This paper presents numerical results from three-dimensional Large Eddy Simulations (LES) of steady flow around a pipeline close to an uneven seabed with staggered gaps underneath it. It was found that even small gaps below a pipeline can lead to great changes in the hydrodynamics along the span, as well as the seabed shear stress. It is observed that the total forces can be calculated by independently considering the gap section and the embedded section; however, the spanwise variation in the configuration leads to much energetic vibrations in sectional forces. The instantaneous flow feature across the span, however, appears to be less affected by the gaps, where a coherent turbulent structure is found to develop in the upstream and break down above the pipeline.


The research on pipeline hydrodynamics has been largely driven by the wide use of pipelines for transporting oil and gas products across ocean floors, which has been well documented in the monograph by Sumer & Fredsøe (1997) and papers by Bearman & Zdravkovich (1978), Zdravkovich (1985) and Lei, Cheng & Kavanagh (1999), among others. Most of the research was concerned with pipelines either embedded in or spanned above the seabed uniformly along the pipeline. Due to either uneven seabed or local scour below the pipeline, however pipeline/seabed contacts are unlikely uniform. It is expected that the non-uniformity of pipeline-seabed configuration will affect the hydrodynamic forces on the pipeline and the associated flow features around the pipeline. To date, little research has been reported with regard to this issue. This motivated the present study.

Problem Definition

As illustrated in Fig. 1, this paper investigates steady flow around a pipeline (represented by a circular cylinder) laid on an uneven seabed, mimicking a situation where the pipeline is partially embedded at certain locations, while free spanned at other locations.

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