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Mitigating vortex-induced vibrations
(05/12/01) Oil and gas exploration and production operators in every deepwater drilling region in the world are confronted with a potentially disastrous phenomenon capable of shutting down a rig or platform as fatigue or even failure strike their risers, costing hundreds of thousands of dollars a day in lost operations and inability to access reservoirs.
The phenomenon is vortex-induced vibrations (VIV), which occur when vortices of water shed past or spin off risers. The vibrations are set into motion by deep currents that are so violent and turbulent that the risers begin to oscillate in a figure eight motion, making them highly vulnerable to fatigue and failure.
Interior view of a segment of a Smooth Sleeves sheath.
Shell International Exploration & Production (SIEP) began to study flow separation and vortex shedding past a cylindrical structure and the consequent problem of vortex-induced vibrations in 1987, and developed numerous mitigating tools during the years since, including such items as ultrashort fairings, gap spars, and helical strakes. More recently, however, SIEP began analyzing VIV at supercritical Reynolds numbers (reyn) through experiments in the Rotating Arm Facility at the US Naval Surface Warfare Center in Caderock, Maryland, USA.
Preparing to install a Smooth Sleeves sheath on an offshore riser.
These studies, which examined the effect of surface roughness on VIV and drag of flexible cylinders at critical and supercritical levels of dynamic viscosity ranging from 100,000 to 400,000 reyn (689475729.316836 to 2757902917.26735 pascal second), revealed that there is a direct relationship between the relative roughness of a cylinder and the intensity of VIV, and that VIV can be virtually eliminated with the aid of sufficiently smooth cylinders.
Four levels of surface roughness were tested for their effect on VIV and drag of a circular cylinder, and those cylinders with rougher surfaces experienced much greater vibration than those with smooth surfaces.
A section of a Smooth Sleeves sheath after installation.
Smooth Sleeves
Shell's discovery of the ability of smooth cylindrical surfaces to suppress VIV and drag in drilling and production risers and other slender objects such as truss members on spars, led to the development of the companyÕs practical solution, Smooth Sleeves, which have recently been tested successfully in the Trinidad and are now being installed on the US Gulf of Mexico's Augur platform.
A simple solution, Smooth Sleeves consist of large cylindrical fittings that are mounted on the outside of riser struts and act as VIV suppressants.
More information can be obtained from SIEP by email to: shellglobalsolutions@opc.shell.com or phoning 31 70 377 2470.
Source: Shell Global Solutions
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