Simplified t-z models for estimating the frequency and inclination of jacket supported offshore wind turbines

Abstract With the water depth exceeding 30 m, jacket foundation is the suitable supporting structure for offshore wind turbine (OWT). In this paper, it first illustrates that the vertical soil resistance along the pile shaft (namely the t-z springs) significantly affects the inclination behaviour of jacket supported OWT. Then, for different shear amplitudes, normal stresses and normal stiffnesses, a series of sand-pile interface shear tests are performed to study the cyclic degradation characteristics of interface strength. Based on the test results, two simplified t-z models and related calculation methods for estimating the frequency and accumulated inclination of jacket supported OWT with multi-piles are proposed. For the two simplified t-z models, a general interface strength degradation formula is established, in which the cycle number, normal stress, normal stiffness and shear amplitude can be considered. The verifications of the two proposed calculation methods are conducted with field tests of single pile and a centrifuge test of jacket foundation. Afterwards, a series of simulations are performed to study the evolutions of frequency and accumulated inclination under cyclic loadings for a typical jacket supported OWT. It illustrates that the accumulated inclination should be the controlling factor rather than the natural frequency for the design of jacket supported OWT with multi-piles in engineering.