MS2.6 Advances in Control of Structural Vibrations

Offshore wind is a primary source of renewable energy. The international sustainability targets - set in view of the energy transition - require a significant increase of offshore wind capacity. This leads to ever increasing size of offshore wind turbines (OWTs), water depth of installation and distance to shore. As a consequence, engineering challenges continuously arise in the design and installation of OWTs and innovative solutions are required to accommodate the growing offshore wind demand. Currently, OWTs are primarily supported by bottom-fixed foundations – in particular, monopiles correspond to over 80% of installed OWTs up to date in Europe. Monopile installation is mostly performed by means of impact hammers. However, impact piling raises major environmental concerns related to underwater noise emissions, which can be only partially mitigated by employing costly sound-proofing measures. As a result, alternative installation techniques that are environmentally friendly and high-performing are vital for the offshore wind industry. Vibratory pile driving methods pose an efficient alternative to impact piling and have been widely employed in onshore applications. However, their deployment in the offshore environment remains limited. To further boost the potential of vibratory techniques, the Gentle Driving of Piles (GDP) was developed and successfully tested throughout an extensive experimental campaign in 2019. This technique is based on the combination of high-frequency torsional and low-frequency vertical vibrations. The installation tests carried out in the preceding field campaign showcased the potential of the GDP method in terms of installation performance and significantly encouraged the further development of the method. In this paper, the introduction of new and unique features in the context of vibratory and GDP techniques is presented, along with relevant experimental results. Specifically, a lab-scale shaker that accomplishes frequency-amplitude decoupling of the input has been designed, engineered and manufactured; frequency-amplitude coupling is a common constraint in vibratory devices operating based on the counter-rotation of eccentric masses. The present shaker consists of a main optimized aluminium block that connects three linear hydraulic actuators, two positioned horizontally and one positioned vertically, in order to generate torsional and vertical vibrations. The actuators are position-controlled during installation, and their amplitude and frequency are variable and independent to each other. In this experimental campaign, the new lab-scale GDP shaker is used to install piles into a conditioned soil and the respective results are discussed. Particular emphasis is placed on the effect of the different shaker settings on the installation performance, which is studied by means of direct shaker (input) and pile measurements.

This paper proposes a multi-level system modeling for studying the structural behavior of an Offshore Wind Turbine (OWT) with a flexible monopile foundation considering the Pile-Soil Interaction (PSI). This analysis shows that the structural response is affected by a signifi-cant uncertainty due to the randomness of the geometric and mechanical properties of the tower and foundation and by the environmental loads and rotating blades. With a Monte Carlo simulation, the sources of uncertainty of the environmental and exchange zones are generated for a Performance-Based Wind Engineering (PBWE) design of the OWT. This way the structural response, the stresses along the flexible foundation, and the power production can be evaluated in probabilistic terms estimating the best design configuration. These re-sults are compared to the PBWE design of the OWT controlled by a Pendulum Tuned Mass Damper (PTMD) used to mitigate structural vibrations.

This study presents a novel decentralized active control algorithm for vibration mitigation of structures subjected to seismic excitations. In recent years, the advancement in instrumentation schemes, i.e., sensors, actuators, and computing units is the reason behind the growing interest in the decentralized control approach as compared to the more complex and computationally extensive centralized architecture. In this study, the controller framework is designed through a suboptimal H2- based state-feedback control algorithm. The objective of this optimal strategy is to reduce the structural energy and control input energy simultaneously. Further, the concept of decentralized switched control law is included, and the detailed formulations are presented in digital domain. A set of communication channels are deliberated to trade the state information between the adjacent subcontrollers. The performance efficacy of the designed control strategy is numerically illustrated through an eight-story building model equipped with multiple active-controlled actuators. The building model with the same actuator arrangement is considered under the conventional centralized technique and uncontrolled conditions. Comparative assessment with uncontrolled and conventional centralized techniques confirms the better adaptiveness of the proposed control scheme.

This paper addresses the optimal tuning and numerical performance assessment of regenerative tuned mass damper inerters (RE-TMDIs) in three different configurations with non-grounded inerters attached to cantilevered primary structures under Gaussian white noise base excitation. The studied RE-TMDI configurations behave linearly and differ in the placement of the electromagnetic motor (EM), modelled as viscous damping element used for transforming kinetic energy to electricity, with respect to the inerter element. The primary structure is modelled as a linear damped generalized single-degree-of-freedom system, while a connectivity index is used to account for the location of the two RE-TMDI attachment points to the primary structure. A bi-objective optimization problem formulation is adopted and numerically solved for determining optimal RE-TMDI stiffness and EM damping coefficients that minimize primary structure displacement variance and maximize the available energy for harvesting by the EM. Parametric numerical results are reported for different RE-TMDI configurations, connectivity, inertance, secondary mass ratio and relative weighting between the two optimal design objectives. These results demonstrate that improved energy generation and vibration suppression is concurrently achieved with increasing inertance and/or increasing the distance of the host structure locations where the RE-TMDI is attached to. Recommendations are provided establishing the most advantageous RE-TMDI configuration.