and monitoring the line was adequate. SCVWD considers the $330,000 spent to validate electromagnetic results and evaluate the failure margin of the pipeline a good investment compared to the costs of unnecessarily repairing the 59 pipes identified as distressed by electromagnetic inspection (Dion and Zarghemee 2008).
4. Lake Arrowhead Pipeline, Wichita Falls, Texas: RFTC inspection was performed on 5.3 miles of 54-inch-diameter non-shorting strap PCCP in Lake Arrowhead Pipeline and identified 192 distressed pipes of 1708 inspected (11.24%). SGH performed failure margin analysis of the distressed pipes and field inspection to verify the results of RFTC.
Results of verification indicate that RFTC’s estimation of number of broken wires in zones with less than 15 broken wires is accurate within the normal uncertainties, RFTC’s estimation of the number of broken wires in pipes categorized as DA (PPIC terminology for EM signal shift detected across entire length of pipe) appears to be overly conservative, and RFTC’s estimation of the number of broken wires in zones with more than 35 broken wires appears to be conservative. The City of Wichita Falls decided to replace the 44 pipes identified as Repair Priorities 1 and 2 due to their geographic proximity. The cost of the inspection plus the cost of the repairs was only a fraction of the cost of replacing the pipeline. The use of RFTC inspection technology and failure margin analysis enabled the City to prioritize repairs and replacements and to determine that significant useful life remaining in its pipeline (Taylor 2008).
5. 102 inch diameter ECP Aqueduct, Providence, Rhode Island: A pipe failure occurred in 1996 prior to failure margin analysis and a heavily distressed pipe was identified by internal nondestructive inspection in 2005. External inspection confirmed that the distressed pipe contained approximately 150 broken prestressing wires. The maximum expected operating pressure for these pipes was about 65 psi. Risk curves developed specifically for the pipe design of the failed pipe and the distressed pipe showed that the strength limit state curve passes very near to the failed pipe, indicating that the strength limit state curve predicts the failure reasonably well. The pressure and observed number of broken wires in the distressed pipe exceed the damage limit state, but are below the ultimate strength limit state. Observations of a 0.020-inch-wide crack in the concrete core, delamination between the steel cylinder and concrete core, and minor surface rust on the steel cylinder correlate well with the level of distress expected on a pipe exceeding the damage limit state (Ojdrovic et al. 2009).
Summary Failure Margin Analysis Using Risk Curves Technology
The failure margin of PCCP with broken wires depends on the number and location of wire break zones, the number of broken wires in each zone, pipe design, maximum pressure in the pipe, earth load, and pipe and fluid weights. The vast majority of pipes with broken wires do not have a short time to failure. In fact, an objective of pipeline condition assessment is to identify the relatively small number of pipes that have an unacceptably high failure risk and repair them before they fail. Maintaining an acceptable failure risk of distressed pipe with broken wires is accomplished by performing failure margin analysis to determine their failure
risk and by subsequent repair of such pipes. This form of proactive maintenance can result in an overall improved reliability of the pipeline and reduced cost of maintenance and repair.
The failure margin analysis using risk curves technology provides, for each distressed pipe, relationships between the number of broken wires and the maximum internal pressure corresponding to certain limit states on serviceability, damage, and strength (rupture). These risk curves are used to determine the failure margin of the pipe based on the number of broken wires, the cover height, gravity effects, and the maximum pressure in the pipe.
Reliability of pipe depends on the uncertainty in the number and location of broken wires detected, nature of wire breakage (corrosion or hydrogen embrittlement), and the rate of increase in the number of broken wires in the future if the pipe is not repaired immediately. Field verification of distressed pipe reduces the uncertainty in the predicted number of broken wires and in the failure margin analysis. Uncertainty analysis allows importance factors to be assigned to parts of the pipeline where consequence of failure is great. These uncertainties are used to calculate the effective number of broken wires and pipe repair priorities at present and at several years into the future, within which time the pipeline may be reinspected or repaired.
RISK RANKING Description Definition
Risk ranking identifies individual pipes or sections of pipelines that have high failure risk, based on evaluation of parameters that are believed to correlate to pipe distress. Such methods include risk index systems, finite element models that are not based on experimental verification, and criteria based solely on the predicted number of broken wires. Risk index systems evaluate failure margin by assessment of design, construction, operation, and environmental parameters that may result in higher risk of wire breakage. Finite element methods evaluate the effect of broken wires on the performance of the pipe using a model that has not been experimentally verified or calibrated against full-scale testing. Methods using only the number of broken wires to evaluate pipe failure margin and remaining service life are based solely on previous experience with the number of broken wires or rate of wire breakage that resulted in pipe failure in the past.
History
The risk ranking methods have been developed since mid-2000 by various consultants or utilities in an attempt to determine what actions need to be taken after performing inspection of a pipeline and identifying distressed pipes and estimating the distressed level in such pipes. Their success has not been documented and remains unknown.
Physics of Technologies
Risk index systems account for environmental conditions that are aggressive to PCCP and concludes that the likelihood of broken wires is higher in areas where environmental conditions are more aggressive, containing high chloride concentrations, high sulfate concentrations, severe acids, and/or high levels of dissolved carbon dioxide (AWWA Manual M9).
Finite element models developed to capture the behavior of PCCP with broken prestressing wires can identify the stress in the pipe in a relative basis, but have not been shown to be able to capture all failure modes and predict the failure of distressed pipes accurately through experimental means.
Analysis Process
Risk Index System: Risk index systems (e.g., Pipeline Decay Index or Pipe Criticality Index) determine criticality of pipeline section or a pipe based on the varying physical characteristics of the PCCP and environment along the pipeline. Factors taken into account include design parameters, original construction practices, modes of operation, maintenance procedures, results of corrosion surveys, results of internal inspections and results of acoustic monitoring of the line.
Finite Element Analysis: FEA methods are based on a finite-element model of the pipe subjected to internal pressure and possibly gravity loads and prestressing, including loss of prestress. In general these FEA methods define “a measure” of distress as a function of the number of broken wires. Depending on the complexity of their model, they may also provide a measure for failure margin of the distressed pipe, but may not be able to predict neither the failure margin nor time to failure with any accuracy.
An FEA model was developed for the analysis of an aqueduct in Mexico and used to calculate the stresses in the concrete core and steel cylinder at various lengths of prestress loss and for a constant internal pressure (Gomez et al. 2004). No experimental basis or verification data is provided for this model.
One FEA method assumes incremental decrease in the prestress level along the entire pipe in order to establish the sensitivity of pipe design to wire breaks over a large area and determines the effect of prestress loss on performance of the pipe using limit state criteria from AWWA C304.
There are other proprietary methods with no information provided to allow evaluation (Marshall 2009, Loera 2007).
Specified Number of Broken Wires: Using this method, failure margin and remaining service life of a pipe is based only on the estimated number of broken wires or on the observed rate of wire breaks from NDT data. Empirical data of the number of wire breaks that causes a PCCP to fail in the past are generally used to determine the maximum tolerable number of broken wires. Alternatively, pipes are evaluated based on the rate of wire breakage and are replaced if the observed rate increases beyond a threshold.
Benefits and Issues
The benefits of the risk ranking technologies can be summarized as follows:
FEA methods are based on structural analysis, and provide a relative sensitivity of the pipe to loss of prestress.
Risk index method and specified number of wire breaks provide a low cost method for determining the relative failure risk of distressed pipe.
All technologies allow for prioritization of repairs.
Use of index systems and decisions made based solely on the number of broken wires are relatively low cost after the data collection.
The following limitations of the risk ranking technology have been identified:
The correlation between the actual failure margin and predictions of these technologies has not been documented.
Subject to uncertainties in the results of inspection and rate of wire breakage. No published literature discusses how uncertainties in nondestructive testing results are accounted for in the use of these technologies.
Application
Risk ranking methods have been used as a part of condition assessment and asset
Risk ranking methods have been used as a part of condition assessment and asset