Pile-Soil Interaction Under Cyclic Loadings for Offshore Wind Monopiles

Abstract

Offshore wind structures are subjected to cyclic loadings from wind, wave and current which must be accounted for when designing foundations. The effects of cyclic loadings can govern the dimension of the foundations. However, less standard and calculation procedure for designing foundations under cyclic loadings is generally accepted in practical design. A parametric study at the generic clay profiles were carried out by using the conventional beam column approach and the finite element analysis. In addition, a monopile foundation at the layered soil profiles in Korean West Sea were designed taking into cyclic loading effect account. Both design results show that the bending moments and shear forces along the pile by using beam column approach are larger than the results from the FE analysis. The FE analysis gives less pile displacement and rotation than the beam column approach. The reason for these differences can be partially explained by the soil reactions in the upper soil profiles. For the clay layers the lateral soil resistance is generally higher in the FE analyses compared to the cyclic p-y curves. However the lateral resistance in the silt sand layers is greater by using the standard cyclic p-y curves with a drained friction angle than using the constitutive models applying cyclic properties. Based on the results, a practical approach of monopile design for the specific offshore location can be recommended by calibrating and simplifying cyclic p-y curves from the results of FE analyses taking into account the cyclic stress-stain response of the soil.

Site-specific variability of load extremes of offshore wind turbines exposed to hurricane risk and breaking waves

“…Ultimate moment calculations are considered at the mudline only, and moment strength is calculated based on the local buckling and yielding provisions given in DNV RP C202 . Serviceability calculations are made for the monopile rotation at the mudline based on work by Shin et al , requiring that the rotation of the pile at the mudline does not exceed 0.25°. The design approach considered here does not consider fatigue limit states and ignores any effects associated with current, tides and storm surge, as well as wind forces on the tower.…”

“…For the present study, 24 1 h simulations were used in design to promote convergence of the load estimates. The mudline moment M and mudline shear force F from the fast analyses are factored according to the DLC, and design constraints on ultimate strength and serviceability are checked based on DNV guidelines for ultimate strength and research by Shin et al for serviceability. This procedure is followed for each of the 16 considered sites, and the results of the design procedure are given in section 4.…”