Active and Passive Control of Spar Vortex-Induced Motions

Despite designer’s best efforts, Vortex-Induced Motion (VIM) of spars remains a significant problem. While strakes have proven highly effective at suppressing riser VIM, all three straked classical spars in the Gulf of Mexico continue to experience occasional VIM events. Since VIM can cause riser damage, mooring line fatigue, and even work stoppage, its suppression is a major priority. But designers will only successfully eliminate VIM once reliable predictions tools become available. VIM is both roughness and Reynolds number dependent, and spars lie outside the validated range of current computational tools in both these areas. This paper presents the development of Reynolds-Averaged Navier-Stokes (RANS) methods to predict real-world spar VIM behavior. It includes the ability to address rough surfaces and high super-critical Reynolds numbers. Validation is provided against existing experimental data where available. The resulting algorithms were used to assess effectiveness of two alternative control strategies for suppressing spar VIM. The first strategy consists of actively injecting fluid in a direction tangential to the spar. The second strategy is passive, and relies on adding an external sleeve to the spar. Active injection was found highly effective, and even capable of totally eliminating VIM. The passive alternative was also found to reduce VIM, but is not as effective as active injection.