Long Term Effects of Cyclic Loading on Suction Caisson Foundations

Abstract

Significant financial investments are made in the offshore wind energy, in a race to reduce greenhouse gas emissions. The trend is to install larger capacity wind turbines in deeper waters, further from shore. DTI (2001) states that 30% of this investment is represented by the foundation costs. Senders (2008) and Byrne & Houlsby (2003) found suction caisson foundations within a multi-footing configuration to be a viable and more economically attractive solution. But, the available norms and standards provide insufficient guidance. It is the aim of this dissertation to provide an additional method to investigate the long term performance of these foundations for their usage as footings for large (6 MW) offshore wind turbines, embedded in sandy soil conditions. Literature revealed significant research being undertaken to better understand the cyclic performance of different foundation types. Three main model types have been found: empirical (does not assess each individual cycle), constitutive (assess each individual cycle) and hybrid (a combination of the first two). The hypoplastic sand model with intergranular strain concept is employed, based on its ability to accurately assess the two main effects of cyclic loading on sand: pore pressure and strain accumulation, provided that a correct calibration is done. This research looks at the usage of a centric three-leg jacket for an offshore wind turbine, located in the North Sea, with a soil profile characterised by medium dense sand. It is shown that suction caissons present a large advantage in lateral resistance. Due to their increased diameter, the problem is reduced to the assessment of vertical cyclic resistance, confirming the theory that multi-footings take the loads through a “push-pull” system. The loading conditions, defined by vertical mean load and cyclic amplitude, found within a certain range of the caisson capacity, produce no significant strain or pore pressure accumulation. The behaviour within the elliptically shaped boundary is dominantly hypoelastic. Outside of the boundary, hypoplastic strains and pore pressure build-up occur, with consequent strength and stiffness degradation. Due to insufficient data for the calibration of the medium dense sand soil volume, an overestimation of the pore pressure occurs. Thus, no conclusion may be drawn regarding the behaviour outside of the defined boundary. Nonetheless, this boundary is conservatively constructed based on the chosen type of soil, medium dense sand that is more prone to pore pressure build-up than the denser sands found in-situ. The period of application of the load is conservatively chosen to be equal to the wave period, regardless of the observed larger values, closer to the wind period, which would allow for more consolidation time. This research provides a basis for the design of suction caisson foundations for offshore wind turbines. It provides a tool for the identification of loading conditions that are potentially dangerous for the long term performance of these foundations embedded in sand and subjected to vertical cyclic loading.