Kenneth Brian Gomez

Pipeline Material Selection

Material Selection for Subsea Pipelines

Introduction

Pipeline material selection is a critical aspect of subsea pipeline engineering, as it directly impacts the pipeline’s performance, durability, and overall cost. The selection process involves evaluating various materials based on their mechanical properties, corrosion resistance, compatibility with the transported fluids, and environmental conditions. This write-up provides a detailed overview of the key considerations and materials used in subsea pipeline construction.

Key Considerations in Material Selection

  1. Mechanical Properties: The material must possess adequate strength, toughness, and ductility to withstand the operational loads, including internal pressure, external hydrostatic pressure, and mechanical stresses during installation and operation.
  2. Corrosion Resistance: Subsea pipelines are exposed to harsh marine environments, which can lead to corrosion. The material must resist corrosion from seawater, transported fluids, and any external protective coatings or cathodic protection systems.
  3. Compatibility with Transported Fluids: The material must be compatible with the fluids being transported, which may include hydrocarbons, water, and various chemicals. This includes resistance to sour service conditions (presence of hydrogen sulfide) and other corrosive agents.
  4. Environmental Conditions: The material must perform well under the specific environmental conditions of the subsea location, including temperature, pressure, and potential exposure to marine organisms.
  5. Fabrication and Installation: The material should be suitable for the chosen fabrication and installation methods, including welding, bending, and laying techniques. It should also be available in the required sizes and quantities.
  6. Cost: The material’s cost, including procurement, fabrication, installation, and maintenance, must be considered to ensure the project’s economic viability.

Common Materials Used in Subsea Pipelines

1. Carbon Steel

Properties:

  • High strength and toughness
  • Good weldability
  • Cost-effective

Applications:

  • Widely used for subsea pipelines due to its mechanical properties and cost-effectiveness.
  • Typically used with external coatings and cathodic protection systems to mitigate corrosion.

Limitations:

  • Susceptible to corrosion, especially in marine environments.
  • Requires additional protective measures, such as coatings and cathodic protection.

2. Corrosion-Resistant Alloys (CRAs)

Properties:

  • Excellent corrosion resistance
  • High strength and toughness
  • Suitable for sour service conditions

Applications:

  • Used in environments with high corrosion potential, such as sour gas fields.
  • Common CRAs include stainless steels (e.g., 316L, 22Cr, 25Cr) and nickel-based alloys (e.g., Inconel, Incoloy).

Limitations:

  • Higher cost compared to carbon steel.
  • More challenging to fabricate and weld.

3. Duplex and Super Duplex Stainless Steels

Properties:

  • High strength and toughness
  • Excellent corrosion resistance, particularly to pitting and crevice corrosion
  • Good weldability

Applications:

  • Used in environments with high chloride content and where high mechanical strength is required.
  • Common grades include 22Cr duplex and 25Cr super duplex.

Limitations:

  • Higher cost compared to carbon steel.
  • Requires careful control of welding and fabrication processes to avoid issues such as hydrogen embrittlement.

4. Clad and Lined Pipelines

Properties:

  • Combines the strength of carbon steel with the corrosion resistance of CRAs.
  • Clad pipelines have a metallurgically bonded CRA layer, while lined pipelines have a mechanically bonded CRA liner.

Applications:

  • Used in environments with high corrosion potential where the cost of solid CRA pipelines is prohibitive.
  • Commonly used in sour service conditions and deepwater applications.

Limitations:

  • More complex and costly fabrication processes.
  • Potential issues with bonding integrity and differential thermal expansion.

5. Thermoplastic Composite Pipes (TCPs)

Properties:

  • Lightweight and flexible
  • Excellent corrosion resistance
  • High strength and fatigue resistance

Applications:

  • Used in dynamic applications, such as risers and flowlines, where flexibility and fatigue resistance are critical.
  • Suitable for transporting hydrocarbons, water, and chemicals.

Limitations:

  • Higher initial cost compared to traditional materials.
  • Limited track record in some applications.

Protective Coatings and Linings

In addition to selecting the base material, protective coatings and linings are often applied to subsea pipelines to enhance their corrosion resistance and mechanical performance. Common protective measures include:

  1. External Coatings:
  • Fusion Bonded Epoxy (FBE): Provides excellent adhesion and corrosion resistance.
  • Polyethylene (PE) and Polypropylene (PP): Offers good mechanical protection and corrosion resistance.
  • Concrete Weight Coating: Provides mechanical protection and stability on the seabed.
  1. Internal Linings:
  • Epoxy Linings: Protects against internal corrosion and improves flow efficiency.
  • Cement Mortar Linings: Used in water pipelines to prevent corrosion and improve flow.
  1. Cathodic Protection:
  • Sacrificial Anodes: Made of materials such as zinc or aluminum, these anodes corrode preferentially, protecting the pipeline.
  • Impressed Current Cathodic Protection (ICCP): Uses an external power source to provide a continuous protective current to the pipeline.

Case Studies

Case Study 1: North Sea Pipeline

A subsea pipeline project in the North Sea required transporting sour gas from an offshore platform to an onshore processing facility. The pipeline material selection process involved evaluating the following options:

  1. Carbon Steel with External Coating and Cathodic Protection: While cost-effective, this option required extensive corrosion protection measures due to the sour service conditions.
  2. 22Cr Duplex Stainless Steel: Offered excellent corrosion resistance and mechanical properties but was significantly more expensive than carbon steel.
  3. Clad Pipeline (Carbon Steel with Inconel Cladding): Provided a balance between cost and corrosion resistance, with the carbon steel providing strength and the Inconel cladding offering protection against sour gas.

After a detailed cost-benefit analysis, the project team selected the clad pipeline option, as it provided the necessary corrosion resistance at a lower cost than solid CRA pipelines. The pipeline was successfully installed and has been operating without significant corrosion issues.

Case Study 2: Gulf of Mexico Deepwater Pipeline

A deepwater pipeline project in the Gulf of Mexico required transporting oil from a subsea production system to a floating production storage and offloading (FPSO) unit. The key considerations included:

  1. High Hydrostatic Pressure: The pipeline needed to withstand pressures at depths exceeding 2,000 meters.
  2. Corrosion Resistance: The pipeline material had to resist corrosion from seawater and the transported oil.
  3. Flexibility and Fatigue Resistance: The dynamic environment required a material that could withstand cyclic loading and movement.

The project team evaluated several materials, including carbon steel, super duplex stainless steel, and thermoplastic composite pipes (TCPs). After extensive analysis, TCPs were selected due to their lightweight, flexibility, and excellent corrosion resistance. The TCPs were successfully installed using a reel-lay method, and the pipeline has demonstrated excellent performance in the challenging deepwater environment.

Conclusion

Material selection for subsea pipelines is a complex process that requires careful consideration of mechanical properties, corrosion resistance, compatibility with transported fluids, environmental conditions, fabrication and installation methods, and cost. By evaluating these factors and selecting the appropriate materials and protective measures, engineers can ensure the long-term performance and reliability of subsea pipelines in various challenging environments. The case studies presented highlight the importance of a thorough material selection process and the successful application of different materials in real-world projects.