Mohammed Barzan Mohammed

Determination of Hysteretic Soil Damping in Monopile Supported Offshore Wind Turbines through Advanced Cyclic Triaxial Testing  and Numerical Modelling

Offshore wind turbines (OWTs) typically behave as slender and flexible structures with low fundamental natural frequencies that often lie in close proximity to the frequency range of environmental loads (wind and waves) and mechanical loads (mass imbalance at the nacelle-rotor system and blade shadowing effects). As a result, there is a high possibility that resonance could occur which would cause the vibrational amplitudes and stresses within the support structure to amplify and in turn negatively influence the design fatigue life. For this reason, evaluating the total damping is crucial since damping governs the amount of energy that is dissipated per each cycle and causes the stresses to reduce which leads to lower fatigue damage accumulation, and hence this parameter should be estimated accurately since it increases the life span of the OWT, and yields cost effective design. There are multiple sources that contribute to total OWT damping such as Aerodynamic damping, hydrodynamic damping, structural damping, supplemental damping, and foundation (or soil) damping. It is anticipated that soil damping plays a prominent contribution towards total damping and is the largest source of damping in the side-side direction and when the turbine is idle. However, this form damping is not studied well and is often neglected in design.

Soil damping is divided into radiation and hysteretic damping. The former is frequency dependent and is not relevant to the loading frequencies encountered in OWTs which are lower than 1 Hz. The later is strain dependent and plays the main role in dissipating the cyclic energy. This research will predominantly study hysteretic damping so that it could be implemented in modern in monopile design.

In order to tackle the issues and uncertainties related to estimating soil damping, state of the art cyclic triaxial tests will be utilised in combination with advanced numerical finite element modelling. It is expected that gaining better understanding in hysteretic soil damping will considerably yield cost savings through lowering the costs associated with foundation design.

This project is funded by the Sustainable Energy Authority of Ireland (SEAI) as part of the initiative to support the growth of the offshore renewable energy sector in Ireland.

Project Supervisor: Asst Prof. David Igoe