Basel Abdalla

Development and Validation of a Coupled Eulerian Lagrangian Finite Element Ice Scour Model

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.Copyright

Permafrost Thawing-Pipeline Interaction Advanced Finite Element Model

The increasing energy demand has promoted the interest in exploration and field development in the Arctic waters, which holds one quarter of the world’s petroleum reserves. The harsh conditions and fragile environment in the arctic region introduce many challenges to a sustainable development of these resources. One of the key challenges is the engineering consideration of warm pipelines installed in permafrost areas; found mainly in shallow waters and shore crossings. Evaluations have to be made during the pipeline design to avoid significant thaw settlement and large-scale permafrost degrading. In this paper, a three-dimensional (3D) finite element (FE) model was developed to study the interaction between buried pipelines transporting warm hydrocarbons and the surrounding permafrost. This interaction is a combination of several mechanisms: heat transfer from the pipeline, results in permafrost thawing and formation of thaw bulb around the pipeline. Consequently, the thaw settlement of soil beneath the pipeline base results in bending strains in the pipe wall. For safe operations, the pipe should be designed so that the induced strains do not exceed the ultimate limit state conditions. The developed model helps in accurate prediction of pipe strains by using finite element continuum modeling method as opposed to the more commonly used discrete (springs) modeling and hand calculations. It also assesses the real size of the thaw bulb and the corresponding settlement at any time, thus preventing an over-conservative design.© 2009 ASME

The technical challenges of designing oil and gas pipelines in the arctic

The world demand of oil and gas is growing at an ever increasing rate, and as a result, there is a demand to explore new areas for more petroleum production. The arctic region is one of the remaining unexplored areas where such exploration still can be done. According to the US Geological Survey estimates, the arctic region, mostly offshore, holds as much as 25% of the worlds untapped reserve of hydrocarbons where much of the reserve is lying under seasonal or year-round sea ice. The exploitation of these remaining reserves, however, will depend upon meeting the technical challenges of design, construction, and operation of offshore installations. Despite some experience with Arctic oil and gas exploration and production during the last three decades, technology gaps still exist and will have to be bridged in order to enable optimized developments to proceed. In this paper, technical design difficulties particular to arctic pipelines are presented; these include ice gouging, permafrost thaw settlement, strudel scour, and upheaval buckling. An emphasis is then placed on advanced finite element techniques that can be used to address these issues, with an example of such techniques illustrating their ability to model highly complex and nonlinear phenomena.

Numerical Study of Thermosyphon Protection for Frost Heave

Thaw subsidence and frost heave are two different hazards to pipelines in arctic regions. The former is due to the thawing of permafrost induced by a warm pipeline, while the latter is resulted from a cold buried pipeline that causes ice lens growth upon freezing in the direction of heat loss. Some pipelines may be operated in a wide temperature range and thus subjected to both types of threats.Two-phase closed thermosyphons have been employed extensively in Arctic projects to protect the permafrost from thawing. The thermosyphons’ response as a “thermo-diode” is the key to this technology. This paper presents a finite element analysis (FEA) based feasibility study for using thermosyphons with pipelines in arctic regions to reduce the potential for frost heave. There are two major challenges in the numerical simulation. One is the efficient modeling of a thermosyphons which works as a heat pump in winter and stops working in summer. This study proposes an anisotropic conduction model that simplifies the thermal-fluid processes within the thermosyphon without overwhelming computational cost. The other challenge is the frost heave modeling, which was recently achieved based on the framework of the porosity rate function. New developments involved in this paper include the extended application to permafrost and transient temperature boundary conditions.The outcome of this work proves the value of using thermosyphons with pipelines that transfer both cold product. The method introduced here can also be used to optimize the design of new infrastructure and pipelines in permafrost, as well as to assess how thermosyphons work as a mitigation method in existing projects that are affected by frost heave.© 2015 ASME

Extended Porosity Rate Function for Frost Heave

Frost heave is a common phenomenon in the Arctic, where soil expands in the direction of heat loss due to ice lens growth upon freezing. It also occurs if a refrigerated structure is buried in unfrozen frost heave-susceptible soil, and thus special considerations are required when designing chilled or LNG pipelines in the Arctic. In the past decades, many theoretical and numerical methods have been developed to predict the frost heave of freezing soil. Among them, the rigid ice model, segregation potential model, and porosity rate function model are the most popular. These frost heave models work well in predicting the soil response during a pure freezing process, but none of these methods consider a thawing and consolidation of soil, which is the opposite but integrated process when the system undergoes the annual temperature cycle.In this study, efforts are made to extend the porosity rate function to the thawing branch based on reasonable assumptions. With the extended model, a fluctuating surface temperature can be applied on top of the soil surface to simulate a continuous changing ambient temperature. The extended model is realized in ABAQUS with user defined subroutines. It is also validated with test data available in the public domain. As an application example, the extended model is utilized to simulate a chilled gas line buried in frost-susceptible soil to estimate its frost heave over a multi-year operation.Copyright

Understanding Hybrid Subsea Foundation Design

Hybrid subsea foundations (HSF) are combined foundation systems of mudmats and piles. The primary motivation of combining these two foundation types is to provide greater resistance to large horizontal loads in addition to vertical loads, for which use of mudmats alone will require it to be of impractically large size. The contribution from the piles in the lateral capacity helps to limit the size of the mudmat, which is critical in subsea environment. In a brownfield situation, this is sometimes a hard limit with only limited space available to place a new mudmat in the existing field layout. Also, in some cases, the HSF may prove to be a more economical option for resisting large horizontal loads compared to, for example, to suction piles. While the authors are aware of some scattered project-specific design and use of subsea mudmat-pile hybrid foundations by individual contractors and operators, there is no industry-wide publicly known best practice currently available. These designs of HSF appear to be generally based on simplified analytical approach that require superimposition of conventional shallow and deep foundation capacity calculation methods, hence violates the static and kinematic compatibility requirements fundamental for a sound and robust prediction procedure.This paper attempts to provide some insight into the behavior of mudmat-pile foundations as a hybrid integrated system numerically using finite element modeling and analysis (FEA). The interactions between the mudmat and the piles in an HSF are complex and hence a FEA-based approach is considered most suitable. The FEA model in this study included the mudmat, the corner piles, the pile-mudmat connections and the seabed soil. Sensitivity of the HSF capacity to the size of the piles (length and diameter), the connection type of the piles to the mudmat, and the number of piles are selectively investigated and the results presented. Based on these results some pertinent observations relevant to design of HSFs are also given.While the study is of limited scope, it offers important insights into the effects of the primary design variables on HSF’s capacities. Therefore, the authors hope the information herein will be of benefit to practicing subsea engineers who might have to face choices to consider mudmat-pile hybrid foundations as a real option for their projects.Copyright

FEA-Based Stability Analysis of Mudmats: Coupled Soil-Structure-Flowline Interaction Model

The traditional method of foundation stability assessment for subsea structures is to calculate the bearing capacity factor of safety using classical approach given in the API-RP-2A/2GEO. This classical approach can be overly conservative for foundations under complex loading conditions (e.g., multiple interacting loads). A typical example is pipeline end manifold or flowline sled, which can be subject to self-weight, structure-soil interaction, and multiple interface loads from flowline and jumpers under operational condition.A more rigorous 3D-FEA based assessment approach is developed in this paper to achieve more accurate bearing capacity estimates for a flowline sled supported by mudmat. This fully combined global model comprises the structure (with sliding mechanism), soil foundation, jumpers, and flowline as realistically as possible so as to capture the more accurate interactions among the different parts of whole sled-soil system. The use of such advanced numerical modeling has proven to improve the mudmat bearing capacity factor of safety.© 2013 ASME