Predicting accumulated deformation of offshore large-diameter monopile using hollow cylinder apparatus

Abstract

Offshore wind turbine structures are commonly subjected to cyclic lateral loading, from waves and wind. The cyclic loading can cause an accumulated deformation of monopile. This study investigates cyclic accumulated deformation of offshore monopiles by using soil cyclic shear tests and numerical simulation. A series of soil axial-torsional bi-directional cyclic load tests are conducted to obtain the cyclic behavior of sand with different relative densities. The test results are analyzed in terms of soil secant modulus from accumulated shear strain, and the results are then used to improve a degradation stiffness method. This improved method was verified by comparing with a pile test. Finally, the cyclic accumulated deformation of large-diameter monopile was studied in loose over dense sand, medium-dense over dense sand and dense sand by using this improved method. The results show that the secant modulus decreases rapidly in the initial cycles of loading, and then the rate of decrease diminishes and remains below an asymptote. The depth of the max bending moment increases with increasing the cyclic number.