Short-Term Monitoring of Offshore Wind Turbine Jackets Supported by Suction Caissons

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4. Short-Term Monitoring of Offshore Wind Turbine Jackets Supported by Suction Caissons

The installation method of suction caisson foundations for jackets supporting Offshore Wind Turbine Generators (WTG) could alter soil conditions. In particular, the penetration of the skirt into the soil, as well as transient flow of pore water through the soil, may degrade the soil especially if cyclic installation with alternating under- and over-pressure is adopted. While the first causes remoulding of the soil, the latter may induce soils hydraulic failure. Such changes may affect the structural response. The entire installation sequence, including foundation installation, tower mounting, grouting the void under the lid and scour protection placement, will have implication on the WTG response. In this paper, the philosophy and results of a planned and executed short-term structural monitoring programme on the Seagreen Offshore Wind Farm (OWF) are presented. Eleven jackets were monitored for 19 to 186 days during the installation sequence. The acceleration data were processed to derive modal properties. The effect of the installation method, grout installation, turbine placement and scour protection on the natural frequencies of the structure are investigated. The natural frequencies were found to be comparable independent of the installation procedure. The effect of grout altered the first natural frequency by about 2%, the tidal variation showed an impact as well of approximately 2%, and the placement of scour protection resulted in a change of circa 1%. Installation of the tower and rotor-nacelle-assembly (RNA) significantly affected the first natural frequency, by about 90%. An approximately linear relation between the water depth and the natural frequency is observed.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

11 - Suction installed foundations and anchors