Long Term Deformation of Cyclic Loaded Monopiles in Sand-Paper

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The study investigates long-term deformation of offshore wind-turbine monopiles in sand under repeated lateral wind and wave loading. Accumulated pile rotation can threaten turbine operation, so the authors develop a finite-element (FE) approach to model soil stiffness changes over many cycles.<br /><br />The method uses a cyclic stiffness model derived from drained cyclic triaxial tests. Ramberg–Osgood backbone curves and an extended Masing rule define separate reloading and unloading secant stiffnesses, which evolve with accumulated residual strain. The model is incorporated into a three-dimensional FE analysis and assessed against centrifuge tests of laterally loaded piles.<br /><br />The simulations broadly reproduce measured peak and residual displacements and the bending-moment distribution, including the depth of maximum bending moment. However, they underestimate displacement accumulation, particularly its rate. The authors attribute this limitation to the model’s treatment of cyclic response as hardening and its inability to represent plastic deformation in the upper soil; further model development is needed.<br /><br />A parametric study examines pile embedment lengths under 10,000 cycles. Greater embedment reduces rotation accumulation, but the benefit approaches a limit: beyond about 40 m in the studied case, rotation histories become similar despite differences in pile-tip deflection modes. The findings suggest that conventional pile-length criteria based on tip conditions may not adequately account for long-term cyclic rotation. Overall, the proposed FE framework offers a basis for predicting monopile response under cyclic loading, but improved representation of upper-soil plasticity is needed for more accurate accumulation estimates.

Keywords

offshore wind turbines

monopile deformation

cyclic lateral loading

finite element analysis

soil stiffness degradation

residual rotation

centrifuge tests

bending moment distribution

Ramberg-Osgood model

pile embedment length