MS12.4: Nonlinear Dynamics for Engineering Design

The nonlinear dynamics of a novel concept of Floating Offshore Wind Turbines (FOWT), able to harvest wind energy in deep-harsh sea waters due to its light-structure design, is investigated under regular sea wave excitation by varying wave height and period parameters to evaluate its heave a pitch system displacements. The system designed by T-Omega Wind is modelled as a multibody structure in CAD and its motions are simulated in a real-time multiphysics Marine Simulator at The National Decommissioning Centre at the University of Aberdeen. Simulations are performed for ``High" and ``Low" Sea States to predict the system's behaviour and calibrated with the system damping coefficients. Simulations are validated with experimental results performed with a 1:60 scaled prototype at the Kelvin Hydrodynamics Laboratory (KHL) of the University of Strathclyde. Moreover, feasibility studies of the system, specific dynamic studies and simulations are performed with a designed towing system, which allow the system to be towed to the port for maintenance. The study identified different types of dynamical responses while varying wave parameters proved the modelling approach having achieved good agreement with experimental results. The study confirms that the investigated FOWT concept can operate even under harsh-sea conditions, which is manifested by the maximum heave and pitch amplitudes observed.

Behaviour of a typical multiple impact jarring tool is examined to understand the effect of design and operational parameters on the tool's performance. Low-dimensional models were developed comprising lump masses, springs, and viscous dampers, which can mimic the tool's main function of generating high-amplitude impact forces from the energy stored in the pre-compressed spring. The magnitude and frequency of these impacts are controlled by an overpull and cam mechanisms, respectively. Numerical analysis is performed by direct numerical integration and path-following methods, showing that the system can exhibit co-existing attractors and chaotic behaviour. Parametric studies are carried out to determine the range of design and operational parameters that would enhance the jarring tool performance by generating preferable impact force patterns.

In this work, the vibration attenuation capabilities of a negative stiffness device, termed NegSV, are studied. This nonlinear mechanism can be mounted to multi-storey frame structures, exploiting the partial mass resonance concept, where the top part of the frame effectively acts as a resonator defined with respect to the lower part. An appropriate geometrically nonlinear configuration is used for tuning the stiffness of a specified storey to achieve the aforementioned effect. Mitigation of vibration can subsequently be succeeded in terms of reducing the accelerations and inter-storey drifts in the lower parts of the structure, reducing base shear demand. An optimization of the design is further pursued with the use of a suitably architected objective function, where a set of optimal parameters can be determined exploiting the nonlinear properties of the NegSV. Significant improvements are observed in the dynamic response of the modified structure, compared to the original, showing potential towards structural protection in realistic scenarios.

Nonlinear vibrations of a rotating hub-beam system are analysed in the paper. A model of the beam is based on extended Euler-Bernoulli theory taking into account large beam’s displacements and related geometrical nonlinearities. The derived equations of motion take into account beam’s bending in two directions, elongation, twist, preset angle of the beam and varied angular velocity of the hub. Then, the model is successively reduced for longitudinal-flexural oscillations and next to transversal oscillations with angular velocity and the preset angle as bifurcation parameters. Externally excited oscillations of the beam and vortex induced oscillations generated by fluid flow are studied around selected resonance zones. Vortex induced oscillations are included in the model by an additional van der Pol equation coupled with the main structure.