De Backer, G.

De Rouck, J.

Vantorre, M.

Open Marine Archive

Wave Energy
Ocean waves contain huge amounts of -
unexploited renewable energy, offering gigantic 
opportunities. This energy can be absorbed by 
wave energy converters (WECs). Ocean energy 
is a renewable energy type that is becoming 
more and more important. 
Point absorber systems are wave energy 
converters consisting of oscillating bodies with 
small dimensions compared to the wave length.
Purpose
Numerical simulation of the behaviour of a
heaving point absorber in relative motion to 
a floating platform in order to:
-find an optimal buoy geometry.
-define the absorbed power for a certain wave 
climate.
-assess the effect of restrictions imposed on
buoy motion.
Research funded by Ph.D. grant of the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen), Belgium
Numerical modelling of wave energy
absorption by a floating point absorber system
Griet De Backer
Promotors: Julien De Rouck - Marc Vantorre
Ghent University - Department of Coastal Engineering
Technologiepark Zwijnaarde 904 -9052 Zwijnaarde - Belgium
Email: Griet.DeBacker@ugent.be
Buoy Geometry
Two buoy shapes are investigated:
-A cone (top angle 90 °) + cylinder
-A hemisphere + cylinder
Size and draft of the buoy are varied:
-Diameters: 3 – 3.5 – 4 – 4.5 – 5 m
-Draft: 3 different drafts between 2 and 
4.5 m
Platform Size
The considered platform measures are: 
40 m  x 40 m x 20 m
Linear Model
-The hydrodynamic coefficients for heave in 
the equation of motion are determined with the 
boundary element method software Wamit.
-Energy extraction is modelled by an external
damping force, proportional to the velocity. 
-Motion control is realized by adding
supplementary inertia msup which allows for
adapting the natural period of the floater.
Restrictions & Absorbed Power
-A restriction on the displacement of the floater 
is applied to decrease the probability of 
slamming, a phenomenon that may occur 
when the buoy looses contact with the water 
surface.
-A second restriction is imposed by the limited 
stroke of the mechanical system connecting the 
point absorber to the platform. 
These restrictions reduce the power that can be 
absorbed, as can be clearly seen in the figures 
on the right. 
For a given stroke and slamming criterion, 
optimal values of external damping and 
supplementary mass can be selected in order to 
maximize the power that can be absorbed, 
which is shown in the figure below.
Discussion & Future Work
-The cone-cylinder shape has a slightly better
performance than the hemisphere, but the 
difference is only 3 to 5 %.
The graphs show that the slamming criterion is 
less restrictive than the stroke criterion, at least 
when the draft is not too low. 
-When the restrictions get more rigorous, power 
absorption drops faster in higher energy
classes. 
-In a next step these numerical simulations will
be validated by scale model tests. Future work
comprises also the investigation of the mutual
interaction between the point absorbers in order 
to find an optimal configuration of the point
absorbers in the platform.
Power that can be absorbed if optimum control is achieved
as a function of the significant wave height (Hs) class. All
buoy shapes have a diameter of 4 m. The maximum 
allowed stroke was 10 m.
Absorbed power [W] as a function of external damping 
and supplementary mass with implementation of 
slamming and stroke limitations for Hs values between 1 
and 1.5 m. The buoy is a cone-cylinder with diameter of 
4m and draft of 3 m. The area enclosed by the contour 
lines has to be avoided in order to satisfy the restrictions. 
Absorbed power [W] for the same buoy geometry but for 
a more energetic wave class: Hs between 2 and 2.5 m. 
Floater
Power 
take-off
(PTO)
Supplementary
inertia
External
damping
0
10000
20000
30000
40000
0-0
.
5
0.5
-
1
1-1
.
5
1.5
-
2
2-2
.
5
2.5
-
3
3-3
.
5
3.5
-
4
4-4
.
5
4.5
-
5
Hs [m]
P a
bs
 
[W
]
Cone - diam: 4m - draft: 2.5m Cone - diam: 4m - draft: 3m
Cone - diam: 4m - draft: 4m Hemipshere - diam: 4m - draft: 2.5m
Hemisphere - diam: 4m - draft: 3m Hemisphere - diam: 4m - draft: 4m


Numerical modelling of wave energy absorption by a floating point absorber system [POSTER]

De Backer, Griet

Vantorre, Marc

De Rouck, Julien

Ghent University Academic Bibliography

Numerical modelling of wave energy absorption by a floating point absorber system

Numerical modelling of wave energy absorption by a floating point absorber system [POSTER]

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Ghent University Academic Bibliography