11.4.1.1 Mussel Longlines: Design
In Project No. 3 (Roter Sand) different anchored longlines were installed in order to
test their suitability under open sea conditions in terms of material and functionality
and to obtain insights on how to connect these systems to wind farm foundations.
Various test devices of horizontal ropes anchored to the seafloor with buoys to
provide flotation and vertical droppers for spat collection and/or mussel grow-out
were deployed offshore in multi-use with the wind farm “Energiekontor” 17 nautical miles off Bremerhaven.
The first system had a polypropylene (PP)-based design (Fig. 11.20a) and was
deployed by the Water and Shipping Agency (WSA) Bremerhaven in 2002. The
3-stranded polypropylene-based longline (ø = 32 mm) was made of two 35 m long
segmental parts connected to each other. To keep the longline afloat each segment
was equipped with one 80 kg buoyancy barrel in the centre and with 115 kg barrels
at the coupling to the anchor-line. Further, a few 35 kg ball-like surface marker
floats were fixed to a 5 m rope, which was connected to the longline. Three steel
barrels filled with concrete (200 kg) tightened the longline to the sea bottom at the
coupling to the anchor-line and in the centre. Another design was the steel
hawser-based design (Fig. 11.20e), which was deployed a year later. The
3-stranded steel hawser-based longline (ø = 20 mm) had a similar set up as it was
described for the polypropylene line, however, the longline consisted of more
sections: seven 10 m long segments with each three 10 kg submersed floats in the
centre and at each coupling elements 21 kg submersed floats. All floats were
connected to a short wire element, which itself was spliced perpendicularly into the
hawser and squeezed by one of the hawser’s strands. An alternative attachment was
the use of flat steel panels tightly fixed to the longline to which both, the floats and
the collectors, were attached. Due to the weight of the steel wire, the longline had
more floats than the polypropylene-based longline and no additional weights were
attached to the longline. At all coupling elements pencil-like fenders (28 kg) were
attached to mark the longline at the surface. The third system was based on the
initial PP-design (Fig. 11.20f) but was equipped with V-shaped collector pairs.
All longline designs and moorings were based on the same fundamental setup
having longlines operating in a submerged mode at a depth of about 5 m horizontally below the surface to avoid the destructive effects of surface waves. All
longlines had a 10 m “undisturbed end” at both sides, which could not be retrieved
when sampling or harvesting. Buoys were fixed along the longline in combination
with suspended mussel spat collectors. The three-stranded collectors (Galician type,
2.5 m long), equipped with horizontal inserted pegs to prevent attached mussels to
be shaken off through current and wave forces, were suspended every 2 m perpendicular from the longline in a parallel manner in the first two longline set-ups. In
the third design V-shaped pairs consisted of two 2.5 m side pieces and were
connected in series in a suspended manner and in perpendicular fashion from the
longline. Each collector had an additional weight of approximately 3 kg at the
bottom end and was on the top end hooked into the longline by shackles to ease
the maintenance during harsh conditions. Tufts of unravelled polypropylene lines
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In Project No. 3 (Roter Sand) different anchored longlines were installed in order to
test their suitability under open sea conditions in terms of material and functionality
and to obtain insights on how to connect these systems to wind farm foundations.
Various test devices of horizontal ropes anchored to the seafloor with buoys to
provide flotation and vertical droppers for spat collection and/or mussel grow-out
were deployed offshore in multi-use with the wind farm “Energiekontor” 17 nautical miles off Bremerhaven.
The first system had a polypropylene (PP)-based design (Fig. 11.20a) and was
deployed by the Water and Shipping Agency (WSA) Bremerhaven in 2002. The
3-stranded polypropylene-based longline (ø = 32 mm) was made of two 35 m long
segmental parts connected to each other. To keep the longline afloat each segment
was equipped with one 80 kg buoyancy barrel in the centre and with 115 kg barrels
at the coupling to the anchor-line. Further, a few 35 kg ball-like surface marker
floats were fixed to a 5 m rope, which was connected to the longline. Three steel
barrels filled with concrete (200 kg) tightened the longline to the sea bottom at the
coupling to the anchor-line and in the centre. Another design was the steel
hawser-based design (Fig. 11.20e), which was deployed a year later. The
3-stranded steel hawser-based longline (ø = 20 mm) had a similar set up as it was
described for the polypropylene line, however, the longline consisted of more
sections: seven 10 m long segments with each three 10 kg submersed floats in the
centre and at each coupling elements 21 kg submersed floats. All floats were
connected to a short wire element, which itself was spliced perpendicularly into the
hawser and squeezed by one of the hawser’s strands. An alternative attachment was
the use of flat steel panels tightly fixed to the longline to which both, the floats and
the collectors, were attached. Due to the weight of the steel wire, the longline had
more floats than the polypropylene-based longline and no additional weights were
attached to the longline. At all coupling elements pencil-like fenders (28 kg) were
attached to mark the longline at the surface. The third system was based on the
initial PP-design (Fig. 11.20f) but was equipped with V-shaped collector pairs.
All longline designs and moorings were based on the same fundamental setup
having longlines operating in a submerged mode at a depth of about 5 m horizontally below the surface to avoid the destructive effects of surface waves. All
longlines had a 10 m “undisturbed end” at both sides, which could not be retrieved
when sampling or harvesting. Buoys were fixed along the longline in combination
with suspended mussel spat collectors. The three-stranded collectors (Galician type,
2.5 m long), equipped with horizontal inserted pegs to prevent attached mussels to
be shaken off through current and wave forces, were suspended every 2 m perpendicular from the longline in a parallel manner in the first two longline set-ups. In
the third design V-shaped pairs consisted of two 2.5 m side pieces and were
connected in series in a suspended manner and in perpendicular fashion from the
longline. Each collector had an additional weight of approximately 3 kg at the
bottom end and was on the top end hooked into the longline by shackles to ease
the maintenance during harsh conditions. Tufts of unravelled polypropylene lines
11 The German Case Study: Pioneer Projects of Aquaculture …
289