How are pipes made? The fabrication process
Late last year we wrote about the history of pipes and how important they have been for human development. In this article we take a more in-depth look at the pipe fabrication process.
Seamless or welded steel pipes
The oil and gas industry uses steel pipes in a variety of sizes to store and transport hydrocarbons. These pipes are either seamless, formed from a steel billet that is heated and pierced, or welded, made from long flat pieces of steel that are curved into a tube shape and sealed along the seam. Both types of pipe go through three processes – first raw steel is transformed into a more workable form, then pipes are fabricated on a production line in a pipe mill and finally pipes are cut and modified according to a customer’s requirements.
Blooms and slabs
In the initial stages of pipe fabrication iron ore and coke are melted, blasted with oxygen to remove the carbon and poured into moulds to create raw steel ingots. Ingots are further processed by rolling to create “blooms” which will go on to create the billets that become seamless pipe and “slabs” which will become welded pipe.
Welded pipe
Once formed into slabs, the steel is further processed through heating, coating, descaling and rolling and formed into narrow strips called skelp. The skelp is coiled and sent to be made into welded pipe.
Electric resistance welding (ERW)
The steel skelp is unrolled and heated then passed through grooved rollers, which make the ends curl up and form the pipe shape. The unwelded pipe is passed through welding electrodes – the metal resists the current, which causes heat and molten pools where the edges come into contact with each other and that process forms the weld.
High frequency induction welding (HFI)
HFI welding is similar to ERW but uses an induction coil to create the current. HFI welded pipe is considered more suitable for use in the oil and gas industry because the process creates a stronger and better quality weld that is more suitable for pipes under high fatigue and stress conditions.
Longitudinal submerged arc welding (LSAW)
Longitudinal submerged arc welding is a process in which the weld and the arc zone are “submerged” under a layer of fusible flux. LSAW pipe is formed from steel plates rather than coils and is used for pipes with diameters larger than 24 inches or that require a particularly high wall thickness.
Seamless pipe
Seamless pipe is made when square steel billets are heated and moulded into cylinder shapes called “rounds”. The round is heated to white-hot in a furnace and rolled under high pressure. As the billet stretches out a hole forms in the centre and a bullet-shaped piercer point is pushed through. Larger diameter pipes then go through the mandrel mill process where after piercing the pipe shell is rolled through a mandrel mill with mandrel bars inside. This process creates a more standard outside diameter (OD) and controlled wall thickness (WT). Smaller diameter pipes are made using the plug mill process.
Oil and gas industry usage
All these types of pipe are in use throughout the oil and gas industry. Seamless pipes are popular because the lack of seam indicates fewer stress points and they have superior mechanical strength. They can also be formed into more precise and predictable shapes. Seamless pipes are significantly more expensive than welded pipes, however, and have long lead times, because they are made in very few places. Although the seam in a welded pipe creates a potential weak point constant advances in welding technology have improved quality enormously. Welded pipes also have more consistent wall thickness and much shorter lead times.
Measuring pipes with OMS tools
Quality control
Once pipes have been fabricated they are cut and modified to the customer’s specifications before going through a variety of quality control processes and inspections.
OMS offers a range of pipe and weld inspection services, which you can read more about here. Since our inception OMS inspectors have inspected hundreds of thousands of pipes and welds for energy industry clients throughout the world.
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Updated 28.03.2023
[5 minute read]