This paper presents measurements of the mechanical loads that splitter fairings on vertical and inclined risers will experience while in the wave zone or due to current drag. A 21″ pipe was utilized for the measurements, which corresponds to prototype size for a production riser and a 1:2.5 scaled model for a 54″ drilling riser. The program had three objectives, (a) to determine the nature of wave and current loading on a riser segment that was fitted with a fairing to provide sufficient internal strength and stiffening; (b) to determine the wave and current loading on the fairing connection system to assist in designing reliable latching systems, and (c) to investigate if the measured loading on the fairings from forced oscillation experiments can be used to simulate loading resulting from wave motion.
Of primary interest was the effect of wave loading on the fairings in prototype or near prototype conditions. One significant impediment to conducting such experiments in a model wave basin was the very limited ability of the basin to generate full scale wave in terms of period or height. However the horizontal component of the oscillatory wave velocity can be simulated by oscillating the fairings at prototype wave periods and amplitudes to obtain the same relative flow condition. One of the fundamental questions that remained prior to the research was whether the novel forced oscillation technique could actually replicate the real world wave induced loads and some experiments were directed at confirming this.
While exploratory in nature, these forced oscillation experiments demonstrated that the test apparatus and methodology were able to replicate the corresponding measurements from the wave experiments, thus providing reliable data to assist in predicting hydrodynamic loading on fairings due to current, forced oscillatory motion, and waves. The basin carriage was successfully used to force the fairings in a prescribed oscillatory motion. The analysis of the measurements of the forced oscillation experiments showed that the inertial force correlated well and was linearly proportional to acceleration while the viscous force was a linear function of velocity. It was demonstrated that the forced oscillation experiments could provide reasonably good estimates of the global loading resulting from wave action. This implies that the experimental technique of replacing wave motion with forced oscillation can be used for design. As a result, prototype or near prototype scale models of fairings can be tested in wave conditions well beyond the capability of model basins. While the global loads due to waves could be predicted from the forced experiments it appeared the latching load were about 70% higher while in waves relative to those recorded during the forced oscillation experiments. This discrepancy could be attributed to free surface effects.
