Evaluation of Fairing Performance by Three-Dimensional Tests Using a Flexible Riser Model

Abstract

Due to its low drag and suppression effectiveness, fairings are considered to be an attractive alternative to helical strakes for mitigation of riser Vortex Induced Vibrations (VIV). The Norwegian Deepwater Programme has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV and galloping. Furthermore, emphasis is put on easy handling and installation. By use of 2D small scale tests in a towing tank and in a large cavitation tunnel, a total of 10 different fairing designs have been evaluated and screened in terms of hydrodynamic performance [1]. Three fairing design candidates were promoted for further development. This paper describe the work and findings when studying the effectiveness of promoted fairings on a 3D flexible riser model with respect to galloping and VIV. Three different fairing profiles have been studied in the present model tests. The tests were carried out in the sub-critical Reynolds number regime with Rn up to about 53000. Four different fairing coverages were tested for each design: full coverage, 60 % coverage, 40 % coverage and full coverage of the lower half of riser. Differences in performance have been noticed for the fairing profiles, and the findings have laid the ground for further development of the most promising candidates for real life applications. Further, it is concluded that the combination of 2D and 3D tests, as well as combination of small scale and prototype scale tests, has been an efficient strategy in development of VIV suppression devices.