Hybrid foundations for offshore wind turbines

Nowadays offshore wind turbines represents a valid answer for energy production but with an increasing in costs mainly due to foundation technology required. Hybrid foundations composed by suction caissons over which is welded a tower supporting the nacelle and the blades allows a strong costs reduction. Here a monopod configuration is studied in a sandy soil in a 10 m water depth. Bearing capacity, sliding resistance and pull-out resistance are evaluated. In a second part the installation process occurring in four steps is analysed. considering also the effect of stress enhancement due to frictional forces opposing to penetration growing at skirt sides both inside and outside. In a three dimensional finite element model using Straus7 the soil non-linearity is considered in an approximate way through an iterative procedure using the Yokota empirical decay curves.

"Offshore tower or platform foundations: numerical analysis of a laterally loaded single pile or pile group in soft clay and analysis of actions on a jacket structure"

Laterally loaded piles are a typical situation for a large number of cases in which deep foundations are used. Dissertation herein reported, is a focus upon the numerical simulation of laterally loaded piles. In the first chapter the best model settings are largely discussed, so a clear idea about the effects of interface adoption, model dimension, refinement cluster and mesh coarseness is reached. At a second stage, there are three distinct parametric analyses, in which the model response sensibility is studied for variation of interface reduction factor, Eps50 and tensile cut-off. In addition, the adoption of an advanced soil model is analysed (NGI-ADP). This was done in order to use the complex behaviour (different undrained shear strengths are involved) that governs the resisting process of clay under short time static loads. Once set a definitive model, a series of analyses has been carried out with the objective of defining the resistance-deflection (P-y) curves for Plaxis3D (2013) data. Major results of a large number of comparisons made with curves from API (America Petroleum Institute) recommendation are that the empirical curves have almost the same ultimate resistance but a bigger initial stiffness. In the second part of the thesis a simplified structural preliminary design of a jacket structure has been carried out to evaluate the environmental forces that act on it and on its piles foundation. Finally, pile lateral response is studied using the empirical curves.

Numerical modelling of axially loaded piled foundations with physical tests validation

L'oggetto della tesi è la modellazione numerica di pali di fondazione per turbine eoliche in ambiente offshore. Il metodo di modellazione comprende l'interpretazione di prove CPT per l'estrapolazione dei parametri geotecnici del terreno, la creazione di un modello dell'interazione tra struttura e terreno tramite il software agli elementi finiti Abaqus, la sua verifica, l'ottimizzazione dell'estensione del modello del terreno e della mesh e la simulazione numerica del comportamento di pali caricati assialmente in compressione. Grazie al confronto con i risultati di test su modelli fisici, eseguiti nel laboratorio dell'istituto Fraunhofer IWES di Hannover, di tre pali aventi la stessa geometria ma installati in punti con diverse condizioni di compattazione della sabbia, è stata possibile la messa a punto di una strategia di simulazione più accurata possibile e la sua validazione. A conclusione del lavoro è stato eseguito un esempio di design di una fondazione jacket per una turbina eolica offshore.