Подготовка поверхности монтажных стыков трубопроводов и корродированных сварных швов

Pipeline field joints — the sections of bare or rehabilitated pipe at weld connection points and coating cutback zones — are among the most demanding surface preparation scenarios in the oil and gas industry. They are exposed to corrosion, often in places where abrasive blasting is impractical or not permitted, and the surface condition at a field joint directly determines the integrity of the pipeline's corrosion protection for the next operating cycle.

This guide covers the preparation requirements for pipeline field joints and corroded weld seams, the constraints that make blasting impractical in many field joint scenarios, and the step-by-step MontiPower® Two-Step procedure for reaching cleanliness comparable to SSPC-SP 10 in field conditions without grit.

Why are field joints particularly difficult to prepare?

A pipeline field joint presents several surface preparation challenges that do not exist at a fabrication facility:

The heat-affected zone. Welding creates a heat-affected zone (HAZ) on either side of the weld bead, where the thermal cycle changes the metallurgical structure of the steel and produces oxide scale at the surface. This HAZ oxide differs from mill scale on parent metal and is often tightly adherent, so it needs an aggressive mechanical action to remove. Any method that leaves HAZ oxide in place risks coating adhesion failure at the weld — the highest-stress location on the joint.

Weld spatter. Manual metal arc (MMA) and flux-cored arc welding produce weld spatter — small solidified metal droplets fused to the pipe surface next to the weld bead. Spatter must be removed before coating because it creates sharp points under the coating film that thin the film and become early corrosion sites. Because it is fused to the surface, spatter resists most cleaning tools unless approached at the correct geometry.

Differential corrosion at the joint. On an in-service pipeline undergoing rehabilitation, the field joint area typically shows a different rust grade than the adjacent coated pipe. The coating cutback zone and bare weld area may have been exposed for years, resulting in ISO 8501-1 rust grade C or D at the joint while the coated parent pipe remains at grade A or B beneath its coating. The preparation method must address the worst-case surface condition at the joint.

Restricted access and confined geometry. Field joints are prepared in the trench during construction, or at elevated or restricted locations during above-ground pipeline maintenance. Blast equipment — compressors, blast pots, containment shrouds — cannot always be deployed in these geometries. In classified hazardous areas, any spark-producing work, including dry abrasive blasting, is controlled by the site's zoning, risk assessment and permit system.

Which preparation standards apply to field joints?

The surface preparation standard for pipeline field joints depends on the field joint coating system and the pipeline owner's specification. The values below are typical starting points, not universal requirements:

The governing documents are the pipeline owner's coating specification and the field joint coating standard it references, such as ISO 21809-3 and DNV-RP-F102 (pipeline field joint coating and field repair of linepipe coating). AMPP SP0169 (formerly NACE SP0169) covers external corrosion control of underground or submerged piping and defers to the coating specification for preparation. Always confirm against the project specification — do not assume a universal standard applies across pipeline operators or coating systems.

Note that SSPC-SP 10 and SP 5 are blast-cleaning standards. A power tool result is described as comparable to these grades and accepted by the responsible inspector against the specification.

Why is abrasive blasting often unavailable at field joints?

In-trench construction. During new pipeline construction, field joints are prepared in the trench or at the skid immediately before coating. Deploying a blast pot and compressor in a 1.5–2 m trench, with grit containment and recovery, is operationally difficult on a moving construction spread where coating must keep pace with welding. Hand-held mechanical tools that produce no grit and need minimal setup are a practical alternative.

Hazardous areas. Gas compressor stations, processing terminals, offshore platforms and petrochemical facilities contain areas classified as Zone 1 or Zone 2. Dry abrasive blasting can generate sparks and static, so in these areas it is normally restricted to permitted hot work or shutdown windows. The Bristle Blaster® Pneumatic has been evaluated under ATEX 2014/34/EU as Category 2 equipment for zone 1 gas (group IIA) and zone 21 dust; the marking II 2G c IIA T4 X applies to specific belts in combination with the original pneumatic drive unit. It does not replace the site permit: whether work may proceed is decided by the operator's zoning, risk assessment and permit system. For Tercoo® ATEX status, confirm directly with MontiPower.

In-service rehabilitation. Rehabilitating the external coating of an in-service gas or oil pipeline above ground means working around the operating pipe. Grit blasting next to live equipment adds the risk of grit entering instrumentation, valve bodies and flanges near the work area. Grit-free mechanical preparation removes that risk, although the removed rust and coating still become dust that must be controlled.

The Two-Step procedure for pipeline field joints

The Two-Step MontiPower Method separates bulk removal from profiling. Both steps run on the same Bristle Blaster® drive unit.

Step 1: Surface assessment and classification

Before beginning preparation, assess each field joint visually and record:

• ISO 8501-1 rust grade of the bare metal area
• Presence of weld spatter, HAZ scale and condition of the weld cap
• Any existing coating at the cutback zone — condition and adhesion
• Evidence of mechanical damage, gouging or previous repair
• Oil, grease or other contamination, which is removed by solvent cleaning (SSPC-SP 1) before any mechanical work

Photograph each joint before preparation. This documentation is typically required by the pipeline owner's inspection protocol and provides baseline data for the quality record.

Step 2: Tercoo® disc pass — corrosion and contamination removal

For field joints with rust grade C/D, weld spatter, HAZ scale or deteriorated coating remnants, fit the Tercoo® disc to the Bristle Blaster® drive unit and work in systematic passes over the entire joint preparation zone. The Tercoo® pass removes:

• Heavy rust and laminar corrosion at the bare metal zone
• Weld spatter from the parent pipe surface adjacent to the weld bead
• HAZ oxide scale on both sides of the weld
• Loosely adherent coating fragments at the cutback edge

Work methodically around the pipe circumference. On small-diameter pipe, complete the lower quadrant before repositioning. Then change to the Bristle Blaster® belt on the same drive unit and follow immediately in the same working area.

Step 3: Bristle Blaster® belt pass — cleanliness and anchor profile

After the Tercoo® pass has cleared the bulk corrosion and contamination, fit the Bristle Blaster® belt and work the same area in overlapping passes around the circumference, with light, consistent contact. The hardened wire tips strike the cleared steel and rebound, producing cleanliness comparable to Sa 2½ / SSPC-SP 10 and an anchor profile of 65–85 µm Rz on standard API 5L steel in documented tests. Results vary with steel grade, rust grade, belt and technique.

Pay particular attention to the weld bead and the immediate HAZ — the irregular topography at the weld cap needs multiple passes and careful tool positioning to reach the whole surface. Because the bristle tips follow the surface, the Bristle Blaster® can prepare the weld bead profile where flat abrasive discs cannot reach consistently.

Step 4: Visual and profile verification

After the Bristle Blaster® pass, verify the prepared surface against the joint coating specification:

• Visual cleanliness: compare with the ISO 8501-1 Sa 2½ reference photographs or SSPC-VIS 1, and with the written definition of SSPC-SP 10, which allows random staining on no more than 5% of each unit area.
• Anchor profile: use Testex Press-O-Film® X-Coarse replica tape (range 38–115 µm / 1.5–4.5 mil) per ASTM D4417 Method C at three to five locations around the joint circumference; subtract the 50 µm (2 mil) incompressible film unless the gauge does it for you. Compare against the range specified by the field joint coating manufacturer.
• Soluble salt: measure per ISO 8502-6 / ISO 8502-9 at the joint and parent pipe areas and compare with the project specification maximum. A commonly cited limit for buried service is 20 mg/m² chloride (2 µg/cm²), but the specification governs.

Record all readings on the joint quality record. Do not proceed to coating application until all parameters are within specification.

Step 5: Apply the field joint coating promptly

Freshly prepared steel can flash rust within hours, and sooner in humid or marine conditions. Apply the joint primer or base coat within the window specified by the coating manufacturer, and keep the steel temperature at least 3 °C above the dew point. If the joint cannot be coated within the specified window, the surface must be re-prepared.

Documented field applications

For more pipeline work, see oil, gas and pipeline applications.

Frequently asked questions

Can the Two-Step Method meet a pipeline owner's specification that references AMPP SP0169 or a company standard?

SP0169 defers to the coating specification for surface preparation and does not prescribe a method. Where the coating data sheet and owner allow mechanical preparation to a result comparable to SSPC-SP 10, the Two-Step Method can be used. Documentation is the same as for blasting: visual records, replica tape readings and salt measurements. Confirm with the owner's inspection engineer beforehand.

How is the Bristle Blaster® used on the curved surface of a pipe?

On large-diameter pipe the belt contacts the curved surface almost as on a flat plate. On small-diameter pipe the operator follows the curvature, so the contact area is narrower and more overlapping passes are needed for full coverage. Narrow 11 mm belts help on tight radii, at weld caps and around fittings.

What is the correct soluble salt limit for a buried pipeline field joint?

The limit is set by the pipeline owner specification or the field joint coating manufacturer. A commonly cited reference is 20 mg/m² chloride, measured by Bresle patch and conductivity to ISO 8502-6 and ISO 8502-9. Some specifications also limit sulphates. Where the project does not define a limit, the coating manufacturer's data sheet is the binding reference.

Does the Two-Step Method work on pipe in the ditch during new construction?

Yes. The Tercoo® disc and Bristle Blaster® belt run on the same hand-held drive unit, which can be used in a trench, overhead on elevated pipe, or in other restricted positions that are impractical for blast equipment. The pneumatic drive needs only a compressed air supply, which is often already present on the coating spread.