Engineers traditionally calculate wave forces on offshore structures assuming that the wave system is unidirectional. However, it has been known for some years that for the same point spectrum, spread in the directions of travel of the component wave trains of a wave system results in lower forces, in particular in lower extreme forces, compared with the unidirectional case. As far as the present author can tell from the literature, two physical mechanisms appear to be work. The first is governed by the statistics of the wave particle velocities and accelerations, and is the subject of this paper. There also seems to be some effect due to a change in the eddy-shedding regime in spread seas. As will be shown later, this would appear in terms of the rms measured forces, but the author can find no results clearly stated in these terms. If such an effect exists, it is certainly not well enough established to be used in design, and it will not be considered further here. The statistics of the horizontal component of the water particle velocity vector under random waves are considered. A spread in the direction of travel of the component wave trains does not affect the rms value of the modulus r but it affects the shape of its probability distribution in such a way that the probability of extreme values is reduced. For small and moderate directional spreads the modulus for a given probability of exceedance is reduced by a factor which tends to F, as the probability of exceedance tends to zero, where Fs is the spread factor (which is typically 0.9 in temperate storms). However, the convergence on this asymptote is not complete for probabilities of engineering importance, particularly for wide directinal spreads. The standard deviation of the probability distribution of the direction of the vector is reduced roughly in inverse proportion to r. The same conclusions apply to particle accelerations.
The statistics of particle velocities and accelerations under waves with directional spread
Statistische Untersuchungen der Partikelgeschwindigkeiten und -beschleunigungen von Wasserwellen mit richtungsabhängiger Ausbreitung
Ocean Engineering ; 25 , 6 ; 481-496
1998
16 Seiten, 5 Bilder, 13 Quellen
Article (Journal)
English
Meerestechnik , Offshore-Gebiet , analytische Untersuchung , Strömungsrichtung , Partikelströmung , Strömungsgeschwindigkeit , Rechenmodell , Wellengleichung , statistisches Verfahren , Wahrscheinlichkeitsverteilung , Wirbelablösung , Wasserkraft , Konvergenz , Standardabweichung , Vektor , Beschleunigung , Richtungsabhängigkeit
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