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.

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This paper addresses the problem of estimating the air gap for a large semi-submersible production platform. Although it has a great impact on the design of the floating unit, many times the minimum deck height is still defined from simplified methods that incorporate relatively large safety margins. The reason for this is the intrinsic complexity of the associated hydrodynamic problem. Nonlinear effects on the incoming and scattered waves are usually relevant and sometimes non-linear effects on the motions of the floating hull may also play an important role. This discussion is illustrated by means of wave basin tests performed with the model of a large semi-submersible designed to operate in Campos Basin. Significant run-up effects on its squared-section columns were observed for the steepest waves in several design conditions. Also, the unit presented relatively large low-frequency motions in heave, roll and pitch, which also affected the dynamic air gap measurements. In order to evaluate the difficulties involved in modeling such phenomena, simplified tests were also performed with the model fixed and moored in regular waves of varying steepness. Wave elevation in different points was measured in these tests and compared to the predictions obtained from two different numerical methods: a BEM code that incorporates 2nd order diffraction effects (WAMIT) and a VOF CFD code (ComFLOW), the latter employed for fixed model tests only. Results show that a standard linear analysis may lead to significant errors concerning the air gap evaluation. Extending the BEM model to 2nd order clearly improve the results as the wave-steepness increases. Although the VOF analysis is considerably time-consuming, simulations presented very good agreement to the experimental results, even for the steepest waves tested.