Effective Improvements to Reliability Based Corrosion Management

Reliability methods have being adopted by oil and gas operators for integrity management decisions. These methods explicitly account for all relevant uncertainties and are designed to provide consistent safety. Consequently, a risk or reliability based approach is a very appropriate basis for decision making in the face of uncertainties. However, as in the effective use of any powerful methodology the sensitivities of the method to assumptions and limitations of applicability need to be well understood. This paper presents how improvements were made to reliability based integrity program by understanding its limitations and sensitivities. First the inputs that have the highest impact on the results were identified. These inputs are the most appropriate areas for improvement and data gathering. It is also very important to understand how the results are to be used and for what purpose. The results of this particular inline inspection based reliability assessment are used to make better excavation and repair decisions. A defect-based and joint-based decision making process is essential for determining with sufficient confidence if each defect and joint is in a safe condition. Consequently, the improvements are focused on discriminating between the myriad of defects found during an inline inspection run. Distinct field characteristics of corrosion growth are also taken into account in these improvements. The paper presents the implementation of effective area methods for future integrity probabilistic evaluations. It also describes the benefit of applying defect-specific growth rates. Finally, case studies are presented to demonstrate the effectiveness of the changes.

A Probabilistic Model for Predicting Internal Corrosion Threat in Dry Natural Gas Pipelines

A probabilistic model is developed in this work to predict the internal corrosion (IC) threat due to water condensation in dry natural gas pipelines. The model involves the understanding of tariff limits (TLs) for water and other corrosive species in natural gas; a consensus definition of an extremely unlikely condition for IC threat; a statistical analysis of field operating temperature, pressure, and water content (WC) data from a number of operators; and a known but modified relation of the saturated WC vs. operating temperature and pressure. By setting the limit of the probability of water condensation at 2% of the time that the pipe surface is wet, the maximum WC allowed in the natural gas can be determined for any given temperature and pressure. Practical operating charts have been developed for guiding pipeline operators to understand and minimize IC threats in dry gas (DG) pipelines. This paper presents the probabilistic modeling approach and discusses some model results.

A Precipitation-Induced Landslide Susceptibility Model for Natural Gas Transmission Pipelines

Landslides related to heavy rainfall can cause extensive damage to natural gas transmission pipelines. We have developed and implemented a geographic information system (GIS) model that evaluates near real-time precipitation-induced landslide susceptibility. This model incorporates state-wide precipitation data and geologically-based landslide classifications to produce rapid landslide risk evaluation for Pacific Gas & Electric Company’s (PG&E) gas transmission system during winter rain storms in California. The precipitation data include pre-storm event quantitative precipitation forecasts (QPF) and post-storm event quantitative precipitation estimates (QPE) from the United States National Oceanic and Atmospheric Administration (NOAA). The geologic classifications are based on slope, susceptible geologic formations, and the locations of historic or known landslide occurrences. Currently the model is calibrated using qualitative measures. Various scientists have developed large landslide databases with associated rainfall statistics to determine rainfall thresholds that trigger landslides. With a sufficient number of landslides, we can more precisely determine minimum rainfall thresholds using similar methods.

Approaches to Qualify Strain-Based Design Pipelines

The importance of using strain-based design pipelines is growing due to the increasing number of projects in challenging environments such as permafrost, offshore ice hazards, active seismic areas, and in high temperature/high pressure operations. To ensure pipeline integrity in environmentally sensitive areas and overall cost effectiveness, a strain-based design approach needs to consider all key interrelated design aspects including strain demand, design methods, material selection, strain capacity validation, and impact on construction and operation. To that end, significant research and development efforts have been made by the industry to facilitate the qualification of strain-based designed pipelines. This paper describes methods developed for the qualification of strain-based design pipelines, and demonstrates how recently developed strain capacity prediction tools, calibrated by full scale testing, can facilitate concept selection, material qualification and integrity verification of such pipelines.

Evaluation of UOE and Spiral-Welded Line Pipe for Strain Based Designs

Strain-based designs for Arctic pipelines place stringent demands on properties of the pipe body as well as the girth weld and associated heat affected zone. The pipe body must demonstrate good work hardening behavior in addition to satisfactory strength and toughness properties. Girth welds are required to overmatch the strength of the pipe body; both the weld and heat affected zone must also provide good toughness. In this study, X80 line pipe produced using the UOE and spiral welding processes were compared. The UOE process provides some degree of work hardening resulting from cold expansion. This extra hardening renders the UOE pipe more responsive than the spiral pipe to aging effects associated with pipe coating. However, the UOE pipe has an advantage in balancing LPA (longitudinal to pipe axis) and TPA (transverse to pipe axis) strengths. Greater strengths in the TPA orientation afford the capacity to meet specified minimum requirements of the pipe grade and lower strengths in the LPA orientation facilitate overmatching by girth welds. The two types of line pipe offer both advantages and disadvantages for strain-based designs. It must be emphasized that good work hardening characteristics can be maintained in the UOE pipe when the coating process involves a low temperature, which is an objective of modern coating technologies. It was also observed that aging effects did not affect toughness properties significantly.

A Study on the Standard System of Pipeline Operation

Standardization, as the basis of pipeline, plays an important role in the sound development of pipeline operation and ensures its safety and efficacy on technology. It has become an important and pressing task to build up an appropriate standard system for the application of pipeline operation, in view of which, this article has made an in-depth analysis of the present standardization status of pipeline operation application and the problems in existing standard systems, put forward the principles for the standard of pipeline operation within the oil and gas industry and the established standard system for the application of pipeline operation. This project is intended to make the standard work better and move R&D into the development of standard system.

Development of Failure Criteria for Predicting Tearing of Wrinkled Pipe

Onshore steel pipelines, particularly buried in cold region, often subjected to extreme geo-environmental conditions, where significant inelastic deformation may occur resulting in localized wrinkles. Under continued deformation, there is a possibility of excessive cross-sectional deformation at wrinkle locations, eventually leading to fracture or damage in the pipe wall. A recent field fracture and failed laboratory specimens under monotonic load history address the necessity of conducting a comprehensive research program to better understand this unique failure mode. Initial results have indicated that even under monotonic loading, significant strain reversals can occur at sharp fold of the wrinkle. These strain reversals were identified as one of the key factors to trigger this unique failure mechanism. This paper addresses the development of failure criteria used in the finite element (FE) model of plain pipes subjected to sustained monotonic axial and bending deformation with or without internal pressure. In conjunction with the strain reversal criterion, the critical equivalent plastic strain was used as the fracture or damage initiation limit in the numerical analyses. Results obtained from the full-scale test of an NPS16 pipe were used to calibrate the FE model. Results obtained from the numerical analyses have shown that the proposed criteria predict the onset of fracture at sharp fold of the wrinkle with reasonable accuracy.

Enhanced Leak Detection Risk Model Development

Increasing concerns and attention to pipeline safety have engaged pipeline companies and regulatory agencies to extend their approaches to pipeline integrity. The implementation of High Consequence Areas (HCAs) has in particular had an impact on the development of integrity management protocols (IMPs) for pipelines. These IMPs can require that a risk based assessment of integrity issues be applied to specific HCA risk factors. This paper addresses the development of an operational risk assessment approach for pipeline leak detection requirements for HCAs. A detailed risk assessment algorithm that includes 25 risk variables and 28 consequence variables was developed for application to all HCA areas. The significant likelihood and consequence factors were chosen through discussions with the Leak Detection Risk Assessment Model Working Group and subject matter experts throughout Enbridge. The leak detection algorithm focuses on sections of pipe from flow meter to flow meter, as these are the locations that impact the leak detection system used by Enbridge. Each section of pipe is evaluated for likelihood, consequence and risk. When a high or medium risk area has been identified, an evaluation of potential Preventive and Mitigative (P&M) measures will be undertaken. A P & M Matrix has been developed to identify potential mitigation strategies to be considered for higher risk variables, called risk drivers, in the model. The matrix has been developed to identify potential risk mitigation strategies to consider for each variable used in the HCA Leak Detection Risk Assessment. The purpose of the matrix is to guide the user to consider actions identified for variables that drive the risk for the particular location. Upon review of the matrix, the user determines feasibility of the risk mitigation strategies being considered to identify an action. The paper will describe the consultative process that was used to workshop the development of this algorithm. Included in this description is how the process addressed various methods of leak detection across a wide variety of pipelines. The paper closes with “development challenges” and future steps in applying operation risk assessment techniques to mainline leak detection risk management.

On the Applicability of Extreme Value Statistics in the Prediction of Maximum Pit Depth in Heavily Corroded Non-Piggable Buried Pipelines

There exists a large number of works aimed at the application of Extreme Value Statistics to corrosion. However, there is a lack of studies devoted to the applicability of the Gumbel method to the prediction of maximum pitting-corrosion depth. This is especially true for works considering the typical pit densities and spatial patterns in long, underground pipelines. In the presence of spatial pit clustering, estimations could deteriorate, raising the need to increase the total inspection area in order to obtain the desired accuracy for the estimated maximum pit depth. In most practical situations, pit-depth samples collected along a pipeline belong to distinguishable groups, due to differences in corrosion environments. For example, it is quite probable that samples collected from the pipeline’s upper and lower external surfaces will differ and represent different pit populations. In that case, maximum pit-depth estimations should be made separately for these two quite different populations. Therefore, a good strategy to improve maximum pit-depth estimations is critically dependent upon a careful selection of the inspection area used for the extreme value analysis. The goal should be to obtain sampling sections that contain a pit population as homogenous as possible with regard to corrosion conditions. In this study, the aforementioned strategy is carefully tested by comparing extreme-value-oriented Monte Carlo simulations of maximum pit depth with the results of inline inspections. It was found that the variance to mean ratio, a measure of randomness, and the mean squared error of the maximum pit-depth estimations were considerably reduced, compared with the errors obtained for the entire pipeline area, when the inspection areas were selected based on corrosion-condition homogeneity.

Considerations of Linepipe and Girth Weld Tensile Properties for Strain-Based Design of Pipelines

Pipelines in certain regions are expected to survive high longitudinal strains induced by seismic activities, slope instability, frost heave, and mine subsidence. Material properties, of both pipes and girth welds, are critical contributing factors to a pipeline’s strain capacity. These factors are examined in this paper with particular focus on the modern high strength pipes (grade X70 and above) usually made from microalloyed control-rolled TMCP steels. The examination of the tensile properties of pipes includes some of the most basic parameters such as yield strength, strength variation within a pipe, and newly emerging issues of strength and strain hardening dependence on temperature. The girth weld tensile properties, particularly yield strength, are shown to be dependent on the location of the test specimen. There are strong indications from the tested welds that strain hardening of the welds is dependent on test temperature. The effects of strain aging on pipe and girth weld properties are reviewed. This line of reasoning is extended to possible strain aging effects during field construction, although experimental evidence is lacking at this moment. The paper concludes with considerations of practical implementation of the findings presented in the early part of the paper. Recommendations are made to effectively deal with some of the challenging issues related to the specification and measurement of tensile properties for strain-based design.