Dynamic behaviour of suction caissons

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6 Chapter 1 – Introduction Figure 1.4: Tripod foundation. 1.2.3 Tripod Foundation The tripod concept originates in the numerous steel tripod foundations for the offshore oil and gas industry. The tripod is a welded steel structure with a centre column supported by three piles in separate pile sleeves. The tripod concept is sketched in Figure 1.4. The tripod can be supported by either vertical or inclined piles. The design criterion for the piles in a tripod construction is usually the axial bearing capacity, contrary to the monopile. The main design parameters for the tripod structure are the height of the welded joint and the base radius (offset of pile sleeves). Short comments on the concept: Suitable for larger water depths. Minimum of preparations are required at the site before installation. Known technology from oil & gas industry. The tripod in Figure 1.4 is a traditional steel structure with piles. There are several alternative solutions to this concept. Suction caissons may be used instead of piles, and the main structure may be constructed as a jacket construction (Tjelta 1995) or as a concrete section, with the possibility of ballasting the foundation, see e.g. Vølund (2005). Morten Liingaard
1.2 Foundations for offshore wind turbines 7 Figure 1.5: Suction caisson. 1.2.4 Suction caissons Recent research and development projects (Byrne 2000; Feld 2001; Houlsby et al. 2005) have shown that suction caissons (see Figure 1.5) may be used as offshore wind turbine foundations in suitable soil conditions. Suction caissons (also denoted as bucket foundations or skirted foundations) have previously been used as anchors and foundations for several offshore platforms. Here, the suction caissons are mainly subject to vertical and horizontal loads. On the other hand, when suction caissons are applied as monopod foundations for wind turbines, they must be able to sustain a significant overturning moment. At greater water depths the monopod solution may become uneconomical and a foundation concept with three or four smaller suction caissons may become appropriate (see the previous subsection). The overturning moment is then stabilized by the opposing vertical reactions of the suction caissons, see (Houlsby et al. 2005; Senders 2005). The suction caisson is installed by using suction as the driving force and does not require heavy installation equipment. Lowering the pressure in the cavity between the foundation 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. Hence, the penetration resistance is reduced. A fully operational 3.0 MW offshore wind turbine was installed on a prototype of the suction caisson foundation at the test field in Frederikshavn, Denmark in late 2002. The project is described in details in Ibsen et al. (2005). Short comments on the concept: Hybrid of pile and gravity based foundation. Fabrication/material costs comparable to that of the monopile concept. Non proven technology for large water depths. Decommissioning is a relatively simple process. December 4, 2006
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76 BIBLIOGRAPHY Brincker, R. and P.
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78 BIBLIOGRAPHY Kitahara, M. (1984)
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80 BIBLIOGRAPHY Morten Liingaard
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Dynamic behaviour of suction caissons

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