Session: 04-03-02 Rigid Pipelines II

Paper Number: 79292

79292 - The Impact of Extreme Turbidity Flow on Subsea Pipeline Design in a Deepwater Canyon Crossing 

As pipelines and related subsea developments are being proposed for frontier areas, it has been increasingly necessary to route them through what would normally be considered to be sub-optimal areas, such as deep-water canyons, along the continental slope, or through areas where unavoidable geohazards may exist. Subsea landslides due to seismic events or wave/current erosion of the canyon cliff may cause a debris flow and, subsequently, a turbidity flow. These flows are amongst the most significant geohazards to pipelines and umbilicals crossing the canyon.  During a subsea landslide, pipelines may experience varying impact loadings, from intact soil at the very early stage of the incident to later fully-fluidized conditions (i.e., debris flows and turbidity currents).

When subjected to turbidity flow, pipelines may experience destructive impact loadings and lateral deformations, as the flow velocities for such events can be significantly larger than those associated with deepwater bottom currents. The turbidity current loading is typically applied over some defined length and height, while the deformation of pipeline is restrained by transverse and longitudinal soil resistances in adjacent passive zones. In some cases, the pipeline may come to a stable deformed shape where the continued active hydrodynamic loading is equilibrated by the tension in the pipeline and the passive soil resistance in the adjacent zones. To optimize pipeline routes, mitigate subsea geohazards, and ensure pipeline integrity, it is of crucial importance to understand the responses of pipelines subjected to potential flow impacts and to assess the efficacy of potential mitigations. If the pipeline is in sandy soil, the associated potential soil scouring and resulting long spans will lead to a very challenging pipeline design.

In this paper, finite element (FE) analyses were performed with the commercial software ABAQUS to investigate pipeline behavior when subjected to turbidity flows.  A fluid-mechanics-based approach to soil and fluid was used to simulate the hydrodynamic loads imparted to the pipeline due to turbidity currents. By applying active flow loading over certain lengths and having a given velocity, and with axial and lateral soil resistances applied to the zones just outside of the turbidity flow, assessments of the pipeline were performed, including the local buckling check, lateral deflection, stress/strain in the pipeline, and the anchor load increase at subsea structures in the close vicinity. Particular attention was paid to the overall responses of the spanning pipeline under various configurations of subsea turbidity flow impact. Different analysis techniques were found suitable for various turbidity flow mitigation options.

Although the solution is limited by some of the idealizations, this sensitivity-based approach that focuses on a small number of key parameters provides a useful starting point for pipeline design.  Both static and dynamic non-linear FE models are utilized to investigate the behavior of the pipeline when subjected to the turbidity flow both with and without mitigation, and an appropriate analysis methodology is established for detailed pipeline design.

Presenting Author: Qiang Bai TechnipFMC

Authors:

Qiang Bai TechnipFMC
Xinhai Qi Genesis
William Pearl TechnipFMC
Joanne Shen TechnipFMC
Mark Brunner TechnipFMC