Spiral welded pipe, also called helical seam pipe, is used for large diameter water, gas, heating and slurry lines because the helical seam distributes stress evenly around the circumference and the process can produce diameters that are impractical with straight-seam forming. Correct installation and a defined maintenance routine decide whether such a line reaches its design life, since most premature failures trace back to handling damage to the coating, poor field welding of the circumferential joints, or inadequate attention to the soil and structure interface.
Handling, Hoisting and Transport
Use nylon slings or belt slings in contact with the pipe. Steel wire rope damages the anti-corrosion coating and creates corrosion initiation sites.
Use a spreader beam for long lengths so that the pipe is not bent or ovalised during lifting, and avoid dragging coated pipe across the ground.
Place wooden or rubber pads between bundled pipes, limit stacking height, and store pipe on cradles so that the ends and the coating stay clear of the ground.
Transport on vehicles fitted with fixed side frames or stakes, and secure the load so that it cannot roll. Over-length loads require a transport permit and an approved route.
Cover the pipe and keep the interior dry during the rainy season, and cap the ends for sea freight and long storage.
Joint Preparation and Field Welding
Clean oil, rust and oxide from the pipe ends before assembly. The groove is usually prepared at about 30 degrees with a tolerance of plus or minus 5 degrees, with a root face of 1 to 2 mm ground to a metallic finish using an angle grinder.
Keep misalignment within 10 % of the wall thickness, with the absolute cap stated in the project specification. Use external alignment clamps to maintain concentricity; never force the pipe into line to close a gap.
Apply preheat according to the carbon equivalent and wall thickness of the material, typically 100 to 150 degrees C for heavy-wall carbon steel pipe. Use low-hydrogen consumables for the root pass and control interpass temperature within the limits of the qualified procedure.
Do not weld when the relative humidity exceeds 90 %, or in rain or snow without a shelter, in line with the field welding code GB 50236. Welding procedure qualification follows NB/T 47014.
Repair any coating damaged by welding heat or clamps before backfilling, using the field joint coating system specified for the line.
Buried Laying Requirements
Trench width is usually the pipe outside diameter plus 0.3 to 0.5 m of working space, with a sand bedding layer of about 200 mm placed before the pipe is lowered, following the practice of GB 50268.
In rock or stony ground, over-excavate and replace the bedding with soft soil so that the pipe is not point-loaded, which would concentrate stress in the wall and damage the coating.
Where the line crosses a road or railway, install a steel casing pipe and seal both ends to prevent water ingress. The casing is sized so that the carrier pipe passes with clearance, and the annular space is vented.
Backfill in layers with controlled compaction, and place marker tape above the pipeline. Fit thrust and anchor blocks at bends, tees and terminations where the internal pressure generates unbalanced forces.
In-Service Inspection and Corrosion Monitoring
Internal inspection with an instrumented pig provides ultrasonic wall thickness mapping of the whole length, which is the most cost-effective way to find internal corrosion and erosion in large diameter lines.
External corrosion can be surveyed without excavation by the transient electromagnetic method, and coating defects located by direct current voltage gradient survey or current mapping.
Concentrate attention on the weld seams, bends, casing ends and the transition between buried and above-ground sections, where corrosion and stress concentration coincide.
Compare readings with historical records to calculate corrosion rate rather than relying on absolute values, and excavate and sample for verification when a rate rises.
Measure cathodic protection potential to confirm that the coating and the protection system still work together.
Leak Response and Failure Investigation
Isolate the upstream and downstream valves immediately, evacuate personnel and set up a warning zone.
For gas or oil service, make the area explosion-proof and, where the procedure requires it, inert the section with nitrogen before any hot work.
Install a clamp to plug the leak or cut out and replace the damaged section, then pressure test and reinstate the coating.
Hold a failure analysis meeting, examine the fracture surface and the local wall thickness, and check whether other lengths from the same production batch carry the same risk.
FAQ
Q: What are the precautions for hoisting and transporting spiral pipe?
Use nylon or belt slings to protect the coating, fit spreader beams for long lengths, place pads between bundled pipes, secure the load on a vehicle with fixed side frames, and cover the pipe against rain. Over-length loads need a permit and an approved route.
Q: What groove preparation is required before welding?
The groove angle is normally about 30 degrees with a tolerance of plus or minus 5 degrees, with a root face of 1 to 2 mm ground to a metallic finish. Misalignment should stay within 10 % of the wall thickness, and preheat is applied according to the carbon equivalent and wall thickness of the material.
Q: What are the construction requirements for underground laying?
Trench width is generally the outside diameter plus 0.3 to 0.5 m, with a sand bedding layer before the pipe is placed; rocky ground is over-excavated and replaced with soft soil; road crossings use a sealed steel casing; and backfill is placed in compacted layers.
Q: How is corrosion of in-service spiral pipe detected?
By internal inspection with an instrumented pig for wall thickness mapping, and by external survey methods such as the transient electromagnetic method and coating surveys. Weld seams, bends and soil transitions receive the closest attention, and readings are compared with historical data to establish corrosion rate.
Q: What is the emergency procedure for a leak?
Close the upstream and downstream valves, evacuate the area and establish a warning zone, take explosion-proof measures for hydrocarbon service, then clamp the leak or replace the pipe section. A failure analysis follows, including a review of other pipe from the same batch and any necessary repairs or replacement.
Q: When should welding stop on site?
When the relative humidity exceeds 90 %, when rain or snow falls on an unprotected joint, or when the wind is strong enough to disturb shielding gas or arc stability. The field welding code GB 50236 sets out these conditions and they should be written into the construction method statement.
