Operational limits of 2XL monopile installation

Abstract

The offshore wind energy sector is rapidly growing with an increase in the capacity of offshore wind turbines, resulting in increasing dimensions of offshore wind turbines and their foundations. Additionally, there is a tendency to locate offshore wind farms more offshore because of more consistent wind speeds and more space. Sites with a water depth less than 70 metres are suitable for fixed-bottom foundations. A commonly applied fixed-bottom foundation is the monopile due their simplicity, ease of installation, and relatively low costs. Over the past years, the diameter of the monopile increased from an average of 5 metres tot 10-12 metres. These ever-growing monopiles need to be installed from floating vessels, such as heavy lift crane vessels, which are not dependent on water depth, instead of jack-up vessels. During installation, heavy lift crane vessels form a multi-body system with the crane and monopile and pile gripper ring, inducing coupled dynamic behaviour, resulting in resonance behaviour.

Currently, monopiles are installed at the side of a heavy lift crane vessel. However, a new technology is in development to install monopiles in a recess at the stern of the vessel. The upending and lowering of stage of monopile installation are considered as critical events and it is obligatory to assess the operational limits of these stages during the planning phase. The purpose of this thesis is to compare side and stern installation of 2XL monopiles from a heavy lift crane vessel during the upending and lowering phase in terms of operational limits. Therefore, a case study is performed, using Hornsea Offshore Wind Farm on the North Sea as a reference project. Time-domain simulations are conducted in OrcaFlex for various significant wave heights and spectral peak periods for both side and stern installation. The operability of the mission equipment is assessed, resulting in a comparison for side and stern installation. It is found that during upending stage, stern installation provides a larger operability window compared to side installation. During the lowering stage, side and stern installation have a comparable operability window. In conclusion, stern installation shows promising results to further develop this installation method.