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Process Study on A694 F65 Pipe Fitting for Underwater Production Facilities

The underwater production system is an essential facility for developing and transporting offshore oil and gas, especially in deepwater offshore oil and gas field development projects. The underwater pipeline terminals, manifolds, and subsea pipelines are connected, and the pipe fittings used mainly include tees, elbows, reducers, etc. The material is ASTM A694 F65. These pipe fittings are mostly made of internally welded nickel-based alloy 625 material to improve corrosion resistance. This article studies the production process of A694 F65 pipe fittings in the underwater production system of a certain deepwater oil and gas field. The seamless forged pipe is first processed into a tee using a mold combined with mud-pressing technology. Secondly, the seamless forged pipe is processed into an elbow and reducer using a mold with hot pressing technology. Focus on the production process’s heat treatment and forming processes and conduct mechanical performance testing to ensure that the performance results meet relevant specifications.

1. Introduction

With the advancement of underwater oil and gas development towards deep water, underwater production facilities face increasingly severe underwater operating environments. In addition, the complexity of oil and gas composition has led to higher requirements for underwater production system components’ strength and corrosion resistance. The underwater manifold mainly consists of pipelines, valves, control systems, flow meters, and other facilities. Due to the spatial distribution and layout requirements of valves, process pipelines, connectors, etc., it is necessary to use fittings such as tees, elbows, and reducers to reasonably plan the design positions of the underwater manifold and pipe ends.

Through exploration and research on the production process of A694 F65 pipe fittings used in the underwater pipe end (PLET) of a deepwater oil and gas field project in the South China Sea, the processing technology, physical and chemical performance results, quality control requirements, and other aspects of the pipe fittings were analyzed, accumulating valuable experience for subsequent projects. According to the forming method, pipe fittings have two main types. The first type is processed into elbows and reducers through mold combined with a hot-pressing process, and the other type is processed into tees through mold combined with a mud-pressing process. This article elaborates on the two processing processes: strength calculation, heat treatment process, physical and chemical properties, and surfacing process.

2. Introduction to PLET/PLEM

The underwater manifold (PLEM) and pipe end (PLET) are important components of the underwater production system, equipment used to receive, collect, and control well flow from different underwater production wellheads. They are “essential” for developing deep-sea oil and gas fields. As a typical deepwater underwater production facility, the design of underwater manifolds needs to consider environmental, installation, inspection, maintenance, production, and other requirements to ensure safe, reliable, and stable operation during their lifespan. Due to the high maintenance cost of underwater manifolds and pipe ends, the production and manufacturing of related components must strictly follow the design and technical requirements for quality control.

The process pipeline fittings used in facilities such as underwater manifolds and pipe ends are mainly made of ASTM A694 F65 forged materials. Forging pipes are processed into pipe fittings using a forming process, and internal overlay welding of CRA materials can be used to improve corrosion resistance.

3. ASTM A694 F65 material

ASTM A694 material is carbon steel and alloy steel forgings used for high-pressure transmission pipe flanges, fittings, valves, and parts. It is mainly used for flanges in high-pressure pipelines such as marine, oil, and natural gas and is generally used for transporting toxic gases containing hydrogen sulfide (H2S). The chemical elements and mechanical properties of ASTM A694 F65 material refer to Tables 1 and 2:

Table.1 Main Chemical Element Content of ASTM A694 F65 Material

Carbon Equivalent = C + Mn/6 + (Cr+Mo+V)/5 + Cu+Ni)/15;

Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V10 + 5B

Carbon 0.10 – 0.14%
Silicon 0.15 – 0.25%
Manganese 1.25 – 1.40%
Phosphorous <0.025%
Sulphur <0.003%
Chromium 0.15%
Molybdenum 0.15 – 0.20%
Copper <0.20%
Aluminium 0.015 – 0.025%
Titanium <0.05%
Vanadium 0.05 – 0.07%
Boron <0.0005%
Nitrogen <0.015%
Carbon Equivalent <0.43%
Pcm <0.24

In the formula:

Al: N should be greater than 2, and the carbon equivalent should be less than 0.41%.

When the precious alloy elements added to the forging during the steel ingot smelting process are certain, selecting an appropriate proportion of aluminum and nitrogen elements to add to the steel ingot can significantly refine the material grain, make the grain more uniform, and strengthen the comprehensive mechanical properties of the material. Therefore, the chemical composition of forgings needs to focus on the Al: N ratio and grain size inspection.

Table.2 Mechanical Properties of ASTM A694 F65 Materials

Tensile Strength Mpa Yield (0.2%) Mpa Elongation % Reduction of Area % Charpy J Hardness HB
>530 (>77KSi) >450 (65KSi) >20 >45 >45 152 – 235HB

The base pipes for producing and processing tees and bends are forged pipes. Their production processes include forging drawing design, steel ingot inspection, forging, heat treatment, rough turning, precision turning, dimensional inspection, non-destructive testing, etc. Focus on dimensional inspection and non-destructive testing after precision turning, which significantly impacts subsequent processes.

4. Production process of tees

The tee production process mainly includes welding material retesting, forged pipe size retesting, mold + mud pressing, quenching and tempering heat treatment, surfacing welding, non-destructive testing, dimensional inspection, physical and chemical testing, etc.

4.1 Raw material re-inspection

The raw materials include forged and welding materials, and forged pipes are mainly subjected to dimensional inspection, such as wall thickness, length, etc.; Welding materials are mainly subjected to chemical composition and mechanical property retesting. Forged pipes should be inspected individually, and welding materials should be retested according to the batch number. It can only be used for subsequent production and manufacturing after passing the re-inspection.

Considering the thinning of the branch pipe wall during the processing of the tee, the wall thickness of the forged pipe is slightly higher than that of the finished tee branch pipe.

4.2 Mud pressing process, tempering, and heat treatment

The mud-pressing process of the tee is a forming process that expands the branch pipe through the axial compensation of metal materials. The process involves the forging of the pipe, which is reportedly cut and cut, followed by normalizing heat treatment before forming to reduce the hardness of the forged pipe. Fill the soil into the pipe blank and compact and level it. Using a dedicated hydraulic press and a dedicated compression mold, and through the movement of the hydraulic press slider, the volume of soil in the forged pipe decreases after being compressed. The pressure of the soil increases as the volume decreases. When the pressure required for the expansion of the tee branch pipe is reached, the metal material flows along the inner cavity of the mold under the pressure of the soil in the forged pipe, thereby expanding the branch pipe.

After the hydraulic press slider reaches the predetermined stroke, the mud-pressing process of the tee is completed. Open the outer mold, exit the inner mold, and clean the internal soil and other media. The specific number of times mud pressing molding needs to be carried out according to the process, but each time before mud pressing molding, heat treatment should be carried out. In the formula, the tee formed by mud pressing is shown in Figure 1.

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Figure.1 Tee mud pressure forming diagram

Conduct a preliminary dimensional inspection on the tee after mud pressing, focusing on checking whether the length of the straight pipe meets the requirements. After passing the inspection, mechanical cutting and tempering heat treatment shall be carried out. Quenching and tempering heat treatment include quenching and tempering, focusing on key parameters such as heating rate, holding temperature, and holding time.

During the production process, take a universal three to conduct mechanical performance destructive tests. The test tee should undergo processes such as normalizing heat treatment and tempering heat treatment before mud pressing simultaneously with the product tee. Referring to DNVGL-ST-F101, the sampling diagram for the mechanical performance test of the tee is shown in Figure 2.

5. Production process of elbows and reducers

The elbows and reducers’ production processes are similar, using specialized molds and hot-pressing methods. The production process mainly includes welding material re-inspection, forged pipe size re-inspection, special mold + hot pressing, quenching and tempering heat treatment, surfacing welding, non-destructive testing, dimensional inspection, physical and chemical testing, etc., with a focus on special mold hot pressing and heat treatment processes.

5.1 Hot pressing process of specialized molds

Hot pressing refers to heating raw materials (seamless or forged pipes) to a certain temperature and using special molds and fixtures for hot pressing forming. This type of process is suitable for producing seamless elbows, reducers, etc., but if there are many specifications, specialized molds and fixtures must be customized for different specifications.

After the raw materials of the forged pipe pass the re-inspection, they are cut according to the cutting diagram. Due to the possibility of removing the product steel seal information at the end of the forged pipe during the bending process, the product information must be transferred to the forged pipe body for subsequent traceability.

After the cutting is completed, heat it to the forming temperature and place it in a special hot-pressing mold to ensure that the center of the forged tube is consistent with the center of the mold. After forming, open the mold and take out products such as elbows and reducers, as shown in Figure 3.

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Figure.2 Sampling Diagram for Mechanical Performance Test of Tees

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Figure.3 Hot pressing forming a diagram of elbow

5.2 Quenching and tempering heat treatment

Conduct preliminary dimensional inspection on products such as formed elbows and reducers. Elbows are marked and inspected on the platform to determine the forming shape of the bend and determine the machining line and end trimming line; In addition, a round ball method is used for diameter inspection to verify whether the parameters such as the ovality and inner diameter of the elbow meet the technical requirements.

Preliminary inspection of the inner diameter and outer diameter size data at both ends of the reducer to determine whether a preliminary correction is necessary; Then, the length of the reducing pipe is inspected, and based on the technical drawings, the cutting line at both ends of the reducing pipe is determined by considering the machining allowance of the groove.

After the initial inspection of the elbow and reducer is qualified, mechanical cutting and tempering heat treatment shall be carried out. Quenching and tempering heat treatment include quenching and tempering, focusing on key parameters such as heating rate, holding temperature, and holding time. After quenching and tempering heat treatment, take an elbow and a reducer for mechanical performance destructive testing. Referring to DNVGL-ST-F101, the sampling diagrams for mechanical performance testing of elbows and reducers are shown in Figures 4 and 5.

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Figure.4 Sampling diagram for mechanical performance test of elbows

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Figure.5 Sampling diagram for mechanical performance test of reducer

  • Normalising – Heat to 890 – 960°C for a time commensurate with ruling section, Air cool (If Required).
  • Hardening – Heat to 890 – 960°C for a time commensurate with ruling section and quench in Water.
  • Tempering – Re-heat to 540-650°C. Hold for a time commensurate with the ruling section and cool in still in air.

6. Machining, surfacing, and product inspection

Based on the product size drawing and marking process card, clarify the size parameters such as branch center, main center, branch height, and product length, calculate the cutting size and groove processing allowance, and perform cutting and sizing mechanical processing.

After the preliminary sizing processing, sandblasting and rust removal are carried out, and subsequent internal welding work is carried out. The welding equipment is vertical. Due to the irregular shape of the tee, the focus is on the fusion between the weld beads for surfacing.

After the completion of the surfacing welding, subsequent processes, such as groove processing, sizing processing, dimensional inspection, non-destructive testing, pickling, labeling, etc., will be carried out.

7. Product mechanical performance test

The offshore oil and gas production system has very high requirements for the reliability of underwater forging products. It has high requirements for toughness indicators such as low-temperature impact and upper limits for yield and tensile strength values.

Using the 8-inch elbow product as an example, the experimental data analysis is as follows through mechanical performance testing.

7.1 Tensile properties

Tensile testing can determine a material’s strength and plastic indicators. Strength usually refers to the ability of a material to resist elastic deformation, plastic deformation, and fracture under external forces. The tensile performance values of the pipe bend are shown in Table 3:

Table.3 Tensile properties of pipe bends

Product Tensile Strength Yield Strength Elongation
Raw material 608 Mpa 503 Mpa 30%
Elbow 625 Mpa 510 Mpa 29%

By comparing and analyzing the performance data of raw materials (forged pipes) and elbow products, the tensile strength, yield strength, and elongation can meet the requirements through the hot-pressing process and re-tempering heat treatment.

7.2 Impact performance

Impact performance is used to evaluate the impact resistance of materials or determine the degree of brittleness and toughness of materials, also known as impact toughness. The impact performance values of the bend are shown in Table 4, which meet the standard requirements.

Table.4 Impact performance of pipe bends

Product Impact energy -1 Impact energy -2 Impact energy -3
Raw material 215J 258J 257J
Elbow 310J 307J 296J

7.3 Grain size

The grain size of the raw material forged pipe is 8.0, and the grain size of the elbow product after hot pressing and re-tempering heat treatment is 10.5.

Grain size reflects the grain size obtained during actual heat treatment or hot working of steel parts and directly affects the structure and properties of products obtained after cooling. Using appropriate heat treatment parameters, the grain size of the product can be refined.

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Figure.6 Metallographic photo of the overlay welding layer on the elbow

7.4 Pitting detection data

After welding the inner wall of the elbow is completed, FeCl3 pitting corrosion testing is carried out, mainly to test the product’s pitting corrosion resistance. The testing method refers to ASTM G48 Method A.

The number of samples is 3. Using a water bath, the temperature of the FeCl3 test solution is 50 , and the soaking period is 72 hours. The calculation formula for the corrosion rate is as follows:

Corrosion rate (g/m2) = W/A

In the formula:

  • A – Total area of the sample (m2);
  • W – Weight loss (g).

Through calculation, the pitting corrosion detection data of the welded elbow product is shown in Table 5:

Table.5 Pitting Detection Data

Product Corrosion rate (g/m2) Average value (g/m2)
Sample 1 0.0741 0.0617
Sample 2 0.0741
Sample 3 0.037

According to the pitting corrosion detection data analysis, the Inconel 625 surfacing layer has excellent corrosion resistance (the test qualification standard is 4g/m2).

8. Conclusion

Pipe fittings such as tees, elbows, and reducers are widely used in underwater production facilities, playing a role in changing the direction, diverting, and reducing the diameter of oil and gas transmission. The changes in oil and gas flow rate and speed often cause stress concentration, fatigue, and other impacts on the pipe fitting, which have strict strength and corrosion resistance requirements. Tees, elbows, and reducers formed using A694 F65 forged pipes as raw materials through mud-pressing and hot-pressing processes can have excellent mechanical properties after heat treatment and tempering. After welding the Inconel 625 layer on the inner wall, it can have good corrosion resistance.

Author: Cai Changsheng

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (



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