Catenary-type risers are used in deep water oil and gas extraction. The dynamics of these massive flexible structures and their stability to external disturbances is an area of active research. In a recent work, we showed how some configurations of catenary-type risers are susceptible to destabilization when they are transporting fluid. One potential means of stabilizing such risers is to use a buoyancy module. Such modules can be placed along a section of the riser and, as a result, alter the static equilibrium configuration. In this paper, we examine the efficacy of using buoyancy modules and their potential stabilizing effect on risers transporting fluids. With the help of a linear stability analysis, a threedimensional nonlinear model for the riser, and an extensive set of computations, we are able to establish parameter regimes where linear stability of the riser transporting fluid can be found.
A class of flexible risers, known as lazy-type or steep wave-type risers, consist of a catenary-type riser equipped with a buoyancy module (cf. Figure 1). The module serves both to decouple the motion of the riser from the motion of surface vessel and decreases the effective gravity of this exceptionally long structure (Bai and Bai (2005)). Research on the statics and dynamics of risers equipped with buoyancy modules has been performed by Santillan et al. (2010), Wang et al. (2014), and Ruan et al. (2014). These works build on the large volume of work on the mechanics of catenary-type risers that date to the pioneering works of Garrett (1982) and Nordgren (1974), which have been extended to include extensive studies on the static and dynamic analysis of a catenary-type riser undergoing large deformation caused by various hydrodynamic forces (cf. Chucheepsakul et al. (2003), Chatjigeorgiou (2008) and references therein). It should also be noted that there have been several efforts to validate the robustness of results from static and dynamic analyses. These efforts include at-sea tests as well as large/small model scale tests by Chung (2010), Song et al. (2011), and Lie and Kaasen (2006), and they provide valuable insights on flexible risers and relevant phenomena such as Vortex Induced Vibration (VIV).
