Converting an existing pipeline designed under ASME B31.4 to transport multiphase commodities, such as natural gas and crude oil, involves several critical considerations. Here's a list of key factors to address:

1. Review Applicable Codes and Standards

• While ASME B31.4 focuses on liquid pipelines, and ASME B31.8 addresses gas pipelines, neither explicitly covers multiphase flow. Ensure both standards are reviewed in detail to identify overlaps or gaps.
• Consider whether API RP 14E, DNVGL-ST-F101, or other standards dealing with multiphase flow in pipelines may provide additional guidance.

2. Multiphase Flow Dynamics

• Multiphase flow introduces unique challenges, such as slugging, erosion, and uneven pressure distributions.
• Conduct a flow assurance study to understand the behavior of the gas and liquid phases under operating conditions.
• Account for the risk of hydrate formation, wax deposition, and corrosion due to the coexistence of gas, oil, and water.

3. Pipeline Design and Integrity

• Operating Pressure and Temperature: Verify the existing pipeline’s capacity to handle the pressure and temperature conditions of the multiphase flow.
• Internal Corrosion: The presence of water and the multiphase nature increase the risk of corrosion. Implement a corrosion management plan, including material selection, inhibitors, and monitoring systems.
• Erosion Risks: High velocities in multiphase flow can cause erosion of internal surfaces. Evaluate the erosional velocity limits (e.g., using the API RP 14E erosional velocity equation).

4. Flow Stability

• Analyze the potential for slug flow, which can cause significant fluctuations in pressure and flow rate, leading to mechanical stresses and operational challenges.
• Consider using slug catchers or separators if slugging is predicted to be significant.

5. Pipeline Modifications

• Assess whether additional facilities are required, such as:Slug catchers to manage flow instabilities. Pressure regulation equipment to maintain safe operating conditions. Pigging facilities for cleaning and flow assurance.
• Evaluate if the pipeline wall thickness, fittings, and supports are adequate for the new operating conditions.

6. Hydraulic and Mechanical Analysis

• Perform a detailed hydraulic analysis of the multiphase flow.
• Evaluate the mechanical integrity of the pipeline, considering fatigue and vibration due to fluctuating loads from multiphase flow.

7. Pipeline Materials

• Confirm that the materials are suitable for the new service, especially concerning chemical compatibility with transported commodities and corrosion resistance.
• Address potential issues related to sulfide stress cracking (SSC) or hydrogen-induced cracking (HIC) if sour gas or corrosive components are present.

8. Safety and Risk Management

• Conduct a risk assessment (e.g., HAZOP or FMECA) for the new operation to identify hazards associated with multiphase transport.
• Update emergency response plans to reflect the new operating conditions and risks.

9. Regulatory Compliance

• Ensure the conversion complies with local, regional, and international regulations beyond ASME standards.
• Document the pipeline's fitness-for-service through detailed assessments in accordance with applicable standards, such as API 579/ASME FFS-1.

10. Operational Considerations

• Monitoring Systems: Install real-time monitoring for pressure, temperature, and flow rates to detect issues promptly.
• Training: Train personnel in the operational differences and challenges posed by multiphase pipelines.
• Maintenance: Update maintenance schedules to reflect the increased complexity of multiphase transport.

Summary

Converting a pipeline to multiphase service involves a holistic assessment of flow dynamics, mechanical integrity, material compatibility, and operational risks. A detailed engineering analysis and possibly modifications to the pipeline and auxiliary systems will be necessary to ensure safety, reliability, and compliance. Engaging experts in flow assurance, corrosion, and pipeline design is recommended for this complex transition.