m-pipe® PEEK performance
The limitations of steel pipe versus m-pipe® composite pipe utilising PEEK
Reviewed by Geoff Small, Victrex market Technology Manager
Many variants of high performance steels have the high strength, stiffness and good performance at high temperatures required for subsea oil and gas pipe. However, corrosion is the significant drawback in the use of steel pipe for offshore deployment, estimated to cost the offshore oil and gas industry more than $1.3 billion every year.
Steel corrosion poses an insidious threat which often seriously undermines the structural integrity of oil and gas pipes. Even titanium is susceptible to crevice and pitting corrosion at temperatures above 100°C, conditions that are common in deep water fields.
Some steels can form passive surface layers when exposed to aggressive media, but these layers can be damaged or even completely removed by external subsea erosion, from buckling, wear or damage to the external pipe. Where fresh metal surfaces are regularly re-exposed, the corrosive wear removal rates of material can be highly significant.
Corrosion of steel can be dealt with by a range of techniques such as coating, painting, sacrificial anodes and chemical inhibitors. However, none of these methods is 100% successful, most are expensive and, in the case of inhibitors, they can represent a seriously detrimental environmental threat.
PEEK – the solution to steel pipe limitations for the oil and gas industry
Polymers have long been used in the oil and gas industry on account of their excellent corrosion resistance. The major barrier to their wider uptake has been the lack of mechanical properties in service environments at high temperatures, as even the highest performing thermoplastic polymers such as PEEK lose significant strength and stiffness around 200°C.
This problem is addressed by the use of fibre reinforcement in the form of high performance thermoplastic / carbon composites, where high strength and stiffness is derived from the fibre reinforcement. The most common continuous fibre reinforcements are carbon fibre and ‘S’ glass, which provide combined mechanical properties that exceed those of steel.
Unlike steel, polymers are also inherently more resistant to chemical attack or corrosion. Most thermoplastic polymers have good all-round chemical resistance to sea water, crude hydrocarbons and a range of solvents. Unlike steels, this resistance is based upon a fundamental lack of reactivity in that environment rather than the formation of passive layers.
Unlike some thermoplastics, which can only withstand temperatures of below 130°C, Victrex PEEK polymer pipe benefits from a high level of mechanical strength retention even when temperatures are high, combined with an exceptional chemical resistance to 200°C. Unlike steel, PEEK does not rely on a passive layer build-up, which makes it suitable for subsea applications which involve sea water, potential wear, abrasion and erosion.
Why m-pipe® is the world’s most reliable subsea pipe
The basic building blocks of Magma m-pipe® are Victrex PEEK, carbon fibre and S-2 glass fibre, one of the highest performing glass fibre systems available. Since all these fibres are effectively inert, the chemical resistance of m-pipe® in aggressive subsea environments relies upon the exceptional chemical resistance properties of PEEK polymer.
In the oil and gas industry steels are susceptible to a wide range of corrosion processes, the most widespread type being where steel comes into contact with water to form rust. Both carbon dioxide (CO2) and hydrogen sulphide (H2S) are also catalytic to the corrosion of steel.
Although dry CO2 is not itself corrosive, it dissolves in water to form carbonic acid, which then reacts with the iron to form iron carbide and hydrogen.
In sour corrosion iron reacts with H2S in the presence of water to form iron sulphide, which may form a passivating scale, but is vulnerable to removal. Another feature of hydrogen corrosion is hydrogen embrittlement.
Both these gases can also be readily formed by bacterial activity, sulphur reducing bacteria being a common example. Oxygen promotes corrosion by accelerating the effects of H2S and CO2.
Erosive corrosion occurs where the passive layers are continuously removed by the flow of fluids with particulates over the surface and may be most significant in areas of high turbulence.
In these common environments, PEEK polymer exhibits remarkably good resistance. In an aqueous environment such as sea water there is little effect of ageing on mechanical properties after prolonged exposure, even at temperatures of 200°C.
14 days exposure to seawater at 200°C produces a 4% increase in modulus and a 5.5% increase in strength. Addition of carbon dioxide to the environment and the presence of carbonic acid make little further change and Victrex have measured no effect on mechanical properties at 100°C. Whilst higher temperatures have yet to be fully evaluated, no significant effect is expected.
In the presence of H2S, PEEK shows similar resilience. In industry standard tests such as NORSOK M710 and ISO 23936 where a standard environment contains 10% H2S in the gas phase, there is no change in mechanicals over several thousand hours at up to 220°C, and even 20% sour gas produces no change in strength.
The physical appearance of the polymer also remains virtually unchanged despite its exposure to this hostile environment.
In order to understand the response to H2S better, Victrex has evaluated the performance of PEEK in a 100% sour gas environment and found that it is only above 200°C that any change in mechanical properties begins to develop.
Once again, the appearance of the polymer is almost unchanged by this exposure.
Finally, when considering erosive corrosion, this is not a criteria which affects PEEK at all.
Firstly, there is no passive layer build up on the exposed polymer and, secondly, PEEK demonstrates extremely high resistance to erosion, exhibiting around twice the erosion resistance of steel based on volume of material lost.
Advanced thermoplastic polymer composites such as those utilised in Magma Global m-pipe® provide equivalent mechanical properties to high performing steels over the entire temperature range of both current and future projected oil and gas industry requirements, including those conditions found in deep water and sour service environments.
In addition, thermoplastic polymer composites also have the significant additional benefits of lighter weight (around one tenth that of equivalent steel pipe in sea water) and none of the detrimental corrosion processes suffered by steel pipe.