Analysis of Shell Deformations of Offshore Wind Turbine Monopile Foundations

Abstract

A significant reduction in the levelized cost of electricity is necessary for offshore wind farms to become a competitive energy source. Amongst other methods, overall project cost reduction can be achieved by optimized support structure design. An upcoming trend is the use of `XL monopiles', which provide a cost effective foundation solution for offshore wind farms in deeper water and are able to support larger turbines. One of the most governing design criteria for offshore wind turbines is the fatigue limit state. Current practice is to check compliance of a structural design with this criterion by means of a large number of simulations. In these simulations a monopile is typically modelled using beam theory. For the large diameter monopiles, structural design has become less slender and more thin-walled. This implies that, in relation to the global bending deformations, shell deformations will have a more pronounced contribution to the total deformations in the structure as compared to smaller diameter piles. The goal of this study is to investigate if these shell deformations have a significant contribution to the stresses in these large diameter structures, during fatigue limit state design load cases. Beam theory is unable to capture this phenomena and thus a more advanced modelling method is required. A semi-analytical structural model of a monopile is developed using coupled circular cylindrical shell segments. Soil-structure interaction characteristics are incorporated based on a translation from three dimensional finite element modelling. With a transformation to generalized coordinates it is shown how the structural model can be connected to a model of a wind turbine's tower and rotor nacelle assembly in a manner that allows for computationally effective numerical calculations. Making use of the continuous description of the model, internal stresses at any location in the monopile's shell can be obtained directly from the deformation behaviour. Additionally, following the shell equations, the monopile's shell deformations can be isolated from its global bending behaviour, which allows for a transparent analysis. The dynamic behaviour of the monopile in various design load cases is analysed using time-domain simulations. These are performed using a state-of-the-art numerical code which accounts for structural-, aero- and controller dynamics. Wave loading is additionally incorporated as a distributed surface loading using diffraction theory. It is found that the shell deformations are sensitive to loading at high frequency bands where typically very little wave energy is present. Assuming Airy waves, as is the standard practice in the offshore industry, only a minor quasi-static contribution of the shell deformations on the monopile's internal stresses is found. The sensitivity to breaking waves, which excite the structure at a wide frequency range, and the sensitivity to the slenderness of the monopile are explored as well. It is found that an increase of the monopile's diameter induces additional shell deformation behaviour.