Abstract

The ability of helical strakes and fairings to effectively suppress vortex-induced vibration is compared. Various approaches to testing are classified, and several test programs are referenced to analyze relevant performance data. Tests that produced the basic performance data of both devices are discussed. An examination is then made for the effects of some of the more important parameters such as Reynolds number, surface roughness, interference, and coverage density. The results, together with the results from a sister paper that reviews installation, maintenance, and economic aspects of helical strakes and fairings, can be utilized for better selection of suppression devices and coverage amounts.

Introduction

The growth of deepwater exploration and production activities during the past 20 years has caused operators to engage a number of issues to: enable drilling and production; minimize financial, health, environmental, and safety risks; improve project economics; and insure asset integrity. One important issue that impacts all of these goals is vortex-induced vibration (VIV). While VIV was a well-known issue before this time, its importance has grown with deepwater activities for several reasons, among them the increased probability of encountering large ocean currents in deepwater and the complexity associated with predicting and suppressing VIV in deepwater. While VIV has been studied since the late 1950s for shallow water drilling and production, these studies focused on predicting first-mode behavior (for a variety of mode shapes, including: rigid body motion; bending of a cantilever beam; and bending of a flexible beam or cable) in ocean currents that were close to being uniform with depth. Deepwater VIV includes modeling fluid-structure interaction at much higher vibration modes and currents that changed substantially with depth.

The most basic VIV tests are usually made for single bare cylinders with a low level of surface roughness (i.e., relatively smooth). Many tests during the period ranging from the mid 1960s through the mid 1980s were performed on spring-mounted rigid cylinders, or short flexible cylinders, in uniform flow in the subcritical Reynolds number range, with the data used to construct various VIV models. (Skop and Griffin 1973; Griffin 1975; Stansby 1976; Griffin and Koopman 1977; King 1977; Griffin 1979; Sarpkaya 1979; Griffin and Ramberg 1982; Schewe 1983; Kim and Vandiver 1985; Tsahalis 1987; Vandiver and Jong 1987; and Zdravkovich 1989 give the reader a good starting point for researching this era.) Subsequent work added long flexible cylinders experiments to test analysis methodologies as exploration activities moved into deeper waters (Vandiver and Chung 1987; Vandiver 1993; Vandiver, Allen and Li 1996; Allen 1998).

While the knowledge of how bare cylinders behave when subjected to currents causing vortex shedding continued to evolve, a parallel effort was undertaken to find ways of suppressing VIV. VIV suppression devices have also been well studied since the 1950s, though many of the studies have been rather academic or focused on wind applications, with few devices applied to offshore structures until the late 1970s (Zdravkovich 1981). While early efforts examined a variety of possible devices, helical strakes have been chosen for most wind applications (Vickery and Watkins 1964), and thus helical strakes became the starting point for serious investigations of VIV suppression in water (e.g., King, Prosser, and Verley 1976).

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