Structural health monitoring (SHM) of offshore wind turbines (WT) is essential to ensure their safe and efficient operation, especially as offshore wind farms are increasingly deployed in deeper waters, where the harsh environmental conditions pose significant challenges. Jacket-type foundations have become the preferred choice for turbine support structures due to their cost-effectiveness and stability, yet they remain vulnerable to damage caused by loose bolts, which can compromise the overall structural integrity. This study proposes a methodology for detecting structural damage caused by bolt loosening using a small-scale experimental offshore WT. Although vibration-based techniques are widely employed for damage detection in mechanical systems, relatively few studies have focused on applying these methods to identify bolt loosening in offshore WT support structures. To bridge this gap, this research applies normality models to monitor the structural health of jacket-type supports, achieving a detection accuracy of 93.7%.