Surface Treatment Improves Flow Assurance and Throughput in Corroded Pipe
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This paper describes a material designed to interact specifically with highly corroded and weathered pipe to enable in-place application and refurbishment. The material is applied extremely thinly on the pipeline interior, such that it might be considered a surface treatment, yet it is designed for permanence and for strong adhesion to even severely corroded surfaces. The water- and oil-compatible, chemically resistant material shows extreme resistance to corrosion, particle abrasion, and delamination under operational conditions, and is designed to reduce surface roughness by several orders of magnitude. The study of the practical effects of using a low-surface-energy, low-surface-roughness coating that can be applied effectively to severely corroded pipelines with a minimum of surface preparation demonstrates how material breakthroughs can allow for revitalization of previously mature intervention techniques.
Improving flow efficiency within transport pipeline systems has always been an appealing and seemingly straightforward method of reducing operating expenses, improving product throughput, and significantly reducing the risk of catastrophic pipeline failure. It is believed universally by pipeline operators that regular cleaning by mechanical and chemical pigging; adoption of preventative methods such as application of factory-applied, internally applied flow coats; or pipelines made of corrosion-resistant-alloy (CRA) materials would improve pipeline performance demonstrably. In practice, however, such innovations have been relegated to niche application because of two inherent fundamental constraints placed upon most major transport pipelines: They must operate continuously to turn a profit, and they are judged on delivering the minimum required flow at the lowest operating and capital investment.
These two principles have led to a general practice of resorting to mitigation measures once corrosion or adhered deposits become a problem that cannot be addressed easily. It is common to hear of operators waiting more than 30 years between interventions. Periodic maintenance techniques, such as introducing wire-brush pigs to mechanically dislodge such internal deposits, can result in impressive initial returns—reportedly more than $1 million per annum, and 5% improved throughput. However, these benefits are decidedly temporary because the treatments do not provide permanent protection against further corrosion or deposit, and much of the benefit is derived from the macroscopic effect of widening the cross-sectional area of the pipeline.
Numerous studies and reviews have demonstrated that pressure drop within a turbulent-flow pipeline can be affected even more significantly through manipulation of the interaction between the fluid and the internal pipeline surface. This is widely seen in the use of the Toms effect, whereby frictional drag can be significantly reduced through introduction of small amounts of high-molecular-weight polymer into the product to be transported. This polymer additive can preferentially adsorb to the pipeline wall, creating a high-rheology boundary layer that minimizes interaction between the rough pipeline wall and the bulk turbulent flow. As an intervention strategy that allows for near-zero downtime and minimal disruption to existing pipeline operations, use of drag-reducing agents (DRA) has been widespread. Nevertheless, DRA have the obvious limitation of introducing a significant recurring operational expense, and are most effective when introduced at the beginning of a pipeline’s lifetime, before any corrosion or deposit formation that could interfere with the formation of the lubricating film layer. Additionally, as surface deposits form and overall surface roughness increases, larger amounts of DRA become necessary to maintain the same flow properties.
Traditional flow coats, which provide a physical barrier between the highly roughened pipe surface and the fluid product, have been used as a longer-lasting alternative in both new and existing pipelines. Typically formed from liquid epoxies, these internal coatings provide an extremely smooth finish below that which might typically be attained by thorough and complete pigging of internally bare steel pipe. Because of their extreme thickness and general chemical inertness, such coatings also serve as excellent corrosion inhibitors through simple physical isolation of the pipeline surface from the liquid product. However, these traditional coatings have proved to be extremely challenging to handle and apply because of the inherent properties of the coating materials themselves. Epoxies, which are chemically compatible with petroleum products, typically combine a short pot life with long cure times (more than 24 hours per layer), requiring extended downtime of the pipeline, especially in colder climates, and can be completely unusable in subsea conditions. Additionally, because of the relatively large amount of material that must adsorb physically to the pipeline interior, surface preparation is both extensive and critical to coating function and longevity. Multiple rounds of sequential chemical and mechanical cleaning are necessary to produce a near-white metal finish, and the surface must be completely dry and passivated, with a low chloride content to prevent underfilm corrosion from forming.
The complete paper describes the application and evaluation of a new omniphobic (water- and oil-repellent) surface treatment, hereafter referred to as omnitreatment. Previous efforts investigating the omniphobic material showed that despite being applied very thinly (thickness of more than 4 mils, 0.004 in.) to heavily corroded surfaces with minimal surface preparation, the coating material was able to impart corrosion protection and hydrate particle repulsion while also imparting surface roughness profiles below that of new commercial steel. On the basis of these two properties, the omnitreatment was selected as a strategy to remediate and retrofit a highly aged wastewater pipeline that could not be relined using existing materials because of three factors:
- The pipeline’s extreme age and fragility, which prevented thorough cleaning
- A complex geometry involving multiple 90° bends and completely vertical sections
- Limited access to the pipeline, which prevented the use of hazardous chemicals and materials with high volatile organic compound content
In preparation for final application of the omnitreatment to this line, a model test loop was fabricated, and the effect of the treatment on water-flow pressure drop was reported through both direct measurement and extrapolation on the basis of surface profilometry of the pipeline interior surface.
Test Loop and Field Trial
A case study presented in the complete paper describes the application of the coating to a fluid-transport system, including a variety of different challenging features, including bends, valves, weld seams, flanged connections, and heavily corroded surfaces. The pipeline systems were all treated by in-situ methodology and quality-control procedures that were optimized in a fabricated test loop and evaluated for flow performance through both modeling and experimental observation. The treatment process and test results are described in detail in the complete paper.
Applying the omnitreatment on a corroded, mud-caked elbow joint resulted in an extremely smooth, uniform surface finish. Applying the omnitreatment to the model pipeline interior did not require any substantial acid pickling or extreme mechanical force. Areas of complex geometry and weld seams and flange joints that prove challenging to typical flow coatings were treated and protected fully. The omnitreatment infiltrated into the highly porous pipeline internal surface and carefully sealed any potential gaps or voids (Fig. 1).
After treatment, the average surface roughness within the treated section had dropped from a measured high of 1800 µinches to a low of between 40 and 120 µinches. It is expected that applying the omnitreatment with a measured surface roughness of 100 µinch may improve throughput by as much as 50% over moderately corroded steel pipes, and as much as 150% over heavily corroded pipelines.
On the basis of the potential benefit of reducing frictional drag and the ability to be easily applied to pipelines with extremely tight bends, a field trial of the omnitreatment was conducted on a roughly 150-ft long, 6-in.-diameter pipeline buried approximately 10 ft below ground. The pipeline was designed to transport nonhazardous mixed wastewater from the interior of a fenced compound to a public wastewater utility line. Repeating the test-loop techniques, the omnitreatment was successfully applied to the pipeline interior. The pipeline passed hydrostatic leak testing and was put back into service within 48 hours, including both setup and breakdown of the applicator equipment. Zero hazardous waste was generated over the course of the field trial, leaving it a viable option for further deployment in easily isolatable, challenging-geometry pipelines.
- Surface roughness comparable to or exceeding that of thick epoxy flow-coat liners can be achieved through an extremely thin surface treatment.
- In contrast to the typical epoxy pipe liner, the omnitreatment navigated extremely tight turns and completely vertical sections, and was able to enter and impregnate highly corroded and weathered pipeline interior surfaces.
- On the basis of measured surface-roughness values for the applied surface treatment, reductions in frictional drag losses could be improved by up to 30% over simple mechanical pigging.
- Further studies on treatment behavior over time will be performed in the near future to inform the long-term economic value of the technology better.
- Omnitreatment technology is a potentially attractive option for further deployment in easily isolatable, challenging-geometry pipelines.
Surface Treatment Improves Flow Assurance and Throughput in Corroded Pipe
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