Guide to pipe measurement and inspection — OMS

Pipe failures can arise for a wide range of reasons, with corrosion or cracking perhaps being the most commonly encountered issue. Corrosion is the process where a metal deteriorates due to an electrochemical reaction called oxidation, or rusting as it is more commonly known. Pipe oxidation can be prevented by taking steps to limit the condition, such as selecting the most appropriate material for use, adding corrosion inhibitors to the pipe treatment process, pipe coating, and in-line cleaning to remove contaminants.

Corrosion in pipes can generally be broken down into three types:

External corrosion: typically arising from external conditions of heat, water or pressure combined with contact with corrosive materials such as acidic soil.
Internal corrosion: resulting from the transportation of a corrosive or pressurised product
Corrosion defects: due to physical characteristics such as shape or form, or from issues resulting from handling or installation

In addition to corrosion, stress and cracking, environmental pressure and damage from installation or excavation can all contribute to the integrity of a pipeline.

However, pipes and other similar assets can be inspected for corrosion and potentially damaging defects, and once identified, corrected to ensure production is able to continue without disruption. This type of pipeline inspection can effectively de-risk a project during the installation phase and also during the operational phase, where periodic checks routinely take place.

When pipes are welded together, defects can occur for any number of reasons. Defective welds can significantly compromise pipe joints, with a knock-on effect of doing so to the entire pipelines integrity.

Weld defects in pipe joints might be due to:

Use of substandard materials
Incorrect coating process
Physical attributes such as dents or shape
Poor welding technique
Incorrect use of welding tools

Weld defects are typically discovered when they are inspected using x-ray or automatic ultrasonic testing (AUT) equipment. By the time defective pipe welds are normally identified, the welded joint is likely to have passed through a series of sequential processes. These processes might take place on land at a spool base or aboard a pipelay vessel at sea – it depends on the location of the final pipeline as to which – but the latter will have significant financial consequences due to delays and the corrective procedure.

If a defect is found at this stage, problematic welds will have to be removed by cutting them out, then the entire process has to start again. By inspecting the weld for the specific type of surface defect earlier in the process, a great deal of lost time can be recovered, and costs saved. Pipelay vessels can cost $100k’s per day, so the saving is considerable.

Furthermore, there are some types of defect that the later processes are less sensitive or impossible to detect, such as coloration for corrosion resistant alloy (CRA) welds, HiLo and porosity.