Gravity anchors astride subsea pipelines subject to oscillatory and combined steady and oscillatory flows

Xu Zhao, Liang Cheng, Ming Zhao, Hongwei An, Wei He

    Research output: Chapter in Book / Conference PaperConference Paperpeer-review

    3 Citations (Scopus)

    Abstract

    This writing presents results of simulating oscillatory and combined steady and oscillatory flows past gravity anchors astride subsea pipelines. It can be considered a companion to a previous numerical study on steady currents past gravity anchors. The gravity anchor system comprises large arch-shaped concrete blocks positioned at intervals astride offshore pipelines, and it is engineered to provide innovative and cost-effective secondary stabilisation for high-capacity gas-transporting pipelines serving in severe metocean conditions, eg. cyclone-prone offshore areas. A free-settling marine object bottom-seated on the seabed, however, the gravity anchor may subside into scour pits around its base due to locally disturbed flow regimes, imposing integrity risks on the pipe. Also, the effect of gravity anchors on hydrodynamic loading on nearby pipeline lengths is of interest. The present study employed a Petrov-Galerkin finite element method to solve the three-dimensional Navier-Stokes equations in direct numerical simulation. Firstly sinusoidal flow oscillating perpendicularly to the pipe beneath gravity anchors on an immobile bed was simulated at a Keulegan-Carpenter number of 10 and a pipe Reynolds number of 1000. Then, a steady current co-directionally superimposed on the aforementioned oscillatory flow was modelled at a ratio of steady current velocity to oscillatory flow velocity amplitude of 1. With sediment transport capacity related to bed shear stresses, the time-averaged bed shear stress amplification around gravity anchors in oscillatory flow was revealed first, and found to be consistent with laboratory observations of scour patterns. The effect of superimposing steady flow onto oscillatory flow on bed shear stress amplification was then explored. Lastly, hydrodynamic forces on pipelines in the vicinity of gravity anchors were gauged. The present work intends to shed light on the initial seabed responses with regard to the scour process around gravity anchors immersed in the oceanic wave boundary layers, as well as the effect of gravity anchors on hydrodynamic loadings on pipelines.
    Original languageEnglish
    Title of host publicationProceedings of the ASME 2012 31st International Conference on Ocean, Offshore and Arctic Engineering: OMAE2012: July 1-6, 2012, Rio de Janeiro, Brazil
    PublisherASME
    Pages597-604
    Number of pages8
    ISBN (Print)9780791844922
    DOIs
    Publication statusPublished - 2012
    EventInternational Conference on Ocean_Offshore and Arctic Engineering -
    Duration: 1 Jul 2012 → …

    Conference

    ConferenceInternational Conference on Ocean_Offshore and Arctic Engineering
    Period1/07/12 → …

    Keywords

    • Navier-Stokes equations
    • Petrov-Galerkin finite element method
    • gravity anchors
    • hydrodynamic loads
    • oscillatory flows
    • pipelines
    • steady flows
    • subsea

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