Application of a New Carcass Design for Prevention of Singing Behaviour in Flexible Risers

The flow of fluid over the inner surface of rough bore flexible pipes may create vortex induced pulsations. For gas risers these pulsations can in some cases be the source of a high amplitude tonal sound. This phenomenon is in the industry referred to as “singing risers”. Under certain circumstances these pulsations can result in large structural vibrations with potential fatigue failure of the connected topside or subsea pipe systems. The singing behavior is dependent on the operating conditions such as product density and viscosity as well as the piping layout of the topside and subsea piping. However, the most important factor is the geometry of the corrugations. In traditional designs the carcass is made from a folded metallic strip. NKT Flexibles has developed a novel approach to carcass manufacturing originally intended for deep and ultra deepwater applications, wherein the carcass is made of helically wound wires rather than a folded strip. Although the main focus for this development was to devise a carcass of superior collapse strength compared to conventional systems, the new carcass design provides a very smooth inner surface, thus mitigating vortex formation which will reduce or eliminate the formation of flow induced pulsations. The susceptibility to singing of the new carcass design is compared to that of a traditional carcass design. This includes the onset of the singing and the flow pressure drop. Due to the much smaller cavities in the new design, the singing tendency is shown to be significantly reduced. In particular, the expected onset velocity is 5–9 times that of a classic rough bore design with 20%–50% of the amplitude. An additional benefit of the new design is that the pressure drop for the pipe is close to that of a smooth bore pipe. Therefore, it is expected that for many applications, the new design will not lead to the generation of high amplitude tonal noise within the desired operating envelope.