Abstract

Several fairing geometries have been tested with a tow carriage to examine their ability to suppress vortex-induced vibration and reduce drag. The tests were performed in the critical and supercritical Reynolds number ranges with fairing chord-to-thickness ratios ranging from 1.33 to 2.52. Stabilizer fins were attached to some of the fairings, while others had a thin tail or came to a point at the tail. All the fairings were allowed to fill with water and were free to weathervane while towed in a horizontal configuration on a flexible pipe. The results illustrate that larger chord-to-thickness ratio fairings provide lower drag than smaller chord-to-thickness ratio fairings. Smaller chord-to-thickness ratio fairings are most effective when the tail is thin and longer chord-to-thickness ratio fairings can benefit from the presence of a stabilizer fin.

Introduction

Fairings have been used by the offshore oil and gas industry to reduce vortex-induced vibration (VIV) since the late 1970s (see Every and King 1980; Gardner and Cole 1982; and Grant and Patterson 1977). Most early designs had fairly large chord-to-thickness ratios, and their use focused on drilling riser applications. The common wisdom was that fairings were unsuitable for production applications, due to the potential for marine growth to lock them in place or prevent them from weathervaning properly with the direction of the ocean current.

In the late 1980s and early 1990s, the authors, while conducting tests in a current tank, discovered that short fairings can be very effective at suppressing VIV. In subsequent testing over the next several years, the authors discovered that these short fairings were also effective at critical and supercritical Reynolds numbers (Allen and Henning 2003). This short fairing geometry was subsequently used for several deepwater production risers as well as several drilling risers. Most research and development over the next few years was focused on designing this fairing properly to insure its performance, fitness for purpose, and integrity. While development work was done to expand its usefulness to a wide range of tubular installation methods, very little research was performed to produce new fairing shapes that can effectively reduce both VIV and drag.

The objective of this set of experiments was to ascertain the effects of various fundamental geometrical features of fairings on their performance, as measured by vibration and drag reduction. While a systematic approach was desired, it was recognized that a large number of parameters could be varied. These included:

  • The chord-to-thickness ratio;

  • The chord-to-diameter ratio;

  • The presence of a fairing nose and its shape;

  • The shape of the tail (pointed or flat with varying possible widths);

  • The curvature of the two sides of the fairing;

  • The presence of a stabilizer fin;

  • The geometry of the stabilizer fin.

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