Marine construction contractor Red7Marine has install three nearshore artificial nesting structures for Ørsted to comply with the terms of the development consent order for the Hornsea 3 offshore windfarm.
The structures were demanded as an ecological compensation measure for a vulnerable seabird species – the black-legged kittiwake (Rissa tridactyla).
Red7Marine has been involved with the project since August 2022 managing the fabrication and installation process, working with Ørsted and its designers. Two of the artificial nesting structures (ANS) are located 1km from the shoreline of South Beach, Lowestoft, and the third is 1.4 km from the shoreline of the RSPB Minsmere Nature Reserve.
Each structure is purpose-built, bespoke and specific to the landscape characteristics of its location. The structures comprise a monopile foundation and internal grouted jacket. Each monopile weighs more than 30 tonnes and is embedded 30 metres below the seabed. The upper pile sleeve is fitted with two vertical steel tubes for vessel berthing and the final component is the topside nesting structure.
The construction sequence involved lifting the monopile from a jack-up barge, pitching it through a bespoke piling gate and using a vibratory hammer to install the pile to tolerance. A self-driving percussion hammer was then used and fitted to the pile head, and the pile was driven to the required toe level. Following this, the topside jackets were grouted into position and the topside structures lifted and bolted onto structure.
Each structure has approximately 500 nesting spaces. The artificial nesting structures are designed to be attractive to kittiwake – they have narrow ledges and vertical sides to replicate the cliffs where they would naturally nest. And there are eight sides to give kittiwake options in case one side is too sunny or too windy. Seven of the eight vertical side walls are fitted with nesting compartments, barn doors have been included on the remaining side of the structure for emergency access (for humans).
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