Fatigue Based Topology Optimization of an Offshore Wind Turbine

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Offshore wind turbines have seen a significant increase in size over the past decades. A critical consequence of this increase in size is a substantial increase in the torque transferred from the wind turbine rotor. This leads to the necessity of larger and heavier drivetrains to adhere to structural failure requirements and subsequently to larger and heavier tower and support structures increasing the total mass and cost of the turbine. Hence, reducing the mass-to-torque ratio of the drivetrain has become a key design challenge.

The company Delft Offshore Turbines (DOT) proposes to replace the conventional drivetrain in the top of the turbine with a hydraulic drivetrain, which has a lower mass-to-torque ratio. Although this concept shows promise, finding a low mass design that fulfils the infinite fatigue life requirement can be challenging. Using topology optimization to minimize mass while constraining the structural requirements could, therefore, be instrumental in the realization of this concept.

The design case by DOT can be classified as rotating machinery. In general rotating machinery is
commonly subjected to periodic loading that varies non-proportionally in time. This results in fluctuating stresses causing material fatigue. A consequence of the non-proportionality of loading is that the time response needs to be computed to evaluate for fatigue, which adds additional computation cost. However, another common aspect found in rotating machinery is that parts are cyclic symmetric, which allows for a potential reduction in computation cost.

In this thesis a method is presented to implement infinite fatigue life constraints into density based topology optimization for structures subjected to non-proportional loading, while cyclic symmetric properties are exploited to reduce computation cost. It was found that when the load case on a cyclic symmetric part adheres to certain conditions, a single static FE-analysis can provide multiple time steps for a quasi-static analysis. Decreasing the computational burden roughly proportional to the unique number of time steps obtained. The largest local variations in stress are estimated using a smooth min/max function and aggregated into a global constraint.

The method was tested on several numerical problems as well as applied to the DOT design case. The results showed that the method was able to properly constrain a global fatigue constraint while minimizing mass, achieving final designs that might not be trivial to find by hand.