Novel flexible pipe overcomes challenges in pre-salt subsea developments

In 2017, the Brazil National Petroleum Agency (ANP) issued a failure mode alert to address stress corrosion cracking (SCC-CO₂) triggered by the presence of CO₂ in high-pressure pre-salt conditions. The problem had been identified as the root cause of broken tensile armor wires on a specific flexible pipe installation. Relatively common in other applications, where carbon steel is subject to high CO₂ concentrations, this failure mode was unknown in flexible pipe—and it presented a major challenge for operators in Brazil’s extensive pre-salt fields.

Fig. 1. Flexible pipe spooled and ready for deployment to one of Brazil’s numerous deepwater fields.

Flexible pipes have played an important role in Brazil’s oil production history. For example, no other oil province has applied flexible pipes so extensively as the Campos basin. There, approximately 2,223 km of risers and flowlines have been installed to connect giant fields, including Marlim, Albacora and Roncador, in water depths that range from 1,500 m to 2,000 m, Fig. 1. It’s no exaggeration to say that flexible piping has been crucial to the development of Campos and other basins as viable production sites.

Flexible technology–advantages. Of course, flexible piping is not exclusive to Brazil. Its widespread deployment is due to its application advantages. With concentric and unbonded layers—each of which contributes to the mechanical strength and chemical resistance needed to withstand deepwater conditions—flexible pipes were designed to ensure collapse resistance, internal pressure capacity, bending stiffness and axial-load capacity, among numerous other advantages to deepwater operations. In addition, this unique design gave operators several logistical advantages. Flexible pipes can be transported on smaller, nimbler and more cost-effective vessels. Manufacture does not require quality-sensitive offshore processes, such as welding or field-joint coatings—both of which have, historically, raised concerns about operational integrity.

As the name suggests, the technology enables operators to make subtle, but important changes at later stages of a project, without incurring severe cost penalties. Once in production, flexible piping gave operators the option of moving subsea lines and re-positioning pipes in response to production needs, or to postpone the exact location of production well placement decisions. As operators started production and built knowledge of reservoir behavior, that flexibility allowed them to optimize both output and field life.

That ability to be easily recovered, inspected, repaired, re-laid and connected at new sites was key. The flexibles proved themselves to be the best way of reducing time to first oil by enabling feasible production in short timeframes, even before reservoir delimitation and subsea layout consolidation. They reduced the risks of drilling campaigns and delivered associated advantages, in terms of time and cost. ANP’s announcement about SCC-CO₂ in 2017, therefore, had implications for the entire industry.

Fig. 2. Using rigid pipes would reduce perceived risks, but the efficiency gains achieved with the flexible subsea infrastructure would be lost.

Addressing SCC-CO₂. One short-term option was for operators to reduce their perceived risks by moving away from flexible pipe solutions and adopting rigid pipes instead, Fig. 2. But by doing so, all the flexibility that had allowed Brazil to develop offshore fields efficiently would be lost. Operational complexity, such as the water depth, bore size, temperatures, pressures and contaminants found in the Santos basin, requires constant review and development of the technology—in this case, the optimization of subsea hardware, as well as installation and operational procedures.

For this reason, the company directed its considerable R&D efforts to the exploration of alternative mitigation measures for its clients in Brazil. In the aftermath of the problem identified in 2017, and while not experienced on its own manufactured pipes, Baker Hughes began work on an extensive program to improve the resilience of the installed fleet and to deliver the next generation of SCC-CO₂-resistant pipes.

Fig. 3. Stress corrosion cracking can even induce failure in a pipe’s steel wires. Laboratory testing determined the improvement had to come from the materials used in pipe manufacturing.

Understanding the problem. As the ANP report noted, SCC-CO₂ is a condition that can induce cracking and even failure in a pipe’s steel wires, Fig. 3. However, three conditions need to be present simultaneously for such cracking to take place: environment (water and concentrated CO₂); high tensile stress exceeding a critical level; and very high-strength materials that are consequently crack-susceptible. If one of these three elements is designed out, cracking cannot happen. Since environmental conditions and high levels of tensile stress were unavoidable, the improvement had to come from the materials used in pipe manufacturing. Also, critical to developing an improved pipe solution is the knowledge that the SCC-CO₂ phenomenon is defined by two stages—nucleation and propagation—and that managing them requires different, but complementary approaches.

In the case of propagation of an existing crack, fracture mechanics can be used to define the remaining life of the asset and mitigation work needed. However, a completely different approach is needed when considering the susceptibility of a pipe to crack nucleation. In this case, multiple small-scale tests using armor wires taken from commercial products can be run for six months to simulate severe environmental conditions. When Baker Hughes ran these kinds of tests, wires were exposed to various combinations of contaminants while loaded at stresses close to the yield point.

Collaboration and composites. The laboratory results showed that it is feasible to design and manufacture a flexible pipe to operate in a SCC-CO₂ envelope without incurring any damage. In fact, the tests showed that in pipes proposed, designed and developed by Baker Hughes, the initiation of cracking would only occur if the loading on the wires and associated stress was raised to double that experienced in the field.

With the results of extensive testing and laboratory work as a foundation, engineers began to develop solutions for clients. One of the most important steps was to build alliances and partnerships with key material suppliers, test houses, installers and external experts, such as the National Composite Centre (NCC) in the UK. This has brought experiences, insights and lessons from other industries to the manufacturing of flexible pipe, adding robustness to the qualification and validation programs.

The outcome of this work is a new hybrid composite material for pipe manufacturing. The new material offers superior gas permeation performance, but without the traditional metallic layer that is most susceptible to CO₂ damage. Not only does the composite pipe reduce the concentration of CO₂ at the tensile armors, and remain unsusceptible to SCC-CO₂, it is also lighter than standard flexible piping. This reduces installation costs even further, and allows operators to deploy risers in a free-hanging catenary, removing buoyancies, accelerating installation time and improving safety.

Fig. 4. New flexible pipe system, with reinforced outer layers, is confirmed to extend service life by 50%, compared to standard flexibles.

Recent pipe designs also have included reinforced outer layers to protect against perforation or damage during installation, while all end-fitting ports and seals can be tested against external pressure to prove their capacity in deep water, Fig. 4. A machined area, that allows ultrasonic testing inspection for detecting flooding, and a visor rated to a 2,500-m water depth are added to end fittings.

Sensors and models. Recognizing that pipes in service were also a concern for operators, new ways of carrying out dissections to define initial cracks (the starting point for fracture mechanics) and calculating the service life of installed fleets were also needed. This required some means of testing to identify whether a given pipe was flooded or not.

Naturally, this is a key challenge for integrity management teams: the pipes are not designed to have this kind of verification performed, once they have been installed, and bringing a riser or flowline to the surface for verification is incredibly disruptive. However, it is an area where sensor technology can deliver exceptional results. Baker Hughes’ proprietary sensor technology is now embedded into current risers for pre-salt, and work continues developing methods for retrofitting sensors to installed pipe.

Such a system can detect any ingress of water from the topside into the riser annulus along its full length. It provides continuous monitoring, rather than one-off inspections, without extra vessels or ROVs. It also can cover up to five separate pipes and monitor all riser sections from the FPSO, up to a 3,600-m range. A database of Baker Hughes’ SCC-CO₂ program outcomes also enables quick identification of products that are not susceptible to this damage mode.

Continuing development. A team of engineers is now developing comprehensive sets of modeling tools that will be calibrated further by the test results, as well as undertaking a wide range of manufacturing trials in an automatic fiber-tape placement module. This work will enable the behavior of any pipe structures to be predicted without repeating full qualification testing. These kinds of testing campaigns will continue, to confirm that all variables—and any combination of variables—that may influence or trigger the SCC-CO₂ damage mechanism—are fully explored, mapped and documented to ensure the industry can develop the mitigation strategies for their particular circumstances.

Recently, the inspection, verification and assurance body, Lloyd’s Register, issued Baker Hughes a design appraisal document (DAD) certification, validating the company’s flexible pipe testing program and results. This follows an extensive three-year testing campaign by Baker Hughes—featuring more than 120 test set-ups, methodologies and characterizations, including dissection of pipes recovered from field operations—and marks a step toward a viable flexible pipe solution for the most demanding, high-CO₂ offshore fields. The certification verifies that on top of the standard 25-to-30-year equipment service life, Baker Hughes can extend flexible pipes’ contracted service life by at least 50%, with the potential of 2.5x life extension.

The DAD confirms that the combination of advanced materials and state-of-the-art manufacturing processes produces a flexible pipe that can withstand the most severe operating conditions while still retaining advantages, including flexibility in field layout, simplified FPSO balcony configuration, and reduced time to first oil. Moving forward, all installed flexible pipes by Baker Hughes in Brazil will be covered by the DAD and offer operators extended operating life before replacement. In addition, the DAD enables Baker Hughes to assess and predict the integrity of flexible pipes operating for several years.

ESG challenge. However, the work in Brazil is part of a bigger picture. The world’s oil and gas sector faces unprecedented challenges to meet social and political demands for greater environmental responsibility and emission reductions in the face of extraordinary price pressures and capital constraints. There is, as a result, no shortage of speculation about what the new normal will look like, from autonomous operations to extremely efficient, carbon-neutral developments.

What is perhaps less widely discussed is the idea that “normal” of any kind will be an increasingly rare phenomenon within the oil and gas sector—or indeed within any industrial sector. Constant innovation, continual development, permanent evolution and a relentless re-assessment of what works and what can be done better, will be the defining features of successful operators and their service companies. Whether it’s the use of advanced sensors, data and analytics, or the modification of manufacturing materials, almost every aspect of the business is open to re-evaluation. The reality is composite flexibles combined with advanced sensors and conventional pipe design and manufacturing offer a viable way to continue to use flexible pipes in pre-salt without concern for SCC-CO₂.