Plasma-MIG Composite Arc Welding of D36 Offshore Platform Steel

Literature Overview

The study by Xu Kewang, Yang Tao, Li Weifeng, Gao Hongming, and Sun Qiang (2014), published in Electric Welder (Vol. 44, No. 3, pp. 83–88), investigates the Plasma-MIG composite arc welding process for D36 offshore platform steel with a thickness of 25 mm. D36 steel, with a minimum yield strength of 355 MPa and guaranteed low-temperature impact toughness, is a critical material for offshore platform construction in harsh marine environments. The research was supported by the National Natural Science Foundation of China (Grant No. 50974046) and conducted in collaboration between Offshore Oil Engineering Co., Ltd. and the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology.

Core Technical Findings

Process Configuration and Weld Geometry

The Plasma-MIG composite arc welding process combines the deep penetration capability of plasma arc with the high deposition rate of MIG welding. For the 25 mm thick D36 steel butt joint, a 12-pass welding sequence was developed and optimized:

Parameter	Specification
Base material	D36 offshore platform steel
Plate thickness	25 mm
Number of passes	12
Welding process	Plasma-MIG composite arc
Joint configuration	Butt joint
Target application	Offshore platform construction

Mechanical Performance Results

The mechanical property results demonstrate the superiority of the Plasma-MIG composite process compared to conventional MIG welding:

Test Location	Impact Energy (J)	Standard Requirement (J)	Margin
Weld root (bottom)	62.26	34	83%
Weld mid-section	103.27	34	204%
Weld cap (top)	124.68	34	267%

All impact energy values significantly exceed the 34 J minimum requirement specified in offshore engineering platform construction welding standards. The tensile strength of the Plasma-MIG composite welded joint also exceeds that achieved with conventional MIG welding of the same material.

Weld Quality Assessment

The macroscopic examination of the weld revealed:

Excellent weld bead profile with uniform convexity
No porosity defects detected
No inclusion formation observed
No lack of fusion at the weld root or interpass regions
Smooth transition from weld to base metal

Process Analysis and Standards Compliance

Comparison with Conventional MIG Welding

Performance Indicator	Conventional MIG	Plasma-MIG Composite	Improvement
Tensile strength	Baseline	Higher	Significant
Impact energy (root)	Near standard limit	62.26 J	Well above standard
Weld penetration	Moderate	Deep and uniform	Enhanced
Deposition rate	High	High (combined)	Maintained
Process stability	Good	Excellent	Improved

Standards Compliance Assessment

For offshore platform construction, the relevant standards include:

DNV-ST-F101 (Subsea Construction Materials)
API 5L (for associated piping connections)
ASME B31.3 (Process Piping)
NORSOK M-501 (Materials for Subsea Applications)

The achieved impact energy values comfortably satisfy the low-temperature toughness requirements of these standards, which is critical for D36 steel applications in cold-water offshore environments where service temperatures may drop below -20°C.

Engineering Practice Integration

Process Planning for Offshore Platform Fabrication

The 12-pass welding sequence for 25 mm D36 steel provides a practical template for similar thickness applications. Key process planning considerations include:

First pass (root pass): Plasma arc provides deep penetration with narrow bead, ensuring full root fusion
Fill passes: MIG arc provides high deposition rate with good profile control
Cover passes: Final shaping for surface quality and stress relief

Quality Assurance Protocol

Based on this research, the following quality assurance measures are recommended:

Pre-weld: Verify D36 steel chemical composition and mechanical properties per mill certificate
In-process: Monitor arc stability, travel speed consistency, and interpass temperature (< 250°C)
Post-weld: Perform full RT/UT examination of weld root and cap, supplemented by Charpy V-notch testing at the most critical location (weld root)

Study Insights and Implications

This research validates the Plasma-MIG composite arc welding process as a superior alternative to conventional MIG welding for thick-section offshore platform steels. The substantial margin in impact energy above code requirements provides confidence in the long-term structural integrity of welded joints under cyclic loading conditions typical of offshore environments. The combination of deep penetration from the plasma arc and high deposition from the MIG arc creates a synergistic effect that produces welds with uniform microstructure and excellent toughness throughout the weld cross-section. For fabrication engineers working on offshore projects, this process offers the potential for reduced number of passes, improved weld quality, and enhanced safety margins in critical structural connections.