Qualification of New Design of Flexible Pipe Against Singing: Testing at Multiple Scales

Abstract

Abstract Flexible pipes for production of oil and gas typically present a corrugated inner surface. This has been identified as the cause of "singing risers": Flow-Induced Pulsations due to the interaction of sound waves with the shear layers at the small cavities present at each of the multiple corrugations. The new flexible technology discussed in this paper, called K-carcass, is made of shaped wires and presents a very different bore profile compared to other folded-strip flexibles. In a preliminary study based on small scale testing, it was showed that the new inner profile geometry of this technology was more robust for Flow-Induced Pulsations compared to the folded strip carcass. A qualification program was initiated to address the different risks associated with the introduction of a new pipe technology. A primary goals of this qualification program was to extend the results of the preliminary study to the actual carcass at field conditions (natural gas at production pressure). In this paper, the method followed to qualify the singing behavior of the new flexible technology as installed in the field at operational conditions is considered. The testing program and the scaling approach used to extrapolate results obtained at different scales towards full-scale pipes are described. The program includes a combination of small-scale and large-scale testing. The small scale singing testing consists of blow-through tests of 2" and 3" pipes with corrugated inner bore with air at low pressures on a setup with well-defined acoustic boundary conditions. The large-scale singing testing consists of similar tests on 3" and 5" pipes with natural gas at close-to-operational pressures. These tests are completed with full-scale tests, where a reference riser and a prototype of new-technology riser are tested with natural gas at high pressure. The combination of scales and gas properties at which the tests are done, and the continuity between the different tests conditions, will allow the validation of theoretical and empirical scaling laws used in the qualification of the new riser in operational conditions. The blow-through tests with the prototype pipe with the new K-carcass tested with natural gas at close-to-operational pressure did not show any singing. In contrast, the reference pipe (with folded-strip carcass) started singing even at low velocities. The test results can be extended to an allowable production envelope for a pipe in field conditions. Such a qualification of new technology has to rely on existing prediction tools. As these tools were developed to analyze the singing behavior of the existing technologies, this combined theoretical and experimental approach reduces the risks associated with the introduction of a new technology.