Developing an upending method that utilizes the buoyancy of a monopile with trapped air

Abstract

Due to the rapidly growing offshore wind market, there is a high demand for capable wind turbine installation vessels. Current wind turbine installation vessels struggle with the installation of large diameter monopiles due to a lack of crane capacity and with the installation of wind turbine components at hub height due to a lack of crane height. Most jack-up vessels currently have a crane capacity of 1600 tonnes or lower. This is adequate for the current average diameter monopile but already too low for the larger diameter monopiles. Due to a lack of crane capacity, these vessels are at risk of no longer being capable of installing wind turbine foundations in the near future. However, if it would be possible to reduce the crane loads during the installation process of monopiles, the commercial viability of these vessels can be extended. Previous research and practical experience have shown that a floating monopile upending process is feasible. A theoretical research with an experiment has been performed on the upending of a monopile where the buoyancy is utilized, but this research does not include the environmental conditions that can impact the monopile during the upending. This research also does not show the maximum amount of buoyancy that can be utilized during the installation process.

This thesis determines the maximum installation capacity that can be reached with a floating upending method, using a 1600 tonnes crane. It also analyzes the impact of the environmental conditions and presents the workability of the upending method. An analytical model has been made to calculate the motions of the monopile while it is partially submerged and hanging in the crane. The analytical model is used to validate the numerical model made in Orcaflex that was developed to consider five different concepts for the upending of the monopile. The best-chosen concept is further developed in the floating upending method.

To make the monopile water and airtight during transportation and upending, both ends of the monopile are plugged. The monopile is transported to the installation site by two tugboats that connect it to a rotating gripper which is placed on the side of the jack-up vessel. Then, the crane upends the monopile while the pile stays connected to the rotating gripper and maintains a certain amount of submergence during the upending which creates the buoyancy force. After the upending, the lower plug is removed by increasing the air pressure inside the monopile whereafter the monopile can be lowered to the seabed. This method is analyzed in the frequency domain for six upend angles whereafter the workability of the method is determined by simulations of the three most critical upend angles in the time domain. The floating upending method increases the installation capacity by 40% while maintaining a workability of 60-70% depending on the monopile diameter. It extends the lifetime of jack-up vessels and is a cost-competitive alternative when compared to mobilizing a floating heavy lift vessel instead.