Wave kinematics and response of slender offshore constructions

It has also proven that slender structures vibrate substantially in hydroelastic vibrations in high waves. The vibrations are decisive for the fatigue life. The hydrodynamics and hydrodynamic coefficients for the vortex induced vibrations in these waves have been verified. It is recognised that the highest waves are near-breaking waves which in general have far higher velocities in the crest than the theorietical waves used in the design of offshore structures. The very high velocities in these waves also excite hydroelastic vibrations. Although the excitation only takes place during the passage of the waves it has been found that vibrational amplitudes of above 1 member diameter develope. It can be seen that the lower modes are excited as standing modes carrying the vibration over from wave to wave. The high velocities also generated transient vibrations which were too short to develope into the standing modes. The measurements have confirmed that the vortex induced vibrations cause an increase in the drag coefficient. On the other hand the drag coefficient can be reduced in the zone underneath the instantaneous surface due to air draw down and cavitation. Based on the measurements a recommended Practice for Design of Slender Offshore Structures have been worked out. In addition to the major findings new methods under for underwater acoustical velocity measurements have been developed. It is a development which can lead to a new generation of monitoring instruments for offshore standard