Graduation of Eva Siderius

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 last decade. The Dutch Government is also ambitious to achieve
biodiversity goals in the Dutch North Sea, by restoring ecosystem functions. The ecological value in
the development of offshore wind farms is therefore considered an important aspect and its
significance is only increasing.

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 at offshore wind farms, to kick-start
oyster reef restoration. No larvae source is nearby, therefore oysters need to be introduced for
oyster reefs to grow at 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 specific vessels), on the scour protection at
offshore wind farms. However, this 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, the 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 by people is selected, which ensures easy deployment (compared to crane installation),
because it requires fewer (to none) equipment. This would decrease the engaged expenses
significantly. Such a new installation method requires a different design for the brood stock
structure. Research is necessary in 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 optimal 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 six basic concept designs are
selected, using a set of design criteria. The selected basic concepts are; Reference block, Xblock,
Tetrapod, Cube framework, Anchor, Open table. The design criteria considered for this selection
result in preferred properties for the design of the concept. However, two set design criteria result in
specific requirements, which should be met; the positioning and stability criteria. The positioning
criteria defines a requirement, that the maximum allowed spreading radius for the brood stock
structures is 5.5 meter. The stability criteria defines a requirement, that the brood stock structures
should remain stable during a storm event with a return period of 10 years. A numerical model is
created, which makes behavioural predictions for the performance of the concepts during the
lifetime of the droppable broodstock structure. The numerical model investigates three relevant
situations; fall from the vessel until the scour protection, the landing on the scour protection, the
stability during storm events. The numerical model is used to select the most suited concept
parameters (e.g. volume, dimensions) for the six basic concepts. Ten concepts emerged; Reference
block, Xblock, Tetrapod, Cube framework, Piebox framework, Anchor long, Anchor short, Open table
1, Open table 2 and Open table 3.

Physical model tests were performed to test the suitability of the ten selected concepts based on
their behaviour during relevant situations. The physical model tests are performed in the wave flume
in the Hydraulic Engineering Laboratory at TU Delft. Three types of tests are executed; fall test, land
test and stability test. In the fall test, the fall of the structure from the vessel onto the scour
protection is investigated, to define the spreading during the fall. This is done by dropping the
prototypes in the wave flume and analysing the fall movement. The results are processed to full
scale using extrapolation. Four concepts comply with the positioning requirement, a maximum
spreading radius of 5.5 meters. 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 to the landing. An indication about the amount of oyster lost can define
the amount of structures needed per scour protection, to obtain the required oyster population for
restoration. No valued requirement (arisen from the design criteria) was defined for the results
obtained in the land test. However, minimal oysters loss and minimal amount of structures are
preferred. Four concepts scored best on these criteria. The stability test investigates stability of the
concepts during extreme hydraulic conditions, by generating storm conditions in the wave flume.
The conditions at which the concepts fail to remain stable are identified. The stability requirement
implied that the brood stock structure should at least remain stable during a storm event with a
return period of 10 years. Four concepts comply with this set requirement.

To select the optimal design for a droppable flat oyster brood stock structure, the ten concepts are
analysed and assessed, using two methods. The first method entails a multi-criteria analysis, which
uses all the defined design criteria to assess the ten concepts. This method determines the Piebox
framework to be the most suited concept, followed by the Open table 2 and the Tetrapod. The
second method entails a requirement analysis, which is fully dependent on the two set requirements
(positioning and stability). Only the Tetrapod complies with both requirements. The Open table 2
only slightly exceeds the positioning requirement. Using these two methods, two concepts are
selected to have an optimal design to act as a droppable oyster brood stock structure for the specific
conditions in which the concepts are investigated. These concepts are the Tetrapod and the Open
table 2 concept.

For application of an oyster brood stock structure, the type of installation method and type of brood
stock structure need to be decided. The crane method ensures ecological value but is expensive. The
drop method ensures easy installation, but oysters are certainly lost during deployment. The choice
of a drop method versus a crane method is therefore a trade-off between financial and ecological
value. Possible follow-up research could focus on a cost-benefit analysis of the two installation
methods, including the corresponding structures, to make a well-informed decision for each
application. When a drop method is applied, a suitable concept must be selected. The most suitable
concepts selected in this research, are determined based on preferences and requirements. These
preferences and requirements have emerged from the design criteria set in this study and the
specific conditions selected, for which the concepts have been tested. For each application, these
preferences and conditions differ and should be adjusted for the specific situation. The selection of
the concepts is to be redefined using this research, to obtain the optimal design for a droppable flat
oyster brood stock structure.