Posters

Abstract

The Ultimate Limit State (ULS) design of offshore wind turbine foundations and support structures often follows an approach of estimating extreme environmental conditions which are used to calculate extreme loads and then in turn an extreme system response. Following this approach makes the implicit assumption that what is deemed a priori as an extreme environmental condition (e.g. extreme wave height or wind speed) leads to the most extreme response. For some more complex loading scenarios this assumption needs to be tested, and extreme conditions may require derivation following an approach based on system response over environmental inputs. We will examine two novel problems faced in the design of offshore wind turbine foundations. Monopile resonance: For large (15MW and beyond) wind turbines, increased loading through wave induced resonance is a problem that is becoming more likely to occur. Generally speaking, as turbines and their foundations increase in size their first natural frequency drops. When the first natural frequency lies within the range of frequencies excited by the incident wave forcing, resonance may occur. While resonance is a consideration in existing load cases (Normal Sea States) used to design offshore wind turbine foundations, it is not currently addressed directly in the derivation of ULS conditions.  Internal monopile pressure fluctuations: To facilitate water replenishment and allow for cable entry, monopiles are equipped with subsurface openings. These openings enable the exchange of seawater between the internal volume of the monopile and the surrounding marine environment, driven by fluctuations in water level, currents, and wave action. This causes the free surface within the monopile to oscillate. A ventilation system must be designed to allow air from the internal volume to flow with the movement of the water level, such that the air does not exert pressure on the seal of the ATP.