Basic Knowledge of Oil Pipes

## Basic Knowledge of Oil Pipes

### I. Basic Knowledge of Oil Pipes

**1. Definitions of Special Terms Related to Oil Pipes**

* **API:** An acronym for American Petroleum Institute.
* **OCTG:** An acronym for Oil Country Tubular Goods, referring to tubular goods specifically used in the oil industry, including finished oil casing, drill pipes, drill collars, couplings, pup joints, etc.
* **Tubing:** Pipes used in oil wells for oil and gas production, water injection, acidizing, and fracturing.
* **Casing:** Pipes lowered into a drilled wellbore to line the wellbore and prevent wellbore collapse.
* **Drill Pipe:** Pipes used for drilling wellbores.
* **Line Pipe:** Pipes used for transporting oil and gas.
* **Coupling:** A cylindrical component with internal threads used to connect two threaded pipes.
* **Coupling Stock:** Pipes used to manufacture couplings.
* **API Thread:** Pipe threads specified in the API 5B standard, including tubing round threads, casing short round threads, casing long round threads, casing buttress threads, and line pipe threads.
* **Premium Connection:** A non-API threaded connection type with special sealing, connection, or other performance characteristics.
* **Failure:** The phenomenon of losing original function due to deformation, fracture, or surface damage under specific service conditions. The main failure modes of oil casing include: crushing, slippage, cracking, leakage, corrosion, adhesion, and wear.

**2. Relevant Oil Industry Standards**

* **API 5CT:** Specification for Casing and Tubing
* **API 5D:** Specification for Drill Pipe
* **API 5L:** Specification for Line Pipe
* **API 5B:** Specification for Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads

**3. Conversion between Imperial and Metric Units**

* 1 inch (in) = 25.4 millimeters (mm)
* 1 foot (ft) = 0.3048 meters (m)
* 1 square inch (sp.in) = 645.16 square millimeters (mm2)
* 1 pound (lb) = 0.45359 kilograms (kg)
* 1 pound per foot (lb/ft) = 1.4882 kilograms per meter (kg/m)
* 1 pound per square inch (psi) = 6.895 kilopascals (kPa) = 0.006895 megapascals (MPa)
* 1 foot-pound (ft-lb) = 1.3558 joules (J)

**4. Oil Casing String Structure**

Refer to Figure 1 for a typical oil casing string structure.

**(Figure 1 is not provided in the original text)**

### II. Tubing

**1. Classification of Tubing**

Tubing is classified into plain end tubing (NU), external upset tubing (EU), and integral joint tubing.

* **Plain end tubing** refers to tubing where the pipe ends are directly threaded and coupled without thickening.
* **External upset tubing** refers to tubing where both ends are externally thickened before threading and coupling.
* **Integral joint tubing** refers to tubing where one end is internally thickened and externally threaded, and the other end is externally thickened and internally threaded, allowing for direct connection without a coupling.

**2. Functions of Tubing**

* **Oil and gas extraction:** After an oil and gas well is drilled and cemented, tubing is placed inside the production casing to extract oil and gas to the surface.
* **Water injection:** When the downhole pressure is insufficient, water is injected into the well through the tubing.
* **Steam injection:** During heavy oil thermal recovery, insulated tubing is used to inject steam downhole.
* **Acidizing and fracturing:** In the later stages of well production or to enhance oil and gas well productivity, acidizing and fracturing fluids or proppants need to be injected into the reservoir. These fluids and proppants are transported through the tubing.

**3. Tubing Specifications**

Commonly used tubing specifications are shown in Table 1.

**(Table 1 is provided in the original text)**

**4. Upset End Shape and Dimensions of External Upset Tubing**

The upset end shapes of commonly used external upset tubing are shown in Figure 2, and the upset end dimensions are shown in Table 2. All external upset tubing is externally thickened.

**(Figure 2 and Table 2 are provided in the original text)**

**5. Tubing Length**

The length range of tubing is shown in Table 3. Domestically, tubing is usually delivered with a fixed length within a certain range of R2, the secondary length. For example, 9.4-9.6m is a common delivery length for tubing. Excessive length differences are not conducive to downhole operations.

**(Table 3 is provided in the original text)**

**6. Tubing Steel Grades**

Tubing steel grades include: H40, J55, N80, L80, C90, T95, and P110.

* **N80** is divided into N80-1 and N80Q. Both have the same tensile properties, but differ in delivery condition and impact properties. N80-1 is delivered in the normalized condition or can be hot-rolled to replace normalizing when the finishing temperature is higher than the critical temperature Ar3 and after tension reducing. Impact energy and nondestructive testing are not required for N80-1. N80Q must be tempered (quenched and tempered) heat treated, and the impact energy should meet the requirements of API 5CT, and nondestructive testing should be conducted.
* **L80** is divided into L80-1, L80-9Cr, and L80-13Cr. They have the same mechanical properties and delivery conditions. The differences lie in their applications, production difficulty, and price. L80-1 is a common type, while L80-9Cr and L80-13Cr are both highly corrosion-resistant tubing, which are more difficult to produce, expensive, and typically used in highly corrosive wells.
* **C90** and **T95** are both divided into Type 1 and Type 2, namely C90-1, C90-2, T95-1, and T95-2.

**7. Grades, Chemical Compositions, and Delivery Conditions of Tubing and Coupling Stock**

The chemical composition requirements for tubing in the API 5CT standard are shown in Table 4. This is a wide-ranging guideline. For steel grades such as J55, N80, and P110, only sulfur and phosphorus content requirements are specified, while other major elements are determined by the manufacturer based on performance and application requirements. Therefore, each manufacturer has its own tubing grade. Table 5 lists the delivery conditions of tubing and couplings for each steel grade. Quenching and tempering is called tempering.

**(Table 4 and Table 5 are provided in the original text)**

**8. Mechanical Properties of Tubing and Couplings**

The mechanical properties of tubing and couplings are specified in Table 6.

**(Table 6 is provided in the original text)**

**9. Tubing Connection Types**

The connection types of plain end tubing, external upset tubing, and integral joint tubing are shown in Figure 3, Figure 4, and Figure 5, respectively.

**(Figures 3, 4, and 5 are not provided in the original text)**

**10. Color Coding of Casing and Tubing and Their Couplings**

Casing and tubing and their couplings should be spray-painted with color codes to distinguish different steel grades. The color code for casing and tubing is a color ring painted at a distance of ≥600mm from either end. The entire outer surface of the coupling is painted with a color, and then a color ring is painted. The color codes for casing and tubing and their couplings are shown in Table 7.

**(Table 7 is provided in the original text)**

### III. Casing

**1. Classification and Function of Casing**

Casing is a steel pipe that supports the wellbore of oil and gas wells. Each well uses several layers of casing depending on the drilling depth and geological conditions. After the casing is run into the well, it is cemented in place. Unlike tubing and drill pipes, casing cannot be reused and is a one-time consumable. Therefore, casing consumption accounts for more than 70% of all oil well pipes. Casing can be classified into: conductor casing, surface casing, intermediate casing, and production casing, as shown in Figure 1.

**(Figure 1 is not provided in the original text)**

* **Conductor Casing:** Mainly used in offshore and desert drilling to isolate seawater and sand, ensuring smooth drilling. The main specifications of this casing are: ∮762mm (30in) × 25.4mm and ∮762mm (30in) × 19.06mm.
* **Surface Casing:** Mainly used for the first drilling to penetrate the loose surface formation to the bedrock. To prevent the collapse of this formation, surface casing is used for sealing. The main specifications of surface casing are: 508mm (20in), 406.4mm (16in), 339.73mm (13-3/8in), 273.05mm (10-3/4in), 244.48mm (9-5/9in), etc. The setting depth depends on the depth of the loose formation, generally 80-1500m. It is subjected to low external and internal pressure, and K55 or N80 steel grades are commonly used.
* **Intermediate Casing:** Used during drilling in complex formations. When encountering complex sections such as collapsing formations, oil zones, gas zones, water zones, lost circulation zones, and salt formations, intermediate casing is required for sealing, otherwise drilling cannot proceed. Some wells have deep and complex formations, and the well depth can reach several kilometers. Such deep wells require several layers of intermediate casing, and their mechanical properties and sealing performance requirements are very high. In addition to K55, N80 and P110 steel grades are more commonly used, and some deep wells even use Q125 or even higher non-API steel grades such as V150. The main specifications of intermediate casing are: 339.73mm (13-3/8in), 273.05mm (10-3/4in), 244.48mm (9-5/8in), 219.08mm (8-5/8in), 193.68mm (7-5/8in), 177.8mm (7in), etc.
* **Production Casing:** When the well is drilled to the target zone (oil and gas bearing zone), production casing is used to seal the entire oil and gas zone and the upper exposed formation. The inside of the production casing is the reservoir. Production casing has the deepest setting depth among all types of casing, and its mechanical properties and sealing performance requirements are also the highest. Steel grades used include K55, N80, P110, Q125, V150, etc. The main specifications of production casing are: 177.8mm (7in), 168.28mm (6-5/8in), 139.7mm (5-1/2in), 127mm (5in), 114.3mm (4-1/2in), etc.

**2. Casing Specifications**

Commonly used casing specifications are shown in Table 8.

**(Table 8 is provided in the original text)**

**3. Casing Length**

The length range of casing is shown in Table 9. Although the API 5CT standard specifies three ranges, in practice, both domestic and international casing delivery lengths use the R3 range. The domestic market generally uses 10.5-11.0m, while the international market generally uses 42-44ft (12.77-13.38m).

**(Table 9 is provided in the original text)**

**4. Casing Steel Grades and Chemical Compositions**

Casing steel grades include: H40, J55, K55, M65, N80, L80, C90, C95, T95, P110, Q125, and V150.

* H40 is the lowest steel grade and is rarely used in the market.
* J55 and K55 have the same level of yield strength, but K55 has a higher tensile strength than J55.
* N80 is divided into N80-1 and N80Q.
* L80 is divided into L80-1, L80-9Cr, and L80-13Cr.
* C90 is divided into C90-1 and C90-2.
* Q125 is divided into Q125-1, Q125-2, Q125-3, and Q125-4.
* V150 is a non-API high-strength steel grade mainly used in deep and ultra-deep wells.

The chemical composition requirements for casing in the API standard are shown in Table 4. Similar to tubing, the API standard for casing chemical composition is also a wide-ranging guideline, and each manufacturer has its own dedicated casing grade.

**(Table 4 is provided in the original text)**

**5. Casing Connection Types**

The connection types of various types of casing are shown in Figures 6, 7, 8, and 9.

**(Figures 6, 7, 8, and 9 are not provided in the original text)**

### IV. Drill Pipe

**1. Oil Well Drill String Structure**

The oil well drill string structure is shown in Figure 10.

**(Figure 10 is not provided in the original text)**

**2. Classification and Function of Drill String Components**

The drill string, consisting of the kelly, drill pipes, heavy weight drill pipes, and drill collars, is the core drilling tool that drives the drill bit from the surface to the bottom of the well. It is also a channel connecting the surface to the bottom of the well. It has three main functions:

* Transmitting torque to drive the drill bit for drilling.
* Applying weight to the drill bit to crush the rock at the bottom of the well.
* Transporting drilling fluid. The drilling mud is pumped from the surface by a high-pressure mud pump, enters the drill string through the internal bore, flows to the bottom of the well to flush rock cuttings and cool the drill bit, and carries the cuttings back to the surface through the annulus between the outer surface of the drill string and the wellbore, achieving the purpose of drilling.

The drill string is subjected to various complex alternating loads during drilling, such as tension, compression, torsion, and bending stresses. The inner surface is also subjected to the erosion and corrosion of high-pressure mud.

* **Kelly:** The kelly is a square or hexagonal pipe, with one square kelly typically used per drill string in China. Its specifications include: 63.5mm (2-1/2in), 88.9mm (3-1/2in), 107.95mm (4-1/4in), 133.35mm (5-1/4in), 152.4mm (6in), etc. The commonly used length is 12-14.5m.
* **Drill Pipe:** Drill pipes are the main drilling tools connected to the lower end of the kelly. As the drilling depth increases, drill pipes are added one by one to lengthen the drill string. Drill pipe specifications include: 60.3mm (2-3/8in), 73.03mm (2-7/8in), 88.9mm (3-1/2in), 114.3mm (4-1/2in), 127mm (5in), 139.7mm (5-1/2in), etc.
* **Heavy Weight Drill Pipe:** Heavy weight drill pipes are transition tools connecting drill pipes and drill collars, which can improve the stress distribution of drill pipes and increase the weight on bit. The main specifications of heavy weight drill pipes are: 88.9mm (3-1/2in) and 127mm (5in), etc.
* **Drill Collar:** Drill collars are connected to the lower part of the drill string. They are extra-thick-walled pipes with high rigidity, providing weight to the drill bit for rock crushing and acting as a guide during vertical drilling. Commonly used drill collar specifications include: 158.75mm (6-1/4in), 177.85mm (7in), 203.2mm (8in), 228.6mm (9in), etc.

**3. Drill Pipe Dimensions**

The dimensions of commonly used drill pipes are shown in Table 10.

**(Table 10 is provided in the original text)**

**4. Drill Pipe Upset End Shape and Dimensions**

The upset end shapes of commonly used drill pipes are shown in Figure 11, and the upset end dimensions are shown in Table 11.

**(Figure 11 and Table 11 are provided in the original text)**

**5. Drill Pipe Steel Grades, Designations, and Delivery Conditions**

The steel grades, designations, and delivery conditions of drill pipes are shown in Table 12. The API 5D standard only specifies the phosphorus and sulfur content for drill pipe chemical composition. Other elements are selected by the manufacturer as long as they meet the performance and application requirements. Domestic and foreign drill pipe manufacturers all have their own dedicated designations.

**(Table 12 is provided in the original text)**

**6. Drill Pipe Mechanical Properties**

Drill pipe mechanical properties are shown in Table 13.

**(Table 13 is provided in the original text)**

### V. Line Pipe

**1. Line Pipe Classification**

Line pipe is the general term for steel pipes used to transport crude oil, refined oil, natural gas, and water in the oil and natural gas industry. Pipelines for transporting oil and gas are mainly divided into three types: trunk pipelines, branch pipelines, and urban pipeline networks. Trunk pipelines typically have specifications of ∮406-1219mm, wall thickness of 10-25mm, and steel grades of X42-X80. Branch pipelines and urban pipeline networks typically have specifications of ∮114-700mm, wall thickness of 6-20mm, and steel grades of X42-X80.

Line pipes can be welded steel pipes or seamless steel pipes, with welded steel pipes being more commonly used than seamless steel pipes.

**2. Line Pipe Standard**

The standard for line pipe is API 5L “Specification for Line Pipe.” However, China issued two national standards for line pipe in 1997: GB/T 9711.1-1997 “Technical Delivery Conditions for Steel Pipes for Pipeline Transportation Systems in the Petroleum and Natural Gas Industries – Part 1: Grade A Pipes” and GB/T 9711.2-1997 “Technical Delivery Conditions for Steel Pipes for Pipeline Transportation Systems in the Petroleum and Natural Gas Industries – Part 2: Grade B Pipes.” These two standards are equivalent to API 5L. Many domestic users require supply according to these two national standards.

**3. PSL1 and PSL2**

PSL is the abbreviation for Product Specification Level. Line pipes have two product specification levels: PSL1 and PSL2, which can also be referred to as quality grades PSL1 and PSL2. PSL1 is higher than PSL2. These two specification levels differ not only in inspection requirements, but also in chemical composition and mechanical property requirements. Therefore, when ordering according to API 5L, in addition to specifying common indicators such as specifications and steel grades, the contract must also specify the product specification level, i.e., PSL1 or PSL2.

PSL2 is stricter than PSL1 in terms of chemical composition, tensile properties, impact energy, and nondestructive testing requirements.

**4. Line Pipe Steel Grades and Chemical Compositions**

Line pipe steel grades are divided into: A25, A, B, X42, X46, X52, X60, X65, X70, and X80, from low to high.

The chemical compositions of line pipes are shown in Table 14 and Table 15.

**(Table 14 and Table 15 are provided in the original text)**

**5. Line Pipe Tensile Properties**

The tensile properties of line pipes are shown in Table 16 and Table 17.

**(Table 16 and Table 17 are provided in the original text)**

**6. Line Pipe Impact Properties**

* PSL1 does not require impact testing.
* For all steel grades except X80 in PSL2, the average Charpy V-notch impact energy at 0°C for full-size specimens is: ≥41J in the longitudinal direction and ≥27J in the transverse direction.
* For X80 steel grade, the average Charpy V-notch impact energy at 0°C for full-size specimens is: ≥101J in the longitudinal direction and ≥68J in the transverse direction.

**7. Line Pipe Hydrostatic Testing and Nondestructive Testing Requirements**

Each line pipe should be hydrostatically tested, and the standard does not allow nondestructive testing to replace hydrostatic testing. This is also a significant difference between the API standard and the Chinese standard.

PSL1 does not require nondestructive testing, while PSL2 requires each pipe to be nondestructively tested.

### VI. Oil Pipe Threads

**1. Types of Oil Pipe Threads**

Commonly used oil pipe threads include the following types:

* API Threads
* Line pipe threads
* Tubing round threads
* Casing short round threads
* Casing long round threads
* Casing buttress threads
* Premium threads

**2. Oil Pipe Thread Gauges**

Gauges are inspection tools for threads. Different thread types and sizes require different gauges. Therefore, thread processing plants need to be equipped with a large number of gauges. Gauges are consumables and should be scrapped when they fail inspection by a higher-level gauge.

Gauges have different classes. Thread processing plants should generally be equipped with working gauges and inspection gauges. Working gauges are used to inspect pipe threads, while inspection gauges are used to inspect working gauges.

Each gauge is further divided into plug gauges and ring gauges. Plug gauges are used to inspect internal threads of couplings, while ring gauges are used to inspect external threads of pipe bodies.

**3. Oil Pipe Thread Profile**

Oil pipe thread profiles are shown in Figure 12.

**(Figure 12 is not provided in the original text)**

**4. Introduction to Premium Connections**

Premium connections are pipe threads with special structures that differ from API threads. Although API threaded connections are widely used in oil well production, their shortcomings are evident in certain oilfield environments:

* API round threaded connections, although they have good sealing performance, the tensile strength of the threaded portion is only 60%-80% of the pipe body strength, so they cannot be used in deep well production.
* API buttress threaded connections, although they have much higher tensile strength than API round threaded connections, their sealing performance is not very good, so they cannot be used in high-pressure gas well production.
* In addition, thread compounds can generally only function in environments below 95°C, so they cannot be used in high-temperature well production.

Compared to API round threaded and buttress threaded connections, premium connections have made breakthroughs in the following aspects:

* Good sealing performance: Through the design of elastic and metal-to-metal sealing structures, the gas-tight resistance of the connection can reach the yield pressure within the pipe body limit.
* High connection strength: Oil casing connected with premium connections has a connection strength that reaches or exceeds the pipe body strength, fundamentally solving the problem of slippage.
* Through material selection and surface treatment process improvements, the problem of thread galling has been basically solved.
* Through structural optimization, the stress distribution at the connection is more reasonable and more conducive to resisting stress corrosion cracking.
* Through the reasonable design of the shoulder structure, make-up operation is easier.

Currently, more than 100 patented premium connections have been developed worldwide, but only about 10 are widely used. Foreign premium connections already used in China mainly include: VAM connections from Vallourec (France), BDS connections from Mannesmann (Germany), NK3SB connections from NKK (Japan), NSCC connections from Nippon Steel (Japan), FOX connections from Kawasaki (Japan), TM connections from Sumitomo (Japan), SEC connections from Tenaris (Argentina), etc. These connection types are also commonly used premium connections worldwide.