Numerical model for suction caisson under axial cyclic loadings

Jacket foundation with multi-caissons has been recommended for the offshore wind turbines (OWTs) with water depth exceeding 30m. For the jacket supported OWT, the response of suction caisson under axial cyclic loadings is critical for the engineering design. In this study, a numerical model is proposed to estimate the differential pressure, displacement accumulation and loading stiffness of suction caisson under axial cyclic loadings. In the proposed model, novel cyclic t-z and Q-z spring models are established and included to account for cyclic loading effects. The proposed model is validated by the centrifuge tests of suction caisson. Afterwards, a series of numerical studies are performed to study the effects of key factors (i.e., cyclic loading amplitude, frequency, soil permeability and loading direction) on the suction caisson responses. It is found that for the suction caisson in typical fine sand seabed, the threshold amplitude of cyclic tension loadings generally ranges between 0.4Pu_d and 0.5Pu_d (Pu_d is the uplift capacity of suction caisson under fully drainage condition). The contribution of differential pressure to the suction caisson capacity can reach up to about 20% for the stable state. The failure risk of suction caisson in permeable seabed will be obviously increased for low-frequency cyclic tension loadings.