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Structure stability analyses in Borssele OWF and OTS

The aim of the Rich North Sea program is to enhance nature within offshore windfarms by stimulating biodiversity enhancement measures. One of these measures is to place a number of artificial reef structures on the seabed within the offshore wind farms (OWFs) vicinity. One of the first locations where these reefs are placed is in the North Sea Farmers Offshore Test Site (OTS), 12 km off the coast of Scheveningen. Additionally, it is researched if these reefs can also be placed in Borssele next to TenneTs substation Borssele Beta.

In this study performed by Waterproof Marine Consultancy & Services BV. the stability of the proposed structures is verified. Thereto, first the extreme MetOcean boundary conditions (waves, flow) are identified. Next, the expected scour in the vicinity of the structures is calculated considering the local morphological circumstances (soil composition, bedforms, depth, etc.). Using both flow flume experiments and analytical calculations the stability of the structures is determined, both for the failure mechanism turning over as well as sliding.

The result of the study is that scour protection should be applied, to prevent the structures from sinking into the bed. The scour protection requirements for the Offshore test site are higher than for Borssele Beta, especially because the water depth at the Offshore test site is relatively limited. Moreover, the study showed that additional measures need to be taken to prevent the structures against turning over / sliding, as for both locations the stability of the structures is not sufficient to be able to withstand a 1/100 year storm.

Recommendations are proposed to increase the stability, during and after instalment, by e.g. increasing the weight of the structures and making them symmetrical in shape.

Author: Waterproof Marine Consultancy & Services BV.

Reference: WP2020_1218

Date: 31/09/2020

Keywords: Ecological structures, North Sea, stability, scour, scour protection

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