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danyduclos (Mechanical) (OP)
25 May 10 16:30
I am currently looking into a piping life expectancy adjustment for accounting and i am not finding many resources on the subject. My objective is to document what would be ther normal life and be able to back it up with some sources.
The piping i'm lloking at is carbon steel fuel oil piping in a marine environment; anyone has infos on where i should look?

Regards
Helpful Member!  metengr (Materials)
25 May 10 19:27
Can you provide more detail as to the specific environment for this pipe system -exposure of the ID surface and exposure of the OD surface? For example, ID surface, fuel oil at x degrees temperature, with a flow rate of x and for the OD surface submerged in sea water or air exposure next to sea water at x degrees, etc.

What was the condition  of the pipe OD surface before service - bare or painted or coated?
danyduclos (Mechanical) (OP)
26 May 10 7:46
The objective is to estimate the life expectancy of the piping network, it is composed of pipes of 3/4'' to 16" diameter. Piping is painted except when it has heat insulation.
The piping is exposed to saline air, located near the ocean.
The client wants somethning like : the piping network should have a life expectancy of XXX with sources in litterature to back up my conclusion.
It is not common for this kind of thing. Having had a check a said piping with visual and ultrasonic inspection, i'd say the expectancy is 25 years, i just need something to back me up... if at all possible...
metengr (Materials)
26 May 10 8:59
In this situation you can try this approach; since the design life of the pipe is controlled by corrosion exposure versus other time dependent damage mechanisms, and some level of inspection had been performed after "x" years of service, one could conclude that the design life of the pipe is in excess of "y" years.

Your rationale for determining what "y" years should be may be as simple as; periodic inspection was used to extrapolate loss of wall thickness over "x" years. Assuming a negligible corrosion rate based on the past inspection over "x" years, the remaining service life will be at least 25 years (use a graph to show the calculated corrosion rate based on past inspection). Once the nominal wall thickness reaches a value below design (using a minimum wall thickness calculation referencing ASME B31.3 or other suitable piping code) minimum wall thickness based on the extraploated corrosion rate form above, this would be the point in time where repair or replacement is necessary.  

The reason 25 years is typically used as a benchmark is because most economic models for equipment use a 25 year "design life" for economic modeling. In reality, the "real design life" could far exceed 25 years, it could be 50 or 75 years.
 
Helpful Member!  SJones (Petroleum)
26 May 10 20:11
All of which is outlined in API RP 581 starting at Part 1, Clause 4.

Steve Jones
Materials & Corrosion Engineer
http://www.oilandgaspeople.com/cv/11499664
http://www.linkedin.com/pub/8/83b/b04
 

stanier (Mechanical)
7 Jun 10 0:13
A Discussion on the Corrosion Allowance topic from another forum.


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.

Fikry Barouky

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.

Harry Hanson

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.

Gerry Craft

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!

Vic Ashworth

 

corrosionman (Mechanical)
7 Jun 10 16:21
I would have thought that on any pipeline the cost of extra wall thickness for corrosion allowance would be a negligible fraction of the total  overall project cost, and it should give confidence in the long term life of the overall pipeline. Certainly there will be areas of localised corrosion - - areas where things happened that were not supposed to happen - - Possibly these are repairable with not too much difficulty, whereas replacement of the entire pipe might not be so easy.And are pipelines insured - - does a thin wall pipe cost only as much as a thick wall pipe?
Corrosionman
SJones (Petroleum)
7 Jun 10 20:13

Quote:

the cost of extra wall thickness for corrosion allowance would be a negligible fraction of the total overall project cost

At $1200 per tonne say, 3 mm on a DN 1050, 300 km pipeline is not a small number. The installation costs also go up for handling and welding.

Quote:

it should give confidence in the long term life of the overall pipeline

Corrosion allowance would be one small factor in the long term life of the pipeline, particularly where chemical treatments for corrosion control are required. It may also not be about the 'repair' when transporting toxic fluids as a repair is too late if it has already leaked.

 

Steve Jones
Materials & Corrosion Engineer
http://www.oilandgaspeople.com/cv/11499664
http://www.linkedin.com/pub/8/83b/b04
 

stanweld (Materials)
8 Jun 10 8:55
When I worked for an EPC firm, our general corrosion allowance philosophy for piping was:

1. Carbon steel with 1/4" C.A - Owner mandated carbon steel to keep initial cost down. Severe corrosion with design life past warranty.
2. Carbon steel with 1/8" C.A. - moderate corrosion with design life of 10 years minimum.
3. Carbon steel with 1/16" or less C.A. - Slight corrosion with design life > 20 years.

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