Abstract

Short fairings (fairings with small chord-to-thickness ratios) for vortex-induced vibration (VIV) suppression have been used on many offshore structures, such as tension leg platform (TLP) tendons, marine risers, and seabed pipelines. A variety of laboratory tests have been performed to investigate the stability and performance of fairings. It has previously been established by the authors that short fairings not only reduce lift forces, but also generate hydrodynamic damping. In this paper, case studies drawn from four experiments are used to investigate the stability characteristics of short fairings in a range of marine applications. The dynamic stability of short fairings for a range of reduced velocities is demonstrated with the experimental results presented herein. These findings should be of interest to offshore engineers involved in VIV analysis and suppression design.

Introduction

Short fairings for vortex-induced vibration (VIV) suppression have been used on many offshore structures, such as tension leg platform (TLP) tendons, marine risers, and seabed pipelines. Understanding the performance of these fairings is critical to offshore designers in their efforts to attack the VIV problem in order to design risers that are safer and more economical. A variety of laboratory tests have been performed to investigate the stability and performance of fairings. The authors have previously found that short fairings not only reduce lift forces, but also generate hydrodynamic damping [1,2]. Dynamic instability in a different fairing design, characterized by the increase of responses with excitation, was observed by other researchers in model tests [3]. Dynamic stability, defined in a classical sense, is the fact that the response of a system increases with time, which is caused by negative damping in the system [4]. In the context of VIV, dynamic stability can be described below: as the flow speed increases, the VIV motion of a cylinder rises to a certain level, and then the motion interferes with the vortex shedding process and begins to break up the symmetric pattern of alternate vortices. The motion magnitude does not increase even if the flow speed continues to rise, thus the process is self-limiting. It is well known that a circular-shaped cylinder, typical of most marine tubulars, possesses this self-limiting characteristic. When VIV suppression devices (such as fairings) are attached to a cylinder, the dynamic characteristic may be influenced. For example, if not properly designed, fairings can rotate around the cylinder and form a "dynamically" unsymmetrical cross section that could entail lift forces significant enough to actually increase vibration amplitude beyond that of a bare cylinder. These motions may not be self-limiting. Therefore, the dynamic characteristics should be investigated whenever the circular-shape is altered. Case studies drawn from four experiments are used to investigate the dynamic stability and the performance characteristics of short fairings in a range of marine applications. The first experiment consisted of a faired 5-9/16- in. diameter pipe with a length of 6.5 ft, towed horizontally in a high-speed channel. The Reynolds numbers in this test ranged from 0.7 to 1.4 million.

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