Hydrodynamics of a monopile

The hydrodynamics of a monopile

The offshore wind industry is growing rapidly and the installation costs of offshore wind farms are for a large part governed by the installation of the support structures, which are currently primarily monopiles.

As the turbines grow in size, the monopiles are becoming larger as well. Aside from the static weight, also other factors become important for offshore cranes. The internal water column of monopiles being lowered to the seabed can play an important role in the dynamic forces acting on the pile because this water mass can be excited and show resonance behaviour called sloshing.
The limiting criteria during the installation operation of monopiles from a jack-up are governed by the wave-induced motions and loads. Minimizing downtime is key, making the need of accurate predicting of the motions of these structures in waves of high importance.

In general, the motions for large diameter cylinders are calculated using a linear radiation-diffraction analysis (traditionally used for monohull vessels). In the calculated hydrodynamic properties, sloshing-induced peaks are visible due to free-surface motions of the internal water column. In order to validate the output from 3D diffraction software, several towing tank tests were performed at TU Delft.

From the tests it became clear that sloshing, which is a non-linear phenomenon, was indeed present. Sloshing appeared indeed around the hand-calculated natural frequencies. Calculating the effects of sloshing correctly is complex. With this research it is possible to identify frequency ranges for which the common calculation methods are valid. However, for determination of forces outside these ranges, more research is needed.

Related pages