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.
In this paper, a methodology is proposed to use FPSO Fluminense motion-based criteria to establish the operational window for the disconnection and safe removal of a flexible riser with structural damages in parts of the tensile armour wires. A global model was established in Orcaflex to describe the FPSO and flexible riser responses considering two sea states (Hs = 2.0 and 2.5 m / Tp = 12.5s) and different headings from 0° to 45°. The results are then post-processed to calculate the statistics of top tension for 20-minutes time intervals and assess its correlation with FPSO motion (Heave, Pitch, Roll and Heave Rate). Finally, limit values for FPSO motion could be calculated using a high-level confidence interval in the regression analysis and comparing with top tension allowable limit considering the broken wires. A strong correlation was found between FPSO movements and top tension on the riser to be removed for all variables except for roll amplitude. Therefore, it was decided not to establish a roll limit in this case, but to use the roll measurements as an indication of the actual heading relative to incoming waves. A linear regression model and a confidence interval were calculated considering 99.9% of confidence to calculate the upper motion limit that would provide top tensions greater than the allowable limit. The calculated limit was then compared to the data measured by onboard MRU to support risk assessment in real-time. Based on historical data, the operational window could be improved by 7% using motion limit criteria and only 2% of historical data provided a rigid no-go (all limits reached).
This paper explains the efforts to assess the integrity status and to extend the life of the subsea system connected to a Floating Production Storage and Offloading vessel (FPSO) moored offshore in Brazil, at 700m water depth, set up in 2003 as the first IOC operated assets in Brazil. The work performed covered the assessment of flexible risers, flowlines, umbilicals and rigid gas export line, comprising studies on operational history, simulations, in-loco special inspections (AUT, EMAT and Direct and Indirect flexible riser tensile armour inspections) and actual FPSO and risers' movement monitoring. Subsea equipment as Christmas trees, manifolds and PLETs were not included, as design life was sufficient. The results found that some equipment was in remarkable good shape, whilst others had compromised integrity. Assessments demonstrated that some METOCEAN and motion data for the FPSO, utilized during the design phase, were non-conservative when compared to the real case, which led to under design of risers' fatigue life. In-loco verification of some equipment integrity revealed damages, probably related to fatigue associated with corrosion. Improvements on the existing system and new monitoring techniques applied to the field, to assure safe operations past its original design life, are also depicted. The work performed was pioneering on life extension of a subsea system for a field operated by an IOC in Brazil and can contribute to other operators facing ageing assets in the country.
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