The "Action Plan for Energy Efficiency" presented by the European Commission has set a goal of reducing energy consumption by 20% by 2020. Offshore wind turbines offer a chance to create clean, renewable energy. Wind parks in Germany are planned along the North Sea and East Sea, of which the North Sea has especially severe meteorological conditions. One possibility for offshore wind turbine foundations is the monopile with large diameter. The wind and wave load acting cyclically on a monopile could exceed 10E6 cycles over the lifetime of the structure. The existing design method does not account for the lateral behavior of a single pile of large diameter subjected to long-term cyclic load. The design guideline of BSH (2003) advocates approval of several cyclic laboratory tests. However, the application of these test results is not presented. The purpose of this study is to find a method for estimating the accumulated displacement of a monopile embedded in sandy seabed under long-term cyclic lateral load incorporates the results of drained cyclic triaxial tests.

A numerical model consisting of degradation stiffness and finite elements method is presented to simulate the behavior of a monopile foundation embedded in cohesionless soil under long-term cyclic loading. Accumulated displacement under cyclic loading can be interpreted as a decrease in the secant stiffness modulus of each soil element in pile-soil finite element model. When elastic strain is negligible, the secant modulus after N cycle E_sN of each soil element can be formulated with plastic strain in the first cycle, ε^a_p,N=1 , and the Nth cycle, ε^a_p,N. KCombined with semi-empirical formulations to describe plastic strain obtained in the cyclic triaxial tests, the degradation stiffness of each element in finite element model can be determined with a stress -dependent variable X and two regression parameters b₁ and b₂.

The regression parameters depend on soil characteristics and are determined from cyclic triaxial test results. Stress dependent parameter can be presented by main principal stress at failure in static test σ_1,f and cyclic stress in the main principal direction σ_1,cyc.

This degradation stiffness is assigned to the elasto-plastic material in a finite element model to simulate the displacement response for the specified cycle number in Figure 1. A complex pile-soil system under long-term cyclic loading can be simulated with degradation stiffness model and accumulated displacement of monopile is quantified based on limited laboratory cyclic triaxial test results. The effects of pile geometry, stiffness, loading condition, and soil density will be investigated further in a series of parameter studies using a degradation stiffness model.

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