A Comparison Between Measured and Predicted Hydrodynamic Damping for a Jack-Up Rig Model

An extensive set of measurements funded by the EU project Large Scale Facilities Program has been carried out on a model of a jack-up rig at the Danish Hydraulic Institute. The test serieswere conducted by MSC and include determination of base shears and overturning moments in both regular and irregular waves.In the paper these results will be compared with theoretical predictions based on two different methods.

The first is time simulation procedure using various stretching methods for the wavekinematics above the still water level and applying both the relative and absolute velocityassumption in Morison's equation for the wave load. The second procedure is a non-linear, single degree-of-freedom method making use of thetheory of diffusion processes.

This procedure is able to account for the spectral content in therandom waves and requires much lesser computational effort than the time simulation methods.In the comparison between the model test results and the theoretical predictions, thehydro-dynamic damping proves to be the most important uncertain parameter.

It is shown thata relative large hydrodynamic damping must be assumed in the theoretical calculations in orderto predict the measured results. This finding is discussed in the light of other results quoted inthe literature and some general guidelines are extracted.