Digital twin mitigates recovery risk of damaged riser, offshore Brazil

As a mitigation measure to recover a damaged 5-in. gas lift riser from the FPSO Fluminense, 4Subsea, a leading provider of digital technologies and services, was tasked by Shell Brazil Petróleo LTDA (SBPL) to propose a digital replica of the FPSO’s riser system to support safe recovery operations, quantifying the loads and avoiding the risk of rupture. More than 30% of the wires on the structurally weakened flexible riser were broken in the outer tensile armor layer, which required a conservative validation of safety factors against riser recovery procedure. Located between Bijupirá and Salema fields in the Campos basin, 250 mi east of Rio Janeiro, the FPSO was moored with an external turret that supported 15 risers and three umbilicals in a 740-m water depth.  

To establish safe operating limits, the virtual copy was composed of both a local and global model, which compared the allowable axial tension, with the loads expected during the six critical steps of the recovery operation. The complex engineering feat was supported by PLSV Sapura Topázio and was designed and performed to minimize risk to the workforce, the asset and the environment. 

METHODOLOGY 

Following identification of partial damage on the armor layers of the Bijupirá gas lift riser, it was put out of service, and a recovery plan initiated.1 The flexible, rough bore riser was 1,095 m long and had an effective diameter of 131.4 mm and an outer diameter of 194.6 mm. The project to evaluate the condition of the outer tensile layer of risers2 included MODA (monitoring based on optical fiber attached directly on armor wires). Data revealed that of the 61 wires of the outer tensile layer, 16 were broken, three were suspected broken, and a further six were at risk of breaking, leading to a significant reduction of the allowable axial load. The digital twin of the FPSO’s riser system was composed of two parts: 1) a local and a global model, each applying specialized software; and 2) flexible pipe engineering.  

Local model. The local model was analyzed in MARC, a nonlinear implicit general-purpose finite element (FE) program. This calculated the peak stress for the most loaded wire in the outer tensile armor layer across different integrity scenarios. It was also used to determine the influence of bending and to study the level of plastic strain for tensile wires showing stresses above yield by using elasto-plastic material properties.  

As the pipe was depressurized, no internal pressure was considered. Until the pipe was lowered through the guide tube, the maximum axial tension load was 285kN near to the top end-fitting for initial phases of recovery. For the bending case, the initial axial load was calculated to be 200 kN. In the elasto-plastic analyses, the axial load was increased linearly up to the point where the FE model stopped and was no longer able to withstand the applied loading, indicating a level of maximum load. 

Each riser layer is defined by its geometry and linear elastic material properties to show the composition of the layers in the FE model. The carcass and pressure armor layers were modelled as equivalent layers, using three equally thick elements to ensure the correct radial and bending stiffness. Some of the layers were modelled as orthotropic and those layers were implemented in MARC, using a cylindrical coordinate system. The straight riser model is built up layer-by-layer, Fig. 1.  

Fig. 1. "Stripped" finite element model of the riser.

The tensile wires were modelled individually, with a Young’s modulus of E=207,00 MPa and a Poisson’s ratio of 0.3. Broken wires were modelled by physically cutting through them in the FE model. The specified minimum yield stress (SMYS) for the tensile wires was 1,179 MPs. For the elasto-plastic analyses, the ultimate tensile strength (UTS) was modelled, using linear strain hardening up to 10% (1.1x1,179 MPa = 1,270 MPa) at 5% plastic strain. 

The total length of the model was 1,767.8 mm, corresponding to two outer tensile armor pitches. Besides the analyses representing the different tensile wire integrity conditions, two additional models were developed in MARC to study riser behavior for both straight and curved sections. The bend geometry was achieved by moving a rigid cylinder gently towards the riser, Fig. 2. 

Fig. 2. Analysis setup for the bending case.

There were two general boundary conditions for MARC FE analyses. First, all layers are fixed to rigid surfaces at both ends of the FE model, and secondly, the axial tension load is added, using a point load at the end of the FE model.  

In the straight riser analysis, one end is completely fixed, and the other is fixed in X and Y directions for displacement and rotation. In the bent riser analysis, the only difference is that the rotation Rx is kept free at both ends. Post-processing showed that the reaction force in the cylinder is low, indicating the bend is not being restricted by the boundary conditions of the model.

Fig. 3. Riser in Orcaflex model – general view (left) and close up view at the turret with guide tube (yellow) and bend stiffener (right, grey).

Global model. The global model, developed in Orcaflex software, estimated the loads from the FPSO movement on the riser. A general model, based on the RAO of the FPSO Fluminense, was used to calibrate real data from previous projects.3 The seabed was modelled by defining a profile from 705 to 722 m of water depth in the region close to the analyzed riser, while wave analysis included the effect of both swell and wind.  

In Fig. 3, the riser was divided into segments to refine the mesh and provide adequate precision and convergence. The line is connected to the turret topside and is anchored subsea at the Riser X flowline connection. Two different scenarios were developed for the global model. The first studied changes in static top tension for different end-fitting positions, while the second considered the dynamic analyses. For the static analysis, the goal was to identify the most critical steps of the operation in terms of average top tension, which were then compared to the load capacity obtained from the local analysis. This would determine which sea states and relative headings provided top tensions below the load capacity obtained from the local analysis.

Fig. 4. Pullout at different stages, showing different parts of the riser passing through the guide tube curvature.

The highest Hs over a five-month period was reported on the same day as two new broken wires were detected by MODA. This indicated that the sea state can be severe enough to increase damage in the riser. All riser lengths above the bend stiffener were expected to pass through the guide tube curvature and therefore be subjected to a similar bend radius, Fig. 4. A winch component simulated the pull-out procedure onboard the FPSO, where a steady state was achieved. Next, the end-fitting was lifted 0.5m to remove hang-off. The compatible time intervals provided useful insights for the operation. 

RESULTS  

The local analyses performed in MARC were divided into three subsections:  

1. Straight riser – elastic material model. Several tensile wires at the outer tensile wire layer were assumed to be broken. Figure 5 shows contour plots of stresses for the outer tensile wire layer at full axial loading for 28 broken wires. It can be observed that the stress level increases in the tensile wires next to the failure zone. The peak stress for the first intact tensile wire was plotted for all integrity cases: 285kN axial load was applied. This crosses the 80% yield curve at 20 failed wires and the 100% yield curve at 25 failed wires. For more than 25 failed wires, the stress increase in the tensile wires were reduced, which indicated that the cross-section of the riser might be reaching a stability point.
   

   Fig. 5. Contour plot of outer tensile wire layer for wire breakage case with 28 wires broken.
    
2. Straight riser – elasto-plastic material model. The maximum axial tension load calculated when using elasto-plastic material model in the tensile wires is 357kN for 25 broken wires. At this point, complete failure of the pipe is anticipated, analysis therefore stopped due to numerical convergence problems.  
3. Bent riser. To simulate different positions of the failure during bending, four analyses were performed, each with a different failure location relative to the bend. The results indicated that the wires close to the failed area tend to re-orient when bent, as the stresses seems to “flatten out.” There are two possible reasons: sideways wire slip or twist and out of plane bending in the riser.  

Figure 6 presents the reduction in allowable top tension as the bending radius decreases from infinite (straight riser) to 43 m. The value of 280 kN corresponds to the allowable top tension for a straight riser with 25 broken wires considering yield strength as a limit. As bending is imposed, stresses increase and, consequently, a smaller top tension is enough to cause yield. This reduction can vary depending on the position of the failure. 

Fig. 6. Allowable top tension reduction at different bending radius for all four analyses.

Global analyses. During different steps of the operation, static analysis showed the highest static top tension was around 215 kN, while the second highest load was 205 kN. At this stage, the load was smaller, as the catenary angle was also smaller. As the estimated top tensions are slighter, they were not deemed to be a concern. In summary, the dynamic analyses showed that the riser may be subjected to a bend radius as low as 4 m near the guide tube exit during pull-out. At this moment, the safety margin was expected to be limited and preventive actions were put in place to protect personnel.  

CONCLUSION 

It was concluded that two steps of the operation could potentially be affected by the results of the digital twin. The first was during the riser lowering process, when the end-fitting approaches the guide tube curvature. In this moment, calculations indicated that the riser was expected to face the most critical stress stage of the operation, due to the combination of tension and bending. For the selected riser with 25 broken wires (a conservative estimate), there were some safety margins expected when wave height Hs is limited to 2m. For the other steps of the recovery operation, the margins are expected to be higher, and therefore, less restrictions were assumed. 

The second was after the riser exits the guide tube, when the stresses are reduced compared to when the riser was lowered. However, this could result in a reduction of load capacity. The analysis was the basis for several mitigative actions, which were carried out by the operator prior to the physical operation. These included: 

• Preventive/corrective actions on HAZID to consider a complete or partial failure of the riser. 
• A review of the riser’s integrity status over time. 
• Limiting the operational weather window. 
• Preparing for unexpected or unwanted developments during lowering and recovery, with contingencies to protect personnel involved and equipment available for operational execution. 

The operation was successfully executed, with the riser safely recovered for dismantlement in the near future. The complex and hazardous project has shown how digital twins can be leveraged to significantly improve safety, planning and decision-making, mitigate risk and expedite the recovery process safely and sustainably. This transformative technology will allow operators to overcome challenges with greater confidence, ensuring safer, more efficient, with regards to offshore critical operations.  

ACKNOWLEDGEMENT 

This article contains excerpts from OTC paper 32424-MS “Case study: Use of digital twins to mitigate risks in pull-out operations of damaged risers,” presented at the Offshore Technology Conference, Houston, Texas, May 1-4, 2023. 

REFERENCES  

1. Rodrigues, F.D.M., and L. A. Pereira, "Subsea systems life extension of Bijupirá and Salema fields," OTC paper 31725, presented at the Offshore Technology Conference, Houston, Texas, May 2–5, 2022. https://doi.org/10.4043/31725-MS 
2. Santiago, B. M., S. R. Morikawa, W. Carrara, A. Kravetz, and J. A. Martins, "Monitoring flexible risers with optical sensors – operational experience and future perspectives," OTC paper 28080-MS, presented at the Offshore Technology Conference, Rio de Janeiro, Brazil, October 2017. https://doi.org/10.4043/28080-MS 
3. Lobo, D., L. A. Santos, L. A. Pereira, G. Axelsson, and F. D. M. Rodrigues, "Case study: Use of vessel motion-based criteria to establish operational window in critical operations," OTC paper 32511-MS, presented at the Offshore Technology Conference, Houston, Texas, May 1-4, 2023.  https://doi.org/10.4043/32511-MS 

About the Authors

Dr. Daniel Lobo

4Subsea

Dr. Daniel Lobo is a project engineer at 4Subsea, active in the areas of digitalization, monitoring, and analysis. Prior to joining 4Subsea, he worked in the coordination and execution of a research project at COPPE/UFRJ, with an objective to develop models capable of describing drillstring dynamics. Dr. Lobo holds a degree in mechanical engineering and recently completed his D.Sc in drillstring dynamics analysis.

Leury Araujo Pereira

4Subsea

Leury Araujo Pereira has 18 years of experience in the oil and gas industry and has worked across several subsea system projects for TechnipFMC and Shell Brazil. As R&D technology manager for Shell Brazil and as SURF lead, Mr. Pereira developed risers and projects from concept to operations of subsea systems in deepwater fields. In 2022, he joined 4Subsea in Norway as SURF expert services manager, responsible for integrating digital tools for engineering and integrity management. He graduated with a degree in production engineering and also holds an MS degree in physical metallurgy from the Federal University of Rio de Janeiro.

Felipe Duncan Marotta Rodrigues

Shell Brazil

Felipe Duncan Marotta Rodrigues works for Shell Brazil, specializing in subsea systems. He has more than 17 years of experience working with subsea equipment in the oil and gas industry in Brazil. He has extensive knowledge in integrity management of subsea systems, including flexible risers, gained during his seven years with Petrobras as a subsea equipment engineer. He is now the highest technical authority for subsea systems for Shell Brazil Petroleo. Mr. Rodrigues holds a degree in mechanical engineering from the Pontifical Catholic University of Rio de Janeiro.