Pipelines in the arctic offshore must be installed and buried below the seabed to avoid direct contact, and to mitigate the effects of strains induced by soil displacement below the ice keel scour depth. A three-dimensional (3D) finite element (FE) model that utilizes the Coupled Eulerian Lagrangian (CEL) formulation has been developed to provide direct and explicit estimation of pipe stresses and strains. The CEL formulation is novel, and no published work has attempted to explore its capabilities and potential for ice scour modeling to date. The developed model will be helpful in solving some of the uncertainty regarding pipeline burial depth, potentially resulting in major trenching cost savings. In order to gain confidence in this numerical modeling technique, a systematic validation effort was carried out, whereby numerical predictions of subgouge displacements were compared with measured data from centrifuge tests and other published empirical and numerical data. Sensitivity analyses were then performed to investigate the effect of the scouring keel geometry, depth, and attack angle on the induced subgouge soil displacements. Preliminary conclusions were drawn and presented in this paper.
Offshore pipelines in ice environments may be subjected to unique geohazards such as seabed ice gouging. These events involve nonlinear processes including large deformations and strains, contact mechanics, and failure mechanisms. Current pipeline engineering design practice employs decoupled, structural finite element modelling procedures to assess system demand and capacity. The inherent error and uncertainty within this approach drives conservative engineered solutions. Physical modelling and continuum numerical simulation tools complement this engineering framework to improve confidence in predicted outcomes. The relative performance of engineering models, used in current practice, and numerical simulation tools, including structural and continuum finite element modelling procedures, to predict the deformation and strain response of a buried pipeline subjected to an ice gouge event is examined. Refinements to the numerical modeling procedures and establishment of a consistent and compatible reference framework for the performance evaluation differentiate this study from others, which are subsets of the current investigation. For the parameter analysis conducted, within an equivalent reference framework, the outcomes demonstrate key factors, including superposition error and directional load decoupling, that influence model error that may not be as significant as previously considered. The scope and extent of this outcome is not fully understood and requires further investigations to delineate the significance across a wider parameter range.
With the majority of estimated Arctic oil and gas reserves being held offshore, ice gouging will likely be a major consideration in the design of transport pipelines in these regions. The implications of the effects of ice gouging on buried pipelines are well understood. The ability to model this phenomenon using advanced numerical simulation tools has been proven in recent years, and is demonstrated in this paper. The uncertainty that revolves around these tools, due to limitations in the available physical dataset that can be utilized to validate the results, is discussed. In this paper, a limited parametric study on the influence of ice keel attack angle and interface strength on the free-field subgouge displacement field, and subsequent effects on a buried pipeline is presented. The Coupled Eulerian Lagrangian finite element formulation available in ABAQUS/Explicit and the Arbitrary Lagrangian Eulerian formulation in LS-DYNA are used to conduct the numerical experiment. The results are shown and the observations are discussed in detail. Finally, an assessment in terms of the challenges of implementing the numerical tools in an engineering application is provided. Introduction Energy demand has promoted renewed interest in the exploration and field development of offshore hydrocarbon basins in the arctic and ice covered waters of the northern hemisphere. In these harsh environments, pipelines offer a safe and cost effective mode to transport hydrocarbon resources to the marketplace. The presence of ice features and potential interaction with the seabed impose significant engineering challenges for design, construction and operation of subsea pipelines. Key technical issues relate to establishing pipeline mechanical performance criteria and trenching requirements for pipeline protection that meet target safety levels and satisfy logistical and economic constraints. Ice gouge events involve ice keel/seabed interaction, soil failure mechanisms, soil clearing processes and subgouge soil deformations [1,2]. As the magnitude of ice keel/seabed reaction forces can be an order greater than other conventional load events; such as anchor dragging and pullover, then direct ice/pipeline contact is not a viable design option for most ice gouge scenarios [3]. In the 1970's, the use of extremal statistics was considered sufficient to estimate the design gouge depth and corresponding trench depth requirements. Later, in the 1990's, it was realized this approach to avoid direct ice/pipeline contact was incomplete. This conclusion was primarily based on physical modelling studies highlighting the importance of subgouge soil deformations and potential effects on the pipeline mechanical response [4,5]. Early continuum finite element (FE) modelling investigations on ice gouge events did not achieve the research objectives due to technology constraints [1,6]. Two-dimensional simulation models were not representative of the ice gouge clearing processes, and three-dimensional analysis encountered numerical instability due to excessive element distortion.
In recent years, pipe-in-pipe (PiP) systems have been employed in an increasing number of subsea projects. According to the previous studies, the external pressure required to develop the initial local buckle on the PiP system is significantly higher than the pressure required to propagate the buckle along the system. In this respect, it is reasonable to investigate a novel topic where the propagation of buckle is induced by a lateral interference load instead of external pressure (e.g., diagonal fishing gear impact). On this subject, the recent studies showed the progression of plastic damage along a single-walled pipe, which is induced by a lateral load, could significantly lower the load-carrying capacity of the pipe. The present study investigates this finding for a PiP solution under a two-phase loading condition: in phase 1, the PiP solution is subject to 75 mm perpendicular indentation, and in phase 2, the resulting plastic damage in phase 1 is translated and induced longitudinally along the PiP system. Furthermore, using finite element analyses, the effect of combined loading (axial and lateral load) on the load-carrying capacity of the PiP specimen is investigated. The test results show that upon the initiation of damage progression, the load-carrying capacity of the PiP specimen (against the lateral indentation) declines by 10%. Also, the numerical results show that the structural resistance of a PiP specimen against a lateral indentation drops significantly when the inner pipe is subject to axial compression.
Thermal expansion and global buckling is a critical design aspect for subsea flowline systems subjected to high pressure and high temperature (HPHT). In the Gulf of Mexico, HPHT oil/gas production is becoming exceedingly common as drilling and production depths extend deeper. Advanced finite element analysis becomes essential for flowline expansion and buckling design which is highly dependent on pipe-soil interaction behavior. For decades, pipe-soil interaction has been the focus of many research studies and joint industry projects. For HPHT flowline systems, thermal mitigation is decisive for safe design. Thermal mitigation acts to control global buckling at designate locations and avoid buckling in unknown locations. Thermal mitigation results in significant cost savings by lowering the welding class besides the buckling locations and increases safety in terms of local buckling, fracture, and fatigue. One widely used thermal mitigation method involves attaching a buoyancy module around a segment of the flowline. In this paper the Coupled Eulerian Langrangian (CEL) finite element (FE) formulation is utilized to simulate the interaction between soil and the thermal mitigation buoyancy module (TMBM). The paper demonstrates the capability of the CEL FE method to simulate large soil deformation without the numerical difficulties that are commonly associated with other numerical formulations e.g. ALE (Arbitrary Lagrangian Eulerian) or more conventional Lagrangian. Initially, a three dimensional (3D), continuum, FE model is used to establish the variation of initial embedment along the length of the buoyancy and adjoining pipe. The study then establishes the lateral displacement/resistance relationships under different levels of contact pressure and soil embedment for a series of buoyancy-soil interaction segments, also using the CEL FE method. Current practice for global pipeline thermal expansion FEA is to utilize the same friction model for both buoyancy-soil interaction and pipe-soil interaction. The obtained buoyancy-soil interaction model from the current study is to be used as input to the global FE model to more precisely simulate flowline lateral buckling behavior. This paper presents a practical application of the current state of the art in modeling large soil deformations in providing an improved approach for modeling buoyancy-soil interactions in the global FEA of pipeline thermal expansion and lateral buckling.
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