Pipeline inspection guide

A buried pipeline cannot be inspected by eye. Inline inspection (ILI) sends a tool — a 'pig' carrying sensors — through the line with the product flow, and reads the wall from inside. The two methods in this line divide the network between them.

The two methods

Magnetic flux leakage (MFL) magnetises the wall; where metal is lost, flux leaks and sensors map the defect. The MFL tool fleet covers 159–1,422 mm diameters, runs 60+ hours continuously within 0–10 MPa and -10 to 80 degrees C, and passes bends of 1.5D radius or greater. It is the trunk-line method — see the MFL testing guide for its strengths and blind spots.

Eddy current testing induces currents in the wall and reads their disturbance — no magnetisation, which suits small diameters. The eddy current machine here passes DN65 lines and detects holes and corrosion beyond 15% wall loss — the gathering and city-gas networks MFL cannot enter. The method comparison works through the choice defect by defect.

Planning a programme

• Segment the network — by diameter, product and consequence: trunk lines to MFL, small-bore gathering and distribution to eddy current.
• Check the line data — bend radii (1.5D minimum here), deformation limits (15–20% OD), launcher/receiver facilities and operating pressure against the tool envelope before promising a run.
• Decide tool or service — repeat programmes justify owning tools; occasional surveys are usually cheaper as the MFL inspection service, which covers 8–56 inch lines with reporting included.
• Plan the dig programme — inspection output is only valuable if defect positions (referenced to +/-0.1 m of the nearest girth weld) feed an excavation and repair plan. The MFL process notes walk the full sequence.
• Add security runs where needed — lines with theft exposure add the illegal tapping detection configuration to the programme.