Ryan Phillips

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.

Trench Effects on Pipe-Soil Interaction

Trenched pipelines subjected to large lateral soil movements were studied to quantify mitigative effects of trench geometry, backfill soil material and strength on the force-displacement behaviour in cohesive soils. Experimental and numerical models show a good agreement in terms of undrained ultimate forces, which are also consistent with design guidelines and previous studies. Undrained lateral pipe resistance factors, are assessed in terms of soil strength and soil weight in uniform soil. A normalised pipe displacement rate characterizes a transition in the lateral resistance from undrained to drained conditions is presented. The presence of a trench backfilled with material weaker than the native soil softens the lateral load-deformation (p-y) response compared to that of the same pipe buried in native soil. An increase in the trench width increases the pipe displacement to peak load. The lateral interaction force is much lower in a pipe with a wider trench than a narrower trench prior to reaching the peak load. The peak load occurs after pipe touches the trench wall and is controlled by the native soil strength. It decreases slightly with increase in the trench width because of upward movement of pipe prior to reaching trench wall. The mitigative effects of trench wall inclination are also demonstrated. A simple approach to determine the p-y response for a trenched pipeline backfilled with material weaker than the native soil is proposed.Copyright

Combined Axial and Lateral Pipe-Soil Interaction Relationships

A recent study recommended design guideline improvements to assess ground movement hazards for buried pipeline response. A review identified a wide variation of normalised axial resistance factors. This paper presents a three-dimensional finite element parametric study of pipe-soil interaction under combined axial and lateral loading. The normalised axial resistance is found to be very sensitive to the direction of pipe displacement and controlled by the interface friction near pure axial motion. An interaction diagram is developed and a design equation is proposed for combined axial and lateral loading.© 2004 ASME

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

Strain Softening and Rate Effects on Soil Shear Strength in Modeling of Vertical Penetration of Offshore Pipelines

Offshore pipelines play a vital role in the transportation of hydrocarbon. In deep seas, pipelines laid on the seabed usually penetrate into the soil a certain amount. These pipelines might experience significant lateral movement during the operational period. The resistance to lateral movement depends on vertical penetration and berm formation around the pipe. Vertical penetration is a large deformation problem. Finite element modeling of vertical penetration of offshore pipeline in soft clay seabed in deep water is presented in this study. The modeling was performed using ABAQUS finite element software. Soil was modeled in an Eulerian framework and the pipe in a Lagrangian framework. Strain softening behavior and strain rate effects on undrained shear strength of clay was incorporated in ABAQUS FE software using user subroutines written in FORTRAN. The variation of undrained shear strength with depth is also considered. The results are compared with centrifuge test results and also with available solutions.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

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

Liquefaction Study of Heterogeneous Sand: Centrifuge

From past numerical research and small scale laboratory tests, it has been observed that more excess pore water pressure (EPWP) is generated during earthquakes in a heterogeneous sand deposit than in the corresponding homogeneous sand with relative density equal to the average relative density of the heterogeneous sand. This interesting phenomenon is investigated here in large scale experiments using geotechnical centrifuge modeling techniques. A series of liquefaction tests have been conducted at C-CORE’s geotechnical centrifuge facility: Two on variable sand and one on uniform sand deposits. A one level frame structure resting on two strip footings was also placed on that sand deposit to study the effect of soil variability on building foundations. Experimental results such as accelerations, EPWPs and settlements were monitored and measured throughout the tests. Recorded results support the conclusion of previous research that more EPWP is generated in heterogeneous sand deposits than in the corresponding homogeneous sand. The liquefaction mechanism of heterogeneous sands leading to this phenomenon is discussed in this paper.

Effect of Soil Restraint on the Buckling Response of Buried Pipelines

Long-term large deformation geohazards can impose excessive deformation on a buried pipeline. The ground displacement field may initiate pipeline deformation mechanisms that exceed design acceptance criteria with respect to serviceability requirements or ultimate limit states. Conventional engineering practice to define the peak moment or compressive strain limits for buried pipelines has been based on the pipeline mechanical response for in-air conditions. This methodology may be conservative as it ignores the soil effect that imposes geotechnical loads and 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. The authors previously developed a new criterion for local buckling strain of buried pipelines in stiff clay through response surface methodology (RSM) [1, 2]. In this paper the new criterion was compared with a number of available in-air based criteria to study the effect of soil restraint on local buckling response of buried pipelines. This criterion predicted larger critical strain than selected in-air based criteria which shows the significant influence of soil presence. The supportive soil effect is discussed. The soil restraining effect increases the pipeline bending resistance, when the pipeline is subjected to large displacement-controlled ground deformation. A correlation between Palmer’s et al. (1990) conclusion [3] and current study’s results has been made. The critical strain increases as the ratio between axial thrust and pipeline bending stiffness decreases.Copyright

Mitigation of Infilling Around Pipelines With Cyclic Lateral Deflections

The soil response around a high temperature pipeline proposed for construction was evaluated. Infilling of backfill around pipelines at side bends subjected to temperature changes may accumulate during cyclic displacements leading to ratcheting and possibly buckling of the pipeline. A medium-scale laboratory model testing program was used to gain a better understanding of how silty sand backfill migrates around a pipeline when subjected to cyclic lateral loading. Several mitigation techniques using geosynthetics and compressible materials were evaluated in order to both delay and reduce the net downward soil movement from pipe displacement cycles.Copyright