  In a recent paper, the hydrodynamic code NeXSPheRIO was used in conjunction
with STAR analysis methods to study two-particle correlations as function of
Delta_eta and Delta_phi. Both the ridge-like near-side and the double-hump
away-side structures were obtained. However, the mechanism of ridge production
was not clear. In order to understand it, we study a simple model with only one
high-energy density peripheral tube in a smooth cylindrical back-ground, with
longitudinal boost invariance. The results are rather surprising, but the model
does produce the triple-ridge structure with one high ridge plus two lower ones
placed symmetrically with respect to the former one. The shape of this
structure is rather stable in a wide range of parameters.
