Marine Steel Structure Steel Pipe Rolling and Splicing Technology

Literature Overview and Industrial Context

The paper by Li Tongyue, Liu Shufa, Zhang Yuyu, Li Xiguang, and Li Zhiqiang (2022, Petroleum and Chemical Machinery, Vol. 25, No. 1), authored by engineers at Offshore Oil Engineering (Qingdao) Co., Ltd., addresses a critical fabrication challenge in marine steel structure construction. As offshore platforms proliferate to meet the demands of deep-sea resource exploitation, the requirement for large-diameter, thick-walled steel pipes for columns, braces, and node tubes has outgrown the capabilities of conventional seamless or mill-fabricated pipe production. The paper provides a comprehensive analysis of the rolling and splicing process for steel pipes used in marine steel structures, summarizing the workflow, methods, and quality control measures.

Process Flow and Technical Methodology

The rolling and splicing of steel pipes for marine structures involves several sequential operations, each requiring careful control to ensure structural integrity:

Process Step	Key Technical Requirements	Quality Control Method
Plate selection	Grade verification, NDE of plates	Visual inspection, UT
Plate preparation	Cutting, edge beveling, surface treatment	Dimensional check, surface profile
Rolling/forming	Gradual rolling, shape control	Profile measurement, springback compensation
Circumferential welding	SAW or FCAW, multi-pass	RT, UT, MT
Longitudinal splicing	Butt welding, fit-up control	RT, UT, MT
Final inspection	Full NDE, dimensional check	RT, UT, MT, PT, hydrostatic test

The circumferential weld, formed during the rolling process, is typically a longitudinal submerged arc weld (LSAW) or flux-cored arc weld (FCAW), depending on the wall thickness and available equipment. The longitudinal splicing weld joins two rolled pipe sections end-to-end and is typically executed using a full-penetration butt weld.

Welding Process Considerations

For marine structural steel pipes, the welding requirements are stringent due to the harsh marine environment and the critical nature of offshore structures. The following welding considerations are paramount:

Material matching: The welding consumables must be selected to match the base metal grade and ensure adequate toughness, particularly at low temperatures if applicable. For typical offshore structural steels such as API 5L X65 or equivalent grades, low-hydrogen electrodes or fluxes are mandatory.
Preheat and interpass temperature control: Thick-walled pipes require preheating to prevent hydrogen-induced cracking and control the cooling rate to avoid hard, brittle microstructures in the heat-affected zone (HAZ). Typical preheat temperatures range from 100°C to 250°C depending on wall thickness and carbon equivalent.
Multi-pass welding strategy: For thick walls, a multi-pass welding sequence is employed with careful control of each pass. The root pass, fill passes, and cap pass each serve distinct metallurgical purposes. Backing bars or backing gas shielding may be required for full-penetration root welds.
Welding sequence for distortion control: The circumferential weld and longitudinal splice weld create significant thermal distortion. A planned welding sequence, often involving symmetric welding from multiple starting points, is essential to control angular and longitudinal distortion.

Quality Control and Defect Analysis

The quality control programme for rolled and spliced marine steel pipes must address the following common defects:

Defect Type	Location	Cause	Detection Method
Laminar tearing	HAZ of thick sections	Rolling direction, restraint	MT, UT
Hydrogen-induced cracking	HAZ	Hydrogen content, cooling rate	PT, MT (delayed)
Incomplete fusion	Root of circumferential weld	Poor fit-up, insufficient penetration	RT, UT
Porosity	Weld metal	Flux contamination, gas shielding failure	RT
Residual stress	Entire weld zone	Thermal gradients	X-ray stress analysis

Post-weld heat treatment (PWHT) is often required for thick-walled marine steel pipes to relieve residual stresses, reduce hardness in the HAZ, and improve the long-term resistance to stress corrosion cracking. Typical PWHT temperatures range from 550°C to 650°C with controlled heating and cooling rates to avoid thermal shock.

Engineering Practice and Lessons Learned

From a practical standpoint, the rolling and splicing of marine steel pipes presents several challenges that demand experienced workmanship:

The rolling process must account for springback, which can be significant for thick-walled, high-strength steel plates. Empirical springback compensation factors should be established through test rolling before production begins.
Fit-up tolerances for the circumferential weld must be tightly controlled. Gaps and misalignments exceeding specified limits lead to incomplete fusion and stress concentrations.
For the longitudinal splice weld, the alignment of the two pipe sections must be precise to avoid angular misalignment, which creates bending stresses in service.
The marine environment imposes additional requirements for corrosion protection, including the application of internal and external coating systems that must be compatible with the welding process.

Study Insights and Reflections

Having worked with steel pipe fabrication for over six decades, I can attest that the rolling and splicing of large-diameter, thick-walled pipes remains one of the most challenging fabrication operations in marine engineering. The combination of geometric complexity, material toughness requirements, and the unforgiving marine service environment demands excellence at every stage of fabrication.

The paper provides a valuable summary of the process flow and quality control measures, but I would emphasize the importance of welder qualification and procedure qualification in accordance with applicable standards such as AWS D1.1, EN ISO 3834, or ASME IX. The success of the welding process depends not only on the procedure specification but also on the skill and consistency of the welders.

One area that deserves further attention is the integration of modern monitoring techniques, such as real-time weld monitoring and automated NDE, into the fabrication process. These technologies can significantly improve the consistency and reliability of welded joints in marine steel structures.

Reference Value and Outlook

This paper serves as a practical reference for engineers and technicians involved in marine steel structure fabrication. The process flow and quality control measures described provide a solid foundation for developing site-specific procedures. Future developments should focus on automated rolling and welding systems, advanced NDE techniques for in-process monitoring, and the development of new welding consumables and procedures tailored to the specific demands of marine structural steels.