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4″ wet sour gas pipeline

Pipeline Description

4” wet sour gas flow line in Northern Germany.  Pipe dimensions: carrier pipe, (StE 360.7 S) OD 114,3mm, WT=11mm, ID 86,3mm.  Liner is Alloy 825 (material no. 2.4858) WT 3mm.  Total length is 600m.

Why is it „un-piggable“?

Ultrasonic inspection tools, that could handle the wall thickness, were not available for 4”.  Further, limitations were reported whether the surface between carrier pipe and liner will reflect sufficiently.  An MFL inspection is not feasible since it does not refer to the liner material.  Further the 11mm ferritic wall thickness cannot be magnetised by a 4” MFL tool.

Target of the inspection

Perform a high resolution inspection for liner wall thickness and in particular lift-off of the liner from the internal surface of the carrier pipe.

Concept for a solution

3P Services’ “DMR” sensors (direct magnetic response) were applied on this inspection.  Basically these sensors measure the distance between their own location and the next ferritic surface.  If continuously guided along the internal liner pipe surface, the DMR sensors directly measure the thickness of the liner without influence of the carrier pipe wall thickness.  If dis-bonding and liner lift-off occurs, then an apparent thickening of the liner is indicated.  This tool measures generally all kinds of internal coatings, layers and accumulations of non-ferritic materials.ion.


3P Services designed a DMR inspection tool for the dimensions of the flowlines. Two identical units, each carrying 24 DMR sensors were built and tested in a section of the original pipe.  The 24 sensors cover the circumference by approx. 60%, which was acceptable.  Upon the final testing, the tools were approved by the Client and it’s Authority and mobilised to site.  The tools were propelled by water at ambient temperature.  Both units of the DMR inspection tool were run to achieve a double set of data.


The first inspection was a baseline survey performed in December 2000 prior to commissioning of the pipeline.  In December 2003 the line was taken out of operation, flooded with water, and re-inspected.  Both tools during both inspections recovered complete inspection data that allowed an interpretation over the entire length of the pipeline.  Liner thickness could clearly be identified over the entire pipeline length at excellent repeatability. The inspection was approved by the Authority and the Client’s targets were achieved, budget and time schedules were met.