The tripod or jacket is a type of foundation for offshore wind turbines. The tripod is generally more expensive than other types of foundation. However, for large turbines and higher water depth, the cost disadvantage might be compensated when durability is also taken into account.
History
Start of the offshore wind industry The exploration of offshore wind energy started with the introduction of monopile foundations for wind turbines in a range from 1 up to 3MW in water depth of about 10 to 20m during the 1990s. Germany has been facing water depths up to 40m, when it joined this new field of renewable energy. At the same time the 5MW turbine class appeared. One representative of this new turbine generation was the Multibrid M5000 with a rotor diameter of 116m, later 135m under the labels Areva and Adwen. The first prototype of this machine was erected in Bremerhaven in 2004 onshore. Already in this stage Bremerhaven had supported the development on behalf of BIS Bremerhavener Gesellschaft für Investitionsförderung und Stadtentwicklung mbH.
Development of the tripod foundation Since the new century, there has been a search for a feasible foundation for the upcoming large turbines and greater water depths, in light of the available geotechnical assessment methods, fabrication processes, pile driving equipment and logistic and installation equipment. One result was the Tripod foundation. The first design was drawn by OWT – Offshore Wind Technology in Leer (Germany) in 2005. The Tripod was integrally designed with the tower from this early beginning. The three-legged structure reaches from the sea bed up to typically 20m above the sea water level, keeping the bolted flange on top safely apart from the crest of the waves. This section allows to be outfitted onshore with all functionalities needed in terms of boat landing, cable guiding and last but not least corrosion protection systems. The central column is designed as an open system allowing an unrestricted water exchange in each tide cycle. This circumstance is beneficial when the corrosion protection system has to be designed for the inner surfaces.
The Tripod is fixed with midsized pin piles at the sea bed. The piles might be pre piled or post piled. A suction bucket foundation was designed as well. The first tower section, called S3, is foreseen to be mounted offshore on top of the Tripod with a bolted flange connection. This section contains the outer service platform and the entry door. This section is independently accessible for electrical equipment and cold commissioning procedures. Additionally it provides simply height, what can be saved on Tripod side. The height of a Tripod amounts already about 60m for 40m water depth. In 2006 a Tripod onshore demonstrator was designed by OWT for Multibrid GmbH, manufactured and erected in Bremerhaven, Germany, by WeserWind GmbH Offshore Construction Georgsmarienhütte. This was the beginning of a long lasting collaboration between the turbine developer and manufacturer Multibrid, the foundation designer OWT and the fabricator WeserWind. Meanwhile, the design covers the demands of an offshore turbine foundation to sufficient extent, the fabrication was even challenging regarding size and shape of the structure. That time WeserWind was supported in terms of fabrication and assembly by its sister company IAG Industrieanlagenbau Georgsmarienhütte GmbH, a member of the Georgsmarienhütte group as well. The first operation of the turbine was accompanied by the research project IMO-Wind. The first steps in condition monitoring have been undertaken including the determination of stress curves, the so-called "Hot spot" Survey, in order to enable the comparison with calculation models.
Large scale deployments In 2008 Tripods were built as a substructure for six Multibrid M5000 offshore wind turbines in the Alpha Ventus project. Alpha Ventus was planned as a first test field for the exploration of offshore wind energy in German waters. The project organisation has been Deutsche Offshore-Testfeld und Infrastruktur GmbH & Co. KG, DOTI. It was founded in 2006 by EWE AG (47,5%), E.ON Climate & Renewables Central Europe GmbH and Vattenfall Europe Windkraft GmbH (each 26,25%) assisted by Stiftung Offshore Windenergie. The German Federal Ministry of Environment BMU supported a number of research projects, which were summarized in the RAVE initiative (Research at Alpha Ventus). A broad basis of experience and knowledge was gained for the construction, commissioning and operation for future offshore wind farms. The Tripods were fabricated by Aker Kvaerner in Verdal, Norway. A horizontal assembly of the Tripods was realized in accordance to the local fabrication experience of the yard, coming from large oil and gas jacket fabrication, with subsequent upending and of course upright sailing from Norway to the offshore terminal in Eemshaven. The transportation of the Tripods to the location was done by Taklift 4 from Royal Boskalis one by one. The year 2010 marked the next milestone in rolling out the M5000 turbine with the Tripod foundation. The two projects Borkum West II and Global Tech I decided to erect their farms using this technology platform. 40 Tripods were ordered by each project in first instance nearly at the same time. Anticipating this demand WeserWind has developed a serial production approach for Tripods in the years before, together with Dr. Möller GmbH / IMS Nord, Bremerhaven. The key parameters of this approach are the upright assembly concept, the setup of an assembly line with up to nine work stations, the transportation of the growing structures on behalf of heavy load rail carriers along the assembly line and the integrated load out operation to a tailor-made pontoon. Based on this concept Georgsmarienhütte released the investment program building this assembly shop with two parallel lines at Lunedeich, Bremerhaven. The building was operational in the beginning of 2011 and in June the first Borkum-West II-Tripod was completed.
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