Motion Responses Analysis for Tidal Current Energy Platform: Quad-Spar and Catamaran Types
doi: 10.1007/s13344-020-0061-1
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Abstract: One approach to support floating tidal current turbines is by using a moored catamaran, a barge type platform. Considering its low draft, one might expect that it performs best at typical straits with sea states of small wavelets to small waves. The problem is that the high rotational motion responses of the catamaran due to wave loads tend to reduce the turbine performance. This paper looks for a possibility to deteriorate these rotational responses by introducing a platform with four buoyant legs referred to as a quad-spar considering its good stability performance. The platforms are moored by four catenary cables as their mooring system. The motion response modeling was undertaken by Computational Fluid Dynamic (CFD) simulation based on three-dimensional potential flow theory. Considering sea states of straits with typical tidal current energy potentials, the environmental load was set on random wave with the significant wave height, Hs, of about 0.09 to 1.5 m and the wave period, T, of about 1.5 to 6 s corresponding to the wave frequency,
$ \omega $ , of about 1.1 to 4.2 rad/s. This study found that lower motion responses can be satisfied by the quad-spar, in which its yaw, roll and pitch responses are on average about 5%, 44%, and 38%, respectively, compared to those of the catamaran. This result indicates that the quad-spar is more effective in reducing rotational motion responses needed to keep a high performance of the tidal current energy system.-
Key words:
- floating platform /
- tidal current energy /
- CFD simulation /
- catamaran platform /
- quad-spar platform
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Table 1. Verification of numerical method
Parameter of
catamaran ( $ \lambda /L) $Experimental maximum pitch
RAO (°/cm)Numerical maximum pitch
RAO (°/cm)Percentage of difference
(%)0.8 0.79 0.85 6.7 1.0 0.95 0.98 3.6 1.2 0.99 1.02 2.9 1.5 1.12 1.16 3.4 1.8 1.38 1.45 4.4 2.0 1.26 1.30 3.1 2.2 1.22 1.28 4.5 2.5 0.98 1.02 3.8 3.0 0.90 0.95 5.1 Table 2. Parameters of catamaran and quad-spar as power station
Parameter Symbol Value Unit Demi-hull separation S 14.4 m Breadth of demi-hull B 2 m Height of demi-hull Hc 2 m Diameter of quad-spar Dq 2.06 m Height of quad-spar Hq 5.62 m Height of deck Hd 1 m Breadth of deck Bd 18.4 m Length of deck Ld 24 m Draft of Catamaran Tc 1.2 m Draft of quad-spar Tq 4.82 m Diameter of turbine D 4 m Length of span Ls 5.5 m Table 3. Perfomance parameters
Parameters Twin turbines-loaded catamaran Twin turbines-loaded quad-spar Total weight (kg) 118060 118060 $ {K}_{xx} $ (m) 1.21 1.52 $ {K}_{yy} $ (m) 1.21 1.52 $ {K}_{zz} $ (m) 0.02 0.1 Defeaturing tolerance (m) 0.1 0.1 Maximum element size (m) 0.4 0.4 Number of nodes 29658 30769 Number of elements 29721 30821 Table 4. Coordinates of anchor
Number of mooring cable Coordinate of anchor (m) x y z 1 64.14 33.2 −35 2 64.14 −33.2 −35 3 −64.14 33.2 −35 4 −64.14 −33.2 −35 Table 5. Properties of mooring system
Terms Specification Unit Number of mooring cables 4 − Length of mooring cables 75 m Diameter of mooring cables 58 mm Weight of mooring cables/ unit length 67 kg/m Pretension of mooring cables 35.55 kN Table 6. The difference of maximum tension in Cable 3
${H}_{\rm{s} }$ (m) Maximum tension (kN) Percentage of
difference (%)Catamaran-typed platform Quad-spar platform 0.09 1.70 1.57 7.87 0.18 1.79 1.61 10.28 0.27 1.89 1.65 12.56 0.35 1.97 1.69 14.47 0.44 2.07 1.74 16.46 0.53 2.17 1.78 18.22 0.62 2.27 1.82 19.82 0.71 2.37 1.87 21.24 0.8 2.47 1.91 22.52 0.89 2.56 1.96 23.67 0.98 2.66 2.00 24.69 1.07 2.75 2.06 25.35 1.16 2.87 2.11 26.73 1.25 3.02 2.16 28.53 1.34 3.18 2.22 30.28 1.43 3.34 2.28 31.73 1.5 3.48 2.34 32.70 Table 7. Steel plate estimates
Type Estimation of floater area
(m2)Number of steel plate requirement Estimation of steel plates prices needed (US$) Twin turbines-loaded catamaran 270.63 30 5139.46 Twin turbines-loaded quad-spar 158.74 18 3083.68 -
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