In order to address the response of a long cylindrical riser in current, design codes (like Vandiver, 1983) require lift and drag coefficients of 2-D sections. Uncoupling the flexible response from the loading these design codes will determine the flexural response based on sectional stiffness and mass coefficients. However the physics of VIV behavior indicate that the hydroelastic response is significant. Therefore a study has been initiated to research the 3-D response of a part of a long riser in current by experiments. A circular steel pipe of 16 mm diameter and 12.6 m long was towed horizontally in a towing tank at speeds between 0.5 and 3.0 m/s and with pretensions between 0.5 and 2.5 kN. Measurements were made of the drag loads, the acceleration and the bending moments. State-of-theart fibre optical measuring techniques were deployed to obtain detailed insight into the complex VIV response of the test pipe. The first analysis of the test data shows a very complex response, including strong coupling between cross flow and in-line motions, beating, multimode response, traveling wave response, etc. Further analysis will continue and additional results will be published later.
One of the great challenges in the offshore industry is still the assessment of the motions of a circular cylinder in waves and current for application to risers or riser bundles in water depths up to 3,000 m (approximately 10,000 feet). Here the fatigue life of the riser is often dominated by Vortex-Induced Vibrations (VIV). The riser may respond in several possible modes, but it is most often uncertain whether the riser will predominantly respond in one or a few modes or that it will respond in several modes simultaneously (Blevins, 2001; Triantafyllou et al., 1999 and Vandiver, 1983 and 1993).