The following discussion that occurred on the NACE corrosion network indicates what some corrosion experts know of the subject.
The Question: Sun, 28 Jan 2001 10:02:35
We, like many other pipeline owners, use a corrosion allowance (typically 3mm) in the design of our flowlines and trunklines (carrying oil, gas or water). Since nearly all of the corrosion that we experience is very localised (typically small pits), this begs the question: Is Corrosion Allowance a necessary , cost effective & useful addition to the pipeline design? Can the money not be better spent on internal lining, supplementary inhibition or more inspection & monitoring?
Ali N Moosavi, PhD
Answer 1: Sun, 28 Jan 2001 13:15:41
Taking into consideration the techno-economics and reliability of the structure, corrosion allowance is the better option. Internal lining or use of inhibitor may work. But their selection needs time and money and their reliability is always doubtful. To make it more reliable you have to introduce monitoring and /or frequent inspection. These are additional burden of time and cost.
Nausha Asrar, Ph.D
Answer 2: Sun, 28 Jan 2001 13:59:17
I agree with Mr. Asrar because localized corrosion might happen any time whether you monitor the lines uniformly or randomly since you can not go to inspect every section of your line due to its high cost. Therefore, we have to all agree that we have no guaranteed method of preventing or monitoring any or all the localized sections like pitting and so ever. Moreover, corrosion allowance with the corrosion rate help us to estimate the life of our piping and/or equipment under various process circumstances that normally improve or shorten their life.
Mohammed H. Al-Anazi
Answer 3: Sun, 28 Jan 2001 15:26:52
I am aware of a growing number of pipeline projects which are going away from the conventional and more conservative approach of using corrosion allowances. These have typically been in deep waters (and some not-so-deep waters) and are stretching the demands for reliable online corrosion monitoring capabilities in remote areas. Reliable, real time (at least once or several readings per day) are necessary to provide the basis for doing away with the corrosion allowance. The basis for this also include using multiple corrosion monitoring techniques as a check and balance. It also requires that you have a premium performing chemical system available for mitgating corrosion to ultra-low levels as most of these projects are 20 year life minimum.
Dr. R.D. Kane
Answer 4: Sun, 28 Jan 2001 15:54:55
I do agree with Ali Moosavi. Corrosion allowance has insignificant contribution when localised corrosion (pitting) is considered as the only corrosion effect. I have seen cases where corrosion allowance reached 6mm and perforations took place in less than two years in a flowline that was designed for 20 years. But to be more specific you should consider an overall wall thickness to stand erosion and wall thinning effects due to the flow characteristic, e.g. turbulent flow, suspended particles and particles size, and also the possibility of having uniform corrosion in the system. For flowlines, the possibility of using RTP (Reinforced Thermoplastic Pipes). pipeline is recommended industry-wide after it has been proved to be working without corrosion problems, however, other problems associated with the material properties are yet to be rectified.
Answer 5: Sun, 28 Jan 2001 16:07:33
Corrosion allowance was never meant to provide enough steel to let a corrosion problem go unattended. A corrosion allowance does provide a little time so that one can detect corrosion problems and devise a remedy.
Answer 6: Mon, 29 Jan 2001 11:00:43
Corrosion allowance has been an accepted design criteria and tends to keep local designers and regulators happy. As was previously said the mode of failure is localized and corrosion allowances do not give you much protection. I have been tracking corrosion failures for twenty years in the Gulf of Mexico. I think we are starting to see more emphasis on locating the areas of high risk and adopting more detail design methods to control corrosion failures such as appropriate metallurgy and non-metallics (including composites, coatings and linings).
I am afraid because we don't typically investigate our failures, and if we do, we do not like to share them with others. Thus we have very little data to make design decisions on. We are also starting to put to much emphasis on localized monitoring and risk base evaluation. From my experience I would spend more time on upfront control. Materials and Corrosion engineers are brought in on design of projects often too late and then materials upgrades tend to look very cost prohibitive.
Charlie F. Speed
Answer 7: Mon, 29 Jan 2001 13:35:10
When having a cracking possibility (as is the case when we have pitting) and using a Fracture Mechanics approach we know that the plane stress conditions have tendency to change to plane strain when the thickness increases, A crack in this plane strain situation will grow, while in plane stress has the tendency to arrest, indicating that the thicker the material highest the cracking risk. Monitoring more that inspecting, I believe, is the newest and more correct strategy to follow.
Enrique A. Martinez.
Answer 8: Tue, 30 Jan 2001 11:16:09
In the water and wastewater industries in the USA, it is my opinion that most engineers have increasingly shifted away from specifying a corrosion allowance over the past twenty years or so. A corrosion allowance would be very cost effective if corrosion took place uniformly over a pipeline. Of course, we have learned this is not the case, since most corrosion failures I have seen are the results of localized pitting and penetration of the pipe wall. I believe the thinking is now that a corrosion allowance is very expensive in terms of buying a very little bit of increased service life. It seems that the money is better spent on proven, cost-effective corrosion mitigation efforts. All the more so when considering life cycle cost analysis in light of the fact that many water and wastewater systems are expected to have a useful service life of fifty to one hundred years and beyond.
Answer 8: Thu, 1 Feb 2001 08:35:58
In order to "approach" the position of reducing or eliminating the Corrosion Allowance, we need to place a major commitment to changing the way systems operate (i.e. thinking outside the box of how corrosion engineers normally think). This includes investing in a combination of systems including system design, chemicals and high reliability monitoring. In some operating applications, this new mindset is critical to economic, long term operations. This is what has lead to a few major companies efforts and their input and scoping of our new JIP effort in deep water pipelines.
Dr. R.D. Kane
Answer 9: Thu, 1 Feb 2001 13:01:18
Where can I buy these amazing 'corrosion mitigation effects' that reduce the corrosion rates to zero indefinitely and will allow me to do away with a corrosion allowance? I suspect that they may not exist and that I would have to rely on material in the pipe wall above that required for pressure retention to take up the slack. Another name for a corrosion allowance in my book!