Design of a New Foundation for Offshore Wind Turbines

The gravitation platform and the monopile have in the previous major offshore wind turbine projects been dominating. A four-year research and development project has proven the bucket foundation to be feasible in suitable soil condition in water depth from near shore to app. 40 meters. A prototype was installed at the test field in Frederikshavn in late 2003, with a 3 MW wind turbine in normal operation. The R&D work is continued the complete the bucket concept and having the design standards for the construction and installation methodologies recognised. The design saves about half of the steel weight as compared to a traditional pile foundation, it is much easier to install and it can easily be removed when the wind turbine is taken down. However, the new design is suffering from uncertainties in the accumulated fatigue in the both the steel structure and the surrounding earth material. Therefore an on-line monitoring system has been utilized on the 80 m high operating test 3 MW wind turbine. It is explained how the system is being used to obtain mode shapes and modal parameters during different operating conditions, and how the response measurements are being used to improve the estimation of fatigue. INTRODUCTION In recent major offshore wind turbine projects, Middelgrunden, Blyth, Horns Rev, Samso and Nysted, two foundation principles have be dominating, the gravitation platform and the monopile. In future project with bigger wind turbines and/or greater foundation depth tripod foundations or even jackets may be feasible. This paper describes the development of a prototype foundation of a novel principle, the bucket foundation. The bucket foundation is a new type of foundation, which has been developed over the past 3 years and today it is used for a Vestas V90-3.0 MW offshore wind turbines as a prototype. This offshore wind turbine was erected in November 2002, next to the harbour of Frederikshavn, Denmark, as shown in Figure 1. The turbine is a part of a test field concerning offshore wind turbine research consisting of four 2-3 MW offshore prototype wind turbines. In the test field a series of model tests with different foundation solutions are going to be carried out. The test field for offshore wind turbine research has been built in connection with a joint research and development program between Centre for Wind Energy Systems, Aalborg University and MBD Offshore Power. This program deals with the foundation of wind turbine in general. The test field has been constructed by laying an outer pier in the sea. The basin where the wind turbine has been erected consists of a natural Figure 1. Vestas V90-3.0 MW offshore wind turbine on the bucket foundation in the test field in Frederikshavn Denmark. seabed. The first project in the research program is the development of the bucket foundation. It is an innovative foundation solution with great potential and an economically/environmental sound construction for offshore wind turbines considering a life cycle analysis. The bucket foundation differs in its mode of operation from the wellknown “Suction bucket” or ”Suction caisson”. The suction caisson has been used as foundation for a number of different offshore constructions, for instance the Norwegian jacket Draupner E. This platform, was installed with 4 suction caissons, as shown in Figure 2. During the installation process the caissons penetrate into the seabed due to the weight of the structure and also due to the fact that suction is applied to the 4 caissons. The static system ensures that the caissons are loaded only with vertical forces from the wave loading. The stability is ensured because there is not enough time for the caissons to be pulled from the bottom during a wave period. The stability of the foundation relies on that negative pore pressure is generated inside the caisson. Comparing the bucket foundation to the suction caisson, the only thing they have in common is that they are installed in the same fashion. Both of them use suction as the driving force during installation. Lowering the pressure in the cavity between the bucket and the soil surface causes a water flow to be generated, which again causes the effective stresses to be reduced around the tip of the skirt and the penetration resistance is reduced. Figure 2: Suction bucket below the Norwegian jacket Draupner E Assuming the wind turbine is to be founded on one large bucket, the static mode of operation is very different from that of the suction caisson. When the bucket foundation has been installed, the loads from the wind on the wind turbine will cause the foundation to be influenced by a large moment. The stability of the foundation is ensured by a combination of earth pressures on the skirt and the vertical bearing capacity of the bucket. THE PROTOTYPE IN FREDERIKSHAVN The prototype in Frederikshavn is designed with a diameter of 12 meters and a skirt length of 6 meters. The water depth is 4 meters, and as the sitting is in a basin, no wave and ice loads are applied. The steel construction weight app. 140 tons, and was placed late October 2002. The actual installation period lasted app. 12 hours, where the soil penetration period lasted 6 hours, using a computer system to perform the inclination guidance and control of the suction pressure and penetration rate, see Figure. 3. Det Norske Veritas (DNV) has certified the design of the prototype in Frederikshavn to B level. The Vestas V90 3 MW turbine was erected on the foundation in December 2002. Figure 3: Installation of the prototype in Frederikshavn by suction. a) before installation. b) after installation . 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