SECOND-ORDER DIRECTIONAL SEAS AND ASSOCIATED WAVE FORCES

Most available methods for wave force computation incorporate either the nonlinearities of the ocean surface for a single fundamental component or the random and/or directional characteristics using the superposition of linear wave components. An exception is the intuitive "hybrid" method 1 which combines elements of linear and nonlinear waves. The present paper describes and applies a method, which is correct to second order in wave height, for calculating waves and wave forces on an offshore structure due to a directional wave spectrum. Starting with a prescribed linear spectrum of directional waves, a set of random phases is generated and the second-order spectrum computered with phases defined by all contributing pairs of first-order components. With one realization of the spectrum thus complete up to the second order, the wave profile and water particle kinematics can be simulated in the time domain. The wave forces are also computed in the time domain taking full account of their nonlinear and directional properties. The resulting wave forces at any level vary in direction and magnitude. The total wave forces summed over all piling of a structure are less than those for a unidirectional train of waves with the same one-dimensional spectrum. Several examples are presented to illustrate reductions in maximum wave forces due to the directional distribution of the waves. It is found that for a single piling the maximum force decreases by a factor ranging from 1.0 to 0.61 as the directional spread increases from unidirectional to omnidirectional. For a four-pile group on a square array of 300 ft. spacing, the corresponding decrease in the factor is from 1.0 to 0.51 for a Bretschneider spectrum with a peak period of approximately 12 seconds. The results of this complete model are compared with the more intuitive and approximate hybrid method and are found to agree quite well. Force spectra are presented and discussed for the inline and transverse directions.

  • Supplemental Notes:
    • Proceedings of the Eleventh Annual OTC held in Houston, Texas April 30-May 3 1979, Volume 4.
  • Corporate Authors:

    Offshore Technology Conference

    6200 North Central Expressway
    Dallas, TX  USA  45206
  • Authors:
    • Sharma, J
    • Dean, R G
  • Publication Date: 1979

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Subject/Index Terms

Filing Info

  • Accession Number: 00195813
  • Record Type: Publication
  • Source Agency: Offshore Technology Conference
  • Report/Paper Numbers: Volume 4 Proceeding
  • Files: TRIS
  • Created Date: Jul 31 1979 12:00AM