Risers Stability Under External Pressure, Axial Compression and Bending Moment Considering the Welded as Geometrical Imperfection

In Mexico, PEMEX manage more than 60,000 Km of oil and gas land and marine pipelines. Therefore, their structural integrity must be carefully assessed. The current development of offshore structures requires the design of better risers capable of assuring an optimal level of security. For that, some factors must be taken into account, which in critical conditions might become risky. It is common to find situations, in the performance of fixed offshore platforms, in which risers might be found empty due to maintenance or repairing. This condition could represent an unfavorable situation that in many cases might become highly critical given the conditions they are found in. Several studies had been completed with the aim to define a design methodology for the local buckling of pipes. In this paper, the bending capacity of the empty steel marine pipelines (risers) with girth-welded, placed to the legs of the offshore platforms very near to the deep sea, is studied by numerical models. The objective is to study the behavior and local buckling of these pipes submitted to combine loads, bending, external pressure and axial force, considering the girth-welded as the geometrical imperfection and to estimate the bending capacity of the pipes. Critical pressure of buckling, and modal configurations are evaluated by theoretical methods and numerical approaches such as finite element method (FEM). The numerical results are compared with theoretical methods.