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Effects of Free Spans on On-Bottom Stability of Offshore Pipelines: A Global Approach

[+] Author Affiliations
Olav Aamlid, Knut Vedeld, Håvar Sollund

Det Norske Veritas, Ho̸vik, Norway

Paper No. OMAE2010-20988, pp. 999-1009; 11 pages
doi:10.1115/OMAE2010-20988
From:
  • ASME 2010 29th International Conference on Ocean, Offshore and Arctic Engineering
  • 29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 5, Parts A and B
  • Shanghai, China, June 6–11, 2010
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-4913-2 | eISBN: 978-0-7918-3873-0
  • Copyright © 2010 by ASME

abstract

On-bottom stability design of pipelines normally assumes that the pipeline rests on a flat seabed without any free spans. This paper presents on-bottom stability calculations accounting for the presence of free spans. It should be noted however that the calculations presented in this paper are only meant to be indicative, and not for design purposes. The absolute static stability criterion in DNV-RP-F109 is given on a form that directly compares the hydrodynamic loads on the pipeline to the resistance provided by the pipe soil interaction. Hence, by following this line one can estimate the effect that the presence of free spans has on both the load and the resistance. It is known that hydrodynamic loads, lift and drag, are higher close to the seabed than away from it. This will obviously be beneficial from an on-bottom stability point of view. The effect that the free spans have on the pipe soil resistance is not so obvious. Often, one assumes that this resistance consists of two components, pure Coulomb friction and so-called passive resistance due to penetration. The Coulomb friction part is herein assumed to be independent on the span fraction as the total weight remains the same, whereas the soil models in DNV-RP-F109 predicts an overall reduction in passive resistance due to the presence of free spans. For the on-bottom stability calculations it turns out the approach predicts a beneficial effect for pipelines on a sandy seabed. However, under most conditions on clay, where the beneficial passive resistance is more important, the prediction is that the presence of free spans is not beneficial.

Copyright © 2010 by ASME

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