Numerical modelling of submarine landslide impact on offshore free-spanning pipelines

Abstract

Offshore pipelines are one of the most efficient and reliable modes of transportation of oil and gas. In shallow water conditions, the common practice is to bury the pipeline through trenching and backfilling. However, in deep water environments, pipeline burial through trenching is not very practical or cost-effective; therefore, the pipelines are often laid on the seafloor. Depending upon topography and seafloor environments, sections of as-laid (or surface-laid) pipelines might transform into the free-spanning pipeline. The suspended section of the pipeline might experience the impact of submarine landslides those frequently occur in continental slopes. The impact of debris flow, which originates from submarine landslides and travels in the downslope direction at high speed, might cause severe damage and even break out of these pipelines. Quantifying the impact forces on free-spanning pipeline sections is one of the key requirements in the design. In the present study, debris flow impact is numerically modelled using two software packages: (i) Abaqus finite element (FE) and (ii) ANSYS CFX based on a Computation Fluid Dynamics (CFD) approach. Implementing appropriate models for soil and water together with new approaches for modeling pipe–soil–water interface behaviour, the process of impact, including soft clay flow around the pipe, is successfully simulated using the above-mentioned approaches. Overall, the modelling of this large deformation process is computationally expensive. However, the CFD approach in ANSYS CFX is more computationally efficient than the Coupled Eulerian-Lagrangian (CEL) approach in Abaqus FE software. The role of free-water suction in the channel behind the pipe, the effects of seabed shear strength and gap between pipeline and seabed on drag force are investigated. The drag force depends on not only the shear strength of the debris but also soil flow mechanisms around the pipe, which is influenced by the gap and seabed shear strength.