Validation of a high-cycle accumulation model for suction caisson foundations

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Suction caisson foundations are composed of steel cylinders closed by a plate at the top. They are installed into the seabed by means of a suction applied to the inner of the caisson. An offshore wind turbine (OWT) can be founded on a single (mono-caisson) or several (multi-caissons) of these caissons. Compared to conventional foundations for OWT (e.g. monopiles, gravity base foundations) the installation of suction caissons is faster and generates less noise, which is beneficial for the marine fauna. Furthermore, compared to the conventional foundations, suction caissons need less material and their deconstruction at the end of their lifetime is easier. Foundations of OWT are subjected to a high-cyclic loading by wind and waves, which may lead to an accumulation of deformations in the soil and thus to a tilting of the OWT, up to a possible loss of the serviceability. For predictions of the long-term deformations of suction caisson foundations under such loading conditions no validated method exists. The aim of this research project is thus to validate a high-cycle accumulation (HCA) model, which has been already proven for other types of OWT foundations, for suction caisson foundations. This validation is based on a back-analysis of 1g and centrifuge model tests performed at the „Centre for Offshore Foundation Systems“ (COFS) of the University of Western Australia in Perth. In the finite element simulations special elements with u-U formulation are used to capture the complex interaction between the caisson, the soil and the pore fluid. The parameters of the HCA model for both sands used in the model tests are calibrated based on drained cyclic triaxial tests. The element tests are performed at two different stress levels corresponding to the 1g and centrifuge model tests. The simulations are done with and without consideration of the caisson installation, in order to quantify the effect of installation-induced changes in the soil state. The validation of the numerical method, particularly the HCA model used within it, can be concluded in case of a satisfying agreement between the simulations and the model test measurements. In a sensitivity study the effect of a variation of different parameters, among them the state of the soil and the parameters of the constitutive model, on the numerical prediction for the model tests is investigated. Finally, the validated numerical model is used for parametric studies on suction caisson foundations in real scale. Based on the results of these studies simplified design charts for the serviceability state are developed, which can be used to estimate the long-term deformations of suction caisson foundations for various boundary conditions.