Flutter Instability in Riser Fairings

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Abstract Model tests of two different types and two different scales were performed with test cylinders fitted with a freely-rotating riser fairing. At lower flow speeds, both tests showed the fairings to be effective in suppressing vortex-induced vibration (VIV) of the cylinder and in reducing drag. In both tests, however, large lateral cylinder oscillations developed once the flow exceeded a certain critical speed. Motion amplitudes in one test significantly exceeded those of a bare pipe undergoing VIV. A simple two-dimensional model of aircraft wing flutter was used to better understand the cause of the oscillations and to relate the onset of the motion to the fairing's specific design characteristics. The model predicted the observed threshold speeds with reasonable accuracy, suggesting that riser designers can borrow from existing aerodynamic flutter theory to ensure that riser fairings are designed to remain dynamically stable over the range of current speeds expected in service. Introduction and Summary VIV of a bare riser is caused by flow separation and vortex shedding from alternate sides of the bluff circular riser crosssection. Riser fairings are sometimes used to streamline the flow and eliminate or reduce these effects. If separation and vortex formation are avoided, drag is reduced and the transverse excitation forces that lead to VIV are eliminated. Commonly, riser fairings are designed to rotate freely about the riser axis and to passively align with the direction of incident flow so they will effectively streamline the flow regardless of current direction. This rotational degree of freedom introduces the possibility of complex dynamic effects involving coupling between cross-flow translation and rotation. As part of a research program on riser VIV, ExxonMobil performed model tests of two different types to observe and measure the performance of segmented riser fairings intended to eliminate vortex-induced vibrations (VIV) and reduce mean drag. In one test, the freely rotating fairings were placed on a rigid spring-mounted cylinder that was towed horizontally at realistic riser Reynolds numbers. In the second test, a tensioned flexible brass pipe with a length of 9.63m and a diameter of 20mm was fitted with fairings with the same geometry, and tests were performed over a range of speeds in a configuration that simulates linearly sheared current flow. In both cases, the fairings reduced drag and suppressed VIV for low flow speeds. However, in both tests, large pipe lateral motions were observed for speeds exceeding a certain threshold. These motions appeared to involve a coupling between fairing rotation and pipe lateral vibration. In one test, motion amplitudes significantly exceeded those of a bare riser undergoing VIV, suggesting that the fairings could potentially cause damaging riser motions if exposed to current speeds beyond a critical threshold. To understand the phenomenon and the relationship between the threshold flow speed and fairing design characteristics, a simple theoretical model of aircraft wing flutter was used along with the physical characteristics of the fairing to compute threshold speeds for flutter onset. Calculated and observed threshold speeds agreed well, indicating that existing flutter analysis theory may provide riser designers with methods for ensuring that fairings are designed to remain dynamically stable over the range of current speeds expected in service.