An Alternative Carcass Design to Prevent Flow-Induced Pulsations in Flexibles
Authors: González Díez N. , Belfroid S. P. C. , Iversen Solfeldt T. , Kristiansen C. 
Published: October 2021

Flow-induced pulsations (FLIP) are pressure oscillations generated inside of flexibles used in dry gas applications that can cause unacceptable vibration levels and eventually failure of equipment. Because of the design of inner layer of the flexibles, the carcass, the frequency of the pulsations is high, potentially leading to fatigue failures of adjacent structures in a relatively short time.

The traditional carcass is made of a steel strip formed into an interlocked s-shape in a series of preforming and winding steps. To enable bending of the pipe, gaps are present between each winding with a shape that can cause FLIP. The gaps can be reduced, and the profiles optimized, but they will always be able to generate FLIP at a certain gas velocity. To remove the risk of FLIP in dry gas projects and ensure that operator does not get operational constraints, an alternative carcass design has been developed. This is essentially a conventional agraff carcass but with an additional cover strip to close the gap, making the resulting carcass nearly smooth bore in nature. With a smooth bore this carcass can be used for flexibles which have a risk of FLIP or to produce pipes with a lower internal roughness. This alternative design can be manufactured and can therefore build on the large manufacturing and design experience of the traditional strip carcass.

This alternative carcass technology is to undergo a full qualification process, in which the risk of flow induced pulsations is an essential component. With the investigated alternative carcass design, the cavities present in the traditional agraff designs are covered. It is expected that the risk due to the appearance of FLIP is therefore eliminated. Theoretical analysis, numerical simulations and scaled experiments are used to explore the risk for the alternative technology to create FLIP. The theoretical analysis is based on existing knowledge and literature. The numerical simulations and scaled tests are done to generate direct evidence for the end statements resulting from the qualification process. Numerical simulations follow the power balance method presented by the same authors in earlier papers. The same applies to the techniques used for the scaled tests.

The main outcome of the qualification presented here are the pressure drop performance and the anti-FLIP capabilities of the design. The new design performs significantly better than the nominal design carcass for the same purpose. The pressure drop coefficients found are close to those expected for a normal, non-corrugated pipe, and thus the recommendation given by the API 17J standard does not apply to this design. The pressure drop coefficient is dependent on the installation direction of the flexible with respect to the flow. No signs of FLIP are found for the nominal design of the investigated carcass technology. This is the case for either installation direction. This is explained from a theoretical point of view, but also numerical and experimental evidence are provided.