title: Numerical Study of Fluid-Soil-Pipeline Interactions
Creators: name: Chen, Xu
Publisher: name: Griffith University
Year: 2024
Subjects: subject: submarine pipelines
subject: seabed response
subject: scour

description: The stability of submarine pipelines and the surrounding soil is a critical research issue in offshore engineering, especially in complex marine environments. Notably, the primary factors influencing the stability of offshore pipelines are liquefaction and local scour. Thus, the main objective of this project is to explore the mechanisms of soil response and liquefaction induced by waves and currents in the porous seabed adjacent to offshore pipelines. Another goal is to study the local scour around pipelines, taking into account the seabed response. This consideration is crucial as local scour and soil response often interact and combine in real marine environments. It is worth noting that the configurations of dual pipelines, including both the piggyback system and two tandem pipelines, are examined because multiple cylinders have recently become a hot topic for offshore pipelines. A two-dimensional flow-seabed numerical model is proposed. Specifically, the model development incorporates two sub-models: the hydrodynamic model and the seabed model. The hydrodynamic sub-model is governed by the VARANS (Volume-Averaged Reynolds Averaged Navier-Stokes) equation, designed to simulate two-phase incompressible flow motion both outside and within porous media. This model can generate and absorb a wide range of waves in conjunction with ocean currents. The free surface of the flow domain is captured using the Volume of Fluid (VOF) scheme, an Eulerian method. Biot's consolidation equations are employed to solve for the soil responses, with the dynamic wave pressure at the interface between the wave and seabed being considered. This model has been validated against laboratory experimental measurements, demonstrating its reliability in predicting soil responses induced by waves and currents around marine pipelines. Numerical results indicate that the dual pipelines are more likely to be liquefied compared to a single cylinder. Additionally, the configuration of the two pipelines has a significant impact on the seabed response and liquefaction. By adopting the flow-seabed model, a numerical analysis was conducted to study the soil response induced by waves and ocean currents near two tandem pipelines within a trench. The results suggest that the trench structure offers significant protection to the twin pipelines, especially when the trench slope is increased and the backfill thickness is augmented. Following ocean currents can amplify the liquefaction depth beneath the pipelines, whereas opposing currents tend to diminish them. An increased gap ratio between the twin pipes can intensify the instability of the pipelines within the trench. However, when the spacing is sufficiently large, the twin pipelines can essentially be treated as two separate single pipelines. Based on the aforementioned numerical model, this study incorporates a scour model. Herein, the scour sub-model is modified by introducing the modified Shields number and the bed load transport model. As a result, an integrated seepage-scour numerical model, combining the fluid, seabed and scour models, is established to delve deeper into the influence of soil response on the scour process around twin pipelines. The development of the scour process is accelerated, and the maximum scour depth is greater when soil seepage is considered, compared to conditions where soil seepage is neglected. Additionally, as the gap ratio between two parallel pipelines increases, the scour depths around both the upstream and downstream pipelines increase. However, the scour depth around the upstream pipeline is consistently greater than that around the downstream one. Furthermore, the difference in scour depth of the downstream pipeline between the conditions with and without soil response becomes insignificant when compared to the upstream pipe. Moreover, the lower soil permeability and higher degree of saturation tend to accelerate the scour process and result in greater maximum scour depths, as compared to conditions where soil response is not considered.