A Simple Method to Evaluate the Natural Frequency of Jacket Supported Offshore Wind Turbine under Storm-Paper

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The study proposes a practical method for estimating how storms affect the natural frequency of jacket-supported offshore wind turbines, whose foundations must account for pile–soil interaction and cyclic loading.<br /><br />The authors conducted monotonic and cyclic constant-normal-stiffness (CNS) shear tests on sand–steel interfaces. The cyclic tests covered up to 500 cycles and varied soil depth, pile diameter and displacement amplitude. Peak interface strength was mobilized at about 0.5 mm of shear displacement across the tested depths. Cyclic loading reduced interface strength more rapidly at greater depths, with smaller pile diameters (higher normal stiffness), and at larger displacement amplitudes.<br /><br />Using the test results, the authors developed models for cyclic degradation of interface strength and stiffness, incorporating these factors. They then applied the models to the axial t-z springs in a numerical model of a 5.5 MW jacket-supported turbine, while retaining conventional p-y and Q-z springs. The proposed calculation procedure estimates natural-frequency changes after storm loading.<br /><br />In a pile-length optimization example assuming 1,000 storm-loading cycles, the turbine’s post-storm re-operation frequency fell notably for shorter piles. Piles shorter than 29 m caused the frequency to leave the manufacturer’s safe band of 0.24–0.48 Hz; a pile length above 29 m was therefore recommended for the example design. The method offers a relatively simple way to account for cyclic soil–pile degradation in storm-time natural-frequency assessments.

Keywords

offshore wind turbines

jacket-supported structure

natural frequency

storm loading

pile-soil interaction

cyclic degradation

sand-steel interface

constant-normal-stiffness

t-z springs

pile length optimization