Numerical modelling of submarine landslide impact on offshore free-spanning pipelines

Offshore pipelines are one of the most efficient and reliable modes of transportation of oil and gas. In shallow water conditions, the common practice is to bury the pipeline through trenching and backfilling. However, in deep water environments, pipeline burial through trenching is not very practical or cost-effective; therefore, the pipelines are often laid on the seafloor. Depending upon topography and seafloor environments, sections of as-laid (or surface-laid) pipelines might transform into the free-spanning pipeline. The suspended section of the pipeline might experience the impact of submarine landslides those frequently occur in continental slopes. The impact of debris flow, which originates from submarine landslides and travels in the downslope direction at high speed, might cause severe damage and even break out of these pipelines. Quantifying the impact forces on free-spanning pipeline sections is one of the key requirements in the design. In the present study, debris flow impact is numerically modelled using two software packages: (i) Abaqus finite element (FE) and (ii) ANSYS CFX based on a Computation Fluid Dynamics (CFD) approach. Implementing appropriate models for soil and water together with new approaches for modeling pipe–soil–water interface behaviour, the process of impact, including soft clay flow around the pipe, is successfully simulated using the above-mentioned approaches. Overall, the modelling of this large deformation process is computationally expensive. However, the CFD approach in ANSYS CFX is more computationally efficient than the Coupled Eulerian-Lagrangian (CEL) approach in Abaqus FE software. The role of free-water suction in the channel behind the pipe, the effects of seabed shear strength and gap between pipeline and seabed on drag force are investigated. The drag force depends on not only the shear strength of the debris but also soil flow mechanisms around the pipe, which is influenced by the gap and seabed shear strength

Strain-Based Design Methodology of Large Diameter Grade X80 Linepipe

Continuous growth in energy demand is driving oil and natural gas production to areas that are often located far from major markets where the terrain is prone to earthquakes, landslides, and other types of ground motion. Transmission pipelines that cross this type of terrain can experience large longitudinal strains and plastic circumferential elongation as the pipeline experiences alignment changes resulting from differential ground movement. Such displacements can potentially impact pipeline safety by adversely affecting structural capacity and leak tight integrity of the linepipe steel. Planning for new long-distance transmission pipelines usually involves consideration of higher strength linepipe steels because their use allows pipeline operators to reduce the overall cost of pipeline construction and increase pipeline throughput by increasing the operating pressure. The design trend for new pipelines in areas prone to ground movement has evolved over the last 10 years from a stress-based design approach to a strain-based design (SBD) approach to further realize the cost benefits from using higher strength linepipe steels. This report presents an overview of SBD for pipelines subjected to large longitudinal strain and high internal pressure with emphasis on the tensile strain capacity of high-strength microalloyed linepipe steel. The technical basis for this report involved engineering analysis and examination of the mechanical behavior of Grade X80 linepipe steel in both the longitudinal and circumferential directions. Testing was conducted to assess effects on material processing including as-rolled, expanded, and heat‑treatment processing intended to simulate coating application. Elastic-plastic and low-cycle fatigue analyses were also performed with varying internal pressures. Proposed SBD models discussed in this report are based on classical plasticity theory and account for material anisotropy, triaxial strain, and microstructural damage effects developed from test data. The study results are intended to enhance SBD and analysis methods for producing safe and cost effective pipelines capable of accommodating large plastic strains in seismically active arctic areas

Structure-Seabed Interactions in Marine Environments

The phenomenon of soil–structure interactions in marine environments has attracted great attention from coastal geotechnical engineers in recent years. One of the reasons for the growing interest is the rapid development of marine resources (such as in the oil and gas industry, marine renewable energy, and fish farming industry) as well as the damage to marine infrastructure that has occurred in the last two decades. To assist practical engineers in the design and planning of coastal geotechnical projects, a better understanding of the mechanisms of soil–structure interactions in marine environments is desired. This Special Issue reports the recent advances in the problems of structure–seabed interactions in marine environment and provides practical engineers and researchers with information on recent developments in this field

Root size effects on transverse root-soil interactions

For smaller lateral plant roots in coarse-grained soils, a potentially large relative size of soil particles compared to the roots may affect their transverse resistance. Even for the larger roots of trees, particle size effects may be important, e.g. when testing 1:N reduced scale models in a geotechnical centrifuge. The Discrete Element Method (DEM) was used to investigate this problem. A rigid lateral root segment under transverse loading in plane strain was simulated and compared with Finite Element Method (FEM) simulations, where the soil was modelled as a continuum (no particle size effects). Even at the lower root/particle diameter ratios (dr/D50) investigated (6 to 21), particle size effects on transverse capacity were negligible upon push-in, while during uplift, they were observed for (dr/D50) <8, arising from the dimension of the uplifted soil volume above the root. The material properties of roots are also typically diameter dependent. Further simulations of long flexible roots subject to end rotation were performed employing a beam-on-non-linear-Winkler-foundation approach, using p-y curves obtained from the DEM or FEM simulations. Compared with particle-size related effects, diameter-dependent variation of material properties had a much larger controlling effect on root capacity and stiffness as relevant for plant/tree overturning

Investigation of Fracture Toughness Measurement for Pipeline Steels Based on SE(B) and SE(T) Specimens

This thesis deals with issues related to the experimental determination of the fracture toughness resistance curves, i.e. the J-integral (J)-R curve and crack tip opening displacement (CTOD)-R curves, using the single-edge bend (SE(B)) and single-edge tension (SE(T)) specimens. First, the impact of the crack front curvature on the J-R curve measured from the SE(B) specimen is investigated through systematic linear-elastic and elastic-plastic three-dimensional (3D) finite element analyses (FEA) of SE(B) specimens containing both straight and curved crack fronts. Three average relative crack lengths are considered, namely 0.3, 0.5 and 0.7, and three specimen width-to-thickness ratios are considered: 0.25, 0.5 and 1. The curved crack fronts are characterized by a power-law expression. The analysis results suggest that the crack length evaluated from the CMOD compliance of the SE(B) specimen is insensitive to the crack front curvature and that the impact of the crack front curvature on the experimentally-evaluated J values varies with the specimen configurations. For a given specimen configuration, as the crack front curvature increases, the value of J evaluated based on the test standard ASTM E1820-11 without considering the crack front curvature becomes less conservative and tends to overestimate the actual J. New crack front straightness criteria that are in most cases less stringent than ASTM E1820-11, are recommended. The accuracy of the double clip-on gauge method for experimentally determining CTOD is examined through systematic 3D elastic-plastic large-strain FEA of clamped SE(T) specimens. The relative crack lengths of the specimens range from 0.3 to 0.7, and the thickness-to-width ratios are 0.5, 1 and 2. It is observed that the CTOD values determined from the double clip-on gauge method may involve significant errors. This error primarily depends on the crack length, the material property and the loading level. Based on the analysis results, a modified CTOD evaluation equation is developed to improve the accuracy of CTOD evaluated using the double-clip on gauge method

Numerical modeling of large deformation behaviour of offshore pipelines and risers in soft clay seabeds

Deepwater oil and gas development activities have increased significantly in the last few decades to meet the global demand for energy. One of the key components of these developments is the oil and gas transportation pipeline. Deepwater pipelines are often laid on the seabed and may vertically penetrate into the seabed sediment (primarily clay) or remain suspended in the case of uneven seabed profiles. Partially embedded pipelines might displace laterally during operation due to changes in internal pressure and temperature. The displacement of the pipeline depends on soil resistance, which is also related to the initial embedment. The suspended pipelines might also be impacted by soil blocks moving out from submarine landslides. Moreover, in deepwater, steel catenary risers (SCR)—a long pipe of 150–600 mm typical diameter—are often used to transport hydrocarbon from the seabed production system to floating production facilities. The interaction between soil, water and pipes (partially embedded, suspended or SCR) involves significant large deformations, which cannot be modeled properly using traditional Lagrangian-based finite element (FE) techniques and therefore improved numerical modeling is required for safe and economic design. In the present study, simulations of the large deformation behaviour of deepwater pipelines and SCRs are performed using two numerical approaches. First, simulation is performed using the Coupled Eulerian-Lagrangian (CEL) approach available in the Abaqus FE software. In CEL, the soil is modeled as an Eulerian material that flows through the fixed mesh and therefore numerical issues related to mesh distortion at large displacements are avoided. Simulations are performed for undrained loading conditions implementing a strain-rate and strain-softening dependent undrained shear strength model for clay in Abaqus CEL through user subroutines. For partially embedded pipelines, numerical simulations are performed for vertical penetration and subsequent lateral displacements. In addition, dynamic penetration of the pipeline into a deepwater soft clay seabed is simulated. The penetration and lateral resistances are compared with the results of previous physical model tests, and numerical and analytical solutions. Recognizing the limitations of Abaqus CEL and other FE modeling techniques to simulate the role of water, ANSYS CFX—a finite volume software—is used in the second approach for numerical modeling. A technique is developed to implement strain-rate and strain-softening dependent undrained shear strength of clay in ANSYS CFX. The comparison between penetration resistances obtained from CEL and CFX shows that the latter approach can simulate the effect of water in the cavity formed behind the pipe when it penetrates to a sufficiently large depth into the clay seabed, with a transition between shallow and deep failure mechanisms. In the SCR–seabed–water interaction modeling, in addition to undrained remoulding, the reduction of undrained shear strength due to other factors such as water entrainment is considered using “shear wetting”. Cyclic degradation of penetration and uplift resistance, development of suction under the riser during uplift, and the formation of a trench are successfully simulated for a large number of cyclic motions near the seabed, where a significant shear strength reduction occurs, as reported from physical model tests. The impact force on suspended offshore pipelines by submarine landsides is also simulated using both Abaqus CEL and ANSYS CFX. The development of forces on the pipe with its penetration into the soil block shows that the trapped water behind the pipe influences the failure mechanisms and magnitude of force. The suction in the trapped water and flow of free water through the channel formed behind the pipe is simulated using ANSYS CFX. Based on a comprehensive parametric study with calibration against a series of centrifuge test results, a set of empirical equations are proposed to calculate the impact force on suspended pipelines

Behaviour of Wrinkled Energy Pipes Subjected to Axial Cyclic Induced Fatigue Failure

The energy industry has since been faced with the issue of development of wrinkle defect on buried pipelines located in vicious environment. Although, there have been different literatures on how to assess the integrity of pipelines with structural defects such as; corrosion, dents, buckles and welds, limited research data or guideline(s) are available on how to assess the severity of small wrinkle defects. Failure to assess the severity of these wrinkle defects, especially in buried pipelines located in the regions prone to geotechnical movement and extreme seasonal temperature variation, may lead to shutdown of the pipeline operations, and as result lead to loss of revenue. Therefore, this current study is focused on investigating the failure and behaviour of wrinkled pipes with varying wrinkle geometry, when they are subjected to axial cyclic loads representative of extreme seasonal temperature variation and cyclic freeze-thaw. This research program was conducted using both experimental method and finite element analysis (FEA) based approach. This study shows that a pipe with a wrinkle defect subjected to displacement-controlled axial cyclic loading, may fail by fatigue and result to fracture at the crest of the wrinkle due to localized stress and strain concentration. Additionally, the fatigue life of a wrinkled pipe is dependent on the magnitude of internal pressure applied and the type of cyclic loading applied. A methodology based on the strain life approach used in this study, was considered sufficient in analysing the remaining life of wrinkled pipes subjected to displacement-controlled axial cyclic loading

Coastal Geohazard and Offshore Geotechnics

With rapid developments being made in the exploration of marine resources, coastal geohazard and offshore geotechnics have attracted a great deal of attention from coastal geotechnical engineers, with significant progress being made in recent years. Due to the complicated nature of marine environmnets, there are numerous natural marine geohazard preset throughout the world’s marine areas, e.g., the South China Sea. In addition, damage to offshore infrastructure (e.g., monopiles, bridge piers, etc.) and their supporting installations (pipelines, power transmission cables, etc.) has occurred in the last decades. A better understanding of the fundamental mechanisms and soil behavior of the seabed in marine environments will help engineers in the design and planning processes of coastal geotechnical engineering projects. The purpose of this book is to present the recent advances made in the field of coastal geohazards and offshore geotechnics. The book will provide researchers with information reagrding the recent developments in the field, and possible future developments. The book is composed of eighteen papers, covering three main themes: (1) the mechanisms of fluid–seabed interactions and the instability associated with seabeds when they are under dynamic loading (papers 1–5); (2) evaluation of the stability of marine infrastructure, including pipelines (papers 6–8), piled foundation and bridge piers (papers 9–12), submarine tunnels (paper 13), and other supported foundations (paper 14); and (3) coastal geohazards, including submarine landslides and slope stability (papers 15–16) and other geohazard issues (papers 17–18). The editors hope that this book will functoin as a guide for researchers, scientists, and scholars, as well as practitioners of coastal and offshore engineering