Numerical modelling of pipeline and riser seabed interaction

Dong, Xiaoyu (2020) Numerical modelling of pipeline and riser seabed interaction. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Subsea risers and pipelines are widely used in offshore industries especially for the production of oil and gas resources. Due to complex subsea environment, a variety of risks are challenging the operation or serviceability life of subsea pipelines and risers. Subsea riser and pipeline-seabed interaction are proven to have significant effect on its performance. This interaction can be modeled by two main approaches, beam-spring, and continuum approach. Beam-spring model provided the most efficient and economical way to estimate the response of soil. While with more explorations in fields, more sophisticated and accurate models are required and thus continuum models are developed to give more details on the soil behavior around the pipe. Two challenging topics in pipeline and riser seabed interaction were selected, 1- the effect of riser-seabed interaction on fatigue life in touchdown zone, 2- the effect of trenching/backfilling on lateral response of buried pipelines. The first one was modelled by beam-spring approach and the second one investigated by continuum approach. The abstracts of the conducted research works are independently discussed below: A.1. Part I Pipeline-Seabed Interaction Subsea pipelines are often protected by burying in the subsea trenches to mitigate the effects of the functional and environmental loads. Depending on the trenching methodology (pre-lay or post-lay trenching), trenching and laying the pipeline may take place at the same time or in a different period of time. Using the excavated material for backfilling of the pipeline is a common practice and a cost-effective solution. Depending on trenching methodology, construction strategy, and environmental loads, the backfilling material may experience different degrees of remolding resulting in a softer material with a range of shear strengths. The difference between the stiffness of the backfill and native material affects the soil failure mechanisms under the lateral pipeline displacement. The relative displacement between the pipeline and the surrounding soil that may occur due to the ground movements, faults, slope instabilities, ice gouging, etc. exerts forces on the pipeline. The amplitude of these forces on the pipeline depends on several parameters, including the submerged weight of the mobilized backfilling and native soil, the horizontal component of shearing resistance offered by interacted soil, and the suction behind the pipe. And the load-displacement curve becomes important in terms of the design of the embedded pipelines. Under different circumstances, trenched pipelines might be displaced at different velocities (could be from millimeters per year to very high), resulting in different drainage conditions (including undrained condition, partially drained condition, and drained condition). Partially drainage condition in pipe-soil interaction has been a very challenging topic since it requires a coupled analysis with the pore fluid pressure to explore the induced excess pore pressure which affects the responses of the pipe, internal soil deformation, and also the failure mechanism in the soil. However, most of the published works only explored the undrained condition of soil. These parameters in turn depend on several parameters such as the properties of the backfill and the native soil, trench geometry, burial depth and confining pressure, pipeline roughness, pipeline size, loading rate (drained/undrained), soil stress history, the backfill extent of consolidation, and the over-consolidation ratio (OCR) of native soil . In this thesis, a coupled large deformation finite element (LDFE) model using re-meshing and interpolation technique with small strain (RITSS) was developed to give prediction of the pipeline force-displacement response together with the computation of the induced excess pore pressure within large deformations. This coupled LDFE model was proven to have advantages in modelling pipe-soil interaction under drained and partially drained conditions using the ABAQUS built-in coupled pore fluid pressure method, which cannot work with the popular existing LDFE method such as Coupled Eulerian Lagrangian (CEL) method. And the LDFE model was proved to be a strong tool for comprehensive investigation of the progressive failure mechanisms around the pipeline considering the varying pipeline-backfill-trench interaction effects. A.2. Part II Riser-Seabed Interaction Steel catenary risers (SCR) are popular amongst the riser families because of their lower cost and technical advantages such as applicability in a wider range of sizes and water depths. The survey results obtained by remote operating vehicles (ROV) have proved the complex non-linear seabed response to riser fluctuations in the touchdown zone (TDZ), where SCR penetrates into the seabed and cyclically creates trenches often with several diameters deep. The oscillatory motions of SCR in the touchdown zone result in a complex riser interaction mechanism with surrounding media including fluid and soil. Some of the influential parameters contributing to these non-linear hysteretic interactions are: soil stiffness degradation under cyclic loads and riser penetration into the seabed, mobilization of suction force within uplift motions of riser, trench base softening and damping, erosive mechanism by water velocity field around the SCR in TDZ and consequent variation of flow pattern of displaced water, the riser dynamics influenced by internal multi-phase flow regimes and also vessel motions (velocity and frequencies), and vortex-induced vibration (VIV). The existing non-linear hysteretic riser-seabed interaction models have been verified in wave-induced fatigue assessment. However, the effect of non-linear seabed interaction on the riser fatigue under riser vibrations has never been examined. In this work, the performance of the non-linear hysteretic models was investigated in slug-induced fatigue damages in touchdown zone which is a key contributor to fatigue damage. For this purpose, first the nodal and global performance of the most popular models was comprehensively examined, and its pros and cons were thus explored. Then an advanced and novel model was developed to simulate the riser slugging and slug-induced fatigue, which has never been done in the past due to extreme complexity. This model was incorporated into slug-induced fatigue analysis and it was indicated that the model was applicable to these type of analysis with acceptable level of accuracies. The research work showed that the slug-induced vibrations can combine with the wave-induced oscillations and create critical case scenarios. Therefore, it is critical to consider the combined effects of slugging and wave in riser fatigue analysis and fill the knowledge gap.

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