A literature review of current design and analysis methods for offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute (API) p-y method for offshore oil platforms, which presents several issues when extended to OWTs, mostly with respect to the large pile diameters required and the effect of cyclic loading from wind and waves. Although remedies have been proposed, none have been incorporated into current design standards. Foundations must be uniquely designed for each wind farm due to extreme dependence on site characteristics. The uncertainty in soil conditions as well as wind and wave loading is currently treated with a deterministic design procedure, though standards leave the door open for engineers to use a probability-based approach. This thesis uses probabilistic methods to examine the reliability of OWT pile foundations. A static two-dimensional analysis in MATLAB includes the nonlinearity of p-y soil spring stiffness, variation in soil properties, sensitivity to pile design parameters and loading conditions. Results are concluded with a natural frequency analysis.

The installed capacity of offshore wind turbines in Europe is increasing with the monopile being the most common type of foundation. During its lifetime an offshore wind turbine is exposed to high dynamic loads which eventually can result in the fatigue of the substructure. This thesis will show how the linear damage accumulation approach based on the Miner’s rule can be used to estimate the damage induced on the substructure of an offshore wind turbine using measurements from strain gauges. Furthermore, the most important environmental influences will be illustrated and the different stress concentration factors and the size effect introduced in the industry guideline DNVGL-RP-C203 will be analysed towards their effect on the calculated lifetime.

The study presented in this thesis investigates the passive exchange of enclosed water with seawater in an offshore wind turbine foundation. This thesis was undertaken in collaboration with Vattenfall R&D, Älvkarleby, Sweden. The water exchange is studied by utilizing Computational Fluid Dynamics (CFD) simulations. A standard monopile foundation, which is installed in Horns Rev 3 wind farm, is considered for the study. The considered geometry consists of two replenishment holes.  The study aims to develop a methodology to utilize CFD simulations to quantify the exchange rate of water. CFD enables studying the effects of different wave parameters and sea states on the economic exchange rate. However, the secondary aim to develop the methodology for the CFD simulations is also to utilize the available computational resources efficiently.  The CFD methodology incorporates the learning from experiments and utilizes a semi-circular domain to enclose the control volume. The results from a mesh sensitivity study establish that a coarser mesh in the domain and a finer mesh within the monopile, coupled with Implicit LES is appropriate to study the overall effect of wave motion on the exchange rate. Also, the additional term scalar transport, incorporated to study the change in concentration within the monopile, provided an appropriate and computationally efficient tool to visualize the variation in water concentration.