Singing Mitigation in Corrugated Risers by Liquid Injection

Abstract

In the last few decades, flexible risers have increasingly been used in the offshore oil and gas industry. In gas applications these risers can generate high amplitude tonal pressure fluctuations when the gas velocity reaches a threshold value. The resulting pressure fluctuations can then cause high vibration and cyclic stress levels in the associated topside and subsea pipe work. On a number of platforms, riser pulsations have led to serious limitations of the production capability and in at least one case to the fatigue failure of two small bore side branches, resulting into a shutdown of the platform for five months. An acoustic assessment can be performed to determine the risks of a flexible riser to be a source of Flow-Induced Pulsations (FIP) within its intended operating envelope. This assessment is based on models and methods to predict the onset velocity, pulsation frequency and amplitude and is based on actual field measurements on a number of different platforms and on research performed in collaboration with multiple operators and universities. Relevant inputs include the riser carcass geometry, operating conditions and the reflective characteristics of the topside and subsea piping. One critical parameters is the liquid content. The phenomenon is mainly observed in dry gas systems. At even low liquid fractions, corrugated risers are less prone to generate pulsations. A number of laboratory experiments have been performed to evaluate the singing suppression at different liquid loads for both horizontal as well as vertical (upward) flow. Finally, a field trial was executed by Statoil at an FPSO in the Norwegian Sea. One export riser showed singing tendencies. Due to the vibrations, a reduced production limit was set. A field measurement campaign was started to inject liquid into the export risers to provisionally increase production, while in the meantime permanent solutions were evaluated and installed. The mechanisms playing a role when liquid is present in corrugated pipe are reviewed in this paper: the cavities are (possibly partially) filled-up by the liquid, the boundary layers are modified by the added roughness of the wall, liquid droplets can interact with the acoustic waves. All these can reduce the source term and increase the acoustic losses in the pipe, therefore delaying or avoiding the singing.