This thesis investigates the soil-structure interaction and corresponding stability of pneumatic caisson foundations compared to monopile foundations for offshore wind turbines. The monopile foundation has been the industry standard for years, yet pneumatic caisson foundations are being investigated as a possible alternative. This research applies finite element analysis with the software Plaxis 3D to explore the monotonic and cyclic behaviour of the two foundation types, analysing important properties including soil deformation, energy dissipation, and cyclic stability.

The results of the monotonic behaviour analysis show that caissons are experiencing non-linear soil behaviour at lower forces than monopiles. However, their elastic capacity is sufficient to support the environmental loads, resulting in lower displacements under these loads. Monopiles exhibit greater flexibility, energy dissipation, and a tendency to increase soil stiffness over cycles. At the same time, caissons maintain structural stability with small permanent deformation and low energy dissipation, as demonstrated by cyclic loading tests. Pneumatic caissons demonstrate potential as viable alternatives to monopiles, as they offer increased initial rigidity and reduced displacements. Additionally, the pneumatic caisson foundation can provide a solution in environmentally sensitive areas because the pneumatic caissons can be installed with low noise and vibration impacts on the environment. As the load increases to its maximum capacity, the caisson shows a more abrupt failure compared to the monopile. Beyond the load tipping point, where the caisson shows non-linear behaviour, it undergoes significantly more deformations as force increases. The monopile shows a more gradual increase in deformations with an increase in force. As a result, the monopile shows a more gradual failure behaviour.

Practical challenges in the caisson’s production, transportation, and installation need to be overcome. For the production of the big caisson structures, specialised production facilities are required with direct access to the sea. Due to the caisson dimensions used in this thesis, it is too heavy to float and must be transferred using large cranes and barges or semi-submersible vessels. Although this study did not look into it, dimension optimisation or the use of different materials to construct the caisson might save money on transport, particularly if the caisson can float. In order to install the caisson at the correct depth, water must be pumped into its hollow chamber to produce enough downward force to counteract buoyant forces and wall friction.

In summary, the pneumatic caisson foundation offers a viable alternative with advantages in terms of stiffness and lower displacements compared to monopiles. Pneumatic caissons are a promising foundation solution for offshore wind turbines. However, the economic feasibility of the pneumatic caisson method in the offshore environment remains to be examined. ... Read more