Modelling an upend hinge in hydrodynamic simulations of a monopile upend operation

Abstract

The offshore wind energy industry is quite literally reaching for the sky to meet the increasing demand for renewable energy in Europe. Offshore wind turbines are sprouting out of the waves off the coast of many European nations and their size is increasing rapidly. As the turbines become larger and the waters in which they are placed become deeper, the foundations that carry these behemoths must grow as well. The most frequently used foundation is the monopile foundation. Over the years, both the length and the diameter of these monopiles have steadily increased. Installing monopile foundations has, as a result of this, become more and more difficult.
An important aspect of the installation of monopile foundations that has become more challenging is the upending of the monopile. When the monopiles are transported to the site of installation, they are sea-fastened to the deck in horizontal orientation. When they arrive, they need to be rotated to a vertical orientation for installation. This can be done with the aid of an upend hinge. This thesis focusses on the modelling of such a hinge for operability studies.
The goal of these operability studies is to determine the weather and wave conditions in which the operation can be safely performed. Conventionally, the stiffness of the upend hinge is approximated by several linear springs between the vessel and the monopile. The aim of this thesis is to apply dynamic substructuring to existing Finite Element models of a hinge to more accurately describe its dynamic behaviour within hydrodynamic simulations. Hereafter, the response of the dynamically substructured model can be compared to the response of the conventional model.
The findings of the research show that it is possible to use dynamic substructuring to reduce the Finite Element models of an upend hinge and implement them into hydrodynamic simulations of an upend operation. When comparing the conventional model to the substructured model, it can be seen that there is a significant difference between the high-frequency response of the models. However, the responses of the models to prevailing ocean waves are very similar. For operability studies, this response is most relevant. Furthermore, the required computation time is significantly higher for the simulations employing the dynamically substructured models than for the conventional models. Therefore, it is not recommended to apply dynamic substructuring to model this upend hinge in operability studies. For situations where the high-frequency response is more relevant, however, dynamic substructuring may prove a valuable tool to more accurately describe the dynamic properties of an upend hinge or other marine equipment.