Problems in Polyethlene-backed tape coating in gas pipe lines

[09091960]

A vital factor to consider is coating type which has a profound effect on SCC(Stress corrossion cracking) in gas pipe lines. This is especially true when the coating has a tendency to disbond or forms a holidays. This is true for tape coatings, such as polyethylene-backed tapes used predominantly in early 1960s to 1980s.” Tenting occurs between the pipe surface and the tape along the ridge created by longitudinal, spiral, and girth welds. When the tape disbond from the pipe, moisture can accumulate beneath the tape surface. The tape itself has fairly high electrical insulation properties, thus preventing cathodic current from reaching entrapped moisture beneath the tape at the pipe surface. The cracks tend to occur at or near the toe of the seam weld where stress is concentrated and water has access, as well as where the coating has been damaged or disbonded.
Most of the gas pipe lines we are having got tape coating method and constructed in 1970. We are having lot of CP issues.W'll be having our Pigging run in couple of weeks time.What are the remidial action i should take in terms of improving the CP in this sort of conditions.

 

[SJones]

This is probably not the first in line inspection of the pipe; so, in spite of the 'CP issues', how is the pipe actually doing in terms of metal loss? The results of the new run will tell you whether any metal loss problems to deal with. As for SCC, there are a number of other factors to consider beyond the tape coating. Is the soil environment conducive to SCC? Are the operating stresses high enough? In summary, a risk assessment and verification programme is required which may, or may not, result in the need to address the 'CP issues'.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[brimmer]

I do not know what you mean in terms of having CP issues, but if you have disbonded tape, you will undoubtedly have shielded CP conditions somewhere. It would help to some extent to make sure CP criteria (NACE RP0169) are being met, but even so, somewhere in the tape disbondments, adequate CP will not reach the pipe and conditions could lead to corrosion. Seasonal variation in the soil will also affect this. Also environment will play a big role in terms of SCC susceptibility. If you are concerned about SCC there are some things you can do in terms of completing environmental and engineering models to predict where your pipe is more susceptible to SCC and you can complete an SCCDA program. The ILI data should give you an idea of overall coating conditions and of course to what extent your pipe is corroded. SCC will not only occur at the welds but in the pipe body, so if you inspect your pipe (during the ILI validation digs/worst features) look for SCC at all coating disbondments and welds at a minimum (SAW welds more tenting than ERW welds). Have a look at the corrosion deposits on the pipe below the tape, they will give you some idea as to shielding conditions. A first step in terms of SCC would be to determine if your line has SCC, you already know based on coating type can be susceptible to SCC.
More specifically to your CP question, improve CP to meet NACE criteria as above, but shielding will still occur in places leaving pipe susceptible to corrosion/SCC, the only thing you can do is recoat the pipe with a better coating to ensure no shielding. Meeting CP criteria will not necessarily mitigate SCC/corrosion with this coating.
I don't know what stress level this line operates, but this also plays a large role in terms of SCC susceptibility, perhaps your line does not operate at stress levels conducive to SCC?

 

[stanweld]

09091960;
I take it that the cracking experienced is predominantly along the longitudinal weld seam in the HAZ under the bridged location. This was always a weak spot when using the shop tape wrapped systems and there is a hard spot along the HAZ more susceptible to SCC. The HAZ is also most susceptible to developing fatigue cracks during rail shipment.

Is SCC occuring at specific temperature or soil conditions? SCC was most often assocaited with piping downstream from compressor stations. If so you may need to dig up the involved pipe and recoat if condition warrants.

My testing programs always showed poorest cathodic disbonding with the plyethylene cold applied, tape wrapped coatings.

 

[09091960]

Mny thanks for Jones, Brimmer and Stanweld for your inputs in this topic of CP, SSC and Coating issue