Geotechical performance of gravity based foundations for offshore wind turbines in the Gulf of Bothnia’s soil conditions

Abstract

The main purpose of this thesis is to analyze the performance and usability of gravity-based foundations for offshore wind turbines in the soil conditions of Gulf of Bothnia. The thesis offers a comprehensive literature investigation to the local geology, soil conditions and stratigraphy, as well as metocean conditions including wind, wave, and ice conditions. The description of these conditions can be applied in various engineering practices and serve as a thorough overview for multiple other purposes requiring knowledge of Gulf of Bothnia’s properties. The thesis also briefly discusses the impact of glacial ice and its effect on the overconsolidation ratio of the soil.

In the thesis, the performance of gravity-based foundations is studied through three representative soil profiles and two initial foundation designs at 20- and 40-meter water depth. The soil profiles are constructed based on literature and concentrated on areas most likely potential for the utilization of gravity-based foundations. To verify the initial foundation design, a 3D finite element analysis is carried out to study the stability of the foundation with static soil properties in Ultimate Limit State. The strength degradation of the soil due to cyclic loading from wind, wave and ice is studied using cyclic contour data and load histories from literature. The impact of cyclic loading on the acceptability of the initial foundation dimensions is discussed and design recommendations considering cyclic loading are made.

The main results of this thesis revolve around cyclic loading and its effects on the strength of the soil. The results from cyclic strength degradation analysis indicate that cyclic loading can cause significant soil strength reduction: the strength can degrade up to 30 % in low plasticity, highly overconsolidated cohesive soils and up to 80 % in normally consolidated sand. This is important as these types of subsoils are prevalent throughout the Gulf. According to the stability calculation results, the initial foundation geometries are acceptable in Ultimate Limit State using static soil properties. However, cyclic loading could reduce the strength of the soil to the magnitude, that the initial foundation geometry might not fulfil the design requirements for Ultimate Limit State analysis anymore. Because of this, cyclic strength degradation should be accounted for in early design phases. Even though design standards and requirements advice the accounting of cyclic loading, they do not offer straightforward ways of doing so. To fulfil this, the thesis proposes initial cyclic model factors which can be used in preliminary analysis of the effects of cyclic loading on soils strength within the Gulf of Bothnia.

The results offer important insights into the feasibility of gravity-based foundations in the Gulf of Bothnia and provide recommendations for optimal design practices in preliminary analyses. These results contribute to best practices for offshore wind foundation design in challenging conditions, supporting Finland’s renewable energy goals and guiding the safe and durable implementation of wind infrastructure in the region.