Galloping Assessment: Instability Occurence of Slender Structures of Non-Circular Cross Section in Current Flow
Authors: Berhault Christian , Christelle Dassibat , Legras Jean-luc , Wiet Paul , Cinello Alexandre , Deglaire Thomas , Molin Bernard 
Published: April 2012

Bundles arrangements are currently used in the design of riser towers or oilexport lines. Some of them are characterized by a non-circular cross section[6] and therefore may be prone to plunge instability, so-called galloping orplunge instability when exposed to strong current. It is important to be ableto assess, at conceptual design stage, their likelihood of being subject tothis phenomenon.

Galloping is taking place in the low frequency range compared to VIV, but withlarger amplitude, up to several diameters, which could be critical in term ofglobal motion. Galloping occurrence is related to the dissymmetry of the crosssection and then there is a risk for non-circular geometries, such as riserbundles, buoyancy tanks and floater columns. Instability can also occur intorsion or rotation by a coupling effect between transverse oscillations.

Riser Vortex-Induced-Vibrations have been studied for decades, and numerousexperiments have been performed both in-situ and in model test facilities tounderstand and predict the response of a slender cylindrical structure in acurrent. The main reason is the influence of VIV on riser fatigue life.

If galloping and fluttering are well known in aerodynamics [10], no largespecific experiment/study exists for hydrodynamic flows [1], [9]. So it is notevident to assess whether or not galloping may occur for a given riser bundledesign, and, in case of expected galloping, whether there is a potential riskof damage to the individual pipes in the bundle. Until recently, only theBlevins criteria [1] are available to predict the risk of instability but thereare limitations.

Based on recent examples of riser tower, experimental and numericalinvestigations have been done within the CITEPH Gallopan project, with the goalto propose guidelines to help designing a bundle cross section in a way toavoid or reduce the risk of galloping.

Two cross section shapes supported the investigations, the academic squarecross section, for which previous studies have been done [1], and a tokenbundle cross section expected to be subject to galloping. Model tests have beenperformed in two steps:Captive tests and transverse forced oscillation tests in steady current toderive hydrodynamic coefficients (using a multi-DoF motions generator), to beused to check the Blevins instability criteria.Free oscillations in steady current to identify the instability domain inrelation to the reduced velocity and to estimate galloping amplitudes. Aspecific experimental arrangement, based on a vertical pendulum system, hasbeen designed and set-up for this step.

A methodology has been proposed to assess the risk and consequence of gallopinginstability using standard riser numerical tools in which hydrodynamiccoefficients are issued from model tests. This paper presents the main resultsof the Gallopan project in term of methodology based on model tests to analysegalloping occurrence and response for non-circular slender geometries. Usingthese results, it is now possible to develop a galloping-free riser bundledesign.