Pipes for Surface Gathering and Transportation in Oil and Gas Fields

Abstract

This paper examines the materials used in pipeline systems for oil and gas field gathering and transportation. It analyzes the performance characteristics and applications of materials such as carbon steel, stainless steel, and alloy steel. Special focus is given to material performance in challenging environments, including corrosion, high temperatures, and high pressures. The paper also evaluates factors influencing material selection, such as the properties of the transported medium, operating conditions, and construction and maintenance costs. By comparing the advantages and limitations of different materials, the paper aims to provide both theoretical and practical guidance for selecting and optimizing materials in oil and gas gathering and transportation pipelines.

Surface gathering and transportation pipelines in oil and gas fields constitute a significant portion of the overall surface infrastructure, and the choice of pipeline materials directly impacts the overall efficiency of field development. Driven by the "carbon peak and carbon neutrality" policy, numerous carbon dioxide flooding projects have been launched in China. As a result, significant amounts of carbon dioxide are typically found in the associated gas of oil and gas fields, imposing more stringent technical requirements on the selection of materials for gathering and transportation pipelines and equipment. The selection of appropriate pipelines, based on both technical and economic factors, will be a key focus in oil and gas field development.

In response to recent industry developments, several new types of pipes have emerged in the market, including non-metallic and composite pipes, with over ten distinct varieties. As these pipes are used more widely in oil and gas fields, the product range is expanding, leading to a broader range of structural designs. A significant portion of non-metallic pipes is still produced based on proprietary standards set by individual manufacturers, which results in inconsistent manufacturing practices, uneven product quality, lack of standardized design criteria, and inadequate inspection methods. This paper addresses these challenges by summarizing the structural characteristics, size specifications, connection types, and application requirements of several common gathering and transportation pipes used in oil and gas fields, including carbon steel, low-alloy steel, bimetallic composite pipes, polyethylene pipes, steel-reinforced polyethylene composite pipes, and flexible high-pressure composite pipes. Additionally, the paper provides a detailed overview of corrosion evaluation methods and the principles of pipe selection for specific operating conditions, serving as a valuable resource for oil and gas field design engineers.

1. Carbon Steel and Low-Alloy Steel

Carbon steel and low-alloy steel continue to be the most widely used materials for surface gathering and transportation pipelines in oil and gas fields. After decades of use, the manufacturing, construction, and acceptance processes for carbon steel and low-alloy steel pipelines are the most well-established among pipeline materials. These pipelines are classified into high-frequency resistance-welded pipes (HFW), submerged arc-welded pipes (SAW)—including both spiral-seam (SAWH) and straight-seam (SAWL) variations—and seamless steel pipes (SMLS). The application range for each type of pipe is influenced by the manufacturing processes and equipment capabilities, as shown in Table 1. Submerged arc-welded pipes are primarily used for large-diameter pipelines, with the L555M grade representing the highest available grade in China, capable of supporting diameters up to 1422 mm. High-frequency resistance-welded pipes are typically used for medium-diameter pipelines, usually ranging from X mm to Y mm, but the defect rate in the weld and heat-affected zone is higher than that of other pipe types, limiting their use to operating conditions below 6.3 MPa. Seamless steel pipes, produced by hot rolling, extrusion, or cold drawing of steel ingots, are suitable for small- and medium-diameter pipelines and are known for their thicker walls compared to welded pipes.

Carbon steel and low-alloy steel can be classified into two categories based on operating temperature: ordinary steel and low-temperature steel, with the dividing threshold generally set at -20°C. Steel used in environments below -20°C is classified as low-temperature steel, whereas steel for warmer conditions is classified as ordinary steel. Low-temperature steel offers superior resistance to brittle fracture, crack initiation, and propagation when compared to ordinary steel. Its low-temperature performance is typically assessed using the Charpy impact test, which measures energy absorption at low temperatures. For pipes with an outer diameter greater than 508 mm, a low-temperature drop hammer test is also used for evaluation. The use of L555M low-temperature pipeline steel in the China-Russia East Line highlights its suitability for operating conditions as low as -40°C.

Table 1: Size Range of Steel Pipes

Note: The data above are sourced from GB 34275-2017, and the survey reflects the general domestic standards.

1.2 Metal Composite Pipe

A metal composite pipe is a type of composite pipe made from two or more different metals. It typically consists of a base steel pipe with either a lining steel pipe or a cladding layer, each selected to enhance the properties of the materials. The base steel pipe provides structural support, bearing pressure and load, while the lining or cladding layer offers corrosion resistance. Metal composite pipes combine the corrosion resistance of alloys with the cost-effectiveness of carbon steel, providing enhanced corrosion protection and significantly reducing manufacturing costs. To preserve the integrity and corrosion resistance of the inner lining, its thickness typically ranges from 2 to 3 mm. Over the past three decades, metal composite pipe technology has made substantial advancements and is now widely used in oilfields in China, including Daqing, Changqing, Xinjiang, and Southwest China. However, due to the recent development of metal composite pipes in China, there remains a noticeable gap in design, construction, and acceptance standards compared to pipes made from single materials, particularly in the area of joint connections. Welding dissimilar metals in these joints requires advanced technical expertise. As a result, fully automated welding and testing methods will be essential for supporting the wider adoption of metal composite pipes.

1.3 Non-metallic Pipes

Common non-metallic pipes used in surface gathering and transportation for oil and gas fields include polyethylene pipes, steel-reinforced polyethylene pipes, and high-pressure flexible composite pipes. A summary of their structural characteristics and applications follows.

1.3.1 Polyethylene Pipes

Polyethylene pipes are one of the earliest non-metallic pipes. These thermoplastic pipes are produced by melt-extrusion of polyethylene-based materials. Polyethylene pipes are typically categorized by raw material grade, including PE 32, PE 40, PE 63, PE 80, and PE 100. Currently, polyethylene pipes are widely used in municipal water supply and gas distribution pipelines. In developed countries, polyethylene pipes account for up to 90% of the gas pipeline market. For example, since the 1970s, polyethylene pipes have been used in more than 90% of newly constructed municipal pipelines in the United States. Polyethylene pipes are also commonly used in gas gathering pipelines for coalbed methane and tight gas fields. Given that coalbed methane and tight gas fields are often in mountainous and hilly areas, the lightweight and easy-to-install nature of polyethylene pipes significantly reduces construction challenges. Polyethylene pipes for coalbed methane gathering and transportation are available in four series: SDR11, SDR17, SDR21, and SDR26. Design specifications and usage requirements are detailed in Table 2.

Polyethylene pipes are regulated by a comprehensive set of manufacturing, design, construction, and testing standards. Key manufacturing standards include GB/T13663-2018, "Polyethylene (PE) Pipe Systems for Water Supply," and GB/T15558-2015, "Buried Polyethylene (PE) Pipe Systems for Gas." Design, construction, and acceptance standards include TSG D2002-2006, "Technical Rules for the Connection of Polyethylene Pipes for Gas," CJJ 63-2018, "Technical Standards for Polyethylene Gas Pipeline Engineering," and NB/T 10884-2021, "Buried Polyethylene (PE) Pipes and Fittings for Coalbed Methane Gathering and Transportation." Polyethylene pipe connections are made using either electric fusion or hot melt methods. Electric fusion is used for pipes with a diameter of DN65 or smaller, while both hot melt and electric fusion connections are applicable for pipes larger than DN65. Unless specified otherwise, hot melt connection is generally preferred for its cost-effectiveness. Phased array ultrasonic testing is an efficient non-destructive testing method for polyethylene welded joints, capable of detecting various defects in both thermal and electric fusion connections. These requirements are governed by standards such as GB/T38942-2020, "Pressure Pipe Specifications for Public Pipelines," NB/T 10884-2021, "Buried Polyethylene (PE) Pipes and Fittings for Coalbed Methane Gathering and Transportation," as well as relevant local standards from regions such as Shanghai, Guangdong, and Inner Mongolia.

Table 2: Design and use requirements of PE100 pipe

1.3.2 Steel Skeleton-Reinforced Polyethylene Composite Pipe

Steel skeleton-reinforced polyethylene composite pipes are an enhancement of polyethylene pipes, designed to improve pressure resistance. While polyethylene pipes have limited pressure resistance, the incorporation of a steel skeleton within the polyethylene pipe enhances their structural strength. The reinforcement structure of steel skeleton-reinforced polyethylene composite pipes is typically classified into three types: steel wire welded skeleton, steel plate mesh skeleton, and steel wire wound skeleton. The different reinforcement types are illustrated in Figure 1.

Steel skeleton-reinforced polyethylene composite pipes are regulated by comprehensive standards. Manufacturing standards for these pipes include SY/T 6662.1-2012, "Non-metallic Composite Pipes for the Petroleum and Natural Gas Industry, Part 1: Steel Skeleton-Reinforced Polyethylene Composite Pipe," and CJ/T 189-2007, "Wire Mesh Skeleton Plastic (Polyethylene) Composite Pipes and Fittings." Design, construction, and acceptance standards include SY/T 6769.2-2018, "Non-metallic Pipeline Design, Construction, and Acceptance Specifications, Part 2: Steel Skeleton-Reinforced Polyethylene Composite Pipe," and SY/T 6770.2-2018, "Non-metallic Pipe Quality Acceptance Specifications, Part 2: Steel Skeleton-Reinforced Polyethylene Composite Pipe." According to Q/SY 06034-2021, "Guidelines for the Application of Non-metallic Pipes in Oil and Gas Fields," steel skeleton-reinforced polyethylene composite pipes are primarily used for oil gathering and water supply in surface pipelines. These pipes are available in diameters up to 600 mm, with a pressure rating of 4.0 MPa and a temperature limit of 65°C. As the pipe diameter increases, the pressure-bearing capacity of non-metallic composite pipes generally decreases. Connection methods for steel skeleton-reinforced polyethylene composite pipes typically include hot melt or electric fusion connections. After the pipeline connection is completed, a pressure test is conducted before the pipeline is put into service, with no additional non-destructive testing required.

Figure 1: Steel skeleton-reinforced polyethylene composite pipes with various reinforcement structures

1.3.3 High-Pressure Flexible Composite Pipe

High-pressure flexible composite pipes, used in the oil and gas industry, are made from polymer composite materials. These pipes are characterized by high-pressure resistance, corrosion resistance, flexibility, and long service life. A high-pressure flexible composite pipe comprises three layers: an inner transmission layer, a reinforcement layer, and an outer protective layer. The inner transmission layer is made of materials such as PE, PE-X, or PE-RT; the reinforcement layer is made of aramid filaments, polyester filaments, or similar materials; and the outer protective layer is made of PE. Figure 2 shows the structure of the high-pressure flexible composite pipe.

The standards regulating high-pressure flexible composite pipes are comprehensive. Manufacturing standards include SY/T 6662.2-2020, "Non-metallic Composite Pipes for the Petroleum and Natural Gas Industry, Part 2: Flexible Composite High-Pressure Transmission Pipes," and SY/T 6716-2008, "Flexible Composite High-Pressure Transmission Pipes for the Petroleum and Natural Gas Industry," among others. Relevant design, construction, and acceptance standards include SY/T 6769.5-2016, "Non-metallic Pipeline Design, Construction, and Acceptance Specifications, Part 5: Fiber-Reinforced Thermoplastic Composite Continuous Pipes."

According to Q/SY 06034-2021, high-pressure flexible composite pipes are used primarily for oil and gas gathering and transportation, water injection, and alcohol injection in surface systems within oil and gas fields. These pipes are available in diameters up to 150 mm, with a maximum pressure of 32 MPa for water and alcohol injection, 16 MPa for gas transmission and gathering, and an operating temperature of up to 65°C. Due to their flexibility, high-pressure flexible composite pipes can be supplied in coil form. For outer diameters below 150 mm, continuous pipes can reach lengths of up to 150 m. Connection methods for flexible composite pipes include mechanical connections, such as crimped threaded connections and flange connections. These connection methods do not require non-destructive testing and provide lower failure rates and higher efficiency compared to other pipe types.

Figure 2: Structure of high-pressure flexible composite pipe