Introduction
As part of the Interreg Wier en Wind project, a full-scale offshore seaweed cultivation system was installed in the Dutch part of the North Sea, 12 km off the coast of Scheveningen. The system was installed in February 2022 and the seaweeds were harvested in June 2022. During this period, a storm of 5.01 m significant wave height was recorded at the location. This study focuses on the impact of environmental conditions of wave, current and tidal variation, as well as biomass, to the measured load in the full-scale seaweed cultivation system.
Materials and methods
The cultivation system consists of a 50 m x 2.1 m net structure on which the seaweeds grown. A 58-meter polyethylene (PE) pipe is utilized as a floatation device to provide buoyancy to the cultivation line. The distance between the two anchors is 230 m at a water depth of 18 m. A main rope is used to connect the net structure to a shackle that connects to the mooring chain. The main rope is connected to the PE tube using loops that are flexible to move along the pipe. As such, the PE tube does not act as a load bearing component. The general arrangement (GA) of the system is visualized in Fig 1. A force gauge with a sampling frequency of 0.2 Hz was fitted to measure the tension experienced by the main rope.
Results and discussions
During the measurement campaign, the highest peak tension was recorded during the harvesting activities. This value is 44 % higher compared to the highest peak load during the system’s operational condition. The measured load compared to the water level
variations, changes in current magnitudes and significant wave heights (Rijkswaterstaat 2023) is presented in Fig 2. The recorded highest and second highest tensions during operational condition occurred during ebb current and flood current, respectively. This indicates that the tidal variations have dominant influence to the measured peak tension. There are two time windows where the significant wave height is higher than 4.5 m. In those cases, the measured peak load is only up to 66 % of the highest peak tension during operational condition (i.e., excluding harvesting). To analyse the influence of biomass, two time windows are compared to observe the mean measured force, presented in Fig 3. In the first time window (21/02/2022 – 25/02/2022) shown in blue dots, the net structure had just been installed, therefore, no biomass increase is present. This is the opposite for the second time window where the biomass has increased more than 1 month after it was installed. From Fig 3, no clear trend can be observed from the influence of the biomass on the mean measured force.
References
Rijkswaterstaat. 2023. “Rijkswaterstaat Waterinfo.” 2023. https://waterinfo.rws.nl/.
Acknowledgement
This project receives support from the European Union / European Regional Development Fund (ERDF). The partners in the Interreg Wier en Wind project are: GeoXYZ, Ghent University, HZ University of Applied Science, Murre Technologies, North Sea Farmers, The Seaweed Company. Load measurement was facilitated by Aqitec Projects B.V.