Liu Yang1, Dong has been2, Wu Chao1, Zhang Bin1, Wang Lele1, Jiang Zitao2, *，Li Qiuyang1, Yang Rui1, Yin Jia Le1, Hou Juan3
1. National Petroleum and Natural Gas Pipeline Engineering Technology Research Institute; 2. China University of Petroleum (Beijing); 3. China Petroleum Engineering and Construction Corporation, North Division
Communication Author Profile: Jiang Zitao, Senior Engineer and PhD, primarily engaged in research on predicting and protecting against stray currents in oil and gas pipelines.
Most long-distance pipelines in China are made of metal materials. In recent years, with the rapid development of oil and gas storage and transportation industry, the requirements for pipeline materials have been increasing. However, due to the corrosive elements in the internal and external environment of the pipeline, such as CO2、H2S, stray currents, and other factors can cause corrosion of metal pipes, even leading to perforation and leakage, which poses a significant threat to metal pipes.
Non-metallic composite pipes represent a novel piping material that can potentially replace steel pipes. They offer several advantages, including the ability to overcome the corrosion issues associated with high-strength metal pipes, as well as ease of construction, low maintenance costs, and long service life. In the 1980s, China began to apply non-metallic composite pipes in the oil and gas transportation sector. After more than four decades of development, non-metallic composite pipes have been widely used in distribution systems in major oilfields such as the Xinjiang, Daqing, Changqing, and Southwest oilfields.
Currently, China is accelerating the construction of its pipeline network to build a "national unified network," enabling the interconnection of oil and gas long-distance pipelines to ensure the country's energy supply security. With the continued development of interconnection projects, the number of smaller-diameter oil and gas pipelines is increasing, and these pipelines cannot be cleaned and inspected internally, making it difficult to detect corrosion problems on the inner and outer surfaces of steel pipes, which negatively impacts the safety of pipeline operation. Selecting corrosion-resistant pipe materials is a viable alternative solution.
Furthermore, as China's energy structure continues to transition, hydrogen will become a crucial component of future energy. The storage and transportation of hydrogen are key elements in this energy transition. Currently, both domestic and international hydrogen pipelines utilize low-grade seamless metal pipes. The issues of hydrogen embrittlement and hydrogen leakage are severe, which restricts the application of high-grade metal pipes for hydrogen transport. Hydrogen causes degradation of the mechanical properties of metal pipes, such as strength, plasticity, and toughness, significantly reducing the lifespan of the pipes and seriously impacting the safety of the pipeline network.
In recent years, the technology of transporting hydrogen using non-metallic composite pipes has become a focus of research for many research institutions. Among them, enhanced thermoplastic composite pipes (hereinafter referred to as RTP pipes) are a newly developed type of non-metallic composite pipe that has gained significant attention. RTP pipes offer several advantages, including energy efficiency, environmental friendliness, excellent sealing, corrosion resistance, adaptation to uneven settling of the substrate, excellent impact resistance, and high resistance to twisting. These features make RTP pipes a promising new trend in the pipe industry.
Currently, RTP pipes have been successfully implemented on a large scale in oilfield water injection, alcohol injection, and certain transmission pipelines. However, existing standards are not applicable to long-distance pipelines, and the current products do not meet the requirements for application in long-distance pipelines. Furthermore, the inner material of RTP pipes does not meet the requirements for high-sulfur gas and heavy oil high-temperature transportation.
Classification and structure of non-metallic composite pipes
Depending on the material and manufacturing process of the pipes, non-metallic composite pipes commonly used in the oil and gas industry can be divided into: glass fiber reinforced plastic pipes (referred to as "glass steel pipes"), plastic alloy composite pipes, composite material reinforced pipe, and steel pipes (referred to as "CRLP pipes"). Specifically, research and application of glass steel pipes and plastic alloy composite pipes started earlier and are widely used in oilfield gathering systems.
In recent years, with the gradual maturation of non-metallic composite pipe technology, the application prospects of enhanced thermoplastic pipes in hydrogen and offshore oil and gas transport are broad. Table 1 provides a comparison of the advantages and disadvantages of different types of non-metallic composite pipes.
Table 1: Comparison of Advantages and Disadvantages of Non-Metallic Composite Pipes
Fiberglass pipes are a composite pipe formed by continuously winding and solidifying a glass fiber-impregnated resin matrix. Generally, they consist of two layers: glass fiber reinforcement material and the resin matrix (see Figure 1). The resin matrix primarily provides corrosion protection, while the glass fiber reinforcement material is used to bear the loads on the pipe. Additionally, the rigidity of the pipe can be improved by adding materials such as quartz sand to the reinforcement material. Compared to metal steel pipes, fiberglass pipes have advantages such as good corrosion resistance, lightweight, and low operating costs. Currently, fiberglass pipes are widely used in various oil and gas fields for transporting crude oil, natural gas, and other corrosive media.
Figure 1: Schematic diagram of glass fiber pipe structure
Plastic alloy composite pipes consist of a thermoplastic plastic inner layer and glass fiber as a reinforcing layer, forming a non-metallic composite pipe (see Figure 2). Plastic alloy composite pipes have advantages such as corrosion resistance, high strength, and good water conductivity, and are currently widely used in oil field gathering and injection pipelines.
Figure 2: Schematic diagram of the plastic alloy composite pipe structure
CRLP pipes utilize steel pipe as a base, enhancing the hoop strength and crack resistance of the steel pipe by wrapping it with non-metallic composite materials. This improves the overall pressure performance of the pipeline. CRLP pipes are stronger than ordinary steel, allowing for significant savings in pipeline steel, and offer excellent corrosion resistance and fatigue resistance. Studies have shown that, under the same pipe diameter, using CRLP pipes can save 7% to 8% in costs and increase the flow rate by 5%.
The CRLP pipe typically consists of four layers: an inner steel pipe, a transition layer, a composite material reinforcement layer, and an outer protective layer (see Figure 3). The transition layer is typically made of resin material and provides corrosion protection for the steel pipe, while also having a certain degree of adhesion between the steel pipe and the composite material layer, which helps transmit loads. The reinforcement layer can be made of fiber materials such as glass fiber, carbon fiber, or aramid fiber, or resin materials, and works together with the inner steel pipe to provide load-bearing capacity. The axial load is entirely borne by the steel pipe, while the hoop load is shared between the steel pipe and the composite material. The outer protective layer prevents the fiber reinforcement layer from being corroded and damaged during transportation, and also prevents the inner material from deteriorating due to moisture absorption.
Figure 3: Schematic diagram of the CRLP pipe structure
RTP pipe is a high-pressure plastic composite pipe material with a 3-layer structure (see Fig. 4). The inner and outer functional layers are typically made of wear-resistant and corrosion-resistant polyethylene, primarily used for corrosion protection and media isolation. It also has functions of scratch resistance and anti-static properties. The intermediate reinforcing layer provides the pipe with the required strength, and common materials include steel wire, steel strip, glass fiber, and aramid fiber.
Figure 4: Schematic diagram of the RTP pipe structure
Compared to other metal pipes or composite non-metallic pipes, RTP pipes offer several advantages. For example: RTP pipes can have their intermediate structure designed based on the service conditions of the pipe material, while meeting the requirements for internal and external pressure, tension, and bending, they also have good flexibility; small-diameter RTP pipes can be made into continuous pipes in coils, with each coil length reaching tens or even hundreds of meters, which is convenient for pipeline construction; the maximum operating temperature can reach 130 °C, which is 60 °C higher than that of plastic pipes; the service life is 6 times that of metal pipes and 2 times that of plastic pipes; the wall thickness is reduced by more than 70% compared to plastic pipes, and the inner wall does not accumulate, significantly increasing the flow speed of the transported medium; in addition, RTP pipes also have the advantages of energy saving, environmental protection, good sealing, corrosion resistance, and easy construction, which have excellent impact resistance and torsional resistance, and have been applied in onshore oil field water injection, alcohol injection, and part of transportation pipelines. In offshore oil and gas development, traditional metal flexible pipes have problems such as large size and corrosion failure, RTP pipes are considered as alternatives to metal marine pipelines, and have significant advantages in fatigue resistance, corrosion resistance, and light weight.
RTP pipe inner layer material
Currently, the large-scale exploitation of high-sulfur oil and heavy crude oil is the prevailing trend in oil and gas development. As sulfur content and the demand for enhanced oil recovery (EOR) technologies for heavy crude increase, the requirements for the temperature and corrosion resistance of pipelines are becoming more stringent. For RTP pipes, the performance of the inner material is a crucial factor determining its temperature and corrosion resistance. Table 2 provides a comparative analysis of key parameters for commonly used inner materials in RTP pipes.
As shown in Table 2: HDPE, PVC, and UHMWPE have lower costs but poor temperature resistance, limiting their use to lower temperatures. PTFE, FEP, PVDF, and PEEK offer superior performance but are more expensive. PP has poor impact resistance, and PU has poor high-temperature resistance, making them difficult to apply on a large scale in oil and gas fields. POK is a high-performance thermoplastic polymer with good temperature resistance, excellent mechanical properties, and a lower cost, presenting potential for large-scale application in non-metallic composite pipes in oil and gas fields. However, further experimental research is needed to evaluate its performance under complex conditions such as high temperatures and corrosive solvents.
Currently, most RTP pipes still use polyethylene as the inner corrosion-resistant material. However, polyethylene is not suitable for high-temperature conditions and cannot meet the requirements for oil and gas development and transportation. Therefore, there is an urgent need to develop new, high-temperature, corrosion-resistant inner layer materials to replace polyethylene, in order to meet the requirements for pipeline transportation.
Table 2: Key Parameter Comparison for Inner Layer Materials of RTP Non-Metallic Composite Pipes
RTP pipe related standards
Depending on the manufacturing process and product type, a series of standards related to RTP pipes have been established both domestically and internationally. According to statistics, there are over 260 existing international, national, and industry standards related to RTP pipes. Among these, approximately 35 domestic national and industry standards are relevant.
Regarding the RTP hydrogen pipeline standard specifications, the U.S. Department of Energy's Fuel Cell Technologies Office (FCTO) has been conducting standardization work for fiber-reinforced materials. In 2016, ASME B31.12-2019, "Hydrogen Pipelines and Piping," incorporated fiber-reinforced materials into the standard, specifying that their maximum operating pressure should not exceed 17 MPa.
According to the different applicability of standards, RTP pipe-related standards are divided into 5 categories: pipe and pipe fitting manufacturing, product acceptance inspection, engineering design, construction and acceptance, and operation and maintenance. Relevant RTP pipe standards, both domestic and international, are shown in Table 3.
"Foreign RTP (Rigid Thermoplastic) pipe development is earlier and has more mature technology. Comprehensive standards have been established for pipe production, installation, operation, and performance testing. In contrast, domestic development is later. Some researchers have pointed out that the domestic RTP pipe standard system is incomplete. Besides the standards specified in SY/T 6662.4-2014 for steel-supported, continuous thermoplastic composite pipes, there is a lack of standards for product acceptance, design and construction, and material selection guidance. In recent years, domestic standards have become more comprehensive, and standards have been established for different types of pipe materials, such as fiber-reinforced thermoplastic composite pipes and steel wire mesh-reinforced polyethylene composite pipes. These standards cover pipe manufacturing, product inspection, construction, operation, and maintenance."
As domestic RTP pipe technology continues to mature, standardization efforts still face some challenges. First, the applicability of the standards is limited, and they are primarily applicable to oil and gas field station and yard collection and injection pipelines. When compared to GB 50251-2015 "Specification for Design of Gas Pipelines" and GB 50253-2014 "Specification for Design of Oil Pipelines," the content related to RTP pipes in existing design specifications is lacking, particularly regarding pipeline routes, pipeline accessories, instrumentation, and station facilities. Therefore, the current engineering design specifications are not feasible for long-distance pipelines and require a feasibility study and the establishment of relevant standards for RTP long-distance pipelines. Second, due to the differences in the intermediate reinforcement layer materials, each RTP pipe requires a comprehensive set of standards, resulting in many similar sections. The key to future standardization efforts is to consolidate these similar sections into a unified standard. Third, most RTP pipe standards are industry standards, and a national standard should be established as the core and guiding standard. Finally, regarding the determination of properties such as the impact strength, flexibility, stiffness, and creep ratio of thermoplastic pipes, domestic standards have gradually been improved. However, further improvements are needed in areas such as pipe service life, aging tests, and operation and maintenance.
Table 3: Relevant Standards for RTP Pipes
Applications of RTP pipes
Foreign RTP (Rough Terrain Pipeline) development started relatively early. In June 1995, the first RTP pipeline produced by Shell was put into use in the United Kingdom. In late 1996, Shell constructed a 7 km long RTP pipeline with a nominal diameter of 150 mm in an oil field in Oman, to address the severe corrosion and leakage problems of existing oil pipelines. In 2000, Germany laid a 1 km long RTP pipeline with a nominal diameter of 100 mm for transporting non-dry, sulfur-containing natural gas. Currently, RTP pipelines are mainly used in the oil and gas field development sector. The nominal diameter range of RTP pipelines is 75~150 mm, and the pressure range is 1.6~9 MPa. After decades of development, foreign RTP pipeline technology has become relatively mature. According to statistics, the global RTP pipeline market sales reached US$5.4 billion in 2021.
Pipelife is one of the leading pipe suppliers in Europe, with a total length of installed RTP (Rigid Thermoplastic) pipes exceeding 3500 km. based on the applicable pressure, their RTP pipes, "Soluforce," are divided into three types: "Soluforce Light," "Soluforce Classic," and "Soluforce Heavy." Each type can be selected in three different versions: "Standard (ST)," "Gas Tight (GT)," and "High Temperature (HT)." ST refers to standard high-performance pipes, GT refers to sealed pipes that prevent the penetration of transported media, and HT refers to high-temperature pipes. Soluforce can be used for oil and gas gathering, natural gas transmission, condensate pipelines, high-pressure water supply, and offshore oil and gas pipelines.
In addition to Pipelife, Flexpipe Systems Inc. produces "FlexPipe" and "FlexPipe HT" pipes, which utilize high-density polyethylene as the inner layer and glass fiber as the reinforcement layer. These RTP pipes can operate at a maximum pressure of 10.3 MPa and a maximum continuous operating temperature of 82 °C, with a maximum length of 1100 m per roll. They are suitable for transporting various media, including oil, gas, water, CO2, and H2S. The company also produces "Flexcord Linepipe," which uses high-strength steel wire as the reinforcement layer. This pipe can operate at a maximum pressure of 15.5 MPa and a maximum continuous operating temperature of 60 °C, with a maximum length of 615 m per roll. It can improve oil recovery (EOR) and is used for transporting oil, gas, water, H2S, and CO2. Furthermore, the company produces joints for non-metallic pipes, made from duplex stainless steel or nickel-plated alloy. These joints have passed high-temperature, leak-proof, and low-pressure tests according to standards such as CSA Z662 and API Spec 15S.
National Oilwell Varco (NOV) manufactured "Fiberspar" high-strength fiberglass-reinforced pipe has a maximum operating pressure of 24.1 MPa and a maximum working temperature of 95 °C. The pipe can be continuously manufactured to a length of up to 2743 m and is suitable for transporting media such as water, oil, natural gas, brine, CO2, and H2S.
Technip's "Coflexip", a steel-framed, heat-resistant composite pipe with a stainless steel cladding, effectively protects the pipe from external impacts and abrasion. This type of pipe has a maximum operating pressure of 103.4 MPa and can be used to transport media such as oil, gas, water, H2S, and CO2.
Currently, foreign design and construction technologies for RTP pipes with diameters up to 150 mm and pressures up to 9.93 MPa, along with relevant standards and specifications, have been widely applied in oil and gas transmission pipelines and marine oil and gas development auxiliary pipelines. Preliminary research has also been conducted on the design and construction of long-distance transmission pipelines with diameters of 300 mm or more and pressures of 6.4 MPa, but these technologies have not yet entered the engineering application stage.
On the domestic front, some companies have introduced advanced foreign technologies and equipment. In 2009, Nanjing Chen Guang Oupa Composite Pipe Engineering Co., Ltd. introduced the first RTP pipe production line from the German company "Klaus Maffei," becoming the fifth company globally to research and manufacture aramid-reinforced thermoplastic composite pipes.
In recent years, some domestic companies have independently researched and developed RTP pipes, achieving certain results. Ningbo Opea Marine Engineering Equipment Co., Ltd. has six major product series, including steel belt reinforced composite pipes, steel wire reinforced composite pipes, steel yarn/steel rope reinforced composite pipes, aramid fiber reinforced composite pipes, fiberglass reinforced composite pipes, and polyester fiber reinforced composite pipes, with a maximum working pressure of 32 MPa and a maximum working temperature of 130 °C. These products are primarily used for transporting crude oil, natural gas, water injection, and shale gas extraction discharge water.
Changchun Gao Xiang Special Pipe Co., Ltd. produces a range of products, including shallow-sea flexible composite pipes, medium-to-low-pressure distribution composite pipes, and high-pressure transport injection composite pipes, with a maximum operating pressure of 20 MPa and a maximum operating temperature of 90 °C. These products are suitable for high H2S, CO2, Cl- or other highly corrosive environments.
Chinese-produced RTP pipes have been used in oil and gas field projects. In 2005, the Da Hong Port Oilfield of China Petrochemical Corporation used RTP pipes with diameters of 65 mm and 100 mm and a design pressure of 6.3 MPa, with a temperature resistance of 65 ℃; In 2008, the Bohai Oilfield of China National Offshore Oil Corporation used RTP pipes with a diameter of 100 mm and a design pressure of 6.3 MPa; In 2011, the Puguang Gasfield of China National Petroleum Corporation used RTP pipes with a diameter of 80 mm and a design pressure of 6.3 MPa.
Currently, China has mastered the manufacturing and construction technology for RTP pipes with a diameter of DN150mm or less, and has achieved scaled applications in oilfield water injection, alcohol injection, and some pipeline systems. However, there are still gaps in manufacturing technology and pipeline design standards compared to international standards. China has also conducted research on the manufacturing and corresponding design and construction of large-diameter, high-pressure RTP long-distance pipelines with a diameter of 450mm and a design pressure of 6.4 MPa, and has preliminarily mastered the structural design methods and pipeline reliability design methods for steel-reinforced RTP pipe materials. However, there have been no satisfactory results in terms of pipeline connection and repair technology. Currently, existing pipes have small diameters, low pressure and temperature resistance, and the reliability of hydrogen transport is unknown, making it impossible to apply them on a large scale for long-distance pipelines. Therefore, it is necessary to design large-diameter, high-pressure, and high-temperature RTP pipes suitable for the oil and gas industry.
Trends in RTP pipe development
1、Development of non-metallic composite pipes for oil and gas transportation contributes to the construction of oil and gas pipelines.
Currently, the inner diameters of RTP pipes domestically and internationally are largely concentrated below 200 mm. To address the potential future application scenarios, such as underground gas storage facilities and long-distance pipeline projects, there is a need to develop RTP pipes with larger diameters.
Currently, the main standard for pipe manufacturing is SY/T 6662 series. However, it primarily focuses on basic requirements for raw materials and structural design, without specifying key performance indicators such as raw material properties, pipe winding angles, and the number of winding layers. This results in significant variations in the performance of pipes manufactured by different manufacturers, necessitating standardization and regulation.
- Establish appropriate pipe strength indicators based on different regional standards, and combine them with the characteristics of non-metallic composite pipes to propose key technical requirements for pipe laying and bending. Address challenges such as differences in the structural materials of large-diameter composite pipe connections and pipe end roundness.
Under certain temperature and pressure conditions, pipelines may undergo uneven deformation, leading to failure at the connection points. Ensuring the reliability of the connections is a critical issue that needs to be addressed.
Furthermore, compared to steel pipes, non-metallic composite pipes are more brittle and prone to damage from external impacts. It is crucial to determine their resistance to damage and their load-bearing capacity under different damage conditions. Additionally, the main risk factors affecting pipe safety and the required maintenance techniques need to be clarified.
2、The development and utilization of hydrogen energy bring new opportunities for RTP pipes.
The growing demand for hydrogen presents new opportunities for the hydrogen transportation and distribution industry.
Pipeline transportation offers advantages such as high hydrogen delivery capacity, low energy consumption, and low cost, making it an important method for large-scale, long-distance hydrogen transport. The operating pressure of pipelines is typically between 1.0 and 4.0 MPa. The United States has 2500 km of hydrogen pipelines, and Europe has 1598 km. The total length of hydrogen pipelines in China is less than 100 km.
Compared to metal pipe systems, non-metallic composite pipe systems have lower construction and maintenance costs. In recent years, Western countries have focused on developing and using non-metallic composite pipes. RTP pipes are an ideal material for transporting high-pressure gases and fluids over long distances.
In 2019, the port of Groningen, Netherlands, used SoluForce H2T, a reinforced thermoplastic pipe manufactured by PipeLife, to lay a 4 km hydrogen pipeline. SoluForce H2T is a specialized RTP (Reinforced Thermoplastic) pipe designed for hydrogen transport. Its inner and outer layers are made of HDPE (High-Density Polyethylene) material, while the reinforcing layer uses aramid fibers, and the barrier layer is made of aluminum. Research and certification have demonstrated that DN150 mm SoluForce H2T pipes can have a service life of up to 50 years under maximum operating pressure of 4.2 MPa and maximum operating temperature of 65 ℃.
Currently, the application of RTP pipes for hydrogen transport in China is still in the experimental stage. In June 2023, the National Pipe Network Group conducted a full-scale non-metallic pipeline explosion test using 9.45 MPa pipes. The test included three types of non-metallic pipes: high-barrier flexible composite pipes, plastic alloy composite pipes, and small-diameter thermoplastic composite pipes, as well as low-grade metal pipes. This was the first online test conducted in China for DN250 mm, 6.3 MPa hydrogen-transmitting non-metallic pipes and in-service metal pipes.
Currently, there is a lack of specific standards for non-metallic composite pipes for hydrogen transport in China. Further development and standardization are needed. Existing standards primarily focus on short-distance distribution pipelines, and there is a lack of design standards for long-distance pipelines. It is imperative to conduct feasibility studies for the application of hydrogen pipelines and establish and refine standards for non-metallic composite pipe hydrogen transport.
Key technical challenges in RTP pipe technology
Compared to traditional metal steel pipes, RTP pipes offer numerous advantages, including corrosion resistance and ease of installation. As the technology matures, RTP pipes will be widely used in onshore and offshore oil and gas field development, oil and gas transmission, and hydrogen transport. Currently, RTP pipes still face the following urgent issues that need to be addressed:
(1) Connection techniques for RTP pipes. Joints are an important component of the pipe system, and currently, RTP pipe connections are typically made using butt welding joints and compression joints. As pipe diameters and pressures increase, the disadvantages of butt welding joints, such as low strength, and compression joints, such as high cost, are becoming more apparent. Therefore, further research is needed on connection methods for non-metallic composite pipes to provide more options for construction and installation.
(2) Research and development of new RTP pipe technology. Currently, composite pipe inner materials and reinforcement materials have certain drawbacks. The development of cost-effective, high-temperature and high-pressure non-metallic composite materials is the foundation for the large-scale application of non-metallic composite pipes. Furthermore, non-metallic composite pipe technology for offshore oil and gas development has been long monopolized by foreign companies. China needs to narrow the technological gap and solve "bottleneck" technologies to achieve the domestic production of non-metallic composite pipes for offshore applications.
(3) Detection techniques for RTP pipes. While standards have been established domestically and internationally for the manufacturing and quality acceptance of non-metallic composite pipes, there is no unified method for assessing their service life, aging tests, tensile strength, compressive stability, crack and leakage detection.
(4) Management of integrity for RTP pipes. Currently, there is no complete integrity management system in place. The regulatory oversight of pipes is weak, data collection is not standardized, and there is no established integrity evaluation technology. Furthermore, there is a lack of standardized repair methods, making it impossible to perform online repairs on non-metallic composite pipes.
(5) Application Standards for RTP Pipes. The engineering design aspects of RTP pipes are lacking, and there is a lack of complete standards and specifications. The existing standards do not have the feasibility for application in long-distance pipelines. It is necessary to conduct a feasibility study for RTP long-distance pipelines and strengthen the development of standards and specifications.
Faced with numerous technical challenges, the Chinese non-metallic composite pipe industry needs to absorb advanced foreign technologies and conduct independent innovation to achieve the large-scale application of non-metallic composite pipes as soon as possible, providing new directions for the future development of oil and gas pipelines.
Source: Corrosion & Protection, Vol. 23, Issue 9, 2023