Due to the increasing demand of renewable energy to combat climate change, renewable energy is experiencing a rapid growth. To match this demand for renewable energy, especially offshore wind is popular due to its status as a proven technology. To build offshore wind farms in a space and cost-effective way, wind turbines are increasing in size and the farms are shifting towards deeper water. This also has an effect on the monopiles, the most used wind turbine foundation. They are increasing in size and weight to handle the deeper waters and larger turbines. At this moment, monopile installation is limited to approximately 30 meters water depth because of a scarcity of installation vessels with sufficient capacity. To install these large-size monopiles either the fleet of installation vessels must be expanded or new installation methods must be developed.
This thesis addresses the challenge of installing large expected size monopiles without the use of traditional heavy lift vessels, which are reaching their operational limits due to increasing monopile sizes and water depths. The aim of this study is to develop a craneless upending method for a 130 meters long, 13 meters diameter monopile weighing 3500 tonnes, suitable for deployment in waters 50 meters deep.
Five craneless upending methods are evaluated. Two axes of rotation are considered, namely upending by rotation about a fixed point and upending by rotation about a floating point. Furthermore, to rotate the monopiles during upending, a moment can be applied around the axis of rotation and by ballasting parts of the monopile. After considering the four models that follow from combining the given options, a fifth method was developed that includes upending by rotation about a floating point and rotating the monopile by ballasting and applying a moment both.
A numerical model is developed for each concept to analyse the forces, moments, buoyancy, and draft throughout the upending process. The models were verified against each other and validated through a scaled experiment. The experiments highlighted the importance of friction in the upending process and, after adjusting the numerical model to account for the friction, confirmed its validity.
Of the five methods evaluated, the method that incorporates ballasting and applying a moment is found to be the most optimal. Due to combining the two methods of rotation, the water depth and moment required for the upending process can be minimalized.
In this approach, a floating monopile with end caps arrives at the site and is connected to a gripper frame which allows for axial movement and rotation of the monopile. Once the pile is connected, the ballasting phase starts. During this phase, ballast water is pumped into the monopile through the bottom end cap which causes rotation of the pile through the water. When a certain clearance between the monopile and sea bed is reached, de-ballasting starts while applying a moment around the axis of rotation to continue the upending while maintaining the minimum clearance between sea bed and pile.
The findings demonstrate that the presented hybrid craneless upending method is feasible and offers a practical solution to the installation challenges posed for larger monopiles and installation sites at deeper waters.