Experimental Evaluation of Vortex Induced Vibration Response of Straked Pipes in Tandem Arrangements

Abstract

Vortex induced vibration (VIV) due to steady current flow can be a significant driver in the design of offshore riser systems, affecting riser global configuration, component details and overall subsea architecture. Helical strakes are known to reduce VIV but the degree of effectiveness can vary considerably depending on strake pitch, fin height and more importantly, current flow regime. In addition, the amplitude of VIV and the effectiveness of VIV suppression strakes depends on the inclination of flow to the riser (incidence angle) and presence of wake effects from adjacent risers. Test and field data regarding suppression of riser VIV by strakes is not extensively available in the public domain. This is primarily due to the proprietary nature of the tests conducted in industry. In this paper, a program of testing is devised to better understand strake effectiveness as a function of current incidence angle and the presence of adjacent risers. Experiments have been conducted on single and tandem pipe arrangements in air in order to evaluate strake suppression efficiency. Aluminium cylinders are tested in a wind tunnel in the structures laboratory of The University of Western Australia (UWA). Two sets of experiments are conducted: the first to evaluate cylinder VIV response at angles of incidence ranging from 30 to 90 degrees and the second to evaluate VIV response of the downstream pipe in a dual pipe arrangement with varying spacing between the pipes. In both cases the bare cylinders are first tested at varying flow speeds. Helical strakes are then added to the single cylinder, and downstream cylinder in the tandem pipe test, and the vibration response is recorded at varying flow speeds. From the experimentation, it can be seen that downstream cylinder motions are amplified by wake induced instability. This phenomenon is of particular concern for tightly spaced top-tensioned risers (TTR) in wellbays of tension leg platforms (TLP) and deep draft floaters. The VIV motion of the downstream, bare, wake-affected pipe, is magnified to approximately 1.3–2 times the motion of a single bare pipe. When strakes are added to the downstream cylinder, the magnification factor of the downstream cylinder response is largely increased due to the wake of the upstream bare cylinder. However, the actual VIV motions of the downstream cylinder are largely reduced when strakes are incorporated. The present work demonstrates that helical strakes provide an effective means of suppressing vortex induced vibrations of risers in riser arrays, though the degree of effectiveness is reduced in a downstream tubular compared to suppression levels for single pipes.

Investigation on the effectiveness of helical strakes in suppressing VIV of flexible riser

“…In general, the distribution of the strakes along the cylinder is highly correlated to the angle of flow attack towards the cylinder surface. According to Blumberg et al [ 25 ], the angle of flow attack affects the responses of the cylinder and there is an optimum angle (pitch) in reducing the responses of the cylinder. In the present study, the frequency responses of p = 10 D h = 0.10 D strakes possesses the narrowest lock-in region (4.6 < V r < 10.6) based on Fig.…”

The effect of externally applied rotational oscillations on FIV characteristics of tandem circular cylinders for different spacing ratios

“…Assi et al (2010) experimentally studied the effectiveness of free-to-rotate parallel plates as passive vibration control devices acting on the flow-induced oscillation of downstream cylinder located in the wake of stationary cylinder. Blumberg et al (2012) studied the FIV of a single straked sprung cylinder with different values of incidence angles as well as the case where the rear cylinder was positioned behind the fixed smooth cylinder at different spacing ratios. An important observation was the effective mitigation of vibration in the single cylinder, whereas this attenuation was not strong enough for the rear cylinder located in the wake of fixed cylinder.…”