Finite Element Analyses of Offshore Monopile Deflection Accumulation under Harmonic Loading

Abstract

A wide variety of ocean energy converters (OEC) are being considered worldwide, including tidal, wave, and current ocean energy. Regardless of the type of ocean energy device a major technical and cost challenge for these systems is their foundation systems. The specific focus of this paper is for ocean stream energy converters supported on monopile foundations. One of the key design considerations for these OEC devices is related to the long-term performance of the foundation system, which can be subjected to more than 107 cycles of low-magnitude lateral loading during its design life. This very high number of lateral load cycles can result in a gradual long-term accumulation of pile head deformations and rotations. In some cases these pile head deformations can exceed the OEC serviceability limits. Analyses of laterally loaded piles, based on conventional p-y methodologies, typically cannot predict these accumulations of deformations and rotations. This manuscript presents results of 3D dynamic finite element analyses for this soil-structure interaction (SSI) problem. Results from these analyses reveal that this gradual accumulation of pile head deformation could be an important design consideration for certain monopile conditions. These numerical results correspond well with published experimental data. Finite element analysis (FEA) parametric studies, which investigate the influence of different pile installation variables, are presented to shed light on possible design options that mitigate or minimize the accumulation of pile head deformations.