On the force distribution along the axis of a flexible circular cylinder undergoing multi-mode vortex-induced vibrations

One of the main problems in predicting the response of flexible structures subjected to vortex shedding is that a totally reliable model for the fluid loading does not exist. It is also very difficult to measure the distributed force exerted by the fluid along the length of a marine riser without disturbing the system in some way. The methodology described here uses experimental response data obtained in a test programme undertaken in the Delta Flume in Holland during May 2003, linked to a finite element method model (FEM) of the riser. The length-to-diameter ratio of the model used was around 470, the mass ratio (mass/displaced mass) was 3 and the Reynolds number varied between 2800 and 28 000. By using the response data as the input to the numerical model, the instantaneous distributed in-line and transverse forces acting on a flexible cylinder can be studied.<br/

Laboratory measurements of vortex-induced vibrations of a vertical tension riser in a stepped current

This paper presents an initial analysis of measurements of the vortex-induced vibrations of a model vertical tension riser in a stepped current. The riser, 28mm in diameter, 13.12m long and with a mass ratio (mass/displaced mass) of 3.0, was tested in conditions in which the lower 45% of it was exposed to a uniform current at speeds up to 1m/s, while the upper part was in still water. Its response in in-line and transverse directions was inferred from measurements of bending strains at 32 points along its length. Transverse vibrations were observed at modes up to the 8th with individual modal amplitudes up to about 80% of the riser’s diameter. However, in most cases the response included significant contributions from several modes, all at the same frequency. Some evidence was found of lock-in

The loading on a cylinder in post-critical flow beneath periodic and random waves

A major problem encountered in model testing of offshore structures in wave tanks is the difficulty of extrapolating fluid loading on tubular members to full scale conditions. It is well known that in steady flow the drag of a circular cylinder undergoes large changes over the Reynolds number range Re = 10E5. - 10E6. Measurements are presented of the in-line transverse loads on a small element of a vertical and horizontal cylinder in waves at Keulegan Carpenter numbers up to 20 and Reynolds numbers up to 5 x 10E5.. Drag and inertia coefficients are calculated for the vertical cylinder and Morison's equation is found to give a good fit to the measured in-line force. Large transverse forces indicate the presence of strong vortex shedding at post-critical Reynolds numbers. Measurements in regular waves and random waves are found to give similar values of rms force coefficients. Vortex shedding is triggered in random waves when the Keulegan Carpenter number exceeds 7. In both orientations the cylinder loads show little influence of water particle orbit shape. Morison's equation fails to fit the horizontal cylinder data due to the presence of a strong vortex shedding component