Research Papers: Ocean Renewable Energy

Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take-Off and Mooring Systems on Energy Capture

[+] Author and Article Information
Made Jaya Muliawan

e-mail: made.muliawan@ntnu.no

Zhen Gao

e-mail: zhen.gao@ntnu.no

Torgeir Moan

e-mail: torgeir.moan@ntnu.no
Centre for Ship and Ocean Structures (CeSOS),
Norwegian University of
Science and Technology,
Otto Nielsens vei 10,
NO-7491, Trondheim, Norway

Aurelien Babarit

LUNAM Université,
Ecole Centrale de Nantes,
LHEEA Lab. - CNRS UMR6598,
1, rue la Noë,
Nantes 44300, France
e-mail: aurelien.babarit@ec-nantes.fr

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 30, 2011; final manuscript received October 17, 2012; published online May 24, 2013. Assoc. Editor: Hideyuki Suzuki.

J. Offshore Mech. Arct. Eng 135(3), 031902 (May 24, 2013) (12 pages) Paper No: OMAE-11-1047; doi: 10.1115/1.4023796 History: Received May 30, 2011; Revised October 17, 2012

The present paper summarizes analyses of a two-body floating wave energy converter (WEC) to determine the mooring tension and the effect of the mooring system on energy capture. Also, the effect of the power take-off (PTO) is assessed. An axisymmetric Wavebob-type WEC is chosen as the object of investigation. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couple the motions of the two bodies. The analysis is performed using SIMO, which is a time domain simulation tool that accommodates the simulation of multibody systems with hydrodynamic interactions. In SIMO, docking cone features between the two bodies allow movement as per actual operation, and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. Mooring analysis is performed to determine the necessary capacity of mooring lines for each configuration to carry the tension due to the WEC motion in extreme conditions. Hydrodynamic loads are determined using WAMIT. We assumed that the WEC will be operated to capture wave power at the Yeu site in France. The analysis is performed for several regular and irregular wave conditions according to wave data available for that site. Simulations are performed to study the effect of the PTO system, end stops settings and several mooring configurations on power capture.

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Fig. 2

Dimensions of the WEC used in the present analysis

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Fig. 3

Mooring configurations that are used in the simulations

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Fig. 4

(a) Location of the Yeu site specified as “A” [6] and (b) scatter diagram of waves at the Yeu, in France

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Fig. 5

100-year contour line for the wave conditions at the Yeu site

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Fig. 6

RAOs of the WEC for heave, sway, and roll motions when the float and the torus are locked

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Fig. 7

Time series of line tensions (a) line no. 4 of MC1 and (b) line no. 4 of MC4 configurations. Sea state when Hs = 8.16 m and Tp = 12.43 s is applied.

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Fig. 8

Distributions of (a) 1-h and (b) 3-h maximum line tensions for different mooring configurations

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Fig. 9

Panel model in the present analysis

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Fig. 10

Introduction of docking cone springs to let the two bodies move together in sway, surge, roll, and pitch yet move freely in heave and yaw

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Fig. 11

Stiffness parameter applied for every docking cone in simulation

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Fig. 12

Introduction of end stop springs to limit the relative vertical motion between the Float and Torus

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Fig. 13

Restoring coefficient condition applied in each end stop model in simulation

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Fig. 14

Comparisons of (a) PTO force and (b) line tension obtained from Riflex and SIMO for Wavebob-type WEC with mooring but without hydrodynamic interaction between the two bodies

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Fig. 15

Heave RAO of Torus and Float separately and combined heave RAO when they are connected with very high Bpto (Kpto = Mpto = 0)

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Fig. 16

Heave RAO of Torus and Float when they are connected with different Bpto coefficient (Kpto = Mpto = 0)

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Fig. 17

The power function as a function of the Bpto coefficients (Kpto = Mpto = 0) based on simulation sets 1 to 6 in Table 3 and set results from Mouwen [4]

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Fig. 18

The power function as a function of the Kpto coefficients (Bpto = 4000 kN/m s and Mpto = 0) obtained by simulation sets 7 to 10 in Table 3

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Fig. 19

The power function as a function of Mpto coefficients (Bpto = 4000 kN/m s and Kpto = 0) obtained by simulation sets 11 to 14 in Table 3

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Fig. 20

Heave motions of the Torus and Float (a) without end stops and (b) with end stops set with a distance 0.5 m. The simulations are run with an H = 2 m and a T = 12 s without PTO coefficients (Bpto = Kpto = Mpto = 0).

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Fig. 21

Heave RAO of the Torus and Float with different end stop sets. The simulations are run with an H = 7 m and a Bpto = 4000 kN s/m. (Kpto = Mpto = 0).

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Fig. 22

The power function in different end stop sets. The simulations are run with an H = 7 m and a Bpto = 4000 kN s/m (Kpto = Mpto = 0).

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Fig. 23

Heave RAO of the combined bodies (high Bpto is applied with Kpto = Mpto = 0) with MC1 configuration as a function of wave height

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Fig. 24

The power function as a function of Bpto with MC1 configuration for various wave periods where (a) with H = 1 m and (b) with H = 7 m, respectively (Kpto = Mpto = 0)

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Fig. 25

The power function behavior near the maximum power output period in several waves conditions using Bpto = 8000 kN s/m and Kpto = Mpto = 0 when (a) MC1 and (b) MC4 configurations are introduced, respectively. Please refer to Fig. 2 and Table 2 for configuration properties.

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Fig. 26

Comparison of the power function effected by different mooring configurations at wave period (a) 9 s and (b) 10 s, respectively. Simulations performed using Bpto = 8000 kN s/m, Kpto = Mpto = 0.

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Fig. 27

Power production (kW) as a function of Hs and Tp at Yeu by the WEC with the (a) MC1 and (b) MC3 configurations, respectively. Simulation performed using Bpto = 8000 kN s/m, Kpto = Mpto = 0.



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