RBDO AS A DECISION-MAKING TOOL IN THE TUBULAR DESIGN AGAINST COLLAPSE

Resumo

During drilling and lifetime production of an oil well, the casing system is responsible for its
hydraulic isolation and structural integrity, having a high associated cost in relation to the total amount
of the well. The occurrence of failure events in casing tubes can result in large environmental damage
and human losses. Its structural design must comprise the different load scenarios acting on the tubular,
including internal and external pressures, besides axial, bending and torque efforts. Among the possible
failure modes, the collapse of the tubulars, due to the action of external pressure, stands out. The casing
design methodology currently adopted in the oil and gas industry is the Serviceability Limit State (SLS)
verification proposed by API/TR 5C3 (2008), based on a model of four collapse regimes, depending on
the slenderness of the tube. Its formulation does not deal with the uncertainties inherent to projects of
this magnitude, which could improve the structural safety analysis of the tubular, without compromising
its economicity. In this way, it is proposed to apply reliability-based design optimization (RBDO)
techniques in confluence with an Ultimate Limit State (ULS) model in order to find optimal
configurations for the design variables, taking into account the variability of some geometrical and
mechanical parameters, due to the manufacturing process, considering required target safety levels.