Soil–Structure–Wave Interaction of Gravity-Based Offshore Wind Turbines

The world has witnessed a rise in investments in emerging renewable technology. Wind energy is abundant and can be harnessed using technologies with minimal/nil carbon foot print. In this view, numerous wind turbine farms have already been installed in many offshore and nearshore areas around the world. Initial capital investments in such farms is generally high; as such, minimal attention has been paid on optimum design and structural integrity of offshore wind turbines. In particular, the structural design of offshore wind turbines is based on the consideration of coupled dynamic phenomena. Wave loads cause the dynamic oscillation of the monopile, and the dynamic oscillation of the monopile affects the wave loads. The boundary conditions of the gravity-based foundation-monopile-turbine system are mostly affected by the flexural stiffness of the foundation plate, the elastic and creep behavior of the soil, and the inertia (translational and rotational) of the wind turbine mass. Technically, the design of the foundation should consider the dynamic response of the soil and the monopile, and the dynamic response of the soil, as the monopile is affected by the design parameters of the foundation. Published literature has shown that the initial conditions of the system yield transient dynamic phenomena. In addition, it has also been reported that the braking wave at t = 0 causes different dynamic response than the steady-state conditions due to a harmonic wave load.

Overall, since the full-scale testing of offshore wind turbines is difficult and expensive, majority of the published results on numerical methods cannot be verified and the accuracy of many of them is obviously questionable due to unreasonable simplifications and mistakes in numerical modeling. To address this, Professor Dimitrios Pavlou from the University of Stavanger in Norway developed a new integrated method for soil–foundation– structure response of offshore wind turbines in wave loads. His main focus was on the effect of structural parameters on the dynamic response of the system. His work is currently published in the Journal of Offshore Mechanics and Arctic Engineering.

In his approach, an integrated analytical model simulating the aforementioned dynamic phenomena was proposed. Specifically, the researcher derived a solution of the corresponding differential equations for the monopile-soil-foundation system and the boundary and initial conditions. This was achieved with the aid of double integral transforms and generalized function properties. Further, a parametric study was carried out, and results of the effects of the design parameters and soil properties were evaluated and presented.

The author reported that hysteresis of the movement of the turbine mass particularly took place during the transient period of wave loading. In addition, he observed that for the soil-foundation system with small stiffness, larger slope of the monopile cross section on the support z = 0 was yielded. More so, it was seen that heavy turbine mass caused high inertial forces and buckling of the monopile.

In summary, the study successfully presented the development of an integrated model for the dynamic response of offshore wind turbine monopiles. The proposed solution aims to fill gaps of existing researches, i.e., to take into account the soil compliance in the boundary conditions of the foundation, the inertial effects of the soil, the translational and rotational inertia of the turbine mass, the transient response of the monopile in the first braking wave, etc. In a statement to Advances in Engineering, Professor Dimitrios Pavlou said that the solution he presented was advantageous because it was semi-analytical. In fact, he added that credit to the resulting final formulae, providing the dynamic deflection distribution versus the spatial and temporal variables, the parametric study for investigating the effect of predominant parameters was now feasible.