## 1 Introduction  
The market related to wind energy production is very dynamic, with a growing investment trend in the last 20 years mainly aimed at satisfying two needs. The first is to increase the level of productivity of newly installed turbines through the construction of wind towers of greater heights. The achievement of significant heights, however, often has a technological limit, due to the difficulty of the construction, transport and installation of large diameter wind towers, but also the need for adequate foundation systems. The other emerging need is to renew existing wind farms through solutions that make the most of previous investments. In fact, most of the wind turbines in use have an average age between 10 and 15 years. Considering that the life cycle of a wind turbine is estimated to last around 20 years, many installations are nearing their end. In addition, progressive aging causes a reduction in the production capacity of the plant, due to the deterioration of its mechanical and electronic components. Faced with the obsolescence of the plant, the possible solutions available to the wind farm manager consist of decommissioning (dismantling the wind farm), or extending its useful life with targeted interventions to improve the existing farm, or finally in the upgrading of the existing turbine system (repowering) through the adoption of innovative solutions. In this context, the present work intends to provide a novel contribution, with extended research compared to what the authors have shown in a previous conference article [1]. In this context, in recent years, researchers in the field have investigated the potential of many technical strategies in reducing the demand for stress at the base of the wind towers and, consequently, the stress to the foundation system. In fact, these techniques allow the building of high wind towers with relatively small transverse dimensions and less impressive foundation systems. The reduction in the stress demand in the foundation makes the strategy particularly interesting for the repowering of existing plants, where it is convenient not to demolish and rebuild the foundation, but rather to reuse the existing one for the new plant. One of the first studies about the use of external devices for wind turbines to reduce structural demand was that of [PERSON] in 1996 [2]. In that research, a Tuned Mass Damper (TMD) was applied at the top of a wind turbine in order to optimize the tower with regard to material yielding, buckling, and fatigue failure. A few years later, the first scientific work about the use of a semi-active device on wind turbines was published by [PERSON] et al. [3]. The semi-active device was based on the use of magnetorheological (MR) fluid, later designed and tested by [PERSON] [4] with the same purpose of reducing tower vibrations and force demand. [PERSON] [5,6] also pursued his studies about MR for the semi-active control of wind turbines, while [PERSON] and [PERSON] in 2016 [7] studied the effects of the MR in controlling the response of an offshore jacket platform against earthquakes. Hybrid applications of the MR device combined once with a Tuned Vibration Absorber (TVA) and once with a Tuned Liquid Column Damper (TLCD) are proposed by [PERSON], and [PERSON] and [PERSON] [8], respectively. Another semi-active device for similar aims is the so-called STMD (Semi-active Tuned Mass Damper), proposed by [PERSON] et al. [9], by [PERSON] et al. [10], by [PERSON] [11,12], and by [PERSON] and [PERSON] [13], in order to reduce the overall structural vibrations. A semi-active TLCD was used by [PERSON] et al. [14], [PERSON] et al. [15], and by [PERSON] et al. [16], with the goal of dynamic load mitigation. The focus herein is on a particular semi-active (SA) vibration control strategy which is based on the adoption of controllable energy dissipators.