An alternative option to the traditional grouted joint for wind turbines is a direct steel-to-steel connection, also known as slip joint. In a recently published work, a proof of concept of a vibration-assisted installation and decommissioning technique of a slip joint was illustrated. Leveraging on the obtained results, the current study shows for the first time a decommissioning campaign carried out using a vibration-assisted technique applied on a prototype hydraulic wind turbine tower located in the North Sea, and connected to the monopile through a slip joint. The key aspect of the dismounting procedure is a priori knowledge of the resonance frequency clusters corresponding to the slip joint’s cross-sectional modes. Therefore, field hammer tests and experimental modal analysis were carried out inside the wind turbine tower. The identified frequencies and mode shapes were then compared with numerical ones estimated by a finite element model of the investigated structure. The comparison showed that a set of frequency clusters can be directly selected from a detailed numerical model. The preparatory work of the slip joint decommissioning was then executed by installing electric shaker devices, based on the dynamic identification results, and hydraulic jacks mounted inside the wind turbine tower. A first decommissioning trial was carried out in May 2019, while the final decommissioning was performed in August 2019. After analysing the measurements of the hydraulic pressures, displacements and excitation frequencies during the decommissioning campaigns, the results showed that it is possible to disconnect the slip joint if, in combination to a vertical static force, one of the identified cross-sectional mode shapes is excited. The vibration-assisted decommissioning proved to be a successful technique to dismount the connection in a controlled and straightforward manner.@en

The structural failure of grouted connections for offshore wind turbines focused the industrial attention towards different and innovative solutions to guarantee a safe connection between the monopile foundation and the turbine tower. An alternative option to the traditional grouted joint is a direct steel-to-steel connection, also called a slip-joint which was sporadically used for onshore wind turbines. To such regard, a proof of concept is illustrated concerning a new installation and decommissioning technique of a slip-joint. The key aspect of the proposed method is to guarantee a proper fit and sound contact of the slip-joint by means of vibration-assisted settlements. Therefore, the effectiveness of applying a harmonic excitation during the installation and decommissioning procedure is experimentally investigated using a 1:10 scaled model of the slip-joint. During the dynamic tests, the applied static load and the settlements of the joint are monitored using load cells, displacement sensors and strain gauges placed both inside and outside the conical surfaces. For the installation tests, the results show that settlement occurs when applying a harmonic load at specific forcing frequencies. All the vibration-induced settlements tend to stabilize in time, indicating that a sound contact through vibration-assisted installation can be achieved. In a similar way, the decommissioning proved to be effective at certain forcing frequencies. According to all the tests performed during this experimental campaign, both the installation and decommissioning tests showed to be more sensitive to the forcing frequency rather than to the dynamic forcing amplitude.

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A novel pile-driving technique, named Gentle Driving of Piles (GDP), that combines axial low-frequency and torsional high-frequency vibrations has been developed and tested recently. During the experimental campaign, several piles were installed onshore, making use of the GDP shaker. Besides those, a number of additional piles were installed using conventional pile-driving techniques, i.e. impact piling and axial vibratory driving. After the completion of the installation phase, the installed piles have been subjected to impact hammer tests with the following goals. First, the in-situ dynamic properties of the pile-soil system have been identified. Second, the post-installation soil state has been investigated, along with its evolution in time for each pile driving scenario. Preliminary analyses, of the data collected during the impact tests show dissimilar trends in the overall dynamic response between the piles installed with impact hammer and those installed with the axial and the GDP shakers.This observation suggests a difference in the post-installation dynamic behaviour of the pile-soil systems related to different pile-driving techniques. In this paper, a first attempt is made to identify the differences in the overall pile-soil dynamic behaviour of the piles installed by means of the three different pile-driving techniques.

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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.@en