Structural Optimization of the Monopile Installation Frame Design

Abstract

Wind energy plays an important role in the global energy supply and is obtained by wind turbines placed on- and offshore. The expected growth of offshore wind farms will generate a lot of work in the future. Seaway Heavy Lifting is an offshore company which offers Engineering, Procurement, Construction and Installation (EPCI) solutions worldwide for oil, gas and renewables projects.

Offshore wind turbines are most commonly placed on a monopile foundation. The installation of monopile foundations used for offshore wind turbine farms is the main part of the projects Seaway Heavy Lifting is executing. The installation of monopiles is done using an installation vessel, which needs to be anchored during installation. The anchoring is done in order to cooperate with external forces on the side shell due to the installation of the monopile. The installation of the monopile is done using a frame which is connected to the side shell of the vessel. In order to stay competitive in the business, the company has been doing research to how to decrease the amount of installation time of their projects. It is concluded that profit can be gained by reducing the necessary time to anchor the installation vessel.

To install monopiles without anchoring the vessel, the monopile installation frame (MIF) was designed. The MIF can be placed onto the seabed after which the monopile can be hoisted inside of the frame. The frame will support the monopile during hammering. No external forces will be acting on the side shell of the vessel when using the MIF during hammering, which rules out the need for anchoring the vessel. Instead of anchoring, dynamic positioning will be used. Since the installation of monopiles will occur in different water depths, the MIF needs to be modular. An extension piece will be used in order to change the height of the frame.

The goal of this thesis is to obtain a structural optimized design of the MIF. The connections needed to connect and disconnect the extension piece are critical sections of the MIF. During the lifetime of the MIF, fatigue due to waves, wind and current loading will play a role. Therefore, this thesis has focused on the structural optimization of the connection with respect to fatigue loading. A bolted flange connection will be used in order to connect the members, which will be machined and then welded to the tube end. An initial geometry of the connection was designed with help of design rules stated by ir. M. Seidel.

The finite element program ANSYS will be used for the calculation of stress distributions. The decision was made to verify ANSYS, which was done by studying the accuracy of ANSYS, its way of working and to get used to the program. The verification has been done using a reference project.

The fatigue analysis of the connection started first of all with a global load analysis. This was done with help of the program SACS, which uses wave heights and wind speeds together with currents data as input. A calculation model of the MIF was built in SACS. Once the input was completed, the internal forces of the MIF were calculated. The global load analysis is necessary in order to obtain the loads in the members that will be connected by the bolted flange connection. These loads were used as input for ANSYS.

To check whether the initial design could be used as a starting point, the 3 failure modes of a bolted flange connection have been explained and verified for the initial design. Once it was verified, it was used as input in ANSYS in order to study the stress distribution of the model. The initial geometry has a negligible radius between the tube and the flange of the connection. Therefore, it was expected that a high concentration of stresses would occur in the junction between the tube and the flange of the connection. In order to find the stress concentration factor (SCF) in this junction, the maximum stress occurring in the junction needs to be divided by the stress applied to the tube.

Once the SCF was known the fatigue analysis could be performed. The fatigue analysis was done for two details: the junction between the tube and the flange and the welded connection between the tube and the machined part. Firstly, the amount of actual cycles was calculated for a certain time period with help of the wave scatter diagram, after which the corresponding stress ranges during these cycles was obtained. The stress ranges were multiplied with the SCF for the tube-to-flange junction, the SCF was obtained using ANSYS. Once the stress ranges were known, the amount of cycles until failure was calculated using S-N-curves that fit the two studied details. The actual damage to the structure was determined by dividing the actual number of cycles happening by the amount of cycles until failure. With the damage known for a certain time period, the life time of the structure was calculated.

The MIF will be used for a period of more or less 8 years, so the design lifetime was set at 9 years.
The initial geometry had an extremely low lifetime. Therefore, the connection needed to be optimized in order to improve the lifetime. The optimization of the connection was done by increasing the radius of the tube-to-flange junction to lower the SCF. A lower SCF value resulted in a longer lifetime. The design has been optimized until an optimum radius of 36 mm was found. The final design has a lifetime of 9 years.