Modelagem de pull-out de risers flexíveis

Abstract

The oil and gas exploration and production industries face increasing operational challenges, either due to the advance into progressively deeper and more severe environments or to the installation, maintenance, and decommissioning stages of subsea systems. In this context, risers play a fundamental role in offshore systems, connecting subsea equipment to surface production units and enabling fluid transport. As these units reach the end of their service life, decommissioning operations become necessary to ensure structural and environmental safety. This work aims to model and analyze the pull-out procedure of flexible risers in a lazy-wave configuration, which consists of disconnecting the riser from the floating unit—either through free fall or in a controlled manner-followed by retrieval or abandonment, with the objective of understanding the structural behavior during the operation. To this end, nonlinear dynamic simulations were performed using the OrcaFlex software, based on a reference riser available in the literature. Initially, a parametric study was conducted addressing relevant modeling aspects, such as finite element mesh discretization, maximum time step, longitudinal drag coefficient, and the nonlinear moment-curvature relationship of the riser, in order to assess their influence on computational efficiency and on the structural response of the system. Additionally, to obtain more representative estimates of the variables of interest, such as effective tension and curvature radius, a statistical analysis was carried out considering random waves. The results demonstrated the strong influence of the longitudinal drag coefficient on the dynamic response of the system, as well as the importance of adopting a nonlinear moment-curvature relationship, which led to significantly different responses when compared to a constant linear stiffness. The statistical analysis further showed that characteristic values associated with non-exceedance probabilities differ substantially from

those obtained from a single deterministic analysis. Finally, an alternative pull- out scenario with controlled lowering of the riser was evaluated. The results indicated improvements in structural behavior compared to the free-fall scenario, although further studies are still required for a more comprehensive assessment of the proposed alternative.