Cyclic Response of Granular Subsoil Under a Gravity Base Foundation for Offshore Wind Turbines

Abstract

It is essential in the design of gravity foundations under cyclic loading that inadmissibly large reductions of the subsoil bearing capacity due to the accumulated pore water pressure (loss of stability) and tilting of the foundation caused by the accumulation of settlements (loss of serviceability) are ruled out. This paper describes a procedure to predict these phenomena in non-cohesive subsoils. Real irregular loads during the service life of the structure can be idealized into parcels of uniform cyclic loads. Using the “High Cyclic Accumulation Model” proposed by Niemunis [1] the strain accumulation and thus the settlement of the subsoil due to the cyclic loads occurring within the service life of the wind turbine can be estimated. The pore pressure development is analyzed by dividing the mechanism in a pore pressure build-up phase in undrained condition (analytical calculation) and a dissipation phase (3D-FEM-Calculation).

Gravity Based Foundations for Offshore Wind Turbines: Cyclic Loading and Liquefaction

“…. 2.23 Fluctuating moment due to wave loads (top), transient caisson motions and transient pore pressures (middle) and residual pore pressure and residual motions (bottom) (Kudella et al (2006)) 3.1 Implicit and explicit steps to be taken in a coupled approach for pore pressure development (Safinus et al (2011) (Finn et al (1983)) . .…”

“…It is possible to take the dependency of soil stiffness versus mean effective stresses into account, however this leads to an expensive procedure of calculations. Finally, according to Safinus et al (2011) the steps in the hybrid models can be separated into implicit and explicit procedures, with the implicit procedures representing the steps in the constitutive model, and the explicit steps the pore pressure development in an empirical model.…”

“…• Settlements Due to cyclic loading accumulation of strains will occur, resulting in permanent strains and settlements. A large number of cycles can be modelled with a High-Cycle Accumulation (HCA) model (Safinus et al (2011)). This model predicts the strain accumulation of non-cohesive soils for a large number of cycles (N > 10 3 ) with small strain amplitudes ( < 10 − 3).…”

On the modelling of pore pressure developments below cyclically loaded offshore gravity foundations

“…Furthermore, the numerical error increases in each modelled load cycle, resulting in a less reliable pore pressure output. Safinus et al (2011) concluded that the number of load cycles in a constitutive model should not exceed 100 to limit the numerical error. For wave loads, this represents only a small part of a storm and therefore a full constitutive model is not suitable to model a large number of load cycles.…”

“…Pore pressures may have a significant effect on the stability of an offshore gravity foundation for an offshore wind turbine as well. Currently, the design is often based on the static stability check, while the relation between cyclic loading of the subsoil and the stability of the structure receives less attention (Safinus, Sedlacek, and Hartwig 2011; Det Norske Veritas (DNV) 2014).…”