Pinholes can form inside welds. There’s no two ways about it, that’s a patently shoddy example of welding workmanship. The joint isn’t solid. Just as one adverse effect, the strength of a metal-bonded joint diminishes when tiny cavities accumulate inside welds. Okay, having identified the defect, what’s causing this fusion-undermining porosity in the first place? Stopping to look at a MIG welding operation, an inspector ponders this question.
Shield Gas Inadequacies
There’s something wrong with the CO2 gas that’s meant to protect the HAZ (Heat-Affected Zone) from the oxygen and nitrogen-rich atmosphere that we all breathe. Sure, air is a life-sustaining gas for biological entities, for plants and people alike, but that invisible atmospheric stuff is bad for MIG welded joins. That’s where the shielding gas comes in, as a barrier between the air and the weld site. One likely problem here is a lack of CO2, the most common shielding gas in the MIG welding industry. Another possible issue is the presence of a force that’s hampering the formation of the gas cloud. In essence, a strong wind is blowing away the gas.
Stopping Environmental Pinholing
Craters and wormholes are developing on what should be a smooth joint. The gas is flowing, a shelter has been raised around the worksite, so what’s up here? To correct this frustrating problem, stop for a moment. Take a look around. Is the worksite wet or cold? Perhaps one of the work surfaces is dirty or layered with greasy deposits? After raising the wind shelter, let’s take the time to prep the worksite. Clean the surfaces. Are they free of dirt and oil? That’s good, but the prep work isn’t finished yet. If it’s a cold day and the job is being done on an offshore oil rig, humidity is going to impact the work. Try preheating the metal, for that’ll eject the moisture.
MIG (Metal Inert Gas) welders are expected to adopt a forward-thinking mindset. To prevent pinholes, craters, and general porosity issues, this expertly trained individual plans out his job. There’s the proper formation of that gas shielding cloud to ensure. Getting down close to the weld surfaces, the welder checks out the join sites for dirt and oily deposits. Scrubbed clean, there’s still that last potential pinhole causative factor to deal with, so the metal is warmed until all the moisture is evaporated. Needless to say, this approach becomes a reflex action after a while. It’s a course of action that’s taken alongside an exhaustive equipment check. After all, there’s still a chance a simple hose pinch is hampering the CO2 flow.
Girth welds, as the apparel-biased label implies, are those that go around something. Also known as circumferential welds, the fusing process joins pipe rims. Now, thinking like a welding inspector, someone who’s been hired to audit oil and/or gas pipeline junctures, girth weld cracks are a subject of great concern. For, as it turns out, there are pipes everywhere.
Oil and Gas Pipeline Weld Inspections
To be more precise, there are pipes all over an offshore oil rig. The pipelines travel to shore along the seabed, then on land. There are pipes travelling underwater, pipes running for hundreds of kilometres over land, and yet more pipes nestled inside oil refineries. To avoid expensive leakages, environmental damage, not to mention a potentially catastrophic combustion hazard, every pipe girth weld must satisfy a standardized flaw acceptance test, as carried out by a qualified weld inspector.
Girth Weld Cracks Require Immediate Attention
Because pipelines stretch across vast tracts of land, not every little flaw will get immediate attention. That would be a prohibitively expensive practise. No, by applying fitness-for-service models and fracture mechanics principles, a quantitative approach has been developed here, one that addresses current weld discontinuities and those that have a high probability of developing a pipe-weakening flaw. If workmanship standards are unsatisfactory or if a previously unseen weld discontinuity does breach that flaw acceptance threshold, then the girth weld defect isn’t just noted, logged and recorded. To the contrary, an immediate response is essential when such an environment imperilling fault is discovered.
A Final Thoughts Perspective
Girth welds travel circumferentially around pipes. They’re present on long pipeline stretches and sharp bends, where fluid stresses accumulate. In fact, there are a number of stresses at work here, such as longitudinal tension, which introduces the possibility of plastic collapse. Now imagine such defects developing in the field, perhaps below some remote mountain range or in the middle of an arctic wasteland. No matter the locale, though, the same truism applies: an oil or gas pipeline breach will severely impact the environment it stands within.
To deal with such far-ranging hazards, pipeline weld inspectors go looking for girth weld cracks. A drop in pressure has perhaps been detected, or maybe a technician has seen what he feels could be a series of hairline fractures. Not stopping to think about fracture mechanics models, the inspector hits the road and finds an access route to the defect. And it’s exactly the same on an offshore facility or a refinery, minus the days of road travel. The flaw modelling work will come, of course, but that phase takes place after a girth weld defects’ flaw acceptance criteria has been assessed.