Quality and consistency are key for coiled tubing in oil and gas

Rodrigo Signorelli, our Lead Technical Manager for Marine & Energy, shares insight into the alloys used to make coiled tubing for downhole dosing and hydraulic control lines in the upstream oil and gas industry. 

Corrosion resistant alloys are essential as the material used to manufacture coiled tubing for chemical injection lines and control lines. As chemical injection lines, this tubing enables wellhead engineers to dose wells with fluids that enhance separation of sand from oil and gas, reduce foaming, enhance flow rates or inhibit corrosion. For hydraulic lines, for example the tubing provides control for surface-controlled subsurface safety valves (SCSSVs), which shut down production to protect safety if pressure in the line drops. 

The key for these applications is that they must provide complete reliability in subsea and downhole environments that are inaccessible to maintenance crews at up to 10,000m below the surface.
Lines are typically 3.2 to 15.9 mm outside diameter, or even 25.4 mm. They need to withstand the corrosive effects of chemicals and sour environments and also have sufficient mechanical strength to withstand high pressure environments. Fatigue strength is also important to prevent the formation and growth of cracks due to cyclic loading. 

Nickel-based Ultra Alloy 825 is often used for these types of lines in oil and gas. It resists stress corrosion cracking (SCC) and sour media. However, our  grades of Supra 316L, Forta DX 2205 or 2507 may be suitable in some cases, whereas it may be necessary to step up to Alloy 625  in very challenging environments. 

Once the alloy is selected, manufacturers need sophisticated production lines to bend and form strips of alloy into shape. To contain pressure, this type of tube often has relatively thick walls of 0.56 to 2.1 mm. This requires powerful machines to bend the strip into a longitudinally welded tube, which can be further drawn over a mandrel. 

It’s also essential to follow high quality standards and use extensive testing to meet certification requirements and assurance over health, safety and the environment. 


Schoeller Werk’s high production values

Schoeller Werk is one example of a manufacturer that is leading the field. It uses our high quality alloys to produce longitudinally welded tubes capable of containing pressure of up to 2,500 bar. 

After forming alloy strip into tube of up to 2,000 m in length, computer-controlled TIG welding is used to ensure the highest possible quality of the longitudinal seam. The tube is then drawn over a mandrel to control the dimensions and interior smoothness of the tubing. Inspection after the drawing process has found that the microstructure of the welded seam is almost identical to the tube wall – which shows the welded joints will have similar performance to the tube body.

Another test used by Schoeller Werk is air under water (AUW) testing. This checks the soundness of tubing by submerging tubes under water and filling them with pressurised air to 210 bar. A visual inspection for air bubbles quickly shows when tubes are airtight. 

Orbital welding is then used to join individual lengths of tube to provide customers with coiled tubing of 15,000 m or longer. X-ray inspection is then carried out to ensure these welds are airtight and free from defects. A final hydraulic test is performed at up to 2,500 bar. 

The coils may be delivered to the customer at this point. Alternatively, they may be bundled into an umbilical package that also contains multiple control or injection lines, as well as cables and bumper bars. These can be encapsulated together in plastic for ease of handling and mechanical protection. 


Certification and consistent high quality

Another important aspect of producing coiled tube is material certification. This ensures traceability and helps to control risk in upstream operations. When we deliver material for coiled tube, we need to apply a comprehensive package of specifications and certification requirements. Typically, these are unique specifications that combine technical requirements from multiple sources: the operating company, the engineering, procurement and construction (EPC) contractor and the tube producer. 

We use strong project management to meet these requirements. This ensures consistent high quality production covering areas such as:

  • Alloy composition to meet the minimum requirements of the standards, as well as the requirements of the oil and gas industry.
  • Mechanical properties that are ideal for tube-forming while providing the required strength.
  • Tight mechanical tolerances for efficient production – consistent width supports smooth and uninterrupted production, whereas consistent thickness avoids over-specifying the amount of material required for each tube.
  • Surface finish with low roughness to minimize friction during the drawing process and enable production of tubes with smooth internal walls.

Certification also covers requirements from industry bodies such as ASTM and Norsok, as well as 3.1 and 3.2 certification under the EN10204 standard.


The drive towards accurate sustainability data

When it comes to ordering material, carbon footprint is becoming more important as oil and gas operators are facing growing pressure to measure and report on sustainability. 

Until now, OEMs have often used an average carbon footprint, such as the European average carbon footprint of 2.8 kg of CO2 per kg of stainless steel. However, different suppliers have different performance. At Outokumpu, we provide an advantage as our average carbon footprint is around 1.8 kg of CO2 per kg thanks to our use of a high content of recycled steel, as well as sourcing of zero-carbon energy at our mills.

Like any commercial data, there’s a lot more to carbon footprint data than meets the eye. 

One aspect of this is that the carbon footprint of any steel product varies, depending on its alloying content, the technology and production steps used at the steel mill, and the energy mix that is source by the mill to power production. 

An additional factor that makes it hard to compare suppliers is that they may be applying the EN 14040 life cycle assessment differently. The product category rules (PCR) in the standard can be applied in different ways and vary radically if different assumptions are applied. This can create significant variation across Scopes 1, 2 and 3. 

When faced with average data, buyers who want a true comparison should ask their suppliers how they applied the PCR and then crunch the numbers themselves. 

However, at Outokumpu, we are providing customers with more accurate data than is available from other sources. In November 2022, we started publishing the product-specific carbon footprint on the product certificates that we supply when making deliveries to customers. This is based on a rolling average that takes account of all the variations in production that influence carbon footprint and has been verified by engineering consultancy WSP. 

The result is that our customers can get accurate carbon footprint data so that they can have greater confidence in the accuracy of their own carbon footprint calculations. 

Learn more

Oil and gas
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