On the effect of manufacturing imperfections and internal damage on the collapse strength of tubes: a nonlinear perspective

Resumo

The estimation of critical external pressure of tubes is a nontrivial problem, particularly when
considering the presence of damage, such as mechanical or chemical, and, manufacturing imperfections, such as
cross-section ovality and eccentricity. This study presents the findings obtained from a nonlinear finite element
analysis of 2-D tube cross-sections subjected to collapse pressures. The collapse strength of the tubes can vary
significantly with modifications to the parameters related to the geometric disposition of the damage and
imperfections. The study is carried out adopting configurations that are commonly observed in casing tubulars of
oil and gas wells. Therefore, the results can support the design and integrity analysis of casing strings, improving
knowledge about their behavior under well service conditions. The study is performed using Abaqus software with
a Python scripting interface, enabling efficient evaluation of various geometric inputs. The nonlinear solver
employs a load increment approach, causing the system to become unstable upon reaching the critical load. The
load increments and displacements were carefully adjusted to maintain equilibrium using the arc length
methodology (known as the Riks Method in the software). Different equilibrium trajectory shapes can arise
depending on the initial geometric configuration, leading to diverse conclusions. For example, an increase in the
separation angle between the ovality reference location and eccentricity reference location can decrease the
collapse strength of thick tubes but increase it for thin tubes. Several other insights can be drawn from this study.
Finally, a brief case study is presented, comparing the results with widely used equations in the well casing design
practice.