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Wave loads on piles - Spectral versus monochromatic approach
Abstract
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. Copyright © 2008 by The International Society of Offshore and Polar Engineers (ISOPE).
... Only first order gravitational waves are considered in this research because of computation limitations. Moreover, the simplification seems reasonable because according to Barua (2008), the monochromatic (vs. spectral) approach can be applied for relatively shallow waters (less than 15 m depth) though it slightly overestimates wave loads. ...
New results of numerical hydrodynamic analysis of the novel offshore cross-flow vertical axis wind turbine with large floating rotor (WEMU design) are presented. The research goals are to investigate a spatial movement of rotary pontoon in incoming regular and irregular waves as well as their influence upon the water ring. Waves are launched by two components of flow velocity and free surface elevation at inlet boundary. Irregular wave is presented according to Pierson - Moskowitz spectrum by sum of first order waves with random phases. Wave loads acting on the pontoon are predicted and hydrodynamic power loss due to waves is calculated. © 2010 by The International Society of Offshore and Polar Engineers.
The paper concerns hydrodynamic analysis of the novel offshore cross-flow vertical axis wind turbine with large floating rotor (WEMU design). Methods and results of recent CFD calculations of water flow with surface waves and current are described. Water ring arises about the semi-submersible pontoon during rotation. The research goals are to investigate an influence of incoming regular waves and current upon the water ring, to calculate wave loads acting the pontoon, and to calculate additional power loss due to waves and current. The water ring slightly changes the direction of wave propagation. The influence of current flow occurs negligibly small. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).
The power spectra of typical sets of ocean wave data obtained in the open ocean using a cloverleaf buoy are analyzed to determine an idealized form for the spectrum of ocean surface waves. It is shown that most of the single-peaked spectra observed in a generation area can be described well by the spectral form of the JONSWAP type. Two parameters α and γ characterizing the spectral form are calculated for each spectrum measured. Their relations to the dimensionless peak frequency f̄m (=fmU/g) are then determined. These relations are further converted into fetch relations for α and γ through a relation between f̄ and a dimensionless fetch F̄ (=gF/U2). Another spectral form proposed by Toba (1978) is examined and shown to fit as well to the observed spectra at high frequencies This fact shows quasi-equivalence of the JONSWAP spectrum and Toba's spectrum in the high-frequency range. On the basis of the agreements of both spectral forms at high frequencies, properties of the dimensionless constant α&...
The statistical distribution of peak forces for irregular linear ocean waves having a narrow-band spectrum is an exponential-type function, the form of which depends on the average relative balance between the drag and the inertial forces. The distribution is a function of two parameters determined by the drag and inertial coefficients and the physical dimensions characterizing the pile and the spectrum. Formulas and tables are developed for the first through the fourth moments, the median, the mode, the upper fractile, and the average of the highest p-th fraction of the peak forces. The method of moments and the method of maximum likelihood each provide procedures for estimating the parameters from the data. In addition, a graphical method consisting of plotting one minus the empirical distribution function on semi-log paper versus the peak force data and again versus the square of the data is developed. The theory is compared with wave forces measured near Davenport, California, in 50-ft water depths.