Development of Local Dry Underwater High-Frequency Pulsed MIG Welding Power Source

Literature Overview

This 2024 publication in the Welding Journal by Wang Zhenmin, Jia Jianjun, Hu Jianliang, Liao Haipeng, Wu Jianwen, and Zhang Qin from South China University of Technology addresses a highly specialized welding challenge: the development of a dedicated power source for local dry underwater pulsed MIG welding. The research was conducted at the School of Mechanical and Automotive Engineering and the School of Computer Science and Engineering at South China University of Technology, and was supported by the National Natural Science Foundation of China (U2141216), Guangdong Provincial Science and Technology Programs (2021B1515420006, 2021B1515120026), the Guangdong Provincial Marine Economic Development Special Fund (GDNRC[2021]46), and Shenzhen Technology Key Project (JSGG20201201100400001).

Underwater welding has traditionally been limited to low-quality carbon arc and shielded metal arc welding (SMAW) processes, producing welds with poor mechanical properties and high hydrogen content. The development of local dry underwater welding, where a dry chamber is maintained around the weld zone, has opened the possibility of using advanced processes such as pulsed MIG welding to achieve near-surface quality welds. This paper focuses specifically on the power source design for high-frequency pulsed MIG welding in this challenging environment.

Core Technical Content and Methodology

The fundamental challenge of underwater welding is the interaction between the welding arc and the surrounding water environment. Even in local dry welding, where a gas-filled chamber is maintained at the weld site, the boundary between the dry zone and the water introduces unique electrical, thermal, and mechanical phenomena that affect arc stability and weld quality.

Power Source Design Requirements

The authors identify several critical requirements for the welding power source:

High-Frequency Pulsing Strategy

The key innovation is the use of high-frequency pulsing (up to 1000 Hz) to achieve stable short-circuit transfer in the confined dry chamber. At high frequencies, the droplet transfer becomes more regular and predictable, reducing spatter and improving weld bead geometry. The power source must provide precise current and voltage waveforms with minimal distortion and rapid switching capability.

The power source architecture likely incorporates:

1. High-frequency switching power supply: Using IGBT or SiC MOSFET switches operating at kHz frequencies to generate precise pulse waveforms.
2. Closed-loop current control: Real-time feedback control to maintain the desired current waveform despite arc voltage fluctuations.
3. Adaptive pulse parameter adjustment: Automatic adjustment of pulse current, frequency, and duration based on real-time arc voltage and current feedback.
4. High-voltage insulation and protection: Multi-layer insulation, waterproof connectors, and overcurrent protection circuits.

Local Dry Welding Environment Control

The power source design must account for the following environmental factors:

• Chamber pressure: Typically 0.1-0.5 MPa above ambient, affecting arc voltage and gas flow.
• Gas composition and flow rate: Usually argon or argon-helium mixtures, with flow rates adjusted to maintain a stable gas curtain at the chamber boundary.
• Water pressure: External hydrostatic pressure affects chamber integrity and gas flow patterns.
• Temperature gradient: The temperature difference between the hot weld zone and the surrounding water creates convection currents that affect arc stability.

Process and Standards Analysis

Underwater welding is governed by a specialized set of standards that differ from those for above-water welding:

For pressure vessel and pipeline applications, underwater welds must meet the same mechanical property and NDT acceptance criteria as above-water welds. This imposes stringent requirements on the welding process, including:

• Tensile strength within ±10% of base metal
• Toughness (CVN) meeting minimum requirements at service temperature
• No porosity, incomplete fusion, or lack of penetration detected by RT or UT
• Hydrogen content below critical threshold (typically <10 ppm)

Integration with Engineering Practice

The local dry underwater pulsed MIG welding process is particularly relevant to the following applications:

1. Subsea pipeline construction and repair: Welding of subsea pipelines and laterals requires high-quality, reliable welds that can withstand internal pressure and external hydrostatic loads.
2. Offshore platform maintenance: Structural repairs to offshore platforms, including jacket legs, topsides, and mooring structures, often require underwater welding.
3. Submarine pressure vessel fabrication: Welding of submersible hulls, ballast tanks, and equipment compartments requires welds of equal quality to surface-fabricated components.
4. Shipbuilding and repair: Underwater welding of ship hulls, tanks, and structural components during dry-docking or in-water repair.

In practice, the deployment of this technology requires a comprehensive system that includes:

• Diving support: Saturation diving or remotely operated vehicle (ROV) support for welder access and equipment deployment.
• Chamber system: A local dry welding chamber with gas supply, pressure control, and water exclusion capabilities.
• Power supply: The high-frequency pulsed MIG welding power source described in this paper.
• Welding consumables: Submerged arc welding (SAW) or MIG welding wires suitable for underwater conditions.
• Monitoring and control: Real-time monitoring of welding parameters, chamber conditions, and weld quality.

Key Questions and Reflections

Several technical questions arise from studying this work:

• Scalability to thicker sections: The power source specifications suggest applicability to medium-thickness sections (6-40 mm). How does the process scale to thicker sections (50 mm and above) that are common in pressure vessel and pipeline applications? Multi-pass welding strategies and interpass temperature control become critical in this regime.
• Effect of depth on process parameters: Hydrostatic pressure increases with depth, affecting gas behavior, arc stability, and chamber pressure. The paper should address the depth-dependent parameter adjustments required for reliable welding at various depths.
• Long-term reliability of the power source: Underwater environments are harsh, with saltwater corrosion, biofouling, and mechanical damage risks. The power source must be designed for long-term reliability with minimal maintenance, which is challenging in a subsea environment.
• Integration with robotic welding systems: For repetitive welding tasks, such as pipeline girth welds, robotic welding systems would be preferred over manual welding. The power source design should be compatible with robotic control interfaces and automated seam tracking.

Study Insights and Implications

This paper represents a significant step toward enabling high-quality underwater welding using advanced arc processes. The development of a dedicated high-frequency pulsed MIG power source addresses a critical bottleneck in the adoption of local dry underwater welding technology. For engineers involved in subsea construction and repair, this technology offers the prospect of achieving surface-quality welds in underwater environments, potentially reducing the need for dry-docking and extending the service life of subsea infrastructure.

The broader implications extend to the cladding and overlay welding field, where underwater welding of corrosion-resistant alloy overlays on carbon steel structures is an emerging application. The ability to deposit high-quality overlay welds underwater would enable in-situ repair of corroded pipelines, heat exchanger tubes, and pressure vessel components without the need for removal and dry-dock fabrication.

From a standards perspective, the development of this technology will require updates to existing underwater welding standards to incorporate pulsed MIG welding processes and their specific qualification requirements. The integration of high-frequency pulsing into underwater welding standards will also require the development of new test methods for evaluating weld quality under submerged conditions.

Reference Value and Outlook

The research by Wang Zhenmin and colleagues establishes the technical foundation for high-frequency pulsed MIG welding in local dry underwater environments. The power source design addresses the unique electrical, thermal, and mechanical challenges of underwater welding and provides a practical solution for achieving near-surface weld quality. Future development should focus on system integration, field validation, and standards harmonization to enable widespread adoption of this technology in the offshore and subsea industries.

The practical value of this work lies in its potential to reduce the cost and risk of underwater welding operations, improve weld quality and reliability, and enable new applications in subsea construction and repair. For engineers responsible for the design, fabrication, and maintenance of subsea infrastructure, this technology represents a significant advancement in welding capability and a valuable addition to the toolkit of underwater welding processes.