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.
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.
Offshore pipeline on-bottom roughness analysis is generally performed to understand pipe stress and areas of free spanning. In this paper, the predicted pipeline profiles, by finite element analysis (FEA) modeling, are compared with survey data. Several case studies are presented and parametric effects are assessed. The FEA modeling procedures are presented in this paper along with several project experiences. The pipeline on-bottom roughness analysis is conducted to determine the pipe stress and the associated fatigue damage due to internal pressure and temperature fluctuation and vortex-induced vibration (VIV) of free span sections due to seabed unevenness. The analysis presented in this paper can aid the design of pipeline free spans and mitigation, if required. The parametric effects of higher and lower bound soil stiffness, friction resistance, pipeline bottom tension, external / internal pressure, and content density are considered. The predicted pipeline profiles by FEA modeling are compared with surveyed profiles under several loading conditions. A strong correlation between survey data and finite element analysis (FEA) is achieved. This paper shows that the FEA stress prediction is adequate, and the FEA modeling is suitable as an advanced design tool for pipeline on-bottom roughness assessment and free span behavior analysis. A commonly used mitigation method for VIV fatigue is to install VIV suppression strakes. For overstresses due to pipeline bending effect, the common mitigation methods are to install mechanical supports, use heavy wall pipe and correct the seabed unevenness among others. It is important to reliably predict the pipeline profiles in order to precisely assess the pipeline free span response and to provide guidance to determine a proper free span mitigation strategy. The FEA method can be utilized to realistically simulate the pipeline on-bottom roughness behavior affected by the pipe properties, pipe-soil interaction including penetration and soil friction resistance, the internal and external pressure, product content, temperature profile, and the bottom tension from pipe-lay. An accurately built FEA model can provide a reliable prediction for the pipeline profile, free spanning length and gap under all conditions, which are crucial for the stress and fatigue assessment as well as for the design of free span mitigation.
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