Numerical modeling of large deformation behaviour of offshore pipelines and risers in soft clay seabeds

Dutta, Sujan (2017) Numerical modeling of large deformation behaviour of offshore pipelines and risers in soft clay seabeds. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Deepwater oil and gas development activities have increased significantly in the last few decades to meet the global demand for energy. One of the key components of these developments is the oil and gas transportation pipeline. Deepwater pipelines are often laid on the seabed and may vertically penetrate into the seabed sediment (primarily clay) or remain suspended in the case of uneven seabed profiles. Partially embedded pipelines might displace laterally during operation due to changes in internal pressure and temperature. The displacement of the pipeline depends on soil resistance, which is also related to the initial embedment. The suspended pipelines might also be impacted by soil blocks moving out from submarine landslides. Moreover, in deepwater, steel catenary risers (SCR)—a long pipe of 150–600 mm typical diameter—are often used to transport hydrocarbon from the seabed production system to floating production facilities. The interaction between soil, water and pipes (partially embedded, suspended or SCR) involves significant large deformations, which cannot be modeled properly using traditional Lagrangian-based finite element (FE) techniques and therefore improved numerical modeling is required for safe and economic design. In the present study, simulations of the large deformation behaviour of deepwater pipelines and SCRs are performed using two numerical approaches. First, simulation is performed using the Coupled Eulerian-Lagrangian (CEL) approach available in the Abaqus FE software. In CEL, the soil is modeled as an Eulerian material that flows through the fixed mesh and therefore numerical issues related to mesh distortion at large displacements are avoided. Simulations are performed for undrained loading conditions implementing a strain-rate and strain-softening dependent undrained shear strength model for clay in Abaqus CEL through user subroutines. For partially embedded pipelines, numerical simulations are performed for vertical penetration and subsequent lateral displacements. In addition, dynamic penetration of the pipeline into a deepwater soft clay seabed is simulated. The penetration and lateral resistances are compared with the results of previous physical model tests, and numerical and analytical solutions. Recognizing the limitations of Abaqus CEL and other FE modeling techniques to simulate the role of water, ANSYS CFX—a finite volume software—is used in the second approach for numerical modeling. A technique is developed to implement strain-rate and strain-softening dependent undrained shear strength of clay in ANSYS CFX. The comparison between penetration resistances obtained from CEL and CFX shows that the latter approach can simulate the effect of water in the cavity formed behind the pipe when it penetrates to a sufficiently large depth into the clay seabed, with a transition between shallow and deep failure mechanisms. In the SCR–seabed–water interaction modeling, in addition to undrained remoulding, the reduction of undrained shear strength due to other factors such as water entrainment is considered using “shear wetting”. Cyclic degradation of penetration and uplift resistance, development of suction under the riser during uplift, and the formation of a trench are successfully simulated for a large number of cyclic motions near the seabed, where a significant shear strength reduction occurs, as reported from physical model tests. The impact force on suspended offshore pipelines by submarine landsides is also simulated using both Abaqus CEL and ANSYS CFX. The development of forces on the pipe with its penetration into the soil block shows that the trapped water behind the pipe influences the failure mechanisms and magnitude of force. The suction in the trapped water and flow of free water through the channel formed behind the pipe is simulated using ANSYS CFX. Based on a comprehensive parametric study with calibration against a series of centrifuge test results, a set of empirical equations are proposed to calculate the impact force on suspended pipelines.

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