Geotechnical design for Dolwin 2 wind energy platform

The strong expansion of offshore wind energy in the German North Sea and Baltic Sea, requires high-performance AC-DC power converters on offshore platforms, to ensure a safe and efficient feed-in of the electricity produced. NGI has been responsible for the geotechnical engineering for the Dolwin 2 platform.

With DolWin2 the so-far world largest offshore converter is currently developed. It will have a capacity of 900 MW and can serve 1 to 3 wind farms depending on the number a capacity of the installed turbines. A novelty for these type of platforms is the accomplishment of the foundation as a Gravity-Base-Structure (GBS). The support-structure with pontoons on the base rests directly on the seafloor. The outer dimensions of the foundation are 92.3x72.8 m, while the pontoons have a width of 14.3 m only. The water depth is approximately 29 m. The subsoil consists mainly of medium dense to very dense sands, intermitted by a thick, over-consolidated, clayey silt layer.

Various combinations of skirts, ribs, grouting and soil improvements have been studied in the detailed design. The results have been evaluated from a geotechnical and logistical point of view and have revealed particular advantages and disadvantages of the different combinations. Special attention has been given to the cyclic soil behavior of the different solutions. Finally the experiences gained with the application of the German standards for offshore structures were reviewed.

NGI's work is contracted by Aibel and the first wind platform that Aibel is building has been named Dolwin 2. The platform is a collaboration project between Aibel and ABB. Dolwin 2 will be built for a large wind farm cluster in the German sector of the North Sea. It will receive alternating current from wind farms, and convert it into direct current before sending it onshore via subsea cables. Aibel is designing and building the platform, whilst ABB has overall project responsibility and is supplying cables and the conversion equipment.

Dolwin 2 will be towed into position and placed on the sea bed. It has separate living quarters for 24 people, a helipad and two lifting cranes. The platform will be built by a subcontractor, and will be fitted out at Aibel's yard in Haugesund in Norway. In the summer of 2014, Dolwin 2 will be towed out to the Dolwin field.