Technical Realization of Extensive Aquaculture Constructions in Offshore Wind Farms: Consideration of the Mechanical Loads
Authors: Buck B. H. , Berg-Pollack A. , Assheuer J. , Zielinski O. , Kassen D. 
Published: October 2008

The presented study focuses on the development of offshore wind farms in conjunction with open ocean aquaculture within the German Bight. For aquaculture enterprises in the open ocean an extensive cultivation of various species, blue mussels (Mytilus edulis), oysters (Ostrea edulis, Crassostrea gigas) and seaweed (Laminaria saccharina), is considered. However, without the solid foundations for wind turbines, such as monopiles and tripods as anchor or connection points, economic installations of equipment for mariculture would not be possible in view of the high-energy environment in this part of the North Sea. Thus, one of the most important questions pertains whether it is technically possible and economic feasible to use the offshore foundation structures as fixation device for aquaculture operations, such as a longline as one possible culture design. A longline culture is an open-water suspended technology in which cultured species are on-grown on ropes or diverse substrates, such as rope collectors for the catch of mussel spat, suspended from anchored and buoyed surface or subsurface horizontal ropes (longline). For the calculation of additional foundation costs generated through the attached longline system knowledge on the supplement loads and stresses as well as on the required constructive modifications of the foundation must be obtained. The development and the conceptual design of offshore foundation structures are complex and require an interdisciplinary approach. In the presented project we aim to evaluate the induced loads between a foundation structure for offshore wind energy turbines and a longline system and to develop appropriate connection points. As monopile and tripod foundations are the most common foundation structures to date, these both will be considered within the modeling approach which calculates the respective loads from wind and waves. Both foundation structures are in the dimension of 4–5 MW wind turbines, while the monopile design will be calculated for a water depth of 10 m whereas the tripod for a water depth of 30 m. To validate the model outcomes, an entire monopile-longline-monopile construction is set up in the offshore waters 10 nautical miles off the Island of Sylt at a water depth of about 17 m. At this site, the deformation behavior of the entire system will be determined on the basis of analytically calculated loads supported by the measurement of local stresses at the junction of the longline with the foundation. An additional measurement of forces will be performed on selected mussel collectors. To evaluate the observed loads with respect to environmental conditions, namely currents and waves, accompanying sensors will be installed on site. These achieved datasets will be used for the verification of the computer model for mechanical loads at the longline. Finally it is intended to scale up the model to the proportions of a wind farm while considering the interactions in the wavefield in which the longline system will be located.