A stepping frequency is the difference between two successive 'locked-on' frequencies. When a long corrugated riser is connected to rig pipework, the length of the pipework dictates the stepping frequency. The fft graph on the left below shows the noise generated by a long corrugated riser as air is flowed through it. The graph on the right below shows the effect of adding rig pipework. A standing wave is setup in the rig pipework which then feeds back into the corrugated riser and causes the 'locked-on' frequency effect.

FFT vs time plot for a corrugated riser only.

Hear the step changes on Asgard B (3.5MB) FFT vs time plot for a corrugated riser with topside pipework.

When a frequency is heard it is because the vortex shedding process in the riser has setup a standing wave in both the corrugated riser and the topside pipework. When an audible tone is heard on the topside it only changes frequency when an exact number of half waves can fit in the topside length.

Effect on the Subsea Export Riser Base?

The effect of the stepping frequencies has been demonstrated on the topside pipework, it must be assumed that the same effect occurs on the subsea export riser base. The locked on resonance may be able to cause a fatigue failure on the riser base because fatigue failures have already been experienced on the topside.