Abstract
The renewable energy sector is rapidly expanding worldwide, with offshore wind playing an increasingly prominent role. After over two decades of European developments, the offshore wind industry is expanding to America and, beyond China, into the Asia-Pacific (APAC) region. Wind farm developers are currently facing the challenge of designing/building earthquake-resistant support structures for offshore wind turbines (OWTs), which requires reliable seismic analysis of the whole structural system and the soil. In the common case of bottom-fixed OWTs founded on large/stiff monopile (MP) foundations, serious challenges are introduced by the complexity of non-linear MP-soil interaction, which may be accompanied by dynamic amplification phenomena, permanent soil deformations and, possibly, ground liquefaction. Notwithstanding the continual advances in the field of earthquake geotechnical engineering, several gaps must be filled for the optimisation of OWT support structures, to avoid over-conservatism in designing and, consequently, overusing steel for fabrication. Such overuse will be unsustainable from a resource perspective and may prevent economic exploitation of offshore wind energy in seismic regions. This paper describes the background, objectives, and methodology of DONISIS (Design of Offshore moNopiles Including Seismic Interaction with Soil), a recent research project led by Delft University of Technology in collaboration with 17 academic and industry partners and supported through Carbon Trust’s Offshore Wind Accelerator programme. DONISIS will enhance the fundamental understanding of seismic soil-structure interaction mechanisms in tall, MP-founded OWTs, with emphasis on the role of non-linear soil behaviour and pore water pressure effects. The adopted research approach is described herein, particularly with regard to four distinct work packages on: (a) constitutive modelling of soil behaviour during earthquakes, with emphasis on the relevant case of sandy soils; (b) physical testing of seismically loaded OWTs using one of the largest centrifuge facilities in the world; (c) 3D Finite Element (FE) modelling of seismic OWT-MP-soil interaction; (d) engineering seismic analysis of MP-supported OWTs, based on efficient 1D modelling of soil reactions. The knowledge acquired throughout the research programme will feed into the development of a new 1D seismic design model for MP foundations, which will retain high accuracy while allowing fast design computations and swift research-to-practice transition in collaboration with the participating industry partners. The goal is to develop a set of OWT seismic design recommendations and best practice guidelines for seismic MP design.
