Modelagem de queda de risers em configuração lazy-wave

Abstract

Risers are essential elements for the exploration and production of oil and gas in offshore systems, as they transport fluids from wells and connect pipes and other equipment located on the seabed to floating units. Even with great advances in the methods of analysis and design of marine risers, these structures still fail, resulting in their collapse and fall on the seabed. The falling structure may can cause high environmental, in the event of an oil leak, and economic costs, since the impact of the collapsed structure with other neighboring structures and equipment can compromise their lifespan. This work aims to perform the modeling and post-failure analysis of marine structures to understand its behavior right after the occurrence of the rupture during its fall trajectory, with a focus on flexible risers in a lazy-wave configuration. For this, non-linear dynamic simulations were carried out on an example model using the OrcaFlex software. First, the effect of different parameters on the model response was assessed, such as the finite element mesh, the integration algorithm, the time step, the longitudinal drag coefficient, the riser’s slenderness, the use of a non-linear moment-curvature relationship of the riser and the normal stiffness of the soil. This procedure focused on providing a better understanding over the influence of these parameters in the precision and efficiency of the model and in the structural response of the phenomenon. In addition, in order to more reliably determine the maximum velocity, fall time and final geometry on the sea floor, a statistical study was carried out using random waves. This study was important to more reliably evaluate the relevance of the studied parameters and the influence of the current direction, as well as to assist in the design of a safe zone, considering a given probability of non-exceedance. It was observed that the longitudinal drag coefficient and the riser's slenderness have a major influence in the physical response of the problem. In addition, it was found that the use of a non-linear moment-curvature relationship is of great importance to obtain more realistic results, considering that it leads to significant differences when compared to the linear bending stiffness. It was found that a decrease on the riser’s slenderness reduces the randomness of the phenomenon. It was also indicated that the consideration of currents in different directions is important to determine the safe zone. The results and analyzes obtained can help to the occurrence of further damage in the event of accidents in future riser projects.