Solutions to Tackle Unknown Internal Diameters for ILI in Supply Lines

Supply pipeline operators do not always know the exact internal bore of every meter of every pipeline. Most of the time, that does not matter—until inspection time comes around. 

A difference of 10 or 20 mm has little practical consequence in day-to-day operations. The product flows between tanks with no noticeable slowdown or difference between sections.  

Until someone needs to run an ILI tool. 

Suddenly, 10 mm can be the difference between a successful inspection and a lodged tool. 

Ten millimeters—roughly the size of a green pea—does not sound like much, but in supply-line ILI, two factors make it significant: 

1. Supply lines are often relatively small. 

A 6-in. Schedule 80 steel pipe has an internal diameter of roughly 146 mm. Reduce the available bore by 10 mm and you have lost nearly 7% of that diameter—a meaningful change when tool clearance is already limited. 

2. Tight bends make that clearance even more critical. 

An ILI tool might pass through a restricted straight section without difficulty. At a tight bend, however, the tool must also articulate through the curve. Reduced bore leaves less room for that movement, increasing mechanical loading and the risk of a lodged tool. 

Supply lines rarely offer simple geometry. 

They weave through plants around tanks, pumps, roadways, structures, and process equipment. Tight bends, consecutive elbows, valves, vertical sections, and short spool pieces can all appear within a relatively short run. 

That makes uncertainty about minimum bore particularly important. 

How do operators deal with it? 

A conventional approach is staged verification: 

• Review drawings, specifications, and repair history 
• Verify accessible sections using OD and UT measurements 
• Run a conservative gauging pig to prove minimum clearance 
• Use a high-resolution caliper or geometry tool where a detailed bore profile is required 

But difficult supply lines have also driven the development of ILI tools designed to tolerate greater geometric uncertainty. 

Dexon’s Flexi-ILI Dolphin was developed specifically for these environments.  

Its free-swimming, highly flexible design and patent-pending centralization system allow it to negotiate demanding supply-line geometries and accommodate variations in internal diameter while collecting ultrasonic integrity data. 

For suitable lines, this can reduce the need for extensive geometric verification before the main inspection and simplify execution on assets that would otherwise be difficult to inspect. 

Because in supply-line ILI, knowing that a pipeline is “6-inch” is only the beginning. 

What really matters is how much space the tool will actually have when it gets to the hardest part of the line.