Towards an accurate assessment of UOE pipes under external pressure: Effects of geometric imperfection and material inhomogeneity

In deep and ultra-deep waters the diameter of trunk lines coupled with the hydrostatic pressure tends to lead to failure of the pipeline by external collapse. This failure mode is an instability phenomenon that is governed by the geometry of the pipeline and its material properties and takes place through a subtle combination of its properties. As such the exact limiting collapse pressure is very difficult to determine and twenty years of studies seem to have not managed to clarify entirely the matter. present paper the elastic-plastic collapse of test rings from UOE formed pipes under external uniform pressure is analysed both from an analytical and a computational viewpoint and the significance of the actual shape of the out of roundness and of the material inhomogeneity is examined in details. The work has been motivated by a number of accurate experimental tests recently performed on rings cut from UOE pipes and it is found that the material inhomogeneity can affect the collapse modes and produce an occasionally unforeseen reduction in the carrying capacity of the rings prompting a swing between the underlying collapsing modes. Importantly, it is shown that this behaviour can turn quite difficult to follow numerically, also by means of robust strategies such as Riks and improved arc-length. The features of the problem can help to give reason of some apparently anomalous experimental results.