Abstract
Marine slender structures such as risers and power cables can be subjected to vortex-induced vibrations (VIV). This means that the drag force can be significantly amplified compared to that on a fixed body. The amplification of drag is a critical consideration in the design process. The present industry practice using frequency domain VIV prediction tools estimates the drag amplification based on empirical formulations generalized from bare cylinder/pipe tests in uniform flows. A common approach to estimate the drag amplification of cylindrical structures is Vandiver’s equation. When this equation is applied to lazy-wave configurations, several challenges arise. The complex geometry of a staggered buoyancy section means that the excitation can be different for individual buoyancy elements and the bare pipe between them. Furthermore, the drag calculation is taken at a separate step after the response calculation although they are coupled, i.e., larger responses will lead to higher amplified drag. Therefore, using the present practice may lead to large uncertainty or over-conservative drag amplification. This study presents a combined computational fluid dynamics (CFD) and time-domain VIV analysis using timedomain VIV load model to investigate drag amplification in risers with staggered buoyancy elements. A thorough examination is conducted on the effects of oscillation frequency, spacing of buoyancy elements, oscillation amplitudes, and the interaction between buoyancy and risers, highlighting potential advancements in design and fatigue evaluation for lazy-wave configurations.