11.4.1.4 Mussel Longlines: Attachment to a Monopile
For the connection of a longline and a monopile foundation a numerical model was
built up to provide information of the parameters influencing the reactions at the
longline fixation. The analysed load cases were quasi-static calculations for extreme
loads. In support of the model approach, an entire monopile-longline-monopile
construction was set up in the offshore waters 16 nautical miles off the Island of Sylt
at a water depth of about 17 m (Figs. 11.24 and 11.25a–i). Loads at one junction of
the longline with the foundation as well as in between various collector types and
the longline were measured and environmental conditions were assessed by sensors
installed on site over a period of 5 months (May-September 2006).
For the test period a submerged longline design was chosen due to its ability to
withstand strong oceanographic conditions (Buck 2007, Langan and Horton 2003).
The submerged construction consisted of a buoyed, horizontal running backline,
which is commonly described as a back-bone of the entire harness (Figs. 11.24 and
11.25c). This back-bone (polypropylene, 32 mm Ø) was fixed via spliced ends
(including thimbles) to a 1.5 m chain with shackles and swivels (service loads
above 20 tons) at both ends to the test piles in a depth of about 5 m (low tide level).
In this project the investigation of the settlement success of mussel larvae on
artificial collector substrates was not the main objective. Moreover, the concentration of mussel larvae in the water column even in June to July, which is the main
peak season for drifting mussel larvae (Buck 2017), was already determined to be
very low at the study site. Thus, artificial test bodies were introduced, which had a
similar diameter, shape and weight to simulate a fully colonized market-sized
mussel collector. For this purpose, 22 V-shaped test bodies were fastened perpendicular to the longline and served as “non-collecting” collector harness.
The mechanical loads were measured at three strategically important positions
along the line using submersible force sensors (Althen TCA-256). One sensor
acquired the tension of the main leash rated for a load of 200 kN was located
directly at the attachment of the long line. Two additional force sensors each
ruggedized for 10 kN of load were placed above the V-shaped mussel collectors in
order to confirm the charge on a single mussel collector.
Data from the force sensors was recorded on a custom made submersible data
logger (iSiTEC) located in a cage fixed to one pile in 5 m water depth. The measurement was accomplished in a 1 h interval, whereby 180 measurements were
noted within a 3 min burst. This cage also held a battery supplying energy for the
JFig. 11.25 a Location of the test site 16 nautical miles off the island of Sylt (ODAS-foundation);
b fully overgrown collector with mussels (Mytilus edulis); c Drawing of the longline device
connected in between two foundations showing the V-shaped collector types; d former
ODAS-foundation simulating the wind turbine monopile foundation, to which one end of the
longline was connected; e underwater load cell; f steel clamp to connect one end of the longline to
the foundation (the buoys were only fixed to the clamp during transfer from the vessel to the
foundation); g complete longline harness on land; h artificial test body; i drawing of the
underwater data logger. Images AWI/Prof. Dr. Bela H. Buck
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For the connection of a longline and a monopile foundation a numerical model was
built up to provide information of the parameters influencing the reactions at the
longline fixation. The analysed load cases were quasi-static calculations for extreme
loads. In support of the model approach, an entire monopile-longline-monopile
construction was set up in the offshore waters 16 nautical miles off the Island of Sylt
at a water depth of about 17 m (Figs. 11.24 and 11.25a–i). Loads at one junction of
the longline with the foundation as well as in between various collector types and
the longline were measured and environmental conditions were assessed by sensors
installed on site over a period of 5 months (May-September 2006).
For the test period a submerged longline design was chosen due to its ability to
withstand strong oceanographic conditions (Buck 2007, Langan and Horton 2003).
The submerged construction consisted of a buoyed, horizontal running backline,
which is commonly described as a back-bone of the entire harness (Figs. 11.24 and
11.25c). This back-bone (polypropylene, 32 mm Ø) was fixed via spliced ends
(including thimbles) to a 1.5 m chain with shackles and swivels (service loads
above 20 tons) at both ends to the test piles in a depth of about 5 m (low tide level).
In this project the investigation of the settlement success of mussel larvae on
artificial collector substrates was not the main objective. Moreover, the concentration of mussel larvae in the water column even in June to July, which is the main
peak season for drifting mussel larvae (Buck 2017), was already determined to be
very low at the study site. Thus, artificial test bodies were introduced, which had a
similar diameter, shape and weight to simulate a fully colonized market-sized
mussel collector. For this purpose, 22 V-shaped test bodies were fastened perpendicular to the longline and served as “non-collecting” collector harness.
The mechanical loads were measured at three strategically important positions
along the line using submersible force sensors (Althen TCA-256). One sensor
acquired the tension of the main leash rated for a load of 200 kN was located
directly at the attachment of the long line. Two additional force sensors each
ruggedized for 10 kN of load were placed above the V-shaped mussel collectors in
order to confirm the charge on a single mussel collector.
Data from the force sensors was recorded on a custom made submersible data
logger (iSiTEC) located in a cage fixed to one pile in 5 m water depth. The measurement was accomplished in a 1 h interval, whereby 180 measurements were
noted within a 3 min burst. This cage also held a battery supplying energy for the
JFig. 11.25 a Location of the test site 16 nautical miles off the island of Sylt (ODAS-foundation);
b fully overgrown collector with mussels (Mytilus edulis); c Drawing of the longline device
connected in between two foundations showing the V-shaped collector types; d former
ODAS-foundation simulating the wind turbine monopile foundation, to which one end of the
longline was connected; e underwater load cell; f steel clamp to connect one end of the longline to
the foundation (the buoys were only fixed to the clamp during transfer from the vessel to the
foundation); g complete longline harness on land; h artificial test body; i drawing of the
underwater data logger. Images AWI/Prof. Dr. Bela H. Buck
11 The German Case Study: Pioneer Projects of Aquaculture …
297