Structural health monitoring of offshore structures using a modified modal strain energy method (Case study: four-leg jacket substructure of an offshore wind turbine)

Four-leg jacket is one of the most widely used offshore structures that is exposed to various damages during its service life. Unlike land structures, offshore structures are always exposed to waves, corrosion and damages, especially in the splash zone. At shallow to intermediate water depths, these structures are commonly used as the substructure of offshore wind turbines.  Passing their service life has made the structural health monitoring of offshore wind turbines necessary. In this research, a modified modal strain energy method is used for damage detection in deck and splash zone members which are more exposed to periodic loads of waves and high corrosive seawater than other members. The results showed the higher accuracy of the improved modal strain energy method compared to the original method (Stubbs index). Also, using improved method resulted in prediction of single and multiple damages, with low and high intensities with appropriate accuracy.