Vortex induced vibrations of pipes generated by high and steep waves in the crest zone have been investigated by full-scale field testing. An instrumented cylinder has been suspended from a platform bridge in the North Sea. Adjacent to it a newly developed acoustic system capable of measuring the three dimensional wave kinematics was placed. The kinematics were measured all the way up to the instantaneous water surface elevation, i.e. including the wave crest. The paper presents time series of measured water surface elevations and orbital velocities at the instantaneous water surface together with the response of the instrumented pipe in a storm. The sea state was measured to Hs = 6.4 m and Tz = 8.4 sec. It was clearly seen that vortex shedding locking-on takes place in some of the rather high modes at the passage of large waves. Intermittent cross flow vortex induced vibrations from 0.3 diameters up to 0.8 diameters were found in the 8"th and the 4"th mode respectively. The Reynolds numbers and KC numbers were up to 5*105 and KC∼250 respectively based on the maximum surface velocities.
In the design of slender offshore structures such as risers, drilling conductors or well conductors, the vortex shedding induced response will often be decisive for the fatigue life of the structure. Only very few methods for analysing the response in wave motions are available at present, for instance Ottesen Hansen, (1982). Using such types of model for optimal design, precise data for calibration of the load models are essential. There exist a very large number of laboratory tests. They are, however, made at low Reynolds number and with rather short lengths of pipes resulting in an uncertainty in the effects of the correlation lengths. The tests do show however that there is a marked difference between vortex induced vibrations generated by regular wave motion and by irregular wave motion, Kozakiewicz, Sumer and Freds0e (1994) and Sumer and Kozakiewicz (1995).
