In this paper, wave loads on a surface piercing vertical cylindrical pile are presented using the spectral and monochromatic approaches. While the basis of wave load computation for both the approaches is the Morison equation, the spectral method is dependent on the force spectrum - a product of RAO and random sea state described in this paper by JONSWAP spectrum. Horizontal load comparisons are made for a total of 4 cases of low Ursell numbers in the applicability range of linear wave theory. For the cases considered, the results indicate that in deep waters > 15 m, the computation intensive spectral method yields the best load estimate.


Shore-attached pile-supported port structures are increasingly moving to open deep-waters as demands for accommodating deep-draft vessels continue to grow (see e.g. Gaythwaite, 2004; Allsop et al, 2006). Despite often being located in the hydrodynamic shallow water regions in considerations of peak spectral period, such structures are subjected to wave loads from unmodulated random sea states. This requires that the usual practice of applying the simplistic method using monochromatic wave is compared against the more sophisticated spectral method. This paper examines this issue, and presents the computational results for some low Ursell number (around 5) cases. Waves around this Ursell number can be described by linear wave theory. For diffraction parameter (πD/Lp) < 0.5, the basic tool for estimating wave load on a vertical cylindrical pile is the Morison equation (Morison et al, 1950). Figure 1 shows a schematic vertical cylinder in a linear wave field. The random sea state is described in this paper by one-dimensional JONSWAP spectrum (Hasselmann et al, 1973; Goda, 1979). JONSWAP spectrum, developed using North Sea wave measurements is often used to describe offshore waves (Wilson, 2003).

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