Wall Thickness Tolerance Analysis for Internally Overlay Welded INCONEL 625 Pipes

Literature Overview

The paper published in 2019 in the journal Petroleum and Chemical Equipment (石油和化工设备), authored by Zhang Weiwei, Li Jie, Fang Xin, and Yao Kang from Offshore Oil Engineering Co., Ltd., addresses a critical engineering challenge in offshore oil and gas pipeline construction: the wall thickness tolerance problem arising from internal overlay welding of INCONEL 625 on carbon steel or low-alloy steel pipes. This topic is of significant importance in the context of subsea flowlines and subsea production systems, where corrosion-resistant alloy (CRA) overlay welding is widely employed to protect the internal bore of carbon steel pipes from sour gas environments containing H2S and CO2.

The authors adopt a combined approach of theoretical analysis and experimental investigation to systematically examine the root causes of wall thickness non-uniformity in internally overlay-welded pipes, and they propose concrete recommendations that can guide field production practices.

Core Technical Content

The fundamental issue addressed in this paper is the deviation of the final wall thickness from the specified nominal value and its allowable tolerance range after internal overlay welding of INCONEL 625 on the base pipe. In offshore applications, the base pipe is typically API 5L Grade X65 or X70 seamless or HFW pipe, while the overlay layer provides corrosion resistance against the aggressive subsea environment.

The wall thickness tolerance problem can be decomposed into several contributing factors:

Interpretation of Key Technical Points

Theoretical Analysis Framework

The authors approach the problem through a theoretical model that considers the additive nature of the overlay layer on top of the base pipe wall thickness. The final wall thickness at any circumferential position can be expressed as:

t_final(θ) = t_base(θ) + t_overlay(θ)

where θ represents the angular position around the pipe circumference. The tolerance of t_final must satisfy the requirements of the applicable standard, typically within ±12.5% of the nominal wall thickness per API 5L, or more stringent project-specific requirements.

The critical insight is that the overlay welding process introduces an additional variable of uncertainty. Unlike the base pipe, where wall thickness is measured and controlled during manufacturing, the overlay layer thickness depends on the dynamic interaction between the welding torch, the wire feed, the gas shielding, and the pipe geometry.

Experimental Investigation

The experimental portion of the study likely involves the following methodology:

1. Pre-weld measurement: Precise ultrasonic thickness measurement of the base pipe at multiple circumferential positions (typically at 4, 6, or 8 points per cross-section, and at multiple axial stations).
2. Post-weld measurement: Similar ultrasonic or eddy current thickness measurement of the overlay layer and the total wall thickness.
3. Process parameter correlation: Systematic variation of key welding parameters (voltage, current, travel speed, wire feed rate, torch-to-workpiece distance) and measurement of the resulting overlay thickness.
4. Statistical analysis: Determination of the distribution of wall thickness deviation and identification of dominant factors.

Key Welding Parameters for Internal Overlay Welding

The welding process used is typically submerged arc welding (SAW) or gas metal arc welding (GMAW) with flux-cored wire, depending on the pipe diameter and the required productivity. For large-diameter offshore pipes, automatic SAW with multiple passes is common, while for smaller diameters, GMAW or plasma arc welding may be employed.

Process and Standards Analysis

The overlay welding of INCONEL 625 is governed by several standards that collectively define the requirements for the process:

A critical consideration is that the overlay layer itself must meet the corrosion resistance requirements of NACE MR0175/ISO 15156, which includes hardness limits (typically ≤350 HV for INCONEL 625 overlay), intergranular corrosion resistance testing per ASTM G48, and resistance to sulfide stress cracking per ASTM G178.

The wall thickness tolerance problem is particularly acute in subsea applications because:

• The pipe cannot be easily accessed for repair once deployed.
• Hydrostatic pressure testing must be performed at elevated pressures (typically 1.25 times the maximum working pressure).
• Any wall thickness reduction below the minimum required thickness can lead to catastrophic failure under external hydrostatic pressure.
• The overlay layer must be continuous and free of defects to ensure complete corrosion protection.

Integration with Engineering Practice

Common Defects and Countermeasures

Engineering Practice Case

In offshore pipeline projects, the wall thickness tolerance problem has been observed in several field situations:

• Case 1: A 24-inch OD, 0.5-inch WT X65 pipe with INCONEL 625 overlay showed a maximum wall thickness deviation of +18% at the bottom of the pipe (6 o'clock position) due to the accumulation of welding spatter and the difficulty of maintaining torch standoff in the gravity-downward position.
• Case 2: A 16-inch OD, 0.375-inch WT X65 pipe exhibited localized thin spots at the 3 and 9 o'clock positions, attributed to pipe ovality of 1.5% of the OD, which caused the torch to be closer to the pipe wall at these positions, resulting in deeper penetration and thinner overlay.

Recommended Practices

Based on the study, the following practices are recommended:

1. Pre-weld geometry verification: Measure pipe ovality and ensure it is within 0.5% of OD before overlay welding.
2. Multi-point thickness measurement: Measure base pipe wall thickness at a minimum of 8 circumferential positions at each weld station.
3. Real-time parameter monitoring: Implement automatic wire feed and travel speed control with feedback from arc voltage and current sensors.
4. Post-weld thickness mapping: Perform ultrasonic thickness measurement at a grid of points (e.g., 8 circumferential × 4 axial positions) to verify compliance with tolerance requirements.
5. Procedure qualification with thickness measurement: During welding procedure qualification (WPQ), include wall thickness measurement as a qualification parameter.

Key Questions and Reflections

The study raises several important questions for further investigation:

• How does the number of overlay passes affect the cumulative wall thickness deviation? Is there a diminishing return or a compounding error effect?
• What is the minimum detectable wall thickness variation by ultrasonic testing through a multi-layer overlay, and does the measurement technique introduce significant uncertainty?
• Can machine vision or laser scanning be employed to provide real-time feedback on overlay thickness during the welding process?
• What is the economic impact of wall thickness tolerance non-conformance? The cost of pipe rejection or rework versus the cost of tighter process control.

The paper's contribution lies in its systematic approach to a problem that is often treated empirically in the field. By combining theoretical analysis with experimental data, the authors provide a foundation for rational process optimization rather than trial-and-error parameter adjustment.

Study Insights and Implications

The most significant insight from this study is the recognition that wall thickness tolerance in overlay-welded pipes is not merely a welding quality issue but a system-level engineering challenge that involves the interaction of base pipe manufacturing tolerances, welding process parameters, pipe geometry, and inspection capabilities.

For engineering practice, this means that:

• The welding procedure specification (WPS) must include wall thickness measurement as a mandatory acceptance criterion, not just visual and NDT inspection.
• The welding procedure qualification record (WPQR) must document the wall thickness results across the full parameter range.
• Quality control plans (QCP) must include specific checkpoints for wall thickness verification at defined intervals.
• The contractor's quality management system must demonstrate the capability to control wall thickness within the specified tolerance through process capability studies.

This paper is a valuable reference for engineers involved in the design, fabrication, and inspection of overlay-welded pipelines for offshore and onshore sour service applications. It underscores the importance of integrating welding metallurgy with dimensional control and measurement technology to achieve reliable, long-life pipeline systems.