3 SIMPLE THINGS... every oil and gas engineer needs to know about weld inspection.
Tim Clarke of OMS, the global leader in oil and gas pipeline measurement, outlines three key considerations which he believes are critical to pipeline engineers out in the field.
Welding two pipes together - what could possibly go wrong? Pipes are round and welding is a pretty standard process that follows a well-honed formula for success. Unfortunately, real life in the oil and gas industry isn't quite so simple.
Every year, countless hours and millions of dollars in lost revenue are wasted due to pipes with defective welds. An exaggeration you might think? Well, consider the cost of lost production to an oil major when a single pipeline is out of action and no oil can be pumped from an asset valued in billions. You can't just pop a replacement pipe into the water at the drop of a hat. Pipelines need ordering, manufacturing, transporting, coating and finally, fit-up and welding prior to re-laying. Look at the Deepwater Horizon disaster and oil spills in Nigeria in 2008/09. These and other cases prove beyond doubt that getting it wrong in the oil business can damage the environment and prove costly to businesses, compromising the survival of even the largest organisation.
So, if I were to recommend three things to welding engineers, they would be the following:
1. REDUCE PIPE MISALIGNMENT
For numerous reasons, pipes come in a variety of shapes from oval to having multiple lumps and bumps around the circumference. And when two such imperfect pipes are brought together, they very rarely match perfectly.
In addition to the shape challenges, the pipes are not always the same size, which also adds to the mismatch problem. If you don't use some sort of system for pipe selection and fit-up, the welding process must make up the gap between the high and low side of the weld, also known as the 'HiLo'. There are limits to how much HiLo you can accommodate and still achieve a good weld. For critical pipelines, the HiLo tolerance might be as small as 0.5mm. In other cases 1.5mm might become tolerated, even though this is not ideal.
So the first consideration for weld inspection is to check that the HiLo post-welding does not exceed the specification. Using a laser scanner makes this much simpler. In addition, lack of penetration of the weld to achieve full fusion to the high side of the misaligned pipe can also present problems. From time to time, in response to a pipeline failure, we are asked to inspect pipes that have been installed without inspection. Whilst carrying out this type of work, we have often found excessive HiLo, beyond the specification, which has contributed to the failure.
2. MINIMISE EXCESSIVE WELD PENETRATION
Pipes are often welded horizontally, and like it or not, this process is subject to the laws of physics. A weld is the fusion of the parent material from each pipe-end to an additional weld consumable. Here, all three parts need to reach a critical temperature where the material becomes molten in order to fuse together.
The weld is essentially a pool of liquid metal, which under the force of gravity, will sag. At the top of the pipe the weld can easily droop sufficiently to form a bulbous section of weld material that protrudes into the pipe. At the bottom of the pipe, the weld might not fully fuse with either side of the pipe, especially when there is misalignment. Under proper control, and with care and attention, every weld can be performed such that excessive weld penetration does not occur.
To minimise this issue, pre-qualification trials should be conducted prior to starting the project. This will ensure success rather than leave things to chance, as all available parameters will be checked during welding to ensure that the correct process is followed. Although UT is typically used to identify features post-welding, there are limitations to this process.
3. AVOID SUNKEN WELDS OR CONCAVITY
In simple terms, this problem is the opposite of excessive penetration. Here, the weld material does not exceed the level of the pipe wall. Similarly, the weld might not penetrate through the joint between the two pipes at all. Either case has serious implications for the strength of the weld and could also become a site for corrosion to occur, as the fluid being transported through pipes can quite often be highly corrosive.
Welding without inspecting is inviting the problem to be discovered when the impact of the mistake will be significantly higher. Excessive penetration is a significant problem because pipelines need to be inspected through their life using ìpigsî. One thing that presents pigs with major problems are sections of the pipe where there are bumps and lumps resulting from excessive weld penetration. These will damage the pig or in the worst case, cause it to get stuck. In other situations, such as pull tubes for example, the weld might provide an obstruction for a pipe to be pulled through. Weld penetration can be a major issue that should be avoided wherever possible.
To combat this issue, many welding engineers use a Corrosion Resistant Alloy (CRA) layer on the inside of the pipe. This layer will typically be just 3mm thick. If you consider the simple case where the misalignment of a pipe is more than 1mm and concavity is present, then you can understand that the quantity of protective alloy will be considerably reduced. Such a site can then act as a magnet for corrosion, which can compromise integrity and present potentially damaging consequences for the environment and the increased costs as a result of repair or reinstatement.
There are other considerations for weld inspection such as discolouration, porosity, weld width, but the three I have highlighted are central to pipeline integrity. Any engineer has a choice with regards to the hidden, out-of-sight area of the inside of the pipe during welding. They can trust to luck and judgement - or they can use proven technology to inspect and then deal with any problems before they become major issues. At OMS, we have invested heavily in creating the best tools to allow engineers to utilise the best inspection and measurement solutions available.