No abstract

a b s t r a c tHydrodynamic forces on heave plates for a semi-submersible floating offshore wind turbine are discussed herein. A model of one of the platform columns has been built. This allows for the fitting of either a plain solid plate or the real heave plate prototype design. The latter is equipped with a vertical flap at its edge. The influence of the flap on the hydrodynamic coefficients is investigated through a results comparison with the plain solid one. The model plate diameter is 1 m, thus becoming, to the authors' knowledge, the largest for which results have been published. Results from experiments, in which added mass and damping coefficients have been measured, are presented. This experimental campaign also comprised the direct measurement of dynamic pressures on both heave plates, a fundamental magnitude for the structural design, which, until now, had not been experimentally explored for this type of system. For comparative reasons, numerical simulations were also conducted following common industry standards, both with a wide-spread frequency domain panel method (WADAM) and a RANS CFD commercial code (ANSYS CFX). Finally, results are compared with literature and consistent non-dimensionalizations are sought, with the aim of making these results useful for preliminary design purposes. The authors believe this research could benefit the offshore wind industry by improving the hydrodynamic design of the concept.

AZIMUT project (Spanish CENIT R&D program) is designed to establish the technological groundwork for the subsequent development of a large-scale offshore wind turbine. The project (2010–2013) has analyzed different alternative configurations for the floating offshore wind turbines (FOWT): SPAR, tension leg platform (TLP), and semisubmersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a semisubmersible platform to support a 1.5 MW wind turbine. The geometry of the floating platform considered in this paper has been provided by the Hiprwind FP7 project and is composed by three buoyant columns connected by bracings. The main focus of this paper is on the hydrodynamic modeling of the floater, with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 s, accurate computation of the low-frequency second-order components is not a straightforward task. Methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as the Newman's approximation, have their errors increased by the relatively low resonant periods of the floating system and, since the effects of depth cannot be ignored, the wave diffraction analysis must be based on full quadratic transfer functions (QTFs) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software wamit®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.

In the case of SPAR or semi-submersible platforms for floating wind turbines, it is beneficial in some cases to use heave plates that reduce their heave motion amplitude and/or tune their heave natural period. As part of the Hiprwind project, it was decided to study scale effects on the hydrodynamics of this element. To this aim, models of one leg of the platform, equipped with a heave plate without any reinforcements, were built. This model is a simplified representation of the actual one, which incorporates a vertical flap on the heave plate edge. The scales were 1:20, 1:27.6, and 1:45.45, with the former leading to added mass values of the order of 300 kg, becoming one of the largest models for which experiments with heave oscillations have been carried out. Decay tests starting from various amplitudes and forced oscillations tests for a range of frequencies and amplitudes were performed. It is shown in the paper that the influence of the scale factor on the hydrodynamic coefficients is weaker than the effect that the motion amplitude (characterized with the Keulegan–Carpenter (KC) number produces in them. This result is relevant because the selection of a representative KC is an important and somewhat arbitrary aspect to be set in the linear potential simulation codes in order to add viscous damping. What has been shown herein is that a right selection of KC has a larger impact on the models than the uncertainties due to eventual scale effects in the heave-plates dynamics.