Bundles arrangements are currently used in the design of riser towers or oil export lines. Some of them are characterized by a non-circular cross section [6] and therefore may be prone to plunge instability, so-called galloping or plunge instability when exposed to strong current. It is important to be able to assess, at conceptual design stage, their likelihood of being subject to this phenomenon.rnGalloping is taking place in the low frequency range compared to VTV, but with larger amplitude, up to several diameters, which could be critical in term of global motion. Galloping occurrence is related to the dissymmetry of the cross section and then there is a risk for non-circular geometries, such as riser bundles, buoyancy tanks and floater columns. Instability can also occur in torsion or rotation by a coupling effect between transverse oscillations.rnRiser Vortex-Induced-Vibrations have been studied for decades, and numerous experiments have been performed both in-situ and in model test facilities to understand and predict the response of a slender cylindrical structure in a current. The main reason is the influence of VTV on riser fatigue life.rnIf galloping and fluttering are well known in aerodynamics [10], no large specific experiment/study exists for hydrodynamic flows [1], [9]. So it is not evident to assess whether or not galloping may occur for a given riser bundle design, and, in case of expected galloping, whether there is a potential risk of damage to the individual pipes in the bundle. Until recently, only the Blevins criteria [1] are available to predict the risk of instability but there are limitations.rnBased on recent examples of riser tower, experimental and numerical investigations have been done within the CITEPH Gallopan project, with the goal to propose guidelines to help designing a bundle cross section in a way to avoid or reduce the risk of galloping.rnTwo cross section shapes supported the investigations, the academic square cross section, for which previous studies have been done [1], and a token bundle cross section expected to be subject to galloping. Model tests have been performed in two steps:rn1. Captive tests and transverse forced oscillation tests in steady current to derive hydrodynamic coefficients (using a multi-DoF motions generator), to be used to check the Blevins instability criteria.rn2. Free oscillations in steady current to identify the instability domain in relation to the reduced velocity and to estimate galloping amplitudes. A specific experimental arrangement, based on a vertical pendulum system, has been designed and setup for this step.rnA methodology has been proposed to assess the risk and consequence of galloping instability using standard riser numerical tools in which hydrodynamic coefficients are issued from model tests. This paper presents the main results of the Gallopan project in term of methodology based on model tests to analyse galloping occurrence and response for non-circular slender geometries. Using these results, it is now possible to develop a galloping-free riser bundle design.
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