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Multilayered flexible subsea pipelines may experience significant lateral movements due to manmade and environmental geohazards. These pipelines incorporate several structural and protective layers to resist different loads, and may require additional protection such as trenching, rock placement, or burial. In practice, simplifications are considered due to the complexities and uncertainties involved in the multi-layer pipe structure and the surrounding soil, compromising the pipe structure or the soil behavior. These simplifications are applied either on the pipe by assuming a rigid section or on the soil by representing it as elastic springs, which may result in inaccuracies. This study proposes a decoupled methodology combining the Coupled Eulerian–Lagrangian (CEL) model for soil displacement with a small-strain finite element analysis of the flexible pipe. This approach aims to accurately capture cross-sectional deformations and local stresses due to soil movement while maintaining reasonable computational effort. A parametric analysis was conducted to assess the impact of several variables on failure risk. The deformed cross-section was then used for a collapse analysis to determine critical loads at maximum operational depth. The study showed that modeling parameters such as soil strength and internal diameter might significantly influence pipe failure and the risk of collapse. [ABSTRACT FROM AUTHOR]

Prediction of subgouge soil deformation during an ice gouging event is a challenging design factor in Arctic subsea pipelines. An accurate assessment of ice keel-seabed interaction requires expensive model testing and large deformation finite element analysis. Proposing reliable analytical/empirical solutions needs a deep understanding of the key parameters governing the problem. In this study, dimensional analysis of subgouge soil deformations was conducted and eight dimensionless groups of parameters were identified to facilitate proposing potential new solutions. A comprehensive dataset was established for horizontal and vertical subgouge deformations in both sand and clay seabed. Using the identified dimensionless groups, linear regression (LR) models were developed to estimate the horizontal and vertical deformation. Moreover, a sensitivity analysis (SA), as well as an uncertainty analysis (UA), was carried out to identify the superior LR models and the most influential parameter group. A high range of correlation coefficient (R), Nash-Sutcliffe efficiency coefficient (NSC), and variance accounted for (VAF) along with a low range of errors was achieved for the best LR model. The results of the superior LR models were also compared with the existing empirical equations. The study showed that the shear strength parameters of the seabed soil and the ratio of gouge depth to gouge width are the governing dimensionless parameters to model the horizontal and vertical subgouge soil deformations. [ABSTRACT FROM AUTHOR]

Non-linear hysteretic riser-seabed interaction models have been developed and implemented into business software packages within recent years to simulate the riser penetration into the seabed and its influence on fatigue life in the touchdown zone (TDZ). These models have shown significant impact on ultimate fatigue damage and users shall take caution while using the models paying particular attention to the selection of model parameters. The oscillation of steel catenary riser (SCR) in the touchdown zone can be quite complex, where neighbor nodes go under different episodes and magnitudes of penetration and uplift at the same time. Therefore, it is necessary to evaluate the non-linear soil models consistency in nodal performance. This paper comprehensively examined the nodal performance of a popular non-linear hysteretic riser-seabed interaction model through developing a global numerical riser model in ABAQUS and a user-defined subroutine (UEL). The model shows a dominantly strong nodal performance. However, nodal response violations and model malfunctioning were observed in the proximity of trench bottom towards the vessel, which is the most fatigue prone section of SCR in the touchdown zone. Also, it was identified that the model over-estimates the penetration and suction resistance and consequently the fatigue damage in the TDZ. [ABSTRACT FROM AUTHOR]

• Full 3D, nonlinear FE model, with frictional contacts between elements. • Parametric analysis of the damage size effect on tensile armor buckling. • Dynamic explicit analysis, taking into account effects like snap-through. • Damaged outer sheath's impact on compressive critical load is small. • A cut on the external sheath may influence buckling location and act as trigger. A fully three-dimensional finite element model was developed in order to study the axial compressive response of flexible pipes. The model is parameterized and features five layers, with emphasis on the layers responsible for reacting to axial loads. Tensile armor instability may occur in the form of lateral buckling and radial buckling, also known as "birdcaging", which is influenced by several parameters, including defects on the layers that are supposed to restrain and protect the tensile armor, more specifically, the external polymeric layer. By being the outermost layer, the external sheath is responsible for keeping the pipe watertight and it is the most likely to sustain damage from launching, collision from vessels, anchors or ice structures and even damage caused by sea creatures. This article focuses on the effect of damage on the external polymeric layer on the tensile armor instability failure modes and uses a finite element model to simulate that phenomenon. For that purpose, the outermost layer was deliberately damaged with cuts on a key position. The cut dimension is parameterized for this analysis and the results will show that, if the flexible pipe is equipped with a high strength layer and the differential pressure is not taken into account, the external polymeric layer will not play an effective part on protecting the tensile armor layers from instability. However, it may influence the triggering of the instability on a specific position of the pipe. [ABSTRACT FROM AUTHOR]

Abstract: The subsea survey results using remote operating vehicles (ROV) show that trenches with a depth of several riser diameters can be developed underneath the steel catenary risers (SCR). Therefore, an important question in respect of the riser–seabed interaction is, how the trench formation beneath the riser affects the riser fatigue performance in the touchdown zone. A common methodology reported in literature to study the impact of trench formation on riser fatigue life is the insertion of an artificial mathematical expression of the riser profile into the seabed. This study shows that such methodology can be inconsistent and leading to contradictory results. The current paper has employed ABAQUS finite element software and coded a non-linear soil hysteretic model to automatically simulate the variable seabed stiffness and the gradual trench development through the touchdown zone. In this method, the seabed model parameters are initially adjusted to extreme values allowing trench with desired depth to be developed over a moderate number of displacement cycles of the SCR. The design wave scatter diagram is then applied, simulating a generic Spar system, after switching the model parameters to values with normal range. The paper presents the impact of trenches of different depths on the fatigue performance of SCRs in the touchdown zone. [Copyright &y& Elsevier]

To study the compressive behavior of flexible pipes and tensile armor instability, a nonlinear Finite Element model was developed. This fully tridimensional model recreates a five layer unbonded flexible pipe. When the pipe is submitted to compressive loads, tensile armor instability may occur in the form of lateral buckling or radial buckling, also known as "birdcaging". This complex failure mode may be influenced by several parameters, including defects on the layers that are supposed to restrain and protect the tensile armor, more specifically, the high strength tape. This layer is usually made of high a performance textile material that is not directly exposed to the environment. However, damage may be sustained when the outermost layer is subjected to deep cuts and breached. This article focuses on the effects of a damaged high strength tape and its implications on the tensile armor instability failure modes. The study shows that damage in the high strength tape may compromise the cohesion between the layers, significantly reducing the critical instability load and the flexible pipe's stiffness, as well as altering the yielding for the layers. • Full 3D, nonlinear FE model, with frictional contacts between elements. • High strength tape with orthotropic elastic/plastic material model. • Dynamic explicit analysis, taking into account effects like snap-through. • The integrity of the tape is of vital importance for tensile armor instability. • The variation of the dimension of the damage constitutes a limit value problem. [ABSTRACT FROM AUTHOR]

Recent discovered oil and gases in the Arctic area have heightened the need for more attention to ice-seabed interaction during an ice scouring event. The seabed is gouged by these drifting icebergs in warmer months threatening the subsea pipelines transferring the hydrocarbons from offshore to onshore. The simulation of ice scouring seabed needs costly large deformation finite element analysis for the guaranteed operational integrity of the subsea pipelines. In this paper, a cost-effective alternative approach using the self-adaptive evolutionary extreme learning machine (SaE-ELM) algorithm was taken to model the ice-induced seabed scour. Initially, using parameters governing the ice gouging process, 17 SaE-ELM models were developed. Then, a comprehensive dataset was established and properly allocated for training and testing of the developed models. The optimal number of hidden layer neurons and the best activation function were opted for the SaE-ELM network. The premium SaE-ELM models and the most influencing inputs were recognized by conducting a sensitivity analysis. The vertical component of load showed a significant impact on the reaction forces, rather the soil depth and berm height possessed a striking effect for modeling the soil displacements. Ultimately, a set of the SaE-ELM-based equations were presented to estimate the subgouge soil parameters. [Display omitted] • Arctic subsea pipelines are buried for physical protection against the ice scour. • Determining the minimum burial depth needs accurate modelling of the ice-seabed interaction. • Costly and sophisticated are usually required for assessment of the ice-seabed interaction. • A self-adaptive evolutionary extreme learning machine model was developed to model the ice-seabed interaction. • The developed model can be an accurate and cost-effective alternative to estimate the ice-induced subgouge deformations. [ABSTRACT FROM AUTHOR]

[Display omitted] Large lateral displacement of trenched subsea pipelines may be induced by ground movement, landslides, ice gouging, etc. Pre-excavated seabed soil is usually used for backfilling as a cost-effective solution. The large difference between the shear strength of the highly remoulded backfill material and the native soil may significantly affect the pipeline-backfill-trench interaction, the failure mechanisms, and consequently the lateral soil resistance. However, this challenging and less-explored aspect has not been covered thoroughly in design codes. In this paper, the influences of pipeline-backfill-trench interaction on the failure mechanism and resultant lateral soil resistance were investigated by large deformation finite element analyses. Two different methods were considered and compared, including the remeshing and interpolation technique with small strain (RITSS) and coupled Eulerian-Lagrangian (CEL) methods, to assess their relative merits for this problem. A modified Tresca model considering strain-softening effects was incorporated and a parametric study was conducted to investigate the influences of key factors including the trench geometry, stiffness of backfill material, native seabed soil properties, burial depth, and intensity of pipeline-trench bed interaction. The study showed that the ignorance of the pipeline-backfill-trench interaction by using uniform soil may result in underestimation and overestimation of the lateral soil resistance against the pipeline moving inside the backfill and into the trench wall, respectively. [ABSTRACT FROM AUTHOR]

In ice gouging analyses, the seabed is often simplified as a uniform material domain, over-looking the potential complexities inherent in layered seabed formations, which are wide-spread in numerous Arctic regions. This study investigates the pipeline response into the distinct seabed configurations, including layered soft over stiff clay, layered stiff over soft clay, and uniformly soft and uniformly stiff compositions, to ice gouging. This was achieved through comprehensive large deformation finite element (LDFE) analysis, employing a Coupled Eulerian Lagrangian (CEL) algorithm. Incorporation of the strain-rate dependency and strain-softening effects involved the development of a user-defined subroutine and incremental update of the undrained shear strength within the Abaqus software. The pipeline is explicitly incorporated into the model, allowing for a detailed investigation of its response to ice gouging phenomena within these layered seabed scenarios. This research demonstrates that simplifying a layered seabed into a uniform can be misleading. Such an approach can lead to significant errors, potentially overestimating or underestimating pipeline response to ice gouging. By incorporating these complexities, engineers can develop more optimized designs with enhanced safety margins in Arctic environments prone to ice gouging phenomena. • Arctic subsea pipelines are buried for physical protection against the ice scour. • Determining the minimum burial depth needs accurate modelling of the ice-seabed interaction. • Layered seabed strata are not covered in design codes and recommended practices. • The effects of layered seabed on buried pipeline Response to ice gouging was investigated. • Study reveals simplifying layered seabed to uniform domain yields inaccurate prediction of pipeline response to ice gouging. [ABSTRACT FROM AUTHOR]

Large lateral displacement of trenched multilayer pipelines may be induced by ground movement, landslides, ice gouging, etc. The significant difference between the shear strength of the backfill material and the native soil may affect the pipeline-backfill-trench interaction, the failure mechanisms, and consequently the lateral soil resistance. However, this challenging and less-explored aspect has not been covered thoroughly in design codes, even for rigid pipes. This paper investigates the influences of multilayer pipeline-backfill-trench interaction on the deformation of pipe section, soil failure mechanism, resultant lateral soil resistance, and soil pressure on the pipe by large deformation finite element analyses. A decoupled model consisting a Coupled Eulerian-Lagrangian (CEL) approach with a rigid pipe in ABAQUS was created along with a finite element (FE) model of multilayer flexible pipe in ANSYS to assess their relative merits for this problem. A parametric study was conducted to investigate the influences of key factors, including the trench geometry, stiffness of backfill material, native seabed soil properties, burial depth, and intensity of pipeline-trench bed interaction. The study showed that the ignorance of the pipeline-backfill-trench interaction by using uniform soil might result in underestimation and overestimation of the lateral soil resistance at different displacement amplitudes. • Multilayer flexible pipelines are endangered by subsea geohazards. • Full 3D, nonlinear FE model of pipe, with frictional contacts between elements. • Advanced non-linear lateral soil interaction effects in clay. • A decoupled analysis to incorporate the trenching-backfilling effects in pipe response. • Simplification of the soil or pipe may significantly influence the pipe failure. [ABSTRACT FROM AUTHOR]