- Author: Yi Guihu, Zhang Youhui, Ma Yongqing, et al.
(Ocean Petrochemical Engineering (Qingdao) Co., Ltd)
Introduction
Fusion bond epoxy coating (FBE) is a thermally cured, high-performance epoxy powder coating that uses air as the carrier for delivery and dispersion. FBE forms a strong chemical bond with metal surfaces, resulting in a hard, smooth coating with low fluid flow resistance, excellent abrasion, bending, and chemical corrosion resistance, as well as strong anti-anodic delamination properties. Furthermore, FBE does not produce wastewater, waste gas, or solid waste during application, making it an environmentally friendly coating.
Currently, FBE has been widely applied in various professional fields, both domestically and internationally, including water supply, ventilation, chemical, and oil drilling. Through continuous development and refinement, this technology has reached maturity. In some pipeline systems within domestic offshore oil platforms, FBE coating was also used as a corrosion protection method.
However, due to the inherent properties of FBE, all aspects of FBE design, construction, and inspection must meet high standards to ensure that the coating is not prone to failure. The main reasons for FBE failure are typically:① Coating Quality Reasons: - Using lower-cost polyester intermediates in epoxy resin production to reduce costs, resulting in coating failure (Substituting cheaper materials for higher-quality epoxies). - Powder coatings clumping, absorbing moisture, and deteriorating during transport and storage, leading to coating failure.② Construction Quality Reasons: - Substrate surface preparation not meeting the required standards, including but not limited to cleanliness, rust removal level, anchor pattern depth, oil and grease removal, and weld seam treatment, resulting in coating failure. - Coating application process, including coating thickness, and especially the control of the curing process, not meeting the required standards or the recommendations of the powder manufacturer, resulting in incomplete or over-curing, leading to coating failure.③ Coating Selection Reasons: - Selecting the wrong type of epoxy powder for the specific substrate and environment, resulting in coating failure.
These are common issues encountered in FBE design and construction. Currently, there are mature standards and systems available to ensure the quality of FBE. However, research on effectively addressing pipeline patching issues in the overall assembly site is relatively limited and weak. If the quality of the pipeline patching is poor, the patched area is highly susceptible to corrosion, especially galvanic corrosion between large anodes and small anodes. Local corrosion in this area can easily penetrate deeply, leading to perforation at the patching site and causing leaks. Therefore, under the constraints of limited construction environments, equipment, and construction windows at the overall assembly site, the technical aspects of patching are of paramount importance.
1. Anti-corrosion FBE application on offshore oil platform process pipelines
As a corrosion protection solution for offshore oil platform process pipelines, FBE (Fusion Bonded Epoxy) is increasingly being used. However, unlike process pipelines and transmission pipelines, process pipelines have smaller diameters and more complex configurations. In terms of application, offshore oil platform process pipelines should be considered as non-standard coated pipelines. When applying FBE internally, the specific parameters of equipment such as sandblasting, painting, and heating limit the types of pipelines that can be coated. Pipeline construction design must be based on information such as pipe and fitting diameters, lengths, types, wall thicknesses, material properties, and the combination form of the pipeline and fittings, in conjunction with the capabilities of internal pipeline coating. Proper pipe segmentation should be performed during pipeline prefabrication. Excessive segmentation can increase the number of patching points, significantly impacting the integrity and quality of the FBE pipeline, and increasing the risk of corrosion.
2. Analysis of Common Patching Methods for Internal FBE Coating
Given the characteristics of offshore oil platform process pipelines and the difficulty in applying coatings, on-site patching of the internal FBE coating is a critical aspect that affects the overall quality of the internal coating. The following analysis will examine the common methods for on-site patching of internal FBE coatings on offshore oil platform process pipelines, along with their advantages and disadvantages.
2.1 Welding Connection Methods
Welding is the most common connection method, offering simple and easy operation. After welding pipeline assemblies, coating repair must be performed on the weld area (see Figure 1). When patching, equipment such as sandblasting machines and spray guns must be introduced into the inner wall of the weld area of the pipeline. Currently, this patching technology is relatively mature in China and has formed a complete set of equipment, including control units, sandblasting units, spraying units, and detection units. However, due to factors such as the size of internal patching machines, these machines cannot perform internal patching on small-diameter pipes, bent pipes, and pipes with large angles, which severely limits the application of internal patching machines.
"However, offshore oil platform process pipelines have small diameters, complex and variable routes, and certain angles, including vertical orientations. Therefore, if welding connection is used, it is essential to minimize on-site weld joints and position them in locations that are easy to construct. This would necessitate stricter requirements for pipeline distribution design, segmentation design, prefabrication capabilities, overall assembly capabilities, and the construction capabilities of the internal FBE coating contractor. Additionally, due to the limitations of welding, it is difficult to inspect and maintain the internal FBE of the pipelines after the project is put into operation, which would also increase maintenance risks."
Figure 1: Schematic diagram of welding connection method
2.2 Flange Connection Method
Flange connections are a common mechanical fastening method (see Figure 2). Using flange connections effectively prevents damage to internal coatings caused by welding, ensuring the continuity of the internal anti-corrosion coating, and also offers high construction efficiency. During the pipeline prefabrication stage, flanges are pre-welded and installed. After the internal coating is applied, flange connections are only performed during the on-site assembly stage. This installation method is simple, easy to standardize, and facilitates construction scheduling. However, flange connections significantly increase the quality requirements for offshore platforms, which has a significant impact on the quality control of floating platforms. Due to the limitations of flange connections, they also increase the risk of leakage in internal coating FBE pipeline systems. Furthermore, flanges are relatively expensive, which can affect the overall project cost.
Figure 2: Schematic diagram of flange connection method
2.3 Internal Cladding Connection Method
The pipeline connection method, as shown in Figure 3, utilizes an inner lining and outer jacket configuration. The pipeline should be expanded at both ends, i.e., the diameter should be increased (expansion can also be omitted, but this will result in a change in the inner diameter of the pipeline). After the pipeline is prefabricated, FBE coating is applied to the inner surface of the pipeline. During the overall assembly, inner lining and outer jackets are installed at the expansion ports on both ends of the pipeline. After the inner lining and outer jackets are installed, the pipeline is welded to the interface. The material of the inner lining and outer jacket can be non-metallic, such as fiberglass, or the same material as the pipeline.
The inner lining and jacket design is ingenious, offers high strength, and excellent corrosion resistance, making it easy to install and providing a tight fit with the pipe, theoretically reducing the risk of leaks. This can help reduce the quality requirements for offshore oil platforms. However, this design is rarely used in complex structures like process pipelines for offshore oil platforms due to the strict tolerance requirements and dimensional control for both the pipes and fittings. Additionally, it is difficult to inspect and maintain the FBE (flame-retardant coating) applied to the internal surfaces of the pipeline after welding and after the project goes into operation.
Figure 3: Schematic diagram of the inner lining connection method.
2.4 Corrosion-Resistant Alloy Joint Connection Method
The pipeline, connected using corrosion-resistant alloy joints, first involves welding corrosion-resistant alloy rings at both ends of the pipeline. Then, FBE coating is applied to the interior of the pipeline, with the coating overlapping the corrosion-resistant alloy rings (as shown in Figure 4). When connecting the pipeline sections, the corresponding corrosion-resistant alloy welding rod should be used. This method avoids the need for on-site patching of the FBE coating, effectively addressing corrosion issues at the internal coating joints. However, the cost of corrosion-resistant alloys is relatively higher than that of carbon steel, so it is necessary to consider the pipeline layout in advance during the project design and minimize the number of joints.
Some projects adopt carbon steel connection welding with nickel-based alloy overlay to reduce the cost of this part, instead of using corrosion-resistant alloy altogether. The surface treatment at the connection point between the corrosion-resistant alloy and the FBE (flame-retardant primer) should not use steel sand; instead, garnet can be used. However, using garnet for surface treatment makes it difficult to achieve the required roughness or only guarantees the minimum standard requirement, and it is prone to sand entrapment, which cannot be completely removed or avoided. This also affects the quality of FBE in the overlapping area. Furthermore, field welding of corrosion-resistant alloy requires higher quality standards compared to carbon steel.
Figure 4: Schematic diagram of the inner lining connection method.
Conclusion
Protecting offshore oil platform process pipelines is crucial, and internal corrosion protection is a particularly vulnerable area compared to more reliable external protection methods. In particular, the quality of internal corrosion repair patches at the pipeline's connection points is a critical factor for the stable operation of the entire offshore oil platform process system. The various internal corrosion repair methods for offshore oil platform process pipelines described in this article are just a few common methods, and each has its own advantages and disadvantages, and all have certain limitations. Therefore, developing a technology that can completely solve internal corrosion repair, especially for internal corrosion repair patches on offshore oil platform process pipelines, is very important. This requires researchers to intensify their research on internal corrosion repair technologies and quickly put new results into practice.
Please refer to the June 2021 issue of "Coatings and Protection" for the complete content.