grades. Four metal torpedoes served as buoyancy devices at the corners, another 72
floats were pencil-like fenders with 23 kg buoyancy each. The final ring device
(Patent No. PCT/DE2005/000234; Buck and Buchholz 2004b) had a total diameter
of 5 m and consisted of polyethylene tubes with a 10 mm thick wall and a diameter
of 110 mm that were welded to rings (Figs. 11.32a–d, h–l and 11.33a–p). The rings
were weighed down by a steel cable (30 mm in diameter) inserted into the tube and
obtained their buoyancy through eight elongated fenders (23 kg buoyancy each).
They consequently floated at a depth of 1.2–1.5 m. Carrier ropes were suspended
radially and 80 m of culture line could be fastened like cobwebs on each ring.
A crow’s foot was used to fasten the ring on a common mooring system. Due to
permanent chafing of the carrier ropes with the fender ropes and because the fenders
themselves got entangled with each other a modified system was developed. This
consisted of one centre buoy (300 kg buoyancy) with a connected reverse crow’s
foot and a centre guide ring to prevent chafing of the mooring line with the carrier
ropes (Fig. 11.32a, d). Furthermore, all radial splices, which connected the carrier
ropes to the polyethylene tubes, were replaced with metal cuffs (Fig. 11.32b). Three
loops were welded to these cuffs, one to the centre to fix the carrier ropes and the
other two to the bottom and the top of the cuff, to connect both crow’s feet. An
important feature common to all constructions was their ability to adjust the depth
of culture lines to 1–1.5 m as this appeared to prevent possible PAR and UV
damage to the young algae while providing enough light for successful photosynthesis. Moreover, the most turbulent upper meter of the water column could thus
be avoided.
In the following, an overview of technical experience with the different culture
devices in the offshore environments of the German North Sea is given. With the
longline design, only 65 of the 140 culture lines, which were fastened on the
longline system, have been retrieved and 20 of these were evaluated for further
investigation. Due to very stormy weather the farm had not been visited very often.
Every chance during calm weather was used to change the horizontal carrier rope
and supply new culture lines. The study, however, revealed that the weights on the
culture lines were not sufficiently heavy so that they were frequently tossed across
the carrier line resulting in the removal of the young plants by friction and causing
them to become entangled. Other culture lines, consisting of three twisted strands,
were untwisted by the current and turbulences and consequently the individual
strands were torn. Some improved performance of the longline was obtained by
connecting pairs of only 3 m long culture lines at the lower end like a V-shape.
Length data used here were taken from these lines. The ladder construction revealed
problems pertaining to the durability of the frame material, the attached weights
being potential breaking points. Moreover, the 110 L buoys at the corners were very
instable and had to be exchanged several times. These drawbacks were taken into
account during the development of the grid system. The grid system proved much
more stable compared to the “ladder” even though the mooring ropes could not be
adjusted to their optimal length to accommodate the full tidal differences. The use
of elongated fenders instead of ball floats protected the construction by better riding
the swell, which resulted in continuous vertical movement, while the balls used
11 The German Case Study: Pioneer Projects of Aquaculture …
309
floats were pencil-like fenders with 23 kg buoyancy each. The final ring device
(Patent No. PCT/DE2005/000234; Buck and Buchholz 2004b) had a total diameter
of 5 m and consisted of polyethylene tubes with a 10 mm thick wall and a diameter
of 110 mm that were welded to rings (Figs. 11.32a–d, h–l and 11.33a–p). The rings
were weighed down by a steel cable (30 mm in diameter) inserted into the tube and
obtained their buoyancy through eight elongated fenders (23 kg buoyancy each).
They consequently floated at a depth of 1.2–1.5 m. Carrier ropes were suspended
radially and 80 m of culture line could be fastened like cobwebs on each ring.
A crow’s foot was used to fasten the ring on a common mooring system. Due to
permanent chafing of the carrier ropes with the fender ropes and because the fenders
themselves got entangled with each other a modified system was developed. This
consisted of one centre buoy (300 kg buoyancy) with a connected reverse crow’s
foot and a centre guide ring to prevent chafing of the mooring line with the carrier
ropes (Fig. 11.32a, d). Furthermore, all radial splices, which connected the carrier
ropes to the polyethylene tubes, were replaced with metal cuffs (Fig. 11.32b). Three
loops were welded to these cuffs, one to the centre to fix the carrier ropes and the
other two to the bottom and the top of the cuff, to connect both crow’s feet. An
important feature common to all constructions was their ability to adjust the depth
of culture lines to 1–1.5 m as this appeared to prevent possible PAR and UV
damage to the young algae while providing enough light for successful photosynthesis. Moreover, the most turbulent upper meter of the water column could thus
be avoided.
In the following, an overview of technical experience with the different culture
devices in the offshore environments of the German North Sea is given. With the
longline design, only 65 of the 140 culture lines, which were fastened on the
longline system, have been retrieved and 20 of these were evaluated for further
investigation. Due to very stormy weather the farm had not been visited very often.
Every chance during calm weather was used to change the horizontal carrier rope
and supply new culture lines. The study, however, revealed that the weights on the
culture lines were not sufficiently heavy so that they were frequently tossed across
the carrier line resulting in the removal of the young plants by friction and causing
them to become entangled. Other culture lines, consisting of three twisted strands,
were untwisted by the current and turbulences and consequently the individual
strands were torn. Some improved performance of the longline was obtained by
connecting pairs of only 3 m long culture lines at the lower end like a V-shape.
Length data used here were taken from these lines. The ladder construction revealed
problems pertaining to the durability of the frame material, the attached weights
being potential breaking points. Moreover, the 110 L buoys at the corners were very
instable and had to be exchanged several times. These drawbacks were taken into
account during the development of the grid system. The grid system proved much
more stable compared to the “ladder” even though the mooring ropes could not be
adjusted to their optimal length to accommodate the full tidal differences. The use
of elongated fenders instead of ball floats protected the construction by better riding
the swell, which resulted in continuous vertical movement, while the balls used
11 The German Case Study: Pioneer Projects of Aquaculture …
309