Suction Bucket Pile-Soil-Structure Interactions of Offshore Wind Turbine Jacket Foundations Using Coupled Dynamic Analysis

This paper presents a procedure for the coupled dynamic analysis of offshore wind turbine-jacket foundation-suction bucket piles and compares the American Petroleum Institute (API) standard method and Jeanjean's methods used to model the piles. Nonlinear springs were used to represent soil lateral, axial, and tip resistances through the P-Y, T-Z, and Q-Z curves obtained by either API's or Jeanjean's methods. Rotational springs with a stiffness equated to the tangent or secant modulus characterized soil resistance to acentric loads. The procedure was implemented in X-SEA program. Analyses of a laterally loaded single pile in a soft clay soil performed in both the X-SEA and Structural Analysis Computer System (SACS) programs showed good agreements. The behaviors of a five MW offshore wind turbine system in South Korea were examined by considering waves, current, wind effects, and marine growth. In a free vibration analysis done with soil stiffness through the API method, the piles were found to bend in their first mode and to twist in the second and third modes, whereas the first three modes using Jeanjean's method were all found to twist. The natural frequencies resulting from Jeanjean's method were higher than those from the API method. In a forced vibration analysis, the system responses were significantly influenced by soil spring stiffness type. The procedure was found to be computationally expensive due to spring nonlinearities introduced.