Environmentally friendly grid connection of offshore wind farm clusters

For the grid connection of the offshore wind farm clusters “Westlich Adlergrund” and “Arkona See” in the Baltic Sea, grid operator 50Hertz used the ege-com^® Macroduct High-T.

A total of 18.7 km of ege-com^® Macroduct High-T protective pipe for high and extra-high voltage cables was installed trenchlessly using the horizontal directional drilling method. This reduces environmental impact when bringing green energy from wind turbines to the mainland.

The expansion and connection of offshore wind farms are crucial for the success of the energy transition. In the Ostwind 1 project, two of the offshore wind farms located in the Westlich Adlergrund (CWA) cluster were to be connected to the grid. This area is considered particularly suitable for the construction of wind energy facilities. Grid operator and developer 50Hertz is responsible for the realization of the project.

To connect the Wikinger and Arkona-Becken-Südost offshore wind farms of the CWA, three approximately 90 km long submarine cables were laid, which were installed in the Baltic Sea seabed by special vessels. The landing of the 270 mm thick cables, weighing 130 kg per meter, was carried out via a duct system that begins approximately 400 m off the coastline of Greifswald Lagoon. On the land side, the cables are routed as 3 individual cores, also in a duct system, to the 50Hertz substation in Lubmin, about 2.5 km away.

Increased thermal loads occur with high and extra-high voltage cables. Therefore, the ege-com^® Macroduct High-T PE was used for this project. It is a polyethylene protective pipe for high and extra-high voltage cables up to 380 kV. The protective pipe features increased thermal stability due to the special PE-HD material, allowing it to withstand high thermal stresses for a long time. To minimize environmental impact during the installation of the duct system for the 220 kV cable, the pipes were laid using the horizontal directional drilling (HDD) method. Due to its flexibility and unchanged behavior at low temperatures, the pipe is ideal for trenchless installation methods. It is suitable for butt welding, which can withstand the high tensile and bending stresses that occur during the HDD process.

Landing

For the landing of the submarine cables, the duct system was installed using the environmentally friendly horizontal directional drilling method. Since a nature reserve was crossed and the target pits were located in Greifswald Lagoon, special environmental protection requirements had to be observed. For example, no problematic substances were allowed to be discharged into the water. On the sea side, six temporary sheet pile boxes were erected as target pits, collecting the drilling fluid approximately 400 m off the coastline.

The ege-com^® Macroduct High-T PE 800 x 58.5 mm was laid out and welded in the area of Lubmin harbor. Subsequently, the pipes were launched into the water in the harbor and towed by water to the landing bore. For this operation, Lubmin harbor had to be temporarily closed. Matthäi-Wasserbau GmbH was responsible for the hydraulic engineering. During the pulling-in process to excavation pit A, the protective pipe had to be held in position so that wind and waves could not displace it. With up to 700 m, the landing bores were the longest along the entire route.

Land Route

Due to the nature reserves, extended environmental protection measures also had to be taken on land. For example, visual/noise barriers were erected for sensitive bird species. Matthäi Bauunternehmen Westerstede was responsible for the creation and maintenance of the construction sites and access roads, as well as the overall technical coordination.

From excavation pit A, where the submarine cables terminate, the cables were continued on land as 3 individual cores. Therefore, three 250 x 22.7 mm protective pipes were used for the land route, laid in a triangular configuration. The installation also took place using the HDD method, leading in seven further bores to the Lubmin substation, approximately 2.5 km away. The company Beermann Bohrtechnik GmbH carried out a total of 42 bores on the land route (six cable routes with 7 individual bores each).

Meanwhile, the first submarine cable of the Ostwind 1 project has successfully arrived in Lubmin. A high-performance winch installed in excavation pit A pulled the cable into the 700 m long ege-com^® Macroduct High-T PE pipe.