(Tortell 1976) were also connected to the longline to determine the settlement of
mussel post-larvae. The longline was secured at each end with an anchor consisting
of a 4 t concrete block. A chain cable attached to the concrete block was connected
to the anchor-line, which itself held one end of the longline. Additionally, each
concrete block was equipped with a marker-line and a marker buoy (300 kg). The
marker line had a service load of several tonnes that it could be used to tauten the
longline and to retrieve the concrete block at the end of the project. All mooring
components were selected and sized for the use in muddy sea bottom and for long
term durability. All longline segments had spliced eye loops with embedded galvanized thimbles at the ends and at all coupling elements shackles and swivels were
used (Fig. 11.20b–d).
All longline systems and their mooring devices were first modelled and designed
with the WSA. A local cable and wire manufacturer supported the engineering
development for the culture design. All components were selected and procured to
start the preparation and assemblage of the longline during wintertime.
After installation all longlines and their collectors were fully overgrown by
mussels. Heavy mussel growth was found also on all mooring components and a lot
of mussels were recorded on the undisturbed ends of the longline at each side. From
the farmer’s perspective, the offshore site was suitable in terms of settlement and
grow-out. Problems encountered during the test trials included material failure,
heavy fouling on the collector lines and other infrastructural units resulting in large
ballast weights, while heavy predation on mussel seed by sea stars and Eider ducks
also happened. Another difficulty was maintaining proper headline depth and its
management with the right buoyancy due to mussel growth. This added drag forces
to the entire longline, which in turn complicated the handling during hostile sea
conditions.
The PP-longline design was resistant to the high energy environment of the
North Sea, but chafed within the inner parts of the eye loops and therefore some of
the thimbles were lost. These stress forces affect the lifetime greatly. The segmentation set-up in the second longline design allows the replacement of spare
parts. The weight of the suspended collector harness increased significantly with
time, and extra floatation had to be added to support the buoyancy of the longline.
However, the time intervals chosen for service were too long (particularly during
the summer growing season) so that parts of the collector harness including the
floats submersed fully and partly touched the seafloor. Additionally, this process
was exacerbated by the fact that the loss of buoyancy resulted in a further lowering
of the total construction. When the longline was retrieved the concrete blocks were
broken easily out of the clay at the site and raised by the anchor chain. Most of the
mooring system, in particular the longline and its harness, was later inspected
onshore. On a few spots near the eye loops some slight damage of the material was
found, but, after removing the fouling, most of the line looked intact and almost
new.
11 The German Case Study: Pioneer Projects of Aquaculture …
291
mussel post-larvae. The longline was secured at each end with an anchor consisting
of a 4 t concrete block. A chain cable attached to the concrete block was connected
to the anchor-line, which itself held one end of the longline. Additionally, each
concrete block was equipped with a marker-line and a marker buoy (300 kg). The
marker line had a service load of several tonnes that it could be used to tauten the
longline and to retrieve the concrete block at the end of the project. All mooring
components were selected and sized for the use in muddy sea bottom and for long
term durability. All longline segments had spliced eye loops with embedded galvanized thimbles at the ends and at all coupling elements shackles and swivels were
used (Fig. 11.20b–d).
All longline systems and their mooring devices were first modelled and designed
with the WSA. A local cable and wire manufacturer supported the engineering
development for the culture design. All components were selected and procured to
start the preparation and assemblage of the longline during wintertime.
After installation all longlines and their collectors were fully overgrown by
mussels. Heavy mussel growth was found also on all mooring components and a lot
of mussels were recorded on the undisturbed ends of the longline at each side. From
the farmer’s perspective, the offshore site was suitable in terms of settlement and
grow-out. Problems encountered during the test trials included material failure,
heavy fouling on the collector lines and other infrastructural units resulting in large
ballast weights, while heavy predation on mussel seed by sea stars and Eider ducks
also happened. Another difficulty was maintaining proper headline depth and its
management with the right buoyancy due to mussel growth. This added drag forces
to the entire longline, which in turn complicated the handling during hostile sea
conditions.
The PP-longline design was resistant to the high energy environment of the
North Sea, but chafed within the inner parts of the eye loops and therefore some of
the thimbles were lost. These stress forces affect the lifetime greatly. The segmentation set-up in the second longline design allows the replacement of spare
parts. The weight of the suspended collector harness increased significantly with
time, and extra floatation had to be added to support the buoyancy of the longline.
However, the time intervals chosen for service were too long (particularly during
the summer growing season) so that parts of the collector harness including the
floats submersed fully and partly touched the seafloor. Additionally, this process
was exacerbated by the fact that the loss of buoyancy resulted in a further lowering
of the total construction. When the longline was retrieved the concrete blocks were
broken easily out of the clay at the site and raised by the anchor chain. Most of the
mooring system, in particular the longline and its harness, was later inspected
onshore. On a few spots near the eye loops some slight damage of the material was
found, but, after removing the fouling, most of the line looked intact and almost
new.
11 The German Case Study: Pioneer Projects of Aquaculture …
291