Why you need to measure pipe end dimensions
Pipe ends feature in every single joint and hold together some of the longest, deepest and most complex pipeline infrastructures known to man. The integrity of oil, gas and energy pipelines are dependent on pipe ends. Are they the unsung heroes of a sound pipeline installation?
Introduction to pipe end measurement
Pipes, tubes and ducts come in many shapes and sizes. Despite improvements in manufacturing methods, it’s rare for a pipe to be made perfectly round. The illustration below shows the commonly held belief that pipes are these wonderfully smooth and precisely circular-shaped objects. Unfortunately, this isn’t a reality. In fact, we know that pipes are never round, but more about this later.
First, let’s look at the terminology used when measuring pipes, explained in the graphic below.
Measurements across the end of a pipe are typically taken over 3 dimensions:
Outer diameter (OD)
Inner diameter (ID)
Wall thickness (WT)
The outer diameter of a pipe, often referred to as OD, is the total distance from one outside edge of a pipe across a continuous axis to the exact opposite outer edge.
The inner diameter, or ID, of a pipe is the measurement from a point on the inside of a pipe to the other inside edge across the same axis.
Pipe wall thickness, or WT, is the depth of material of the outer substrate of the pipe.
These 3 dimensions are pretty simple to measure, and for many piping applications they’re relatively unimportant. However, where pipes are to be used in situations where they might carry a product that is corrosive or located in a place where atmospheric pressure is high, for example, pipe end dimensions are a much more important consideration.
Why pipe ends need to be measured
Pipelines are generally used to carry substances from one location to another, often over distances from a location where human access is limited. When pipes are installed to create a pipeline infrastructure intended to carry hazardous materials or corrosive liquids such as oil.
The shape of pipe ends become crucial to the integrity of pipeline in high stress areas such as steel catenary risers (SCR’s). These are the pipes that carry oil from the sea bed up to the surface, typically to a floating production storage and offloading unit (FPSO). For example, when pipes are used in deepwater installations, the importance of pipe end shape being a close match to the adjoining pipe can make a huge difference to the welding of a joint. A welded joint that needs approval by a weld inspector will need to conform to industry guidelines, such as those set by DNVGL.
Failure to meet approval can not only have an impact on the installation schedule due to downtime, but it can be incredibly costly too. The joint may need to be re-welded, or worse, the entire pipe is replaced. On board a pipelay vessel where production schedules are critical to maintaining project budgets, the smooth-running of workflow of pipeline engineers in the firing line is very much dependent on good pipe alignment.
The importance of pipe alignment
Despite pipe ends being manufactured to specific tolerances, the fact remains that they are never perfectly round. To the human eye they might appear to be round, or certainly good enough for connecting to another pipe. But when measured using a tool that will accurately record the wall thickness all the way around the diameter of the pipe, the truth becomes clear. Imperfections can be determined when a pipe is rotated, or aligned, to match the next pipe end in the sequence. This type of alignment is often referred to as pipe fit-up. This is the process where pipes are checked against one another to achieve a high to low measurement, or ‘hilo’.
Achieving a hilo measurement that meets the project specifications is a key part of ensuring a weld that will pass weld inspection by a quality assurance inspector. A typical requirement for SCR’s might be to achieve a hilo measurement of 0.5mm, therefore, being able to measure an accuracy greater than this is necessary. If the hilo exceeds project specifications due to measurements being ‘out of spec’, rotation must be recalculated or new pipes brought in to replace them. However, both can be time-consuming unless you know which pipes fit-up best to each other.
How to achieve good pipe fit-up
Limitations of the pipe production process result in variations in the pipe wall thickness. With the tools to measure pipes from every aspect now available, pipeline engineers can meet project specifications safe in the knowledge that they have taken the appropriate steps to ensure the best possible fit-up.
Pipe end measurement technology has evolved from simple calipers to more advanced tools that can digitally measure pipe end alignment accurately with repeatable, traceable results that will satisfy the most stringent project requirements. Pipe ends can also be measured using laser tools which will capture the dimensions of a pipe end in under 12 seconds to create a point cloud of over 2,000 measurements.
These measurements can be read using dedicated software which has been developed to evaluate then compare multiple pipe end characteristics against one another. Pipe analysis software allows pipeline engineers to determine the best possible connections for a set of pipes prior to installation. This means that a pipe sequence can be established with guide marks to indicate the alignment of each pipe in advance of welding, minimising the risk of problems in the firing line.
Proactive measurement of pipe ends can not only have a significant effect on a pipeline installation projects through improved cost efficiency, but it can be the difference between a weld failing or continuing to perform in the field. And we all know what the effects of an oil spill can mean to the environment. The integrity of an entire pipeline has been proven rest on a single joint featuring 2 pipe ends for many well-known installations.
The bottom line is simple: pipes that don’t fit together properly are likely to be weaker than those that do. When you accurately measure a pipe end, you know its dimensions, which can allow you to better match it to another pipe end. This will give you a better fit, a stronger weld and a safer pipeline.
De-risking pipeline installations via a programme of up-front pipe measurement and inspection will greatly limit the likelihood of such catastrophic events. Can organisations afford not to do so?
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[5 minute read]
By James Sale