Material Failure Investigation of High-Pressure Impulse Tubing

Why It Matters

Mercury‑driven LME poses a hidden reliability risk for offshore production, prompting operators to reassess material choices and inspection strategies to avoid costly downtime and environmental incidents.

Key Takeaways

• Mercury caused liquid metal embrittlement in Tungum alloy
• Failure occurred in ½‑inch high‑pressure impulse line
• SEM, EDX, XRD identified intergranular cracking
• Recommend mercury‑resistant alloys like Hastelloy C‑276
• Implement regular integrity inspections on legacy tubing

Pulse Analysis

Offshore oil and gas facilities rely on small‑bore tubing to transport high‑pressure fluids, yet the presence of trace mercury can trigger liquid metal embrittlement—a failure mode that traditional copper‑based alloys, such as Tungum, are ill‑equipped to resist. LME accelerates crack propagation at the grain boundaries, often without visible surface damage, making early detection challenging. As production systems age, legacy materials remain in service longer, increasing exposure to aggressive contaminants and amplifying the risk of sudden leaks that can jeopardize safety and environmental compliance.

The North Sea incident illustrates how a systematic forensic approach can pinpoint the root cause of a catastrophic tubing failure. Investigators combined visual examinations with advanced techniques—scanning electron microscopy, energy‑dispersive X‑ray spectroscopy, metallography, hardness profiling, and X‑ray diffraction—to map the microstructural damage and confirm mercury‑induced intergranular cracking. By applying the five‑whys methodology, the team traced the failure back to inadequate material selection for a mercury‑laden environment, highlighting the limitations of legacy copper alloys under modern operational stresses.

For operators, the findings underscore the urgency of revising material specifications and inspection protocols. Substituting copper‑based tubing with proven mercury‑resistant alloys such as Hastelloy C‑276 or Inconel 625 can dramatically reduce LME susceptibility. Moreover, integrating regular non‑destructive evaluation—ultrasonic testing, eddy‑current scans, and periodic SEM sampling—into integrity management programs ensures early detection of embrittlement signs. Proactive material upgrades and vigilant monitoring not only safeguard production continuity but also align with evolving regulatory expectations for offshore safety and environmental stewardship.

Material Failure Investigation of High-Pressure Impulse Tubing

By Sohail Saeed, Principal Integrity Engineer at Oceaneering · Published in the November/December 2025 issue of Inspectioneering Journal

Introduction

Material selection for small‑bore tubing in offshore oil and gas production systems is critical, particularly when production fluids contain aggressive elements such as mercury. Mercury‑induced degradation mechanisms, including liquid metal embrittlement (LME), have historically led to sudden and unexpected failures in copper‑based alloys, especially in aging offshore facilities where legacy materials are still in service.

A hydrocarbon gas leak was detected on a North Sea offshore production platform and traced to a failing Tungum alloy ½‑inch high‑pressure (HP) impulse line connected to a flow transmitter. This article details the failure investigation, identifies the line’s failure mechanism, and offers recommendations on material selection for similar tubing in environments where mercury is present. It also outlines the methodologies employed, including visual inspection, scanning electron microscopy (SEM), energy‑dispersive X‑ray (EDX) analysis, metallography, hardness testing, and X‑ray diffraction (XRD) analysis.

The incident was subjected to a formal root‑cause analysis and an incident investigation, which included the following steps:

1. Gathering data

2. Forming the investigation team

3. Developing the sequence of events

4. Identifying protective systems

5. Determining root causes (five whys, why tree)

6. Verifying potential causes

7. Developing recommendations

8. Documenting the investigation

9. Reviewing and issuing the report

10. Categorizing the root causes and the incident

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About the Author

Sohail Saeed is a Principal Integrity Engineer at Oceaneering with over 25 years of experience in asset integrity management, materials engineering, and corrosion control across upstream oil and gas operations. He has delivered customized lifecycle integrity solutions for major operators including BP, Chevron, Qatargas, and Vermilion Energy. Sohail holds an MSc in Corrosion Control Engineering.