Thanks to the symmetry of the problem only a quarter of the vessel has been modelled. The domain has been
modelled with all four types of implemented finite elements, starting from a rough mesh and performing then a
mesh refinement. For what concerns the triangular elements different meshes has been tested: one without symmetry
and 2 characterised by different symmetries (only 1 for the quadratic elements). The boundary conditions are taken
as homogeneous essential so that the circumferential displacements uθ are null along the two axes of symmetry.