Abstract

This paper presents an overview of deepwater riser vortex-induced vibration analysis, including vortex-induced vibration prediction, testing considerations, and suppression device testing and selection. Both single and multiple riser configurations are addressed. For each subject the philosophy and thoughts behind various approaches are discussed.

Introduction

Vortex-induced vibration (VIV) occurs anytime a sufficiently bluff body is exposed to a fluid flow which produces vortex shedding at, or near, a structural natural frequency of the body. Deepwater risers are especially susceptible to VIV because:

  1. currents are typically higher in deepwater areas than in shallower areas;

  2. the increased length of the riser lowers its natural frequency thereby lowering the magnitude of current required to excite VIV, and

  3. deepwater platforms are usually floating platforms so that there are no structures adjacent to the riser to which it could be clamped (see Figure 1).

Deepwater risers are usually so long that significant currents will excite a natural bending mode that is much higher than the fundamental bending mode. Since deepwater currents usually change in magnitude (and direction) with depth, it is therefore possible that multiple modes of the riser can be excited into VIV. This makes deepwater riser VIV prediction much more complex than that for short riser spans typical of fixed platforms in shallow water. The VIV response of deepwater risers is further complicated by the presence of adjacent tubulars such as risers and tendons. When all, or part, of a riser is in the wake of an upstream tubular, the VIV of the riser can be substantially altered and often worsened. Furthermore, the presence of adjacent tubulars can cause changes in the drag forces acting on a riser, resulting in the possibility of damaging collisions between tubulars. The option most often chosen to mitigate VIV is to use a vortex suppression device. However, design, testing, and analysis of suppression devices is also a complicated task. In addition, adjacent risers can substantially reduce the effectiveness of a suppression device. The next section discuss deepwater riser VIV prediction for a single riser, and specifically focuses on the most important parameters which must be considered. Following sections discuss the important considerations for "riser interference" (the presence of multiple risers) and for VIV suppression.

Vortex-Induced Vibration Prediction

Accurate estimation of the fatigue life of a deepwater riser experiencing vortex-induced vibration depends critically upon accurate estimation of the response amplitude and frequencies (or mode numbers). Accurate estimations of the response amplitude and mode number are, in turn, dependent upon several "basic" parameters which include:

  • the current profile (both magnitude and shape variation with depth);

  • the frequency and magnitude of the lift force imparted to the riser by the vortex shedding;

  • the excitation and correlation lengths of the lift forces and vortex shedding;

  • the hydrodynamic damping; and el the structural properties of the riser including damping mass, tension, bending stiffness, and the cross sectional geometry (including surface roughness).

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