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Introduction

Welded coiled tubing is a specialized form of tubular steel product widely used in oil and gas well interventions, drilling operations, chemical processing, and heat exchange systems. It is manufactured by forming steel strips into tubes and welding the edges together to create continuous, flexible tubing that can be coiled for easy transport and deployment. This article explores the manufacturing processes, material properties, applications, challenges, and recent technological advancements related to welded coiled tubing.

What is Welded Coiled Tubing?

Welded coiled tubing is produced by shaping long strips of steel into a tubular form and joining the edges by welding, typically using high-frequency induction welding. The resulting tube is seamless in its length, flexible enough to be coiled on reels, and strong enough to withstand high pressures and mechanical stresses encountered in demanding environments.

Manufacturing Process of Welded Coiled Tubing

  • Steel Strip Preparation: Steel sheets, often 40 to 48 inches wide, are slit into strips that correspond to the desired tube circumference. These strips are welded end-to-end to form continuous lengths.

  • Tube Forming: The flat steel strip is progressively bent into a U-shape and then into a full tube by a series of rollers.

  • Welding: The edges of the formed strip are joined by high-frequency induction welding (HFI), which heats the strip edges by electrical resistance and presses them together to form a strong longitudinal seam without filler material.

  • Finishing: Weld flash is removed, and the weld seam is annealed and heat-treated to relieve stresses. The tube is then sized to precise diameter and roundness tolerances.

  • Inspection: Non-destructive testing such as eddy-current inspection and X-ray ensures weld integrity and detects defects.

  • Heat Treatment: The entire tube undergoes induction heat treatment to improve ductility and strength.

  • Spooling: The finished coiled tubing is wound onto large reels for transport and use.

This continuous manufacturing process allows the production of coiled tubing strings that can be thousands of feet long, facilitating their use in extensive well operations without joints that could be weak points.

Material Properties and Welding Technology

High-Strength Welded Coiled Tubing

Recent advances have introduced high-strength coiled tubing grades with enhanced microstructures, such as tempered martensite, which improve fatigue life, hardness, abrasion resistance, and corrosion resistance. Unlike conventional tubing with heterogeneous microstructures at welds, high-strength tubing has uniform properties along its length, reducing stress concentrations and improving performance under cyclic loading.

  • Fatigue Life: High-strength tubing can have up to four times the fatigue life of conventional tubing due to uniform microstructure at welds.

  • Corrosion Resistance: The absence of bainite and presence of fine-grained martensite reduce microbiologically influenced corrosion.

  • Hardness and Abrasion Resistance: Hardness over 36 RC enhances durability in abrasive downhole conditions.

  • Strength: Yield strengths exceeding 140 ksi enable use in high-pressure applications with thinner walls, reducing weight.

Challenges include adapting gripping and shearing equipment to handle the harder tubing materials.

Welding Techniques and Repair

  • Longitudinal Seam Welding: The primary weld along the tube's length is made by high-frequency induction welding, producing a weld consistent with the parent material.

  • Butt Welding: For joining tubing sections or repairing damaged segments, tungsten inert gas (TIG) or metal inert gas (MIG) butt welding is used in the field. Proper welding procedures, including multi-layer welding and controlled heat input, are critical to maintain weld integrity and fatigue resistance.

  • Dual Bias Weld: An innovative weld design to join steel strips at an angle (typically 45°) improves resistance to repeated coiling stresses by distributing stress more evenly along the weld seam.

Field welding requires specialized fixtures and environmental controls to ensure quality under outdoor conditions.

Applications of Welded Coiled Tubing

Oil and Gas Industry

  • Well Intervention: Coiled tubing is used for wellbore cleaning, acidizing, perforating, logging, and fishing operations without pulling production tubing, enabling cost-effective maintenance and repair.

  • Hydraulic Fracturing: It facilitates high-pressure fluid injection for fracturing operations, improving hydrocarbon recovery.

  • Drilling: Coiled tubing drilling rigs allow for efficient drilling of shallow and deep wells, especially in complex well geometries.

  • Plug and Abandonment: Tubing is used to set plugs and perform abandonment operations safely and efficiently.

Chemical and Petrochemical Plants

  • Precise injection of chemicals and fluids under controlled conditions.

  • Maintenance tasks including pipeline cleaning and acidizing.

  • Handling corrosive fluids due to the tubing's corrosion-resistant properties.

Heat Exchange Systems

  • Continuous coiled tubing provides compact, efficient heat transfer solutions in HVAC, radiant floor heating, and industrial cooling towers.

  • The coiled design enhances flow rates and heat exchange efficiency compared to traditional straight tubes.

Water Treatment and Other Industries

  • Used in water distribution and treatment systems for continuous flow with minimal leakage risk.

  • The seamless design and flexibility make it suitable for confined spaces and remote locations.

Advantages of Welded Coiled Tubing

  • Flexibility: Can be coiled and uncoiled easily, allowing deployment in confined and complex environments.

  • Continuous Length: Reduces joints and potential leak points, enhancing safety and reliability.

  • Cost-Effectiveness: Faster deployment and reduced maintenance lower operational costs.

  • High Strength and Durability: Advanced materials and welding improve lifespan and performance.

  • Safety: Reduced risk of leaks and failures due to seamless construction and robust welds.

Challenges and Considerations

  • Fatigue Damage: Coiled tubing undergoes repeated bending and internal pressure cycling, requiring careful fatigue life management.

  • Material Compatibility: Exposure to sour gases (H2S, CO2), acids, and other corrosive fluids demands appropriate material selection and corrosion protection.

  • Welding Quality: Weld seams are critical points; poor welding or repair can lead to failures.

  • Equipment Adaptation: High-strength tubing requires updated gripping and pressure control equipment.

  • Operational Planning: Detailed job planning is essential to optimize tubing size, wall thickness, and handling equipment based on well conditions and fluid properties.

  • Development of self-assembling segmented coiled tubing combining flexibility and strength.

  • Improved welding methods such as friction stir welding to eliminate edge defects.

  • Enhanced monitoring and inspection technologies for real-time weld and tubing integrity assessment.

  • New alloy compositions and thermal processing to further improve corrosion and fatigue resistance.

Frequently Asked Questions

1. What distinguishes welded coiled tubing from conventional pipe?

Welded coiled tubing is manufactured from steel strips formed and welded into continuous tubes with a longitudinal seam, allowing it to be coiled on reels. Conventional pipe is typically rigid and jointed, making coiled tubing more flexible and suitable for downhole operations.

2. How is the weld seam in coiled tubing made and inspected?

The seam is formed by high-frequency induction welding, which heats the strip edges by electrical resistance and presses them together. The weld is then inspected using non-destructive methods such as X-ray and eddy-current testing to ensure integrity.

3. What are the benefits of high-strength welded coiled tubing?

High-strength tubing offers significantly improved fatigue life, hardness, abrasion resistance, and corrosion resistance, enabling longer service life and use in more demanding environments.

4. Can welded coiled tubing be repaired in the field?

Yes, damaged sections can be repaired or extended by butt welding using TIG or MIG processes, provided proper procedures and equipment are used to maintain weld quality and fatigue resistance.

5. What industries use welded coiled tubing besides oil and gas?

Besides oil and gas, welded coiled tubing is used in chemical processing, petrochemical plants, water treatment, HVAC systems, and heat exchangers due to its flexibility, strength, and corrosion resistance.

Article Summary

Welded coiled tubing, manufactured by forming and high-frequency induction welding of steel strips, is a versatile and essential component in oil and gas well interventions, drilling, chemical processing, and heat exchange systems. Advances in high-strength materials and welding technologies have enhanced its fatigue life, corrosion resistance, and operational safety. Its continuous, flexible design allows deployment in complex environments, reducing operational costs and improving efficiency. Proper welding, inspection, and planning are critical to maximizing performance and lifespan. Emerging innovations promise further improvements in durability and application scope.