Reliability-Based Assessment Method of Buried Pipeline at Fault Crossings

The tectonic fault, which is one of the most common geohazards in field, poses great threat to buried pipe segments. Pipes will process to buckling or fracture due to large strain induced by continuously increasing ground displacements during earthquakes. Therefore, it is imperative to conduct safety analysis on pipes which are buried in seismic areas for the sake of ensuring normal operation. However, the highly nonlinearity of pipe response restricts the proceeding of reliability assessment. In this study, a hybrid procedure combining finite element method and artificial neural network is proposed for reliability-based assessment. First of all, the finite element model is developed on ABAQUS platform to simulate pipe response to strike-slip fault displacements. Thus, the strain demand value (the peak strain value obtained by finite element model in each design case) can be collected for database establishment, which is the preparation for neural network training. Thoroughness of the strain demand database can be achieved by a fully comprehensive calculation with consideration of influencing factors involving pipe diameter and wall thickness, operating pressure, magnitude of fault displacement, intersection angle between pipeline and fault plane, and characteristic value of backfill mechanics. Sequentially, Back Propagation Neural Network (BPNN) with double hidden layers is trained based on the developed database, and the surrogate strain demand prediction model can be obtained after accuracy verification. Hence, the strain-based limit state function can be respectively determined for tensile and compressive conditions. The strain capacity term is simply assumed based on published papers, the strain demand term is naturally superseded by the surrogate BPNN model, and Monte Carlo Simulation is employed to compute the probability of failure (POF). At last, the workability of the proposed approach is tested by a case study in which basic variables are referred to the Second West-to-East natural gas transmission pipeline project. It indicates that ANN is a good solver for reliability problems with implicit limit state functions especially for highly nonlinear problems. The proposed method is capable of computing POFs, which is an exploratory application for reliability research on pipes withstanding fault displacement loads.

Management of Ground Movement Hazards: An Overview of a JIP

Ground movements such as landslides, subsidence, and settlement can pose serious threats to the integrity of pipelines. The consequences of a ground movement event can vary greatly. Certain types of ground movements are slow-moving and can be monitored and mitigated before a catastrophic failure. Other forms of ground movements can be difficult to predict. The most effective approach could be hazard avoidance, proactive means to reduce strain demand on pipelines, and/or building sufficiently robust pipeline segments that have a high tolerance to the strain demand. This paper provides an overview of a Joint Industry Project (JIP) aimed at developing a best-practice document on managing ground movement hazards. The hazards being focused on are landslides and ground settlement, including mine subsidence. This document attempts to address nearly all major elements necessary for the management of such hazards. The most unique feature of the JIP is that the scope included the hazard management approach often practiced by geotechnical engineers and the fitness-for-service assessment of pipelines often performed by pipeline integrity engineers. The document developed in the JIP provides a technical background of various existing and emerging technologies. The recommendations were developed based on a solid fundamental understanding of these technologies and a wide array of actual field experiences. In addition to the various elements involved in the management of ground movement hazards, the JIP addresses some common misconceptions about the adequacy of codes and standards, including: • The adequacy of design requirements in ASME B31.4 and B31.8 with respect to ground movement hazards, • The adequacy of linepipe standards such as API 5L and welding standards such as API 1104 for producing strain-resistant pipelines, • The proper interpretation of the longitudinal strain design limit of 2% strain in ASME B31.4 and B31.8, and • The effectiveness of hydrostatic testing in “weeding out” low strain tolerance girth welds.

The Coarse Particle Influence on the Strength of Wax Deposition

Wax deposition has always been an essential issue for flow assurance, especially in subsea pipelines. The coarse particles, which are usually measured in millimeters, will be carried out by oil flow during the deep-water oil fields production. However, due to insufficient understanding of the structure of wax deposits and the complexity of sandy crude oil deposition, the interaction between coarse particles and wax deposits in the pipeline have rarely been investigated. In this paper, the effect of coarse particles on the yield stress of wax deposits has been studied. The sample was mixed at reversible structure temperature so that the impact of shear history could be eliminated, and the rapid particle settlement at high temperature could be avoided. Experimental results have found that there is a critical fraction in coarse particle influences, below which a small number of coarse particles added will lead to a slight increase in bulk yield stress. On the contrary, a dramatic decrease in yield stress when exceeding the critical mass fraction and increasingly marked enhancement of yield stress as the fraction increases. This phenomenon has been explicated microscopically by the structural interaction between coarse particles and wax deposits. The interlock between wax crystals is the major contribution of the structure as the less particle fraction contains. Even though the silica sand is a typical non-colloidal particle, the asphaltene and resin could be absorbed on the surface of particles and forming a cluster of colloidal particles. As the fraction of particles slightly increased, the slip between colloidal particles and wax crystal interlock accelerates structural failure. Nevertheless, more particles involved the overall yield stress may depend on the friction and the adhesive force between solid particles. The subtle changes induced by coarse particles would have a harder deposit, which can hinder pig passing and affect pipeline pigging operations.

Preventing Girth Weld Failure in Pipelines: Measurement of Loads and Application of Assessment Methods

Pipeline failures from circumferential cracking at girth welds continues to affect large diameter oil and gas transmission lines, even for modern lines constructed this century. The key factors that contribute to the failure at girth welds are the dimensions of defects present, the material properties of the pipe and weldments, and the presence of loading that drives crack growth. The mechanisms of failure are well understood, but identifying and measuring the contributing factors can be a challenge. Locating girth welds that are subject to elevated loads will enable operators to focus on sections with an increased threat of failure. In this paper, we consider each of the key factors, how these are identified and defined, and the uncertainties in the measurement process. Specific attention is applied to the presence and quantification of loads and how these influence the potential for failure. This includes sources of active external loading due to ground movement, for example, or loads generated in the pipeline from the construction process. Loads can also be quantified by measuring bending strain from inline inspection inertial measurement units. A more complete picture of pipeline loading can be established by integrating a structural analysis that accounts for the direction of pipeline movement and the presence of axial loads. The relationship between assessing pipeline integrity from ground movement — typically with strainbased methods — and establishing whether the defect can survive the load is explored. The relative contribution of bending and axial loads in the failure of defects is considered. The outcome of the study will assist pipeline operators in prioritising actions that enable the quantification of the all the key parameters. The resultant analysis will provide guidance on the girth welds that have an increased risk of failure and this will enable protective actions to be defined and scheduled accordingly.

Development of Soil Restraints for Buried Pipelines in Muskeg Soils Subjected to Lateral Ground Displacements

A systematic research program was undertaken with the objective of developing quantitative geotechnical parameters to support soil-pipe interaction assessment for buried pipelines in muskeg. For this purpose, a field geotechnical investigation program comprising cone penetration testing (SCPT) with shear wave velocity (Vs) measurements, electronic field vane shear testing (eVST), full-flow ball penetration testing (BPT), and pressuremeter testing (PMT), along with fixed-piston tube soil sampling was undertaken in a muskeg soil terrain. The data from field testing were initially interpreted to obtain typical stiffness and strength parameters for the subject soils. These parameters were then used to numerically simulate pressuremeter tests and the results were compared with those obtained from field pressuremeter testing; the intent was to calibrate a suitable constitutive model to represent the muskeg soil mass. These ascalibrated constitutive model was then applied on numerical models developed to simulate buried pipelines in muskeg soil subject to relative lateral ground movements. The work is aimed at developing a framework to generate soil restraint versus relative ground displacement relations (“soil springs”) to assess soil-pipe interaction of pipelines buried in muskeg soils. Initial results from the research are presented herein, with a comparison made between soil springs developed from numerical analyses and those generated from current practice guidelines.

Stress Analysis of Buried Pipelines Under Thaw Settlement of Permafrost Zone Based on Moisture-Heat-Stress Coupled Analysis

Permafrost thawing caused by the hot crude pipeline is a major threat to the safe operation of buried pipelines in permafrost zone. In this paper, the process of thawing and consolidation of frozen soil is considered, and a three-dimensional (3D) finite element model of buried pipelines in permafrost zone is established using ABAQUS. The calculation of thaw settlement displacement of frozen soil based on moisture-heat-stress coupled was carried out, and the deformation and stress of buried pipelines were analyzed. The effects of ground temperature, oil temperature, thermal conductivity of insulation material and soil distribution along the pipeline on the vertical displacement and longitudinal stress of buried pipelines in frozen soil were studied. Research results show that in thaw-unstable soil, the vertical displacement and stress of the pipeline increase significantly with the increase of the average ground temperature, and change on ground temperature amplitude has a little effect on the vertical displacement and longitudinal stress of the pipeline in thaw settlement zone. It is 1/3 of the vertical displacement of the pipeline without a heat insulating layer. When the thermal conductivity of the insulation material is less than 0.4 W/m °C, the vertical displacement of the pipeline in the thawing zone can be further reduced by reducing the thermal conductivity of the insulation material. When clay and sand appear alternately along the pipeline, the vertical displacement and longitudinal stress of the pipeline can be reduced by reducing the length of clay section. This study has certain reference value for optimizing the design parameters of buried pipelines in permafrost zone and reducing the impact of differential thaw settlement of frozen soils on the safe operation of pipelines.

Geotechnical Lessons Learned From Nineteen Railway Trenchless Crossings During Construction of a Transmission Pipeline

When new pipelines are constructed, they often cross existing major infrastructure, such as railways. To reduce potential service disruption, it is a common practice to complete these crossings using trenchless technologies. Without proper methods and oversight in planning and construction, there may be serious safety and financial implications to the operators of the railways and the public due to unacceptable settlement or heave. If movement tolerances are exceeded, the schedule and financial loss to the railway operators could be in the millions of dollars per day. Recent construction of a new pipeline across the Canadian prairies implemented ground movement monitoring plans at 19 trenchless railway crossings in order to reduce the potential for impact to the track and railway operations. The specifics of the plan varied for each site and were based on the expected ground conditions, as well as permit requirements from the various railway operators, but typically included ground movement monitoring surveys, observation of the cuttings, recommendations for a soil plug at the leading edge of the bore casing, and frequent communication with both the railway operators and the contractors. For all crossings, the expected soil and groundwater conditions were obtained from pre-construction boreholes and confirmed during excavation of the bore bays. Based on the expected ground conditions, appropriate soil plug lengths, if required, were recommended. In general, fine-grained clay/silt-dominated soils needed minimal to no soil plug in order to minimize the potential for ground heave, while coarser-grained sand-dominated soils needed a longer soil plug in order to reduce the potential for “flowing soil” which would cause over excavation along the bore path. Prior to boring, surface monitoring points were established along the tracks to monitor for changes in the ground surface elevation. Additional subsurface points were installed for crossings where the potential for over excavation was higher. These monitoring points were surveyed before, throughout, and following completion of construction, and the frequency of the surveys was increased when the movement was nearing or exceeding specified tolerances. The effort to monitor and reduce the potential for ground movement was a coordinated effort between the geotechnical engineers, railway operators, and construction contractors. The purpose of this paper is to present the lessons learned from the 19 trenchless railway crossings, including the challenges and successes. Recommendations for ground movement monitoring are also provided to help guide railway operators, design and geotechnical engineers, and contractors during the construction of future trenchless pipeline crossings of railway infrastructure.

Qualitative and Semi-Quantitative Model for Estimating the Probability of Failure at River Crossings

Pipeline river crossings are typically managed by using a combination of flood monitoring, ground inspections, integrity assessments, and remediations. Using a probabilistic model to assess the likelihood of failure at river crossings would enable combined consideration of all factors that contribute to the failure threat, provide site rankings to support discrete mitigation prioritizations, allow for evaluation of whether a crossing is acceptable in regard to a risk target, and provide a “check” to the deterministic integrity management methods. This paper describes two models for estimating the pipeline probability of failure at river crossings. The first model is a qualitative scoring model that can be easily implemented by operators and consultants. This model employs a weighting-factors approach to consider the multiple variables that contribute to pipeline exposures and overstress given exposure. The results may be applied to threat rank diverse crossings, as well estimate the probability of failure at a crossing relative to that at historical failure sites. The second model is a semi-quantitative model that 1) estimates the likelihood of a crossing exposure occurring, 2) estimates the associated scour length, 3) assesses the pipelines critical span length, and 4) quantifies the probability that a span length longer than the critical span length could form. This model may be applied to achieve the same goals as the qualitative model, and also compare the probability of failure at a river crossing to a reliability target. Due to the complexity of this model and the paper length limits, it is conceptually described within this paper. The results demonstrated that the model output site rankings correlated reasonably with those estimated by pipeline integrity program managers, the scour depth and length prediction results were consistent with measured historical scours, and the pipeline probability of failure at the assessed river crossings were within expected ranges.

How Do I Ensure “Staff Competency” in My Pipeline Safety Management System?

Standards and regulations are clear: all staff who work on pipelines need to be both “competent” and “qualified.” Standards such as API 1173 are clear about competence within a safety management system: “The pipeline operator shall ensure that personnel whose responsibilities fall within the scope of the PSMS [Pipeline Safety Management System] have an appropriate level of competence in terms of education, training, knowledge, and experience.” The burden of defining and specifying competence falls on pipeline operators, but they have little guidance regarding the required skills, knowledge and levels of competency. Additionally, we are all biased — different operators will have different ideas and emphases on competencies, which will affect their decision-making. The only way to avoid these cognitive biases is to use consensus standards supported by rigorous surveys that capture the required competencies. This paper explores some of the more common biases that can affect decisions and presents the results of a controlled, independent, survey aimed at both specifying and quantifying the necessary competencies needed by a specific engineer working within a PSMS: a pipeline integrity engineer. The paper identifies and ranks these necessary competences. The survey was completed by 100 pipeline integrity engineers from 25 different countries. Its specific objective was to investigate the key skills and knowledge requirements needed in a junior engineering position (i.e., a pipeline engineer with less than three years of relevant experience) working under supervision to be ‘competent’. It listed eight core competencies (identified by subject matter experts) considered essential for a pipeline integrity engineer. Each of these core competencies contained a set of skills. Respondents were first asked to rank the eight core competences, and then rank the skills within the competency. An analysis of the data provides insights into how 100 pipeline integrity engineers view the key skills required to be “competent.” The results of the survey can assist pipeline companies in setting objective competency requirements for their engineering personnel, developing learning programs to address any gaps, and improve the overall safety of their pipeline system.

Comparison of Buried Pipeline Crossing Assessments Using API RP 1102, Analytical Method and Finite Element Approach

API RP 1102 provides a method to calculate stresses in buried pipelines due to surface loads resulting from the encroachment of roads and railroads. The API RP 1102 approach is commonly used in the industry, and widely available software allows for quick and easy implementation. However, the approach has several limitations on when it can be used, one of which is that it is limited to pipelines crossing as near to 90° (perpendicular crossing) as practicable. In no case can the crossing be less than 30° . In this paper, the stresses in the buried pipeline under standard highway vehicular loading calculated using the API RP 1102 method are compared with the results of two other methods; an analytical method that accounts for longitudinal and circumferential through wall bending effects, and the finite element method. The benefit of the alternate analytical method is that it is not subject to the limitations of API RP 1102 on crossing alignment or depth. However, this method is still subject to the limitation that the pipeline is straight and at a uniform depth. The fact that it is analytical in nature allows for rapid assessment of a number of pipes and load configurations. The finite element analysis using a 3D soil box approach offers the greatest flexibility in that pipes with bends or appurtenances can be assessed. However, this approach is time consuming and difficult to apply to multiple loading scenarios. Pipeline crossings between 0° (parallel) and 90° (perpendicular) are evaluated in the assessment reported here, even though these are beyond the scope of API RP 1102. A comparison across the three methods will provide a means to evaluate the level of conservatism, if any, in the API RP 1102 calculation for crossing between 30° and 90° . It also provides a rationale to evaluate whether the API RP 1102 calculation can potentially be extended for 0° (parallel) crossings.