Performance Comparisons of Helical Strakes for VIV Suppression of Risers and Tendons
Authors: Allen Don  W. , Henning Dean L. , Lee Li W. 
Published: October 2008

This paper presents experimental results for various helical strake geometries and discusses their performance coefficients and responses. Experimental results from the presence of surface roughness and/or interference (i.e., the presence of upstream tubulars) are also presented.A substantial amount of the experimental data is from experiments performed on flexible cylinders at prototype (critical) Reynolds numbers for actual offshore tubulars.

Introduction

Helical strakes are one of the most popular devices for suppressing vortex-induced vibrations (VIV) of offshore tubulars. The underlying mechanism of vibration reduction with helical strakes is to disrupt the spatial correlation of vortices by gradually changing the flow separation angle in the longitudinal direction. As a result, the intensity of vortices is weakened, and the lift force is reduced.

The general physics of how helical strakes suppress VIV motion is straightforward. However, the performance of strakes and sensitivity of that performance to various geometrical and flow variations are not well understood, especially for marine riser and tendon applications in field conditions. There are primarily three technical issues to be addressed in deepwater project engineering in relation to VIV strakes. One is to determine strake geometry, such as strake height and pitch; a second is to decide on the strake coverage length; and a third is to determine the need for, and suitability of, marine growth retardation coatings or an underwater cleaning/maintenance program.

The strake test data available to designers are usually pertinent to isolated, rigid cylinders in uniform flows. Risers and tendons in field conditions frequently experience sheared flows and respond in elastic structural modes. In addition, offshore tubulars are often in close proximity and interact with flow in a complex manner. Clearly, direct application of rigid cylinder, uniform flow test data to riser and tendon design can lead to erroneous decisions.

Over the past 10 to 15 years, extensive tests have been conducted at Shell to investigate the performance of various strake designs. These tests were performed either in a circulating current tank, on a rotating arm facility, or in a highspeed linear towing channel.

In these tests, geometric parameters were varied, including the height of strakes, the pitch, and the number of starts. The surface roughness of the strakes ranged from smooth, to medium rough, to very rough. The flow profiles included fully or partially uniform and sheared currents. The Reynolds numbers were in subcritical, critical and supercritical range. The test configurations included straked single pipes, and multiple tubulars with strakes in tandem/offset positions. Responses of tubulars with various strake coverage lengths, placed in low and high speed zones, were also evaluated.

One of the test programs, the rotating arm tests, was conducted at the U.S. Navy's test basin in Carderock, Maryland. The 97-ft long test cylinders were made of fiberglass composite, consisted of five joints connected by inner sleeves, and were mounted horizontally beneath a towing bridge. Accelerometers and load cells were used to monitor the accelerations, tension and drag loads. As the bridge rotates, it drives the cylinder in a circular path in still water.