Today, the monopile is the most widely used offshore wind turbine support structure type, especially in the shallow water of the (Dutch) North Sea. Due to the simple tubular design, its use has been extended to larger and larger wind turbines. However, the increasing size of the monopile also comes with challenges and TNO initiated an explorative study to investigate these.
Very large monopile for Veja Mate offshore wind farm

The monopile is a popular solution to support offshore wind turbines. It basically is a tubular structure, which is relatively easy to design and manufacture. Its main purpose is to carry the wind turbine and transfer the wind loads to the earth. In addition, there are many other requirements to consider: the monopile will be subjected to wave loads, it has impact on the system structural dynamics, it needs to withstand corrosion, and so on. Installation is typically done by hammering the monopile into the soil, although other solutions are under investigation as well.

The largest size monopile currently installed is approaching 11 m diameter, the so called XL monopile. Currently, even larger wind turbines are on the drawing table, that would require next generation XXL monopiles. The increasing size of the monopile comes with challenges, both of more practical nature (such as logistics, installation and manufacturing), as well as related to design/analysis methods (wave diffraction effects, buckling).

In an explorative study performed in 2016, the design limitation have been investigated by performing a preliminary design of a support structure (from seafloor to nacelle) for a 10MW wind turbine in 50m water depth. This was a degree larger than the current designs in the industry with the aim to put our current models to the test. The basic design features a mudline pile diameter of 9.3 m and a total steel mass above mudline of 1850 tons.
The basic design has not been optimized, but was subjected to an ultimate load, buckling and fatigue check. The design passed the ultimate load checks and fatigue check at the mudline. There are indications that buckling could be a challenge, as global buckling for a buckling mode twice the length of the support structure was not passed. However, it is questionable if this mode is representative.
The largest diameter in the substructure is 9.3m, which is within manufacturing capabilities. The total size and weight however poses a serious challenge for installation, both for vessel lifting capacity and hammering equipment. Current state of the art design tool are capable of including soil-structure interaction. However, for large diameter foundations other physical load bearing mechanisms are at play and the PY-curve approach does not suffice for accurate soil-structure modelling. For this diameter monopile, modelling the hydrodynamic loading by diffraction only has a minor influence on the magnitude of the loads and hence Morison’s equation may still be used without being too conservative. For monopiles of more than 10m diameter diffraction effect should be considered, as preliminary load calculations indicate 10-15% fatigue load reduction over the lifetime.

Monopiles will be around for many years to come, even with the ever increasing wind turbines. However, there are challenges to be faced. Please contact us if you would like to hear more about our studies in this field.

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Feike Savenije, MSc