Abstract

Current tank tests have been performed on two equal-diameter, flexible cylinders in tandem with various end spacings or separation distances. While the cylinders experienced transverse VIV in only the first mode, the test Reynolds numbers were well into the transition range, and the drag crisis was clearly observed on the upstream cylinder. Both acceleration and drag were measured to provide some interesting correlations between vibration, drag, and end spacing for each of the two cylinders.

Introduction

During design of floating production platforms in deepwater, it has been recognized that there is the possibility of interference between adjacent production or export risers, or possibly between other riser/tendon combinations such as a drilling riser and a production riser. The two interference consequences of most concern are: a) possible increases in fatigue damage during vortex-induced vibration (VIV); and b) possible contact between adjacent risers. This research project has attempted to further study the vibration behavior of two production risers in tandem, with the hopes of characterizing the changes in drag coefficient with vibration at Reynolds numbers as close to the prototype as possible.

This project is part of a research program in riser interference that has been in progress since 1989. Previous published results for current tank tests performed at low Reynolds numbers, but with high-mode numbers, include: tests with dual equal diameter cylinders in tandem [1]; and tests on dual cylinders in a staggered (offset from tandem) arrangement [2]. In addition to the published test results, low Reynolds number, high-mode number tests have also been performed with: dual unequal diameter cylinders in tandem; dual tandem cylinders with helical strakes; and three equal diameter tests in tandem. High Reynolds number tests have also been performed as part of this research program, and some of the single cylinder results have been published [3-6]. This paper presents results for tandem bare cylinders experiencing (primarily) first-mode VIV at critical high Reynolds numbers. Similar tests have also been performed on tandem cylinders with different surface roughness, helical strakes, fairings, and perforated shrouds. Tests have also been conducted on longer pipe models experiencing critical Reynolds numbers and higher mode VIV. These tests have also been complimented by various tow tests used to determine sectional drag coefficients for cylinders experiencing forced VIV.

This paper presents current tank experimental results for two equal diameter ABS tubes in tandem that were spaced at several different end spacings (separation distances). The cylinders were flexible and experienced VIV in various bending modes, with the first transverse bending mode being the focus of the experiments. Simultaneous drag force and vortex-induced vibration (acceleration) measurements were made. The following sections present a summary of the test results, a description of the tests, and a detailed discussion of the test results.

Test Description
Current Tank.

All of the experiments were performed in the Shell Westhollow Technology Center current tank facility. A ship's propeller driven by a hydraulic power unit circulates the water through the tank.

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