Viv Excitation Competition between Bare and Buoyant Segments of Flexible Cylinders

This paper addresses a practical problem: “Under which coverage of buoyancy modules, would the Vortex Induced Vibration (VIV) excitation on buoyant segments dominate the response?” This paper explores the excitation competition between bare and buoyant segments of a 38 meter long model riser. The source of data is a recent model test, conducted by SHELL Exploration and Production at the MARINTEK Ocean Basin in Trondheim Norway. A pipe model with five buoyancy configurations was tested. The results of these tests show that (1) the excitation on the bare and buoyant regions could be identified by frequency, because the bare and buoyant regions are associated with two different frequencies due to the different diameters; (2) a new phenomenon was observed; A third frequency in the spectrum is found not to be a multiple of the frequency associated with either bare or buoyancy regions, but the sum of the frequency associated with bare region and twice of the frequency associated with buoyancy region; (3) the contribution of the response at this third frequency to the total amplitude is small; (4) the power dissipated by damping at each excitation frequency is the metric used to determine the winner of excitation competition. For most buoyancy configurations, the excitation on buoyancy regions dominates the VIV response; (5) a formula is proposed to predict the winner of the excitation competition between bare and buoyant segments for a given buoyancy coverage. INTRODUCTION It is expected that the existence of buoyancy modules may decrease the fatigue damage rate due to the decrease in vortex shedding frequency for its larger diameter. A bare cylinder will vibrate at a higher frequency than a cylinder fully covered by buoyancy modules of a much larger diameter. When a flexible cylinder with both bare and buoyant regions is excited by the same flow, two different frequencies are excited and a competition exists between lift forces at these two frequencies. The point of this study was to answer the question, “Under which coverage of buoyancy, would the VIV excitation on buoyant regions dominate the response?” There exists some previous research on the excitation competition between bare and buoyant segments of a pipe. Lie et al [1] used the RMS amplitude ratio associated with the buoyant segments divided by the total RMS to determine whether the excitation on buoyant regions dominates the VIV response. While Vandiver and Peoples [2], Li et al [3] and Vikas et al [4] used the frequencies of the peak spectral components to determine whether the excitation on buoyant regions dominates the VIV response. Their preliminary data analysis showed that the coverage of buoyancy modules and the ratio of two diameters play a major role in the excitation competition between bare and buoyant segments. Lie et al [1] stated that the ratio between the lift force on the bare segments and that on the buoyant segments was proportional to the ratio of . Where Lbare and Lbuoyancy are the length of covered bare and buoyant segments, 1 Copyright © 2013 by ASME respectively; Dbare and Dbuoyancy are the outer diameter of bare and buoyancy segments, respectively. Vandiver [5] also came up with a similar formula, where the amplitude ratio between the modal responses due to the excitation on bare segments and buoyant segments was proportional to the ratio of . Both expressions show that when a pipe has two different diameters, the larger diameter region is favored to dominate the response. This paper explores the excitation competition between bare and buoyant regions of a 38 meter long model riser. The power dissipated by the damping at each excitation frequency is the metric used to determine the winner. The distinguishing identifiable feature of the excitation on either region is the excitation frequency. Two excitation frequencies are well known to exist in the spectrum of the pipe with staggered buoyancy modules. The higher frequency is associated with bare regions and the lower one associated with buoyant regions according to the Strouhal relationship f=StU/D. In addition to these two excitation frequencies, a new phenomenon was observed. A third frequency was found to be an unexpected combination of these two excitation frequencies. This is the first known report of such behavior. The main objectives of this paper are to: (1) Explain the existence of the third frequency. (2) Study the effect of the excitation at the third frequency on the VIV response. (3) Determine the winner of the excitation competition between bare and buoyant regions. (4) Propose an expression to predict the winner of the excitation competition for a pipe with staggered buoyancy modules under uniform flow. EXPERIMENT DESCRIPTION Riser and Buoyancy Configurations. The total pipe length was 38 meters. Pipe 2 had a diameter of 30 mm and was made from a continuous length of fiberglass tubing. Pipe 3 used pipe 2 as the inner core. Ninety-three pairs of clamshell modules, 80 mm in diameter, were clamped onto the outside of the 30 mm inner pipe. Each module was 0.4086m in length and was made from two flexible urethane half shells which snapped together around the smaller Pipe 2 and secured with locking clips. Hereafter, Pipe 2 will also be called the medium pipe and the Pipe 3 will be referred as the large pipe. Different buoyancy configurations could be modeled by clamping on the modules, leaving bare gaps between module segments. The details are listed in Table 1. Table 1: Pipe Model Properties Parameter Pipe2 Pipe3 Total length between pinned ends (m) 38.00 38.00 Outer diameter (mm) 30 80 Outer/Inner diameter of fiberglass rod/pipe (mm) 27/21 27/21 The length of 1 buoyancy section (m) NA 0.4086 Bending stiffness of Pipe 2, EI (Nm) 572.3 572.3 Young modulus for Pipe 2, E(N/m) 3.46x10 3.46x10 Mass in air (kg/m) 1.088 5.708 Weight in water (kg/m) 0.579 0.937 Mass ratio 1.54 1.14 The bending stiffness of the urethane shells was considered negligible, compared to the effect of tension on the natural frequencies of the configurations tested. For the staggered buoyancy test, the length of an individual buoyant section was defined as Lb and the gap between two adjacent segments was defined Lc. Figure 1 shows the five configurations of flexible cylinders with staggered buoyancy. In all tests the ratio between buoyancy element diameter and bare riser diameter was 2.67. This non-integer ratio prevents the bare riser vortex frequency from being an integer multiple of the vortex frequency from the buoyancy elements. In order to study the effect of the staggered buoyancy modules on VIV, the configuration matrix was populated with five different length ratios, namely Lc/Lb =2/2, Lc/Lb =1/1, Lc/Lb =3/2, Lc/Lb =3/1 and Lc/Lb =5/2. For example when the length is specified as Lc/Lb =2/2, it means that Lb=Lc=2x0.4086. This identification system has the advantage of identifying the both the length of the buoyant and bare segments, as well as the ratio of length of the buoyant segment to the bare or gap length5. Figure 1. Section configurations of flexible cylinders with staggered buoyancy Test Arrangement. The source of data is a recent model test, conducted by SHELL Exploration and Production at the Lc Lb Lc/Lb=2/2 Lc/Lb=1/1 Lc/Lb=3/2 Lc/Lb=3/1