Dynamic Impedances of Offshore Rock-Socketed Monopiles

Abstract

With the development of offshore wind energy in China, more and more offshore wind turbines are being constructed in rock-based sea areas. However, the large diameter and thin-walled steel rock-socketed monopiles are very scarce at present, and both the construction and design are very difficult. For the design, the dynamic safety during the whole lifetime of the wind turbine is difficult to guarantee. Dynamic safety of a turbine is mostly controlled by the dynamic impedances of the rock-socketed monopile, which are still not well understood. How to choose the appropriate impedances of the socketed monopiles so that the wind turbines will neither resonant nor be too conservative is the main problem. Based on a numerical model in this study, the accurate impedances are obtained for different frequencies of excitation, different soil and rock parameters, and different rock-socketed lengths. The dynamic stiffness of monopile increases, while the radiative damping decreases as rock-socketed depth increases. When the weathering degree of rock increases, the dynamic stiffness of the monopile decreases, while the radiative damping increases.

Coastal Geohazard and Offshore Geotechnics

“…He et al [11] employed ABAQUS to establish the interaction between rock-socketed monopile and layered soil-rock seabed. Based on a combined finite-infinite element model, the dynamic impedances and dynamic responses of large diameter rock-socketed monopiles under harmonic load are analyzed.…”

“…(2) evaluation of stability of marine infrastructures, including pipelines [6][7][8], piled foundation and bridge piers [9][10][11][12], submarine tunnel [13], and other supported foundations [14]; and (3) coastal geohazard, including submarine landslide and slope stability [15,16] and other geohazard issue [17,18]. More details of each contribution are summarized in the following subsections.…”

Study on Meso-Material Parameters of Submarine Weathered Granite Based on Parallel Bond Model

“…The coastal wind resource in China (e.g., Fujian, Guangdong, and other coastal areas) is abundant and the offshore wind turbine (OWT) is being developed rapidly [ 2 , 3 , 4 ]. However, due to the influence of submarine islands and reefs, the topographic and geological conditions are complex [ 5 , 6 ], the rock formations are shallow, and the depth varies greatly. The properties of the overburdened soil are weak.…”

Model Tests on the Frequency Responses of Offshore Monopiles

“…Numerical methods: Zdravkovic et al [15] presented a numerical study for laterally loaded monopiles; Sen et al [16] presented a boundary element formulation for dynamic analysis of axially and laterally loaded single piles and pile groups; Latini and Zania [17] investigated the dynamic responses and the dynamic impedance of suction caissons by analyzing 3D finite element models in the frequency domain; Tao et al [18] studied the influences of frequency, slenderness ratio, thickness-diameter ratio, soil-pile elastic modulus ratio and the existence of a scour hole on the dynamic impedances of monopile foundations; He et al [19] analyzed the dynamic impedances and dynamic responses of large diameter rock-socketed monopiles under harmonic load based on a combined finite element-infinite element model; Ma et al [20] presented a three-dimensional finite element model for analyzing the long-term performance of offshore wind turbines on monopiles in sand; Beuckelaers [21] brought insights into the behavior of monopile foundations and provided various modelling options to capture unloading, ratcheting and gapping effects for laterally loaded piles based on the Pile Soil Analysis (PISA) project.…”

“…The analytic methods [8][9][10][11][12][13][14][15][16] and numerical methods [17][18][19] mostly depend on the assumption that the soil is linear elastic so only the dynamic responses under small stress/strain conditions can be obtained, and the nonlinearity of soil is ignored. When considering the nonlinearity of soil, bearing capacity analysis [30][31][32][33][34][35] and accumulated deformation [36][37][38][39] under static/cyclic loads are mostly studied.…”