Shawn Kenny

Analysis and Design of Buried Pipelines for Ice Gouging Hazard: A Probabilistic Approach

In cold environments, marine pipelines may be at risk from ice keels that gouge the seabed. Large quantities of material are displaced and soil deformations beneath a gouge may be substantial. To meet safety criteria, excessive strains are avoided by burying pipelines to a sufficient depth. In this paper, a probabilistic approach for the analysis and design of buried pipelines is outlined. Environmental actions are applied through distributions of gouge width, gouge depth, subgouge soil deformations, and bearing pressure. Three-dimensional pipe/soil interaction problem is modeled using nonlinear soil springs and special beam elements using the finite element method to estimate pipe response for statistically possible ranges of gouge depths, gouge widths, and burial depths. Relevant failure mechanisms have been considered, including local buckling and a variety of strain and stress based criteria. The methodology presented in the paper was developed and successfully used for several pipeline and electrical cable projects in ice gouge environments. Significantly reduced burial depth requirements have been demonstrated through the application of the probabilistic approach and through the use of strain-based design criteria. Because ice actions are applied through displacements of the soil, more ductile pipes are often necessary to meet reliability targets.

Finite element investigations on the dynamic plastic buckling of a slender beam subject to axial impact

The dynamic plastic buckling response of slender beams, with geometric imperfections, subject to an axial impact is modelled by the finite element method. One of the primary objectives for the current study was the development of an accurate numerical model, which was validated by comparison with experimental investigations. A sensitivity analysis was conducted that considered the element aspect ratio, element formulation, constitutive relationship, boundary condition and type of geometric imperfection. The buckling response was defined by a normalized modal parameter as a function of the effective slenderness ratio. The normalized modal parameter accounted for the axial position and amplitude of the transverse peak buckle with respect to the local stiffness characteristics and natural frequency parameters. For the parameters considered, the influence of modelled boundary conditions and element formulation on the computed buckling response was established. In relative terms, the aspect ratio, imperfection model and contact mechanics was of secondary importance on the modal behaviour.

Lateral Buckling Response of Subsea HTHP Pipelines Using Finite Element Methods

Subsea pipelines are subject to load effects from external hydrostatic pressure, internal pressure, operating temperature, ambient temperature and external reactions (e.g. seabed, structural support). These parameters influence the effective axial force that governs the pipeline global buckling response. Other factors, including installation stress, seabed slope, soil type, and embedment depth, can influence the pipe effective force.Pipelines laid on the seabed surface or with limited embedment may experience lateral buckling. The resultant mode response is a complex function related to the spatial variation in these parameters and kinematic boundary conditions.In this paper, results from a parameter study, using calibrated numerical modelling procedures, on lateral buckling of subsea pipelines are presented. The parameters included pipe diameter to wall thickness (D/t) ratio, pipe out of straightness (OOS), operating temperature and internal pressure, external pressure associated with the installation depth, and seabed lateral and axial friction properties.Copyright

Identification of the Cause of Variability of Probability of Failure for Burst Models Recommended by Codes/Standards

Failure probability of oil and gas pipelines due to external corrosion defects can be estimated using corrosion growth model and the evaluation of remaining strength. Codes/standards have been developed for the assessment of the remaining strength of corroded pipeline. The remaining strength and the operating pressure were considered to develop the limit state equation and consequently the failure probability of the burst models recommended by codes/standards. In the present paper, comparative analyses of the failure probability estimated by the codes/standards were conducted, using Monte Carlo simulation and first order second moment methods. The analysis revealed that the failure probability of the burst models recommended by codes/standards varies significantly for the same defects size. The study further explored the cause of variability in failure probabilities. The study observed that different defect shape specifications (rectangular, parabolic, etc.) and different stress concentration factor derivations (different contributions of l) for burst pressure estimation are responsible for high variability in the probability of failure. It is important to reduce variability to ensure unified risk-based design approach considering any codes/standards.

Numerical Investigation of Oblique Pipeline/Soil Interaction in Sand

Energy pipelines pass through various environmental and geotechnical conditions. They are usually buried and can be subjected to geohazards like landslides, fault movements or large subsidence resulting in large permanent ground deformations along part of their length. The effect of large permanent ground deformations on buried pipelines can be critical for their integrity and safety. Understanding this effect is important for pipeline designers. In the current engineering guidelines the pipeline/soil interaction has been idealized using structural modeling which evaluates the soil behavior using discrete springs with load-displacement relationships provided in three perpendicular directions (longitudinal, lateral horizontal and vertical). These springs are usually independent and during a 3D pipe/soil relative displacement they can not account for cross effects due to shear interaction between different soil zones along the pipe. Some studies in the past including an experimental study by the authors have shown the importance of cross effects between axial and lateral soil restraints on the pipeline during oblique axial/lateral pipeline/soil relative movements. In this numerical study a three-dimensional continuum finite element model is developed using ABAQUS/Standard software. The model has been calibrated against the centrifuge tests conducted by the authors. The numerical model successfully reproduces the ultimate loads and also the shape of failure surfaces observed during physical tests. The numerical model will be used to extend the physical investigation results by parametric studies in future works.Copyright

Comparative Study on the Collapse Response of Flexible Pipe Using Finite Element Methods

Although the scope and use of flexible pipe systems in deepwater developments is expanding, the mechanical behavior for these environments is not fully understood. This is due to the complex response and interaction between multiple layers within the pipe system that introduces significant difficulties and constraints into the engineering analysis. As future developments look to extend the use of this technology to greater water depths and harsher operating conditions there is a need to develop advanced numerical tools that can evaluate the mechanical integrity of these complex hybrid pipe systems. Availability of increasingly advanced computational packages has enabled substantial improvements to be made in the complexity of simulation tools for combined loading, external pressure collapse and fretting.This study establishes a foundation for the development of advanced numerical modeling procedures to assess the collapse failure of composite flexible pipe systems for deepwater applications. Here, a continuum finite element model is constructed using the software package ABAQUS/Standard, and studied using non-linear (arc length) methods. The carcass, pressure armor and corresponding polymer layers are represented in detail and modeled with three dimensional solid brick elements in order to examine the interlayer relationships influencing collapse initiation. In many recent studies, an initial geometric imperfection in the form of general ovality is explored as the predominant bifurcation mode. A similar approach is adopted here, coupled with case studies chosen such as to facilitate validation against existing analytical and numerical data. The importance of element selection, contact mechanics, interface properties and initial imperfections on the system mechanical response and performance is presented and compared to the available literature.© 2014 ASME

Mechanical Damage and Fatigue Assessment of Dented Pipelines Using FEA

Onshore and offshore pipelines may be subjected to mechanical damage during installation and operation due to environmental loads, external forces and third party interference. Pipelines in offshore environment may be prone to mechanical damage from events such as ice gouging, frost heave, and seismic fault movement. For conventional pipelines, the assessment of mechanical damage plays an important role in the development of integrity management programs that may be of greater significance for pipeline systems located in remote harsh environments and that are more prone to anchor drag, seismic loading and ice gouging.This study examines the effect of dents and corrosion loss on pipe mechanical response using continuum finite element methods. ABAQUS/Standard (6.10-1) environment was used to simulate damage events and pipe response. Modelling procedures developed and calibrated against physical and numerical data sets available in public domain were reported previously in Hanif & Kenny 2012, 2013. Once confidence in numerical procedures was established, an analysis model matrix was established to account for a range of influential parameters including pipe/indenter geometry and pressure factor. A nonlinear multivariate regression analysis was conducted to develop strain based empirical tools that characterize the effects of local damage and applied loads on pipeline mechanical response for unconstrained dent conditions. Coupled affect of dent and artificial corrosion loss (in terms of wall thickness reduction in the damage zone) was also analyzed and a sensitivity study was conducted to see the effect of percentage wall loss on pipe response. Finally, operational parameters were varied and resulting stress concentration factors were calculated, that took into account indentations and wall loss, to predict fatigue life of dented pipe segments for both constrained and unconstrained dent conditions.Copyright

Influence of Geotechnical Loads on Local Buckling Behavior of Buried Pipelines

Buried pipelines can be subjected to differential ground movement events. The ground displacement field imposes geotechnical loads on the buried pipeline and may initiate pipeline deformation mechanisms that exceed design acceptance criteria with respect to serviceability requirements or ultimate limit states. The conventional engineering approach to define the mechanical performance of pipelines has been based on combined loading events for “in-air” conditions. This methodology is assumed to be overly conservative and ignores soil effects that imposes geotechnical loads and also provides restraint, on buried pipelines. The importance of pipeline/soil interaction and load transfer mechanisms that may affect local buckling of buried pipelines is not well understood. In this study a three-dimensional continuum finite element (FE) model, using the software package ABAQUS/Standard, was developed and calibrated based on large-scale tests on the local buckling of linepipe segments for in-air and buried conditions. The effects of geotechnical boundary conditions on pipeline deformation mechanism and load carrying capacity were examined for a single small diameter pipeline with average diameter to thickness ratio and deep buried condition. The calibrated model successfully reproduced the large-scale buried test results in terms of the local buckling location, pipeline carrying load capacity, soil deformation and soil failure mechanism.© 2008 ASME

Investigations on the Local Buckling Response of High Strength Linepipe

A numerical modeling procedure was developed, using the finite-element simulator ABAQUS/Standard, to predict the local buckling and post-buckling response of high strength pipelines subject to combined state of loading. The numerical procedures were calibrated using test data from large-scale experiments examining the local buckling of high strength linepipe. The numerical model’s response was consistent with the measured experimental response for predicting the local buckling behavior well into the post-yield range. A parametric study was conducted that examined element selection, mesh topology, second-order effects, geometric imperfections and material properties. The results from this study are presented.Copyright

Probabilistic Design Methodology to Mitigate Ice Gouge Hazards for Offshore Pipelines

For offshore pipelines located in ice environments, the mitigation of ice gouge hazards presents a significant technical challenge. A traditional strategy is to establish minimum burial depth requirements that meet technical and economic criteria. A probabilistic based approach to optimize burial depth requirements based on equivalent stress and compressive strain limit state criteria is presented. The basic methodology is to define ice gouge hazards on a statistical basis, to develop numerical algorithms that model ice gouge mechanisms and pipeline/soil interaction events, to define failure criteria, limit states and target reliability levels and to conduct a probabilistic assessment of pipeline burial depth requirements. Application of the probabilistic design methodology for a generic pipeline design scenario subject to ice gouge hazards is presented. Implications on pipeline design and future applied research initiatives are discussed.Copyright