Mitigation of Vortex-Induced Vibration of Cylinders Using Cactus-Shaped Cross Sections in Subcritical Flow

Flexible cylinders, such as marine risers, often experience sustained vortex-induced vibrations (VIVs). Installing helical strakes on a riser is the most widely used technique to mitigate VIVs. This study was inspired by the giant Saguaro Cacti which can withstand strong wind with a shallow root system. In this study, numerical simulations of flow past a stationary cylinder of a cactus-shaped cross-section in a two-dimensional flow field at a subcritical Reynolds number of 3900 were performed. Results show that cylinders of a cactus-shaped cross-section have a lower lift coefficient without increasing drag compared to those of a circular cylinder. VIV experiments on a single flexible pipe as well as on a set of two tandem-arranged flexible pipes were conducted at different reduced velocities to investigate the effects of the streamwise spacing and wake of the cactus-like body shape on VIV mitigation. Experimental results show that the cactus-like body shape can mitigate VIV responses of the cylinder at upstream position with no cost of increased drag; however, similar to helical strakes, the efficiency of VIV mitigation for the cylinder at downstream position is reduced. Although the cactus-like body shapes tested in this study were not optimized for oscillation suppression, still this study suggests that modification of the cross-sectional shape to a well-designed cactus-like shape has potentials to be used as an alternative technology to mitigate VIV of marine risers.

Forced Vibration Tests for In-Line Vortex-Induced Vibration to Assess Partially Strake-Covered Pipeline Spans

Helical strakes can suppress vortex-induced vibrations (VIVs) in pipelines spans and risers. Pure in-line (IL) VIV is more of a concern for pipelines than for risers. To make it possible to assess the effectiveness of partial strake coverage for this case, an important gap in the hydrodynamic data for strakes is filled by the reported IL forced-vibration tests. Therein, a strake-covered rigid cylinder undergoes harmonic purely IL motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean's towing tank. These tests cover a range of frequencies, and amplitudes of the harmonic motion to generate added-mass and excitation functions are derived from the in-phase and 90 deg out-of-phase components of the hydrodynamic force on the pipe, respectively. Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007 “An Experimental Investigation of In-Line and Combined In-Line and Cross-Flow Vortex Induced Vibrations,” Ph.D. thesis, Norwegian University of Science and Technology, Trondheim, Norway.), the IL VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure IL VIV.

Experimental Study of Hydrodynamic Damping for Water Intake Risers

Water Intake Riser (WIR), conveying cooling water from the sea, is key to liquefaction of natural gas in the Floating Liquefied Natural Gas (FLNG) facility. Due to the wave-induced vessel motion, WIRs may experience resonant vibrations, which influence its fatigue life. In such situations, the estimate of hydrodynamic damping is critical to the prediction of fatigue life. Due to its small motion amplitudes compared to the diameter of WIR, the Keulegan–Carpenter (KC) for motion-induced flow around WIR is normally small (e.g. KC < 5). For small KC values, the effect of steady current on the hydrodynamic damping is not well understood and the current practice of using the relative velocity Morison model for predicting the hydrodynamic damping with in-line steady current is challenged by guidelines such as DNVGL-RP-C205 and ISO-19902. In this study, the hydrodynamic damping of a smooth WIR oscillating in still water or in steady currents is measured with a series of experiments at KC < 5 and the Reynolds number (Re) in the range of 103 ∼ 105. The effect of in-line or cross steady currents on the in-line hydrodynamic damping is investigated and the performance of the relative velocity Morison model for predicting the hydrodynamic damping at low KC is examined. Experiments are also conducted for a WIR with helical strakes in in-line or cross currents. Based on these experimental results, recommendations are made for predicting hydrodynamic damping in the WIR design.