Development of New Jack-Up Substructure Supporting Offshore Wind Turbines in Multi-Layered Soils: Geotechnical Aspects

Few studies have addressed jack-up substructures with spudcans for offshore wind turbines targeting multi-layer seabed conditions, which are frequently found in the Korean seabed. This study analyzed existing guidelines to establish geotechnical design procedures for a newly proposed jack-up substructure supported by tubular legs with spudcans, as well as to present design cases for a target site. This jack-up spudcan was designed for seabed conditions representative of the Korean southwestern offshore seabed, consisting of a sand–clay–sand layer. Analytical procedures from ISO and InSafeJIP guidelines were adopted to estimate the vertical bearing capacity of the spudcan. The yield envelope was determined based on this estimation, and the spudcan size was selected using structural reaction forces. Predictions from theoretical equations were compared with results from centrifuge tests for verification and discussion. Theoretical vertical capacities according to ISO match well with centrifuge results in sand-over-clay layers, while InSafeJIP shows a similar trend in intermediate clay layers. For clay-over-sand layers, only the vertical capacity formula for a single-sand layer case is available in the guidelines, which tends to overestimate the actual capacity for the underlying sand. However, by applying appropriately selected strength reduction factors, the actual foundation behavior can be reasonably predicted for design, but it is still overestimated, requiring further study.