feasibility study resides in the German bight at Veja Mate, which is reportedly
114 km off the shore in the North Sea. The reported results purely base on an
experimental feasibility study since the stakeholder conflicts and the financial
demand for in situ testing would have been too extreme at the point of development. The case study aimed to shed light on the general behavior of two different
fish cage geometries being mounted to a tripile foundation for a 5 MW offshore
wind energy converter (OWEC) at different submergence levels. An aspect of the
analysis comprises the change of wave induced particle velocity and its distribution
around the support structure due to the two fish cages. A second focus has been laid
on the forces exposed to the tripile as a result of the additional fish cage bearing by
means of force measurements under monochromatic waves. Thirdly the additional
effects to potential scour around the tripile are also investigated in order to substantiate earlier findings (Goseberg et al. 2012).
The mechanical system of the fish cage and the OWEC-structure is investigated
under regular wave attack with respect to (a) the deviations in the velocity field
resulting from the fish cage underneath the OWEC, (b) the additional forces
introduced by the fish cage and (c) the potential for scour evolution at the sea
bottom of the OWEC location. Therefore a 110 m long and 2.2 m wide wave flume
at the Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering,
Hannover, Germany is applied. The experiments are scaled at Froude similitude at a
length scale of 1:40. The water depth at the wave maker (piston type) equals 1.0 m,
yet to investigate scour evolution at the OWEC-structure a 0.25 m deep sand pit
(fine sands of d 50 = 0.148 mm) is installed at a distance of 42 m of the wave maker.
At the end of the wave flume a gravel slope acts as passive wave absorption. An
overview of the wave flume, its installations and a schematic of the mechanic
system under investigation is given in Fig. 3.11. The two fish cage models were
made of solid brass. Variations with three different surface properties (net solidity)
were accomplished under the attack of monochromatic waves.
Based on a potential building site in the German bight at Veja Mate, a frequent
(mean) and an extreme wave with a 50-yearly reoccurrence interval have been
chosen. In model scale investigated wave heights are H = 0.04 m with T = 0.95 s
for the frequent case and H = 0.28 m with T = 2.2 s for the extreme wave condition respectively. Surface elevations are measured with capacity-type wave
gauges at various positions. Deviations in the velocity field around the structure and
the fish cage are recorded by either ADV probe inside the fish cage or by stereo PIV
measurements around the fish cage. Wave-induced forces at the constructional
conjunctions between fish cage and tripile legs are taken by force transducers
measuring normal forces. Force measurements are designed to consequently separate horizontal and vertical forces. Therefore all conjunctions between the fish cage
and the tripile legs are designed as pendulum rods whereas the vertical forces are
assumed to be gathered by a single tension rod connected to the upper tripile
intersection. Compression-tension type force transducers (HBM GmbH) with a
capacity of 500 N and an accuracy of 2% of its capacity were used. Scour depth
evolution was manually measured with sediment gauges of 2 mm PVC sticks
which were observed by underwater cameras (Abus GmbH). This optical method
86
N. Goseberg et al.
114 km off the shore in the North Sea. The reported results purely base on an
experimental feasibility study since the stakeholder conflicts and the financial
demand for in situ testing would have been too extreme at the point of development. The case study aimed to shed light on the general behavior of two different
fish cage geometries being mounted to a tripile foundation for a 5 MW offshore
wind energy converter (OWEC) at different submergence levels. An aspect of the
analysis comprises the change of wave induced particle velocity and its distribution
around the support structure due to the two fish cages. A second focus has been laid
on the forces exposed to the tripile as a result of the additional fish cage bearing by
means of force measurements under monochromatic waves. Thirdly the additional
effects to potential scour around the tripile are also investigated in order to substantiate earlier findings (Goseberg et al. 2012).
The mechanical system of the fish cage and the OWEC-structure is investigated
under regular wave attack with respect to (a) the deviations in the velocity field
resulting from the fish cage underneath the OWEC, (b) the additional forces
introduced by the fish cage and (c) the potential for scour evolution at the sea
bottom of the OWEC location. Therefore a 110 m long and 2.2 m wide wave flume
at the Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering,
Hannover, Germany is applied. The experiments are scaled at Froude similitude at a
length scale of 1:40. The water depth at the wave maker (piston type) equals 1.0 m,
yet to investigate scour evolution at the OWEC-structure a 0.25 m deep sand pit
(fine sands of d 50 = 0.148 mm) is installed at a distance of 42 m of the wave maker.
At the end of the wave flume a gravel slope acts as passive wave absorption. An
overview of the wave flume, its installations and a schematic of the mechanic
system under investigation is given in Fig. 3.11. The two fish cage models were
made of solid brass. Variations with three different surface properties (net solidity)
were accomplished under the attack of monochromatic waves.
Based on a potential building site in the German bight at Veja Mate, a frequent
(mean) and an extreme wave with a 50-yearly reoccurrence interval have been
chosen. In model scale investigated wave heights are H = 0.04 m with T = 0.95 s
for the frequent case and H = 0.28 m with T = 2.2 s for the extreme wave condition respectively. Surface elevations are measured with capacity-type wave
gauges at various positions. Deviations in the velocity field around the structure and
the fish cage are recorded by either ADV probe inside the fish cage or by stereo PIV
measurements around the fish cage. Wave-induced forces at the constructional
conjunctions between fish cage and tripile legs are taken by force transducers
measuring normal forces. Force measurements are designed to consequently separate horizontal and vertical forces. Therefore all conjunctions between the fish cage
and the tripile legs are designed as pendulum rods whereas the vertical forces are
assumed to be gathered by a single tension rod connected to the upper tripile
intersection. Compression-tension type force transducers (HBM GmbH) with a
capacity of 500 N and an accuracy of 2% of its capacity were used. Scour depth
evolution was manually measured with sediment gauges of 2 mm PVC sticks
which were observed by underwater cameras (Abus GmbH). This optical method
86
N. Goseberg et al.