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Research Papers: Structures and Safety Reliability

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

[+] Author and Article Information
Carlos Lopez-Pavon

Acciona Energia,
Madrid 28108, Spain
Niobe Tech,
Madrid 28703, Spain
e-mail: carlos@niobetech.com

Rafael A. Watai

Numerical Offshore Tank (TPN),
University of São Paulo,
São Paulo 05508-010, Brazil
e-mail: rafael.watai@tpn.usp.br

Felipe Ruggeri

Numerical Offshore Tank (TPN),
University of São Paulo,
São Paulo 05508-010, Brazil
e-mail: felipe.ruggeri@tpn.usp.br

Alexandre N. Simos

Department of Naval Architecture
and Ocean Engineering,
Escola Politécnica,
University of São Paulo,
São Paulo 05508-010, Brazil
e-mail: alesimos@usp.br

Antonio Souto-Iglesias

Department of Naval Architecture (ETSIN),
Technical University of Madrid (UPM),
Madrid 28040, Spain
e-mail: antonio.souto@upm.es

The so-called low-order methods represent the body geometry by means of flat panels and the velocity potential is considered constant over each panel surface.

One may refer to the discussion provided by Matos et al. [13] on this subject, when dealing with the similar problem of resonant motions of a semi-submersible platform in roll and pitch.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 1, 2013; final manuscript received March 23, 2015; published online April 16, 2015. Editor: Solomon Yim.

J. Offshore Mech. Arct. Eng 137(3), 031602 (Jun 01, 2015) (10 pages) Paper No: OMAE-13-1064; doi: 10.1115/1.4030241 History: Received July 01, 2013; Revised March 23, 2015; Online April 16, 2015

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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Figures

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Fig. 2

Catenary mooring system configuration

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Fig. 3

(a) CEHIPAR's model basin main dimensions and (b) view of the semisubmersible model fixed on the carriage

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Fig. 4

Excerpts of records: (a) wave elevation in meters and (b) surge load in kN/m. Test of bichromatic wave with δω = 0.15 rad/s.

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Fig. 5

Higher-order mesh (dipole patches were used to model the heave plates)

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Fig. 6

Free surface meshes used in the second-order analysis (only half mesh is presented as symmetry was considered)

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Fig. 7

First-order excitation force in surge per unit wave amplitude; numerical and experimental results. Values are in real scale.

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Fig. 8

Second-order low-frequency excitation force in surge per wave amplitude squared; numerical and experimental results: (a) tests with δω = 0.15 rad/s; (b) tests with δω = 0.24 rad/s; and (c) tests with δω = 0.34 rad/s. Values are in real scale.

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Fig. 9

Second-order low-frequency excitation force in surge per wave amplitude squared; tests with δω = 0.15 rad/s; numerical results from QTFs (FIX WAM) and Newman's approx. (NEW WAM); Values are in real scale.

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