Case Study: Use of Digital Twins to Mitigate Risks in Pull-Out Operations of Damaged Risers

Abstract

A digital twin is proposed to quantify the loads and mitigate the risk of rupture during the recovery operation of a flexible riser with severe structural weakness (30% of broken wires in the outer tensile armor layer). The allowable axial tension is compared with the loads expected during operation to establish safe operating limits. The digital twin is divided into a local model and a global model. The local analysis uses a finite element model to investigate how stresses on tensile armor wires change when the number of broken wires increases. It considers all pipe layers and two configurations, 1) straight and 2) with a curvature expected during the recovery operation. The FPSO motion performance is calibrated by offshore field measurements and improved RAOs for the global analysis are provided based on the measured motions. Finally, the allowable axial load from the local analysis is compared with the expected loads from the global analysis. The local model is simulated stepwise with 0 - 28 broken wires and indicated a reduction of 83% in allowable axial load with 25 broken wires. The results were combined with global simulations considering different sea states to provide insights about how close the loads might be to the allowable limit during operation and what loads the riser was subjected to since the last broken wires were detected. This analysis allowed the identification of the most critical steps in operation and helped to design the procedure to achieve ALARP risks. Finally, operating limits were discussed to keep the estimated load inside the established limits.