Flow-Induced Vibration of a Long, Flexible, Straked Cylinder in Uniform and Linearly Sheared Currents
Authors: Frank W.R. , Tognarelli M.A. , Slocum S.T. , Campbell R.B. , Balasubramanian S. 
Published: October 2008

The bare-pipe riser vortex induced vibration (VIV) experiments described in Tognarelli et al [1] were extended to examine the influence of various lengths of strake coverage on riser vibrations. The objective of the tests was to demonstrate whether present riser VIV prediction formulations provide a robust basis for determining the effects of full and partial strake coverage on riser VIV.

Continuous triple-start helical strakes with a height equal to 25% of the cylinder diameter and a pitch of 16 diameters were fitted to a test pipe with a length of 9.6m and a diameter of 20mm. Tests were performed in uniform and sheared currents. The model was instrumented to acquire records of bending strain and lateral acceleration in both cross-flow and in-line directions at a sufficient number of stations to allow accurate reconstruction of static and dynamic bending deflections along the riser. The dependence of these quantities n flow speed and current profile type was compared qualitatively with the physical concepts embodied in numerical models often used in riser design.

In all cases tested, the vibration of the straked riser was significantly less than that of the bare pipe. However, the response measurements indicate that existing VIV prediction methods fail to reproduce even the qualitative characteristics of the motion of the straked riser. The data suggest that current-induced vibration of a riser with these high strakes is driven by a physical mechanism which is different from the vortex excitation of bare risers. A new prediction formulation is needed to properly represent the influence of full or partial strake coverage on riser vibration and fatigue, particularly for demanding applications in which accurate quantification of strake effectiveness is critical for design. The test results described here provide a basis for validating new methods as they are developed through further research.

Introduction

The development of methods for predicting the response of deepwater risers to VIV has been an area of active research in the oil industry for many years. As the industry continues to move into deeper water, continuous improvement is needed in VIV prediction methods to eliminate inaccuracies where possible and quantify the remaining uncertainties in the response predictions.

An additional complication in the VIV analysis of deepwater risers involves the modeling of VIV suppression devices, such as helical strakes or fairings. In deepwater, it is now common for risers to be fitted with some length of VIV suppression. An accurate understanding of the effectiveness of VIV suppression devices is critical in making design decisions, such as the type of suppression device to use and the length of riser to cover with suppression.

In an effort to better understand the dynamic response of deepwater risers to VIV, ExxonMobil undertook a series of tests during the summer of 2003 to measure the VIV response of a long, flexible highly instrumented cylinder. The goal of this test program was to produce a high-quality data set for use in validating and/or developing VIV prediction tools for both bare and straked risers.