Influence of Wave Induced Second-Order Forces in Semisubmersible FOWT Mooring Design

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.

Comparison of Second-Order Loads on a Semisubmersible Floating Wind Turbine

“…The fifth component involves the secondorder velocity potential that can be calculated by a second-order diffraction code applying the perturbation method. As explained by López-Pavón, component V is difficult to estimate [4]. It is noted that DIFFRAC and WAMIT do not determine this component in the same way.…”

“…still water line X 6 component vector with positions of point O X (1) surge m X (2) sway m X (3) heave m X (4) roll rad X (5) pitch rad X (6) yaw rad X   second time derivative of vector X m⋅s - Water Line…”

A time-domain second-order FEM model for the wave diffraction-radiation problem. Validation with a semisubmersible platform

“…28 The hydrodynamics of the semisubmersible concept for FOWTs has received some attention in the 29 recent literature. For instance, [6] and [7] focused on slow-drift and mean-drift forces of semisubmersible 30 platforms, a comparison of a semisubmersible against a SPAR concept can be found in [8,9,10], andsimulations in the Time-Domain (TD) considering different models for the hydrodynamic loads were 1 carried out in [11]. 2 One of the main concerns regarding semisubmersible platforms is the slow-drift forces.…”

“…And although Newman's approximation -which only 10 depends on the first-order solution-could be used for estimating the slow-drift forces, it might not be 11 precise enough as shown in [7]. Hence, second-order effect must be taken into account to accurately 12 compute slow-drift motions, and design the mooring system accordingly.…”

“…More recently, Lopez-Pavon et al [7] and Simos et al [15] focused on the estimation and 12 verification of the second-order wave induced forces on the HiPRWind semisubmersible platform. This 13 work concludes with the following statements: 14 1.…”