Tests With a Flexible Quasifairing to Reduce Riser Drag, Suppress VIV and Limit Drilling Down-Time
Authors: King Roger , Brown Andrew 
Published: October 2008

This paper reports on recent experimental tests with the Ventilated Trouser (VT), an entirely new and innovative quasi-fairing for VIV suppression of cylindrical structures exposed to external fluid flow. The VT concept was developed by Allbrown Universal Components with sole sponsorship by BP, and evolved from the authors' many years experience of VIV, coupled with the practical application of offshore suppression devices.

The VT is a loose fitting sleeve in the form of a light flexible net with integral bobbins in a special arrangement. It is omni-directional, rugged, and made from materials compatible with the offshore environment. Probable applications of the VT are for retro-fitting to drilling risers, tension leg members and production risers, or for deployment at the same time as setting the riser. Unlike strakes or solid fairings, the VT requires nothing to be permanently fixed to the structure, giving greatly reduced fitting times, and significant reductions in drilling down-time. It has no bearings and requires no maintenance. Of particular interest to the Offshore Industry is that the design of the VT allows for uninterrupted emergency pulling of drilling risers.

BP America Production Company commissioned towing tank tests at Solent University, Southampton UK. Using a 0.09m diameter, 2.4m long flexible model riser, suppression performance, design sensitivity, and general handling features were determined. At the highest velocity, equivalent to a Reynolds Number of 1.4x105, VIV amplitude was reduced to ~ 0.02D, and drag coefficient to 1.12. High frequency content was removed from VIV response, thereby ameliorating potential fatigue.

Tests at close to full-scale Reynolds Number proved that the VT is simple to handle, and can simultaneously suppress VIV response and reduce dynamic drag. With minimized storage requirements, low capital cost and zero maintenance, the potential benefits of the VT are significant over conventional suppression devices.

Under a follow-on contract with BP, a full-scale VT for a 46?? riser has been constructed for testing at an onshore site, preparatory to deployment offshore.

INTRODUCTION

The causes and consequences of vortex induced vibration (VIV) are well documented, and will not be re-visited here, as this paper is concerned primarily with VIV suppression. Of the many methods developed to reduce VIV, some, such as helical strakes, axial rod shrouds and perforated shrouds (Airey et al, 1985; Every & King, 1974; Zdravkovich, 1981) function by modifying the boundary layer of the flow around the riser to prevent the correlation of vortex shedding along the length of the riser. These devices can be costly and time consuming to install, create additional drag, and may become less effective if covered by marine growth. Other methods of reducing VIV such as streamlined fairings, wake splitters and flags operate by stabilising the wake. Streamlined, or teardrop shaped fairings that rotate around a riser have been developed to virtually eliminate the shedding of vortices.