No abstract

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.

No abstract

This article presents the development of a numerical tool for seakeeping simulations of marine systems using a time domain boundary element method based on Rankine sources. The formulation considers two initial boundary value problems defined for the velocity and acceleration potentials, the last being used to avoid numerical problems in calculating the time derivatives of the velocity potential. A fourth-order Runge-Kutta method is used for the time marching of the problem, which consists in the integration of the free surface conditions and body equations of motion. Numerical test cases are presented for bodies with simplified geometries, such as an hemisphere and a circular section cylinder. Exciting forces, added mass and radiation damping coefficients, and motions response amplitude operators are compared to analytical and numerical data, presenting a very good agreement. Furthermore, the numerical method is applied to a floating production storage and Off-loading unit and the results are verified with experimental data carried out in the hydrodynamic calibrator of the University of Sao Paulo. By means of these investigations, we have verified that the developments performed so far are correct and new extensions, therefore, may be planned for more complex applications.

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 analysed different floating offshore wind turbines (FOWT): SPAR, TLP and Semi-Submersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a Semi-submersible platform to support a 1.5MW wind turbine. The floating platform geometry 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 hydrodynamic modelling 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 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman’s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) 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.