VIV Model Test of a Bare- and a Staggered Buoyancy Riser in a Rotating Rig
Authors: Lie Halvor , Mo Knut , Vandiver J. Kim 
Published: October 2008

Abstract
The objective of this paper is to present some model test results of vortex induced vibrations (VIV) of long marine risers subjected to shear current. Both a bare riser and a staggered buoyancy riser have been tested in vertical and inclined positions in a rotating rig. The vertical bare riser will be excited by one vortex shedding frequency in the uniform current, while the vertical staggered riser will experience two frequencies. For the sheared current cases (inclined riser) more then one vortex shedding frequency may be present along the riser for both configurations. The interaction between competing frequencies and modes are important items in the paper. The general trend is that the bare riser has transverse displacement spectra with mainly one peak frequency, even for the sheared current conditions. This is in contrast to the staggered riser test, where most spectra have at least two peaks. In some of the staggered riser tests it seems like the riser has locked in to both the vortex shedding frequency related to the riser diameter as well the vortex shedding frequency related to the buoyancy diameter. The relative effect of the buoyancy elements on the motions and the variation in peak frequencies is analyzed.

Introduction
The offshore oil and gas industry is gradually moving into deeper water and the marine riser is a critical component. A riser in deep water will experience the same wave induced loads as a riser in moderate water depth. but it will also experience current forces along a larger part of the length where there are no wave particle motions. Dynamic response due to vortices is important for these risers. It is well known that a flow around an elastic cylinder, such as a marine riser, will create vortices and induce vibrations. The frequency of the transverse load will normally be close to the Strouhal frequency, fs=St U I D. If the flow and/or the diameter vary along the riser, more than one excitation frequency may become active and the response will consist of more than one mode-shape. There are different theoretical models for describing the load and the response from vortices for long pipes in realistic current profiles. Today there is little knowledge of the interaction between frequencies and mode-shapes, and the models are related with large uncertainties. In order to increase the experience, two model tests have been conducted at MARINTEK. In the first model testl, 2, 3 (further denoted as the original test), VIV of a bare marine riser was studied for different shear profiles and speeds by use of a rotating rig. In the second model test, the rig was slightly modified, and both a bare riser and a staggered buoyancy riser were tested4. The results have been further analyzed in Ref. 5. The present paper will mainly present results from the second test. Key findings from the first test are listed in the following, The riser model had a low spatial attenuation resulting in a standing wave type of response pattern. Single frequency type response near the Strouhal frequency was observed even for strong shear. Several modes were present even if the response appeared at a single frequency.