Droppable oyster brood stock structure

Abstract

In 2050, the entire energy consumption in the Netherlands is foreseen to come from renewable sources. Offshore wind energy has the potential to enable this transition to a CO2-emission-free energy supply. Consequently, the development of offshore wind farms in the Dutch North Sea has been expanding enormously in the last decade. The Dutch Government is also ambitious to achieve biodiversity goals in the Dutch North Sea, by restoring ecosystem functions. The European flat oyster is identified as a key species when restoring the biodiversity in the North Sea, because they embody a distinctive benthic community that provides a range of valuable ecosystem services. The environmental conditions in offshore wind farms have shown to be suitable to act as a habitat for the European flat oyster reefs, because of hard substrate and undisturbed areas. Multiple oyster recovery initiatives have been introduced in offshore wind farms, to kick-start oyster reef restoration. No larvae source is nearby, therefore oysters need to be introduced for oyster reefs to develop in offshore wind farms. These oysters are introduced by applying oyster brood stock structures, which are structures to which adult oysters are attached. The brood stock structures are installed by onboard cranes present at vessels, on the scour protection at offshore wind farms. This crane installation method has shown to be complicated and costly, due to the vessel type and equipment required. These complications result in a challenging and therefore limited application. Hence, there is a need to look for an alternative installation method, that ensures a simple and cost-effective application. Deployment of brood stock structures via dropping from a vessel manually is selected, which ensures easy deployment, because it does not require a crane (and a vessel that can hold this crane). This would decrease the engaged expenses significantly. Such a new installation method requires a different design for the brood stock structure. Research is needed into a new design for a flat oyster brood stock structures, which is appropriate for the drop installation method. This leads to the following research objective for this master thesis;

What is the design for a flat oyster brood stock structure, that can be installed at the scour protection at offshore wind farms, via dropping from a vessel, such that it will be stable and integer during deployment and operational lifetime?

Based on literature study and consultation sessions with experts, a set of design criteria for the
droppable brood stock structure are set. Behavioural predictions are made for the performance of the concepts during the lifetime of the droppable broodstock structure by performing calculations.. which are used to select the most suited concept parameters (e.g. volume, dimensions) for the six basic concepts by an iterative process. Ten concepts emerged.

Physical model tests were performed to test the behaviour of the ten selected concepts during the
relevant situations. Three types of tests are executed; fall test, land test and stability test. The physical model tests are performed in the wave flume in the Hydraulic Engineering Laboratory at TU Delft.
The fall test investigated the fall of the structure from the vessel onto the scour protection, to define
the dropping accuracy during the fall.
The land test investigates the landing of the structure on the scour protection, by analysing the
interaction between the prototypes and the stone layer after dropping them in the wave flume. The
interaction observation is used to get insight in the amount of oyster damage encountered during the landing.
The stability test investigates the stability of the concepts during extreme hydraulic conditions, by
generating storm conditions in the wave flume.

To select the design for a droppable flat oyster brood stock structure, the ten concepts are analysed
and assessed, using two methods. The first method entails a requirement analysis and a multi-criteria analysis. Two concepts have been selected as suitable based on these methods, A Tetrapod concept and an Open Table concept.