Vibration-induced settlement of a slip-joint connection for offshore wind turbines

The majority of existing offshore wind turbines typically consist of a monopile foundation, a transition piece with a vertically positioned grouted connection, a turbine tower, and a turbine. Of the 2,653 offshore turbines that were installed by the end of 2015, 80 percent are supported by a monopile.
Despite the current overwhelming dominance of the monopile, its future application is rather uncertain. Offshore wind turbines have continuously increased in size and have moved to deeper waters; these developments require larger and heavier support structures. It is unlikely that floating structures will be preferred to bottom-founded structures, up to a water depth of 80 m. The question thus becomes whether jackets or monopiles will be used under such conditions? The monopile seems to be losing in this competition, as, to meet the requirements a monopile would have to be extremely large; thus, it may no longer fall within industry limits, both in terms of manufacturing demands and the lifting capacity of dedicated installation vessels. One may wonder whether a single monopile would be necessary, or if a set of intelligently connected smaller length monopiles could suffice. The key to the success of such a concept could be the so-called slip-joint connection.
A slip-joint consists of two conical sections made of steel. This connection does not require any grout and, besides being a connection option for the transition piece and monopile, allows monopiles to be comprised of a number of lighter sections of very large diameters. By employing a slip-joint, the applicability of the monopile could be extended to deeper waters and to turbines that have very large rotors and power capacities.
Although the slip-joint connection has been successfully used for onshore wind turbines in the past, it has not yet been used offshore. One of the challenges in using the slip-joint is ensuring a proper fit of the cones despite the imperfections that result from manufacturing tolerances, deformations by pile driving, and the potential damage that may occur during the handling of the cones.
In this thesis, it is proposed that a slight difference in the cone angles be used to address the aforementioned imperfections. A steeper cone angle for the transition piece when compared to that of the monopile is proposed. These slightly different cone angles require the upper cone to deform elastically in order to slide down the lower cone during installation. To facilitate the installation process, it is proposed that vibrations be employed in order to cause the upper cone to slide down under its own weight. In order to use this new method of connecting joints, it will be necessary to investigate the manner in which vibrations influence the relative motions of the two cones that need to achieve stable contact.
The objective of this thesis is to investigate the potential of the use of vibrations in the installation and dismounting of a slip-joint with slightly different cone angles. The research is conducted by means of numerical modelling and experiments.