Mitigation of flow-induced pulsations in flexibles at field conditions with liquid injection

Flexible corrugated pipes as used for the transport of gas in offshore deepwater applications can generate undesired flow-induced pulsations (FLIP). This phenomenon has been also traditionally called “singing risers”. The pulsations generated may result in high-amplitude cyclic stresses in the pipe systems attached to the flexible. Moreover, an HSE risk for the operators around the riser arises due to the tonal noise radiated from the system. The root cause for this phenomenon is nowadays well understood. Namely, vortex shedding at each internal corrugation of the carcass induce pressure pulsations which are eventually synchronized and amplified by an acoustical resonance along the length of the flexible. Despite this established knowledge, mitigation or suppression of the phenomenon once it occurs is hard to achieve. The purpose of this paper is describing field tests executed for a live natural gas export corrugated riser in the North Sea. The tested mitigation measure was liquid injection upstream of the gas export riser. The field tests had several objectives. First, to explore the effectiveness of this technique, for which only one prior experience is documented in open literature. According to very recent studies, the main mechanism contributing to suppression of FLIP by liquid injection is cavity fill-up, though this could not be tested in the field trial. Second, to quantify in more detail the threshold injection rates needed at different gas export rates. Third, to explore the sensitivity to export pressure. The tests consisted of measuring several variables of interest, next to the existing process conditions. These are the dynamic pressure pulsations at the riser hang-off location, as well as vibration levels at selected critical locations. The results are benchmarked with dry runs in which no liquid is injected. The injected liquid performed as expected, not only in mitigating FLIP, but even suppressing it altogether when sufficient liquid is injected. Threshold levels for the injection rates were found that, though not enough for full FLIP suppression, reduced the measured vibrations to acceptable levels. This allowed a significant increase in production capacity.