Dynamic modelling of the blade-hub interface for offshore wind turbine blade installation

Abstract

In the last decades, the urgency for sustainability has dawned on humanity. The increasing pressure for more renewable energy sources is accompanied by the success of the offshore wind turbine. As a result, the installation of offshore wind farms has become a major business. The challenges posed during installation become more complicated with larger turbines. Forces exerted on taller and softer support structures and longer blades limit the installation options of the wind turbine components even more.
Nevertheless, the dimensions of wind turbines have grown explosively over the years and this growth is foreseen to continue.
To overcome the growing challenges of blade installation on offshore wind
turbines, it is crucial to better understand the forces and the induced motions. Insufficient understanding of the behaviour of the support structure and blade during installation of the latter could lead to over-conservatism of an already vastly time-constrained process, resulting in loss of workable weather and unnecessary costs.
The objective of the research conducted in this thesis is to develop a dynamic model of a 20 MW offshore wind turbine to quantify motions at the blade-hub interface during blade installation and examine potential mitigations. When the objective is reached, a good comprehension of the blade behaviour is achieved, allowing for exploration of solutions to reduce the observed motions and increase workability.
A 20 MW wind turbine does not exist as of writing, but with a scaling assessment a configuration was composed. The future wind turbine is estimated to have a hub height of 154.7 m and a blade length of 131.5 m with an estimated weight of slightly less than 90 tonnes. Due to offshore placement of the wind turbine, the length of the support structure is estimated to be extended with a water depth of 35 m. The problem has been further delineated by assuming the weather conditions of a typical North Sea offshore site at the boundary of what is currently possible for blade installation. Installation is assumed to be conducted with a floating vessel, posing even more challenges.
Modelling the installation of a blade is subdivided into examining the behaviour of a wind turbine support structure and a blade suspended from a crane separately. The performed simulations are conducted in 3D, with the support structure and blade being multi-body systems excited by unidirectional
waves, wind and current. Calm, normal, and rough weather conditions are assumed as input conditions to examine the environmental impact. A combination of the results of the models simulating the multibody systems provide the relative motions between the support structure and the blade, resulting in potential impact velocities in the horizontal plane. The model is solved in the time domain, because it allows for a better understanding of the input and output variables...