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A note on the secondary load cycle for a monopile in irregular deep water waves

Published online by Cambridge University Press:  26 June 2018

Bjørn Hervold Riise Open the ORCID record for Bjørn Hervold Riise [Opens in a new window] ,
John Grue Open the ORCID record for John Grue [Opens in a new window] ,
Atle Jensen  and
Thomas B. Johannessen
Bjørn Hervold Riise*
Affiliation:
Department of Mathematics, University of Oslo, PO Box 1053 Blindern, NO-0316 Oslo, Norway DNV GL Oil & Gas, PO Box 300, NO-1322 Høvik, Norway
John Grue
Affiliation:
Department of Mathematics, University of Oslo, PO Box 1053 Blindern, NO-0316 Oslo, Norway
Atle Jensen
Affiliation:
Department of Mathematics, University of Oslo, PO Box 1053 Blindern, NO-0316 Oslo, Norway
Thomas B. Johannessen
Affiliation:
DNV GL Oil & Gas, PO Box 300, NO-1322 Høvik, Norway
*
†Email address for correspondence: bjorn.riise@gmail.com
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Abstract

Laboratory experiments with a bottom hinged surface-piercing cylinder, exposed to irregular deep water waves, are used to investigate high-frequency forcing. The focus is on the secondary load cycle, a strongly nonlinear phenomenon regarding the wave load on a vertical cylinder, first identified by Grue et al. (1993 Preprint Series. Mechanics and Applied Mathematics, pp. 1–30. University of Oslo, available at http://urn.nb.no/URN:NBN:no-52740; 1994 Ninth International Workshop on Water Waves and Floating Bodies (ed. M. Ohkusu), pp. 77–81, available at http://iwwwfb.org). For a total of 2166 single wave events, the force above $3\unicode[STIX]{x1D714}$ (where $\unicode[STIX]{x1D714}$ is the governing wave frequency) is used to identify and split the strongly nonlinear forces into two peaks: a high-frequency peak closely correlated in time with the wave crest when the total load is positive and a high-frequency peak defining the secondary load cycle which occurs close in time to the wave zero downcrossing when the total load is negative. The two peaks are studied by regression analysis as a function of either the Keulegan–Carpenter number ($KC$) or the Froude number ($Fr$). Regarding the secondary load cycle, the best correlation is found with $Fr$. The speed of the travelling edge of the undisturbed wave approximates the fluid velocity. A threshold value separating between small and large forces is found for $KC\sim 4$–5, indicating effects of flow separation. Alternatively, the threshold occurs for $Fr\sim 0.3$–0.4, indicating local wave effects at the scale of the cylinder diameter. The findings suggest that both effects are present and important.

JFM classification

Waves/Free-surface Flows: Surface gravity waves Waves/Free-surface Flows: Wave breaking
Type
JFM Rapids
Information
Journal of Fluid Mechanics , Volume 849 , 25 August 2018 , R1
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Copyright
© 2018 Cambridge University Press 

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