Subsea Pipeline - HSS disbondment - CP shielding 2

[Eyeoftheneedle]

During recovery of an installed pipeline it was noted that there had been some disbondment of some of the Heat Shrink Sleeves (HSS) at the field joints. This had led to some seawater entrapment, which could have potentially been in contact with the steel. As the HSS are noted for their high di-electric properties I understand that there is the possibility of Cathodic Protection (CP) shielding so the location of entrapped water could start to corrode as it is no longer protected by the CP system (sacrificial anodes).

I have a few questions that I can not readily find the answers to and would be grateful for some help with:

1. Is there any way that once the Oxygen 'runs out' in these areas that the oxygen could be replenished?
2. If it can be replenished does that mean that the same mechanism that replenishes the water will mean that the CP is also 'flowing'?
3. Once the oxygen runs out (and it is not replenished) then does acid corrosion or bacterial corrosion 'kick in', or does the corrosion mechanism then stop altogether?
4. If it stops then how do you calculate how what damage has occurred?
5. Is there any non-destructive way of finding out if the field joints that have not been cut back are affected the same way?
6. Is there anything retrospective that can be done to help this?

 

[BigInch]

The O2 will be depleated bonding to the steel and once gone should not be replaced unless the HSS allows a fluid path.

Its doubtful that you will do any more damage than making a very fine coating of top surface rust which will stop further corrosion from occurring underneath it.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SJones]

If the 'leak path' was open, e.g. the disbondment was simply a lifting of the sleeve at the factory coating interface, then CP current will have a conductive path. If the sleeve application resulted in voids that have collected permeated water over a long period of time (the sleeve will simply be a membrane) then CP current will not have got through and corrosion will remain active but probably at a low rate as the volume of electrolyte will be very low (as alluded to above in the 'fine coating of surface rust'). This may explain why the risk of corrosion under disbonded coatings offshore is considered low by some operators.

The use of abrasive blasting, with primer as a third layer of defence, is favoured to address the latter failure mode in shrink sleeves.

ILI will be the only effective way to pick up any ensuing corrosion problems.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Eyeoftheneedle]

BigInch - that's what I suspected (hoped) so glad you confirmed that for me. Thanks.

SJones - good clarification, part of my confusion was getting my head around the fact that if there is water flow then how can there be no conductive path - thanks for helping me with my sanity. ILI (in-line inspection, I presume?) sounds like it may be a good idea but as the pipeline is only recently laid I suspect that there won't be much change from the baseline intelligent pig runs as yet, or indeed any differences that may be found could well be put down to inaccuracies in the ILI technique, especially as any corrosion that has taken place already will be at very localised areas.

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Brilliant guys - responses from 2 of the best first time up...thanks again.

 

[SJones]

Now that a problem with field joints has been identified, it should drive the focus of the next pipeline CP survey to a level where highly localised coating problems can be detected. The results of the CP survey could be used to work out whether the next planned ILI survey can stay as scheduled or whether it needs to be brought forward. It then becomes a toss up as to which works out lowest cost as the specialist CP surveys aren't cheap in themselves, particularly if the line is on the deep side. I'm assuming that the post lay CP survey was not of the sort to identify the field joint problems since it took a retrieval to find them.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04