The Thornton Bank offshore wind farm covers a total surface area of approximately 13.8 km² and is located around 27 km from the port of Zeebrugge in Belgium. The wind farm (Phases 1, 2 and 3) consists of 54 WTGs with a combined capacity of ca. 325 MW, and with an individual WTG capacity of 5 MW (Phase 1) and 6 MW (Phases 2 & 3) each. The wind farm is equipped with one offshore transformer station from where 2 export cables are transferred to the onshore grid system.
The second export cable was laid parallel to the phase 1 export cable and crossed international shipping lanes and the PEC telecom cable before coming ashore via directional drilling operations under the dunes. The cable route length is approximately 35.5 km, with a target burial depth of 1.5 m in sandy soils and 1 m in clay. Furthermore, approximately 1 km of spare cable was laid and buried next to the export cable.
The contracted work comprised the following:
The cable was laid by the Stemat Spirit. The onshore landing was carried out in cooperation with the marine department of Visser Smit Hanab. The Stemat Spirit beached at Ostend, from where a winch pulled the cable ashore. In the shallow water section near the beach, the Stemat Spirit was moved by means of anchors. At a distance of approximately 8 km from the beach, the vessel changed into DP mode. One of the challenges during the cable-laying operation was the monitoring of the cable and touchdown point. This was done by a separate vessel using multibeam and echoscope. As the results were visible in real-time on the Stemat Spirit, it was possible to adjust the position of the vessel during operations. At the offshore substation, the cable was laid in an S-shape to create the overlength needed on the platform. The pull-in was monitored and supported by divers. The cable bight was lifted with airbags in order to decrease the pull-in force. Approximately 17 km of the route was pre-dredged, because of the hard soil conditions and boulders. The other sections were buried with two jetting tools. A barge was used in the shallow waters and a DP vessel in the deeper sections. Thanks to the close co-operation between all parties, the export cable was successfully installed.
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