SSI analyses of monopiles for offshore wind turbines

Abstract

Monopiles with typical diameters in the range of 3.5m to 5m are often used as the foundation concept for offshore wind turbine structures. The current design procedure of these structures are the p-y method although this method was was developed for piles with a higher slenderness ratio. The thesis presents soil structure interaction(SSI) analyses of monopiles in Drammen clay profiles with varying over consolidation ratio by means of the commercial Finite element program Plaxis 3D Foundation. The piles are subjected to static horizontal forces and overturning moments and the response from these loads are compared to the more conventional p-y method, included in the pile program Splice.The calculations are performed in a serviceability limit state with loads which generates moderate displacements of the pile head in the FEM model, before the effect of cyclic loading on the capacity is investigated in the pile program Splice. The main findings in this study are:(1) Large deviations in pile head displacements are observed in all soil profiles although a trend of increasing agreement is observed with increasing OCR. Deviationsmay partly be explained by lack of additional soil springs in Splice. The program does not account for the shearing of the soil inside the monopile at the pile tip.(2) The recommended value of "50 for normally consolidated sediments might be to high to predict reasonable displacements of the pile in Drammen clay. (3) The initial stiffness of the claysare consequently underestimated by Splice compared to Plaxis 3D Foundation. However, for overconsolidated clay, the stiff clay p-y curves indicate a better agreement with the Finite Element program. (4) In contrast to an analysis with cyclic p-y curves, the capacity of the monopile under consideration increases under cyclic loading in normally consolidated Drammen clay. In depth, the cyclic p-y curve softening does not seem to coincide with the Drammen clay properties.