Combined Seamless Steel Pipe for Pipeline Applications

Literature Overview

This patent disclosure, published in the journal "Steel Pipe" (2017, Vol. 46, No. 4, p. 83) by Wang Yuansun, describes a combined-type seamless steel pipe designed for pipeline applications. The invention presents a novel approach to pipeline construction by assembling two semi-circular steel pipe sections into a complete pipe, incorporating locking mechanisms and flange connections for field assembly of arbitrary length pipelines.

Technical Description of the Invention

Structural Configuration

The combined seamless steel pipe system consists of:

Component	Description	Function
First semi-circular steel pipe	Half-section with first mating edges at both ends	Primary structural element
Second semi-circular steel pipe	Half-section with second mating edges at both ends	Secondary structural element
First mating edge	Located at both ends of first semi-circular pipe	Connection interface
Second mating edge	Located at both ends of second semi-circular pipe	Connection interface with E-groove compatible convex surface
First locking device	Located at one end of first mating edge	Secures the two half-sections
Butt flange	Located at one end of the assembled pipe	Pipeline connection interface
Welding layer	Between the connection surfaces	Structural continuity
Second locking device	Located on the butt flange	Flange connection security

Assembly Principle

The assembly sequence follows a logical progression:

The two semi-circular sections are aligned along their mating edges.
The convex surface on the second mating edge engages with the E-groove on the first mating edge, providing initial mechanical interlock.
The first locking device secures the two half-sections into a complete circular pipe.
A welding layer is applied between the connection surfaces to provide structural continuity and pressure containment.
Butt flanges with the second locking device enable end-to-end connections between pipe sections.

Engineering Analysis

Advantages of the Combined Design

Advantage	Technical Explanation	Engineering Benefit
Material economy	Semi-circular sections can be rolled from flat plate more efficiently	Reduced material waste
Arbitrary length	Field assembly allows customization	Reduced inventory requirements
Transport convenience	Smaller diameter sections for transportation	Reduced logistics costs
On-site assembly	Modular construction approach	Reduced construction time

Welding Process Considerations

The welding layer between the two semi-circular sections represents the critical structural element of the combined pipe. From a welding engineering perspective, several factors must be addressed:

Joint geometry: The butt joint between the two half-sections is essentially a longitudinal weld, similar to LSAW (Longitudinal Submerged Arc Welding) or HFW (High Frequency Welding) joints in conventional pipe manufacturing. However, the field conditions impose constraints on welding equipment and procedures.
Weld procedure qualification: The welding procedure must be qualified according to applicable codes (ASME B31.3, GB/T 20801, or ISO 15614) to ensure weld quality under field conditions.
Preheat and interpass temperature: Depending on the steel grade and wall thickness, preheat requirements must be met to prevent cold cracking, particularly for high-strength or carbon-equivalent-sensitive steels.
Post-weld heat treatment: For thicker wall sections or higher strength grades, post-weld heat treatment may be required to relieve residual stresses and temper the heat-affected zone.

Potential Defect Modes

Defect Type	Cause	Detection Method	Countermeasure
Lack of fusion	Inadequate welding parameters	RT/UT	Optimize WPS parameters
Porosity	Contamination or excessive travel speed	RT/UT	Clean surfaces, control parameters
Cracking	High residual stress or material sensitivity	MT/PT/RT	Preheat, PWHT, low-stress welding sequence
Angular misalignment	Poor fit-up	Visual/RT	Precision alignment fixtures
Undercut	Excessive arc length or travel speed	Visual/MT	Parameter control, backfill

Manufacturing and Quality Control Framework

FMEA Analysis for Assembly Process

Process Step	Potential Failure Mode	Severity	Occurrence	Detection	RPN	Recommended Action
Half-section alignment	Angular misalignment	9	4	7	252	Use precision alignment fixtures
Locking device engagement	Incomplete lock	8	3	6	144	Verify lock engagement with torque specification
Welding of joint	Lack of fusion	10	4	5	200	Qualify WPS, monitor parameters
Flange connection	Leak at bolted joint	8	3	7	168	Use calibrated torque wrenches, gasket inspection
Hydrostatic test	Pressure leak	10	2	5	100	100% hydrostatic testing per code requirements

Material Specifications

The semi-circular steel pipe sections should conform to applicable standards:

Application	Material Standard	Typical Grade	Minimum Yield Strength
General pipeline	GB/T 8163 / API 5L	Q235 / B	235 MPa / 241 MPa
High-pressure pipeline	GB/T 9948 / API 5L	Q345 / X65	345 MPa / 450 MPa
Offshore pipeline	NACE MR0175 / ISO 15156	X65 / X70	450 MPa / 500 MPa

Critical Reflections and Study Insights

The combined seamless steel pipe concept represents an innovative approach to pipeline construction that addresses practical challenges in transportation, storage, and field assembly. However, several concerns must be addressed before widespread adoption:

Long-term structural integrity: The welded longitudinal joint between the two half-sections must demonstrate equivalent fatigue life and fracture toughness to a true seamless pipe. Fatigue crack initiation at the weld toe is a critical concern, particularly for pipelines subject to cyclic pressure loading.
Inspection accessibility: The internal weld joint must be accessible for non-destructive testing. Internal calipers or ultrasonic testing methods must be employed to verify weld quality.
Corrosion resistance: The weld joint may exhibit different corrosion behavior compared to the base metal, particularly in aggressive environments. Coating continuity across the weld is essential.
Code compliance: The combined pipe must be recognized by applicable pipeline codes. Current codes (ASME B31.3, GB/T 20801, ISO 13623) may not explicitly address this configuration, requiring a code case or alternative standard justification.

The concept has merit for specific applications where conventional pipe lengths are impractical, such as remote locations, underground installations with limited access, or specialized process piping. However, rigorous qualification testing and code recognition are prerequisites for implementation in critical infrastructure.