Design of Transition Pieces for Bucket Foundations for Offshore Wind Turbines

Using bucket foundations for offshore wind turbines is an alternative solution to monopiles and other foundation types installed in shallow and transitional water depths (up to 50−60 m) due to its greater stiffness, fair simplicity and high speed of installation. However, as only a few prototypes have been installed so far, this concept still requires further investigation before potential commercialization.

The current Ph.D. project deals with material and shape optimization of an intermediate transition section, the so-called “transition piece” (TP), connecting the wind turbine column with the suction bucket used as a monopod foundation for an offshore wind turbine. Traditionally, this part is made of steel triangular flange-reinforced shear panels (stiffeners) which require expensive production, corrosion protection, and suffer from fatigue at the welded joints due to cyclic loading produced by the combined action of wind and waves. The aim of the project was to find an alternative design of this TP utilizing FEA and an alternative material called Compact Reinforced Composite (CRC®), or “The New Concrete”, invented at Aalborg Portland A/S in 1986, which is an Ultra-High-Performance Fibre-Reinforced Concrete (UHPFRC) with dense main reinforcement. A study on workability of UHPFRC material, compressive strength, and non-destructive Computed Tomography (CT-scanning) tests for analysis of fibre orientation, distribution and presence of cracks and air voids were carried out on a batch of UHPFRC samples cast at AAU. Firstly, a conical design of the TP was proposed using three material models: CRC®-steel composite, pure CRC® and steel shell. Carrying capacity in the ULS was checked, and the eigenvalue buckling analysis of the shells was performed. Additionally, a non-linear sensitivity analysis was carried out, and the material models were compared in terms of sensitivity to small geometric imperfections that might appear during the fabrication process. This study showed that a TP structure made of pure CRC® was less sensitive to shape imperfections and was recommended in the further investigations. As a next step, shape optimization of the TP was performed by carrying out preliminary elastic calculations and non-linear FEA. Two CRC® material models with varying compressive-tensile strength were suggested for comparison. Based on the results, introduction of cutaways was suggested to save material, and, potentially, minimize scour and wave loading on the TPs. Several shapes of TPs were selected for scour evaluation under constant unidirectional currents, and “clear water” and “live bed” regimens were generated in a small-scale facility. Development of the scour hole geometry, the scour volume and the equilibrium scour depth were investigated. Correspondingly, scour protection measures were addressed. Additionally, small-scale hydrodynamic loading tests were carried out on these models in small-scale deep water environment with regular and irregular wave conditions to study the effect of implementation of cutaways and propose the shape that could minimize wave forces acting on the TPs.

Based on the x Nezhentseva results of the present investigation, it is believed that mass production of the transition pieces for offshore wind turbines made of UHPFRC is recommended.