OMA-Based Modal Identification and Response Estimation of a Monopile Model Subjected to Wave Load

The Operational Modal Analysis (OMA) has shown potential to provide reliable identification of offshore structures, being exposed to harsh environmental conditions that may degrade their integrity over the designed lifetime. Therefore, this study aims to employ OMA techniques to estimate the response of a scaled structural model that represents an offshore monopile-supported platform exposed to irregular wave loads. An experimental investigation was undertaken including a polycarbonate monopile of 0.90 m height that supports a steel deck. The monopile structure, considered to be fixed at its bottom, was placed inside a wave flume and the wave-induced vibration responses were measured along with the wave elevations. The recorded responses were used to conduct OMA-based identification of the modal properties of the scaled experimental model. To support the experimental investigation, a parallel track of numerical study was followed by developing a finite element (FE) model of the scaled structure. The FE model was appropriately updated to achieve a better matching between the experimental and numerical modal properties. The experimental measurements from wave gauges were also used along with the linear wave theory to estimate the wave kinematics, the latter being calculated by the Wheeler modification. Such a wave kinematics calculation enabled estimating the hydrodynamic forces or the so-called Morison forces while conventional assumptions were made for the definition of the necessary hydrodynamic coefficients, namely the drag, CD, and inertia, CM, coefficients of the Morison equation. The calculated forces were eventually applied to the FE model and the numerically obtained displacements were compared with the ones derived through the experimental investigation. This comparative assessment was further enriched by accounting for several data sets, being associated with varying combinations between significant wave height and peak period. The discrepancy that was found between the simulated and measured responses (i.e., displacements) constitutes an indication that the use of the code-recommended and widely adopted values for Morison’s drag and inertia coefficients, namely CD = 1.0 and CM = 2.0, can yield conservatism to the calculation of the hydrodynamic forces that, in turn, may over-estimate the response of the structural systems being exposed to wave loads. Further research is, though, required to strengthen the validity of the current findings.