Date of Award

2025

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Aaron Bradshaw

Abstract

This study addresses the challenge of accurately estimating monopile foundation stiffness for offshore wind turbines, a critical factor influencing system natural frequencies and resonance avoidance. At the Coastal Virginia Offshore Wind (CVOW) site, stiffness values were calculated using a range of analytical and numerical methods, including subgrade reaction models, elastic solutions, impedance functions, and 1-D p-y curve-based tools (RSPile and MoDeTo), as well as a 3D finite element model (PLAXIS 3D) developed by the Norwegian Geotechnical Institute (NGI). These estimates were compared to stiffness values obtained from three months of structural health monitoring data analyzed using stochastic subspace identification (SSI-DATA) and finite element model (FEM) updating. Because the FEM-updated soil springs represent an average foundation stiffness over the three-month observation period, a method was developed to estimate representative mudline shear forces and moments using strain gauge and tilt data. These loads were then applied to each model to simulate a consistent nonlinear response for comparison. Among the methods evaluated, the 1-D lateral and rotational spring model using MoDeTo provided the closest match to SSI-derived stiffness (74% lateral, 86% rotational), while conventional API-based RSPile models underestimated both. Analytical approaches varied in accuracy, with Shadlou and Bhattacharya outperforming others but still underestimating lateral stiffness by more than 50%. The results underscore the limitations of simplified models under nonlinear loading and highlight the value of FEM updating using monitoring data for accurate, site-specific calibration.

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Available for download on Thursday, May 27, 2027

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