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Research Papers: Ocean Renewable Energy

Modeling of a Semisubmersible Floating Offshore Wind Platform in Severe Waves

[+] Author and Article Information
Irene Rivera-Arreba

Delft University of Technology,
Wind Energy Section,
Delft 2611JG, The Netherlands
e-mail: i.riveraarreba@tudelft.nl

Niek Bruinsma

Deltares,
Delft 2629 HV, The Netherlands
e-mail: niek.bruinsma@deltares.nl

Erin E. Bachynski

Norwegian University of Science and Technology,
Department of Marine Technology,
Trondheim NO-7491, Norway
e-mail: erin.bachynski@ntnu.no

Axelle Viré

Delft University of Technology,
Wind Energy Section,
Delft 2629 HS, The Netherlands
e-mail: a.c.vire@tudelft.nl

Bo T. Paulsen

Deltares,
Delft 2629 HV, The Netherlands
e-mail: bo.paulsen@deltares.nl

Niels G. Jacobsen

Deltares,
Delft 2629 HV, The Netherlands
e-mail: niels.jacobsen@deltares.nl

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the Journal of Offshore Mechanics and Arctic Engineering. Manuscript received December 3, 2018; final manuscript received May 22, 2019; published online June 26, 2019. Assoc. Editor: Amy Robertson.

J. Offshore Mech. Arct. Eng 141(6), 061905 (Jun 26, 2019) (11 pages) Paper No: OMAE-18-1207; doi: 10.1115/1.4043942 History: Received December 03, 2018; Accepted May 22, 2019

Floating offshore wind platforms may be subjected to severe sea states, which include both steep and long waves. The hydrodynamic models used in the offshore industry are typically based on potential-flow theory and/or Morison’s equation. These methods are computationally efficient and can be applied in global dynamic analysis considering wind loads and mooring system dynamics. However, they may not capture important nonlinearities in extreme situations. The present work compares a fully nonlinear numerical wave tank (NWT), based on the viscous Navier–Stokes equations, and a second-order potential-flow model for such situations. A comparison of the NWT performance with the experimental data is first completed for a moored vertical floating cylinder. The OC5-semisubmersible floating platform is then modeled numerically both in this nonlinear NWT and using a second-order potential-flow based solver. To test both models, they are subjected to nonsteep waves and the response in heave and pitch is compared with the experimental data. More extreme conditions are examined with both models. Their comparison shows that if the structure is excited at its heave natural frequency, the dependence of the response in heave on the wave height and the viscous effects cannot be captured by the adjusted potential-flow based model. However, closer to the inertia dominated region, the two models yield similar responses in pitch and heave.

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Figures

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Fig. 1

Schematic overview of the boundary conditions implemented at the computational domain in the Navier–Stokes solver, where I and II, in shaded gray, correspond to the relaxation zones

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Fig. 2

Possible configurations of the mooring system based on its implementation in openfoam

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Fig. 3

Numerical domain setup, with the dimensions indicated with letters in meters: cylinder diameters (D) and wavelengths (w.l.): a = 3 m = 5.80D; b = 6 m = 11.65D = f; g = 9 m = 17.50D; e = 1.8 m; c = d = 0.9 m. The width of the domain is 5 m.

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Fig. 4

(Left) Decay test in heave and (right) decay test in pitch

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Fig. 5

(Left) Heave response for the pitch decay test and (right) stability diagram for the undamped Mathieu’s equation

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Fig. 6

(Left) Time series of the free surface elevation at x/D = 0 and (right) first harmonic amplitude a1, normalized by the first harmonic amplitude of the analytical solution a1,SF

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Fig. 7

Overview of the background mesh for the floating cylinder, where waves propagate at 0 deg, from the COG of the cylinder and from left to right

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Fig. 8

Response of the floating cylinder due to an incoming regular wave in (left) heave and (right) pitch. The heave time series is normalized by the wave height. The time is normalized by the incoming wave period.

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Fig. 10

OC5-semisubmersible models in the two numerical frameworks: (left) the potential-flow model and (right) Navier–Stokes model

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Fig. 9

Overview of the OC5-semisubmersible Navier–Stokes setup, without moorings

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Fig. 11

Moored decay tests in (left) heave and (right) pitch. The displacements are normalized by the maximum initial values and the time by the experimental natural periods.

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Fig. 12

Mesh convergence study for the (top) free surface elevation, (middle) heave, and (bottom) pitch responses of the moored OC5-semisubmersible to an incoming regular wave of 12.1 s period

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Fig. 13

Comparison of the (top) free surface elevation, (middle) heave, and (bottom) pitch response of the moored floating platform with the experimental data to an incoming regular wave of 12.1 s period

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Fig. 14

(Top) Free surface elevation, (middle) heave, and (bottom) pitch response of the moored floating platform to an incoming regular wave of 17.4 s period

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Fig. 15

Response amplitude operator in heave and pitch for the experimental and the potential-flow models

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Fig. 16

(Top) Free surface elevation, (middle) heave, and (bottom) pitch response of the moored floating platform for (left) 8 s and (right) 12.1 s regular waves period

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