Exotic alloys hold the key to long-lasting tubing, explains Brian Mercer
The drive to exploit oil and gas fields in increasingly hostile, deepwater environments makes the job of producing ‘fit and forget’ tubing – tubing that will last for 30+ years – ever more critical. Every operator knows that checking for, and replacing, corroded tubes is time consuming and costly, not only in terms of the replacement operation itself but also in terms of lost production during the downtime.
The pursuit of this ‘holy grail’ has led manufacturers such as the UK-based Fine Tubes and its US-based alliance partner, Superior Tube, to develop high-performance exotic alloy tubing solutions for their customers. The move is away from standard 316L type grades towards more corrosion-resistant duplex and super duplex stainless steels, as well as titanium grades and nickel alloys.
UNS NO8904, commonly known as 904L, is a low carbon, high alloy austenitic stainless steel that is widely used in applications where the corrosion properties of conventional chrome nickel stainless steels are not adequate. The addition of copper to this grade gives it resistance – in particular, to sulphuric, phosphoric and acetic acids. 6 Mo (UNS S31254), on the other hand, is a super austenitic stainless steel with a high level of molybdenum and nitrogen, providing high resistance to pitting and crevice corrosion as well as high strength compared with conventional austenitic stainless steels.
Duplex stainless steels, combining approximately 50% austenite and 50% ferrite, are resistant to the cracking caused by chloride stress corrosion in austenitic grades and have improved resistance to localised corrosion. As such, typical applications for this alloy group include heat exchanger tubes, umbilicals, structural tanks, pipes, fittings and seawater handling systems.
One of the great advantages of nickel-based alloys is their excellent corrosion resistance in both aqueous and high temperature applications. One example, Alloy 625, is a nickel-chromium alloy used for its high strength, excellent fabricability and outstanding corrosion resistance in environments of unusual severity as well as its resistance to high-temperature effects such as oxidation and carburisation. Similarly, Alloy 825, an austenitic nickel-iron-chromium alloy with additions of molybdenum, copper and titanium, provides exceptional corrosion resistance in both oxidising and reducing environments and is resistant to chloride stress-corrosion cracking and pitting.
Titanium, because of its strength, unique density and corrosion resistance, has found applications in many industries. With a tensile strength greater than that of austenitic or ferritic stainless steels, it is also exceptionally corrosion resistant, making it a highly desirable material for high performance applications such as offshore drilling rig components and subsea equipment.
It has to be said, of course, that advanced materials call for advanced manufacturing processes – which is why Fine Tubes and Superior Tube have invested in production and quality control facilities that include:
- Cold pilger mills
- Cold draw benches
- Tube welding mills – in-line weld mills
- Controlled atmosphere heat treatment
- Pickling and passivation plant
- NDT ultrasonic and eddy current testing
- Hydrostatic testing
- Radiographic examination
For the instrumentation equipment that controls measuring devices and supplies hydraulic pumps and tools, primarily seamless precision tubing in straight lengths is used. This type of tubing is also used in onshore control panels, topside processing facilities, offshore subsea manifolds and templates up to 2,000m under
The greater lengths that can be achieved with this tubular technology have resulted in reduced operating costs, improved recovery methods and reduced capital expenditure by connecting remote and satellite wells to fixed or floating central operating platforms.
Using special orbital welds, coils of up to 13,500m can be manufactured for downhole hydraulic control and chemical injection lines, subsea hydraulic power and chemical injection lines, and smoothbore control lines used in fibre optic applications. Depending on requirements, coiled tubing can be manufactured in four main forms: seamless; welded; welded and sunk; welded and floating plug drawn.
As the drive to develop ever more remote oil and gas fields continues, so the demand for tubing that will continue to perform to specification under the most extreme conditions will increase. The key to meeting the demand for ‘fit and forget’ tubing undoubtedly lies in the ability to satisfy the need for high performance alloys whilst ensuring innovation production techniques drive cost competitiveness.
For more information visit www.engineerlive.com/iog
Brian Mercer is with Fine Tubes in the UK and Superior Tube in the USA