A compliant guyed system for deep-sea installations of offshore wind turbines: Concept, design insights and dynamic performance

Abstract The paper explores the potential of a cost-efficient and easy-to-install foundation scheme for Mega-Turbines in seismic regions. The envisioned system comprises a compliant tower, tethered to the ground with an array of four pretensioned cables moored to the seabed by suction anchors. To facilitate installation, a single pinned connection is implemented at the tower base, which is founded on a circular shallow footing. Compared to conventional solutions, the studied scheme offers the necessary support without resource to massive and difficult to construct foundations. By eliminating the moment transfer at the tower foundation, its design requirements are substantially reduced. The efficacy of the proposed solution is demonstrated by investigating the performance of the NOWITECH 10 MW reference turbine installed at 50 m water depth, in the seismically active region of North-Western Adriatic. The entire soil–foundation–structure system is modelled in 3D, employing the finite element method. After confirming the acceptable performance of the guyed system against ULS and SLS conditions (even when allowing 3P mechanical vibrations to resonate with the oscillation of the turbine tower), its seismic response is evaluated. Under seismic loading, the guyed system is shown to be resilient, sustaining the seismically–induced loading with controlled displacements at the cable connection point, protecting the tower from excessive distress and undesirable inelastic deformations. The anchoring is also shown to have acceptable performance, with the maximum attained tension being much lower than the pullout capacity of the suction caisson; the seismically induced deformations are negligible. Finally, the seismic performance of the taut mooring lines is also shown to be satisfactory. Although a condition of “complete slack” may arise momentarily at the lee-ward cable, structural stability is not compromised.