The Offshore Wind industry is in motion, and has been so since its inception merely 30 years ago. Wind turbines have continuously increased in size, as have the steel foundations on which they are installed. For the next generation Wind Turbines the question rises how to install the turbine towers on the foundations.
HVR Engineering has been active in the Offshore Wind Industry since the early days with the design of the foundations for projects such as the Princess Amalia Windfarm, and regularly as an advisor for monopile design verifications, for projects such as Veja Mate and the Gemini Windfarms.
Pile driveability is an often underestimated aspect of any interface connection
Throughout the years HVR Engineering specialized in the design of the top section of the monopile and the drivability hereof by Hydraulic impact hammers. It was therefore that Mr. Jasper Winkes from C1 Connections approached HVR Engineering to analyse worst case scenarios that could occur during pile driving on a new design for a monopile connection. “Pile driveability is an often underestimated aspect of any interface connection,” says Mr. Winkes. “Both hammer and pile imperfections, combined with the highly dynamic behaviour of pile driving, have a large effect on the stresses in the pile top. HVR Engineering is leading the industry when it comes to detailed analysis of this interaction, and since they are also involved with the design of hammer sleeve anvils, they are the experts when it comes to the driveability of monopiles.”
The next generation Offshore Wind Turbines will have very high overturning moments, an issue that is already seen with turbines installed in Asian wind farms where Typhoons can have a massive impact on the load of the connection between foundation and tower. As a results, next generation monopiles are designed with top diameters of seven to eight meters to accommodate equal sized turbine towers. Industry standard L-flange connections are facing limitations with these diameters in combination with high loads.
C1 Connections has developed the C1 Wedge Connection, an innovative solution to connect the monopile foundation to either the Transition Piece, or directly to the Wind Turbine Tower. “This solution creates a pre-stressed symmetric connection with special shaped holes in the top of the pile,” explains Mr. Winkes. “Fork shaped upper and lower members enclose special fasteners using wedges that create the symmetric pre-stress. Testing has demonstrated that the connection has an ultimate capacity that exceeds that of conventional bolted L-flange connections and that the connection is not fatigue sensitive. Further benefits lie in an enhanced safety of the connection and the fast installation process.”
NON LINEAR DYNAMIC ANALYSIS
Due to the novelty of this connection, a thorough analysis of the driveability of the wedge connection was studied by HVR Engineering to evaluate if a monopile with a wedge connection interface at the top can be driven with a large hydraulic impact hammer.
By executing refined 3D non-linear dynamic analysis in ANSYS, HVR Engineering analysed the effects of the top pile geometry as a result of the forces rising in the connection during pile driving. Furthermore, an in-depth analysis was executed to identify how local stresses were affected by the manufacturing tolerances of the components. With the knowledge from the analysis, an optimized design of the pile top was proposed by HVR Engineering that reduces the stress occurring during installation at the most critical locations by 60%. This had a significant effect on improving the fatigue performances of the connection.
“The design verification and optimization performed by HVR Engineering has given valuable insights in the optimization of the C1 Wedge Connection when the connection will be used on the top of a monopile,” says Mr. Winkes. “When we see that currently some wind farms are considering grouted connections again, it is more than clear that a connection with a higher capacity than an L-flange is sought after by the industry. With the analysis performed by HVR Engineering we feel confident that the C1 Wedge Connection can be installed with conventional impact hammers and that we have a solution for the work-horse of the industry: The Monopile.”
The analysis of the C1 Wedge Connection is one of many projects in the recent years performed by HVR Engineering, and another contribution to Energy Transition with the involvement of the development of the next generation in Offshore Wind.
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