Identification of Stress Corrosion Cracking 3

[09091960]

Hi All,

Factors that considered to be effecting in a pipe line to have SSC is that its operating stress, operating temperature, distance from compressor station, age of pipeline and coating. Under these extreme conditions will the SSC cracks appear in a colony or else can it be just a single crack occurring from a corrosion pit? Further cracks appearing from a mechanical damage, arc strike can also be categorized under SCC?

 

[strider6]

I think you should define what type of mechanisms are you talking about. SSC is sulphide stress corrosion and is related to the presence of H2S, water and stress and you've to follow the requiremnts of NACE /ISO 15156.
SCC is stress corrosion cracking due to Chloride, CSCC, or caustic.

http://www.corrosionist.com/Corrosion_Type_Stress_Corrosion_Cracking.htm

S



Corrosion & Rust Prevention Control

 

[metengr]

Factors that considered to be effecting in a pipe line to have SSC is that its operating stress, operating temperature, distance from compressor station, age of pipeline and coating..

SCC requires three factors working simultaneously - susceptible material, tensile stress and environment. Take any of these three factors away and you have no SCC.

Under these extreme conditions will the SSC cracks appear in a colony or else can it be just a single crack occurring from a corrosion pit?

Normally, you have multiple cracks and branching even emanating from a single corrosion pit or multiple pits.

Further cracks appearing from a mechanical damage, arc strike can also be categorized under SCC?

No. An arc strike can cause subsequent SCC if the conditions I mentioned above are present.

 

[09091960]

Hi Metenger,

Quote:

"No. An arc strike can cause subsequent SCC if the conditions I mentioned above are present."

Therefore in an arc strike or else from a mechanical damage due to subsequent SCC cracks of colonies can appear. Will this final out come can be catogarised under SCC?

 

[metengr]

No. SCC is really a stand alone failure mechanism. Corrosion pit(s), stress riser or a locally hardened zone containing a small crack(aka arc strike) can initiate SCC given the three conditions I mentioned in a previous post. On the flip side, SCC can ocurr without the need for pits or an arc strike. In other words, an arc strike may initiate SCC but it can be an initiator for other types of failure mechanisms as well - fatigue crack propagation, brittle fracture. So, if you lump arc strike under SCC you might as well lump arc strike under other failure mechanism categories like brittle fracture, and fatigue crack propagation. See my point....

 

[09091960]

Appreciate your valuable feed back on this issue

 

[brimmer]

09091960,

To add to this, I believe you are asking about external SCC, or EAC (Environmental Assisted Cracking). Generally speaking, it is the external caustic environment which causes this on pipelines (H2CO3, HCO3, CO3 environment). SSC is something different.
SCC usually appears in colonies, however you can get single cracks. There are two basic forms, low pH (non-classical, transgranular), and high pH (classical, intergranualar). High pH SCC was first discovered in the U.S. in the 60's, low pH in Canada in the 70's. I am not sure which predominately they have in Australia.
Your factors listed above are correct, prioritize segments based on these factors. For coatings, tape and asphalt will be susceptible, newer coatings such as extrudued polyethylene and epoxies will not be susceptible.
I have attached some photos of SCC so you can see what some of this looks like if you have not seen it.

 

[stanweld]

brimmer,
Thanks for the great photos.

 

[09091960]

Brimmer,
Another excellant reply. In Australia there were incidents of low Ph SCC. I'm interested of your photo on circumferencial SCC which needs very high axial loads to occur. Will the cracks initiated due to a stress raiser in the material or else can this be due to normal operating conditions (> 60% SMYS)

 

[brimmer]

09091960,

Well, both can cause it to answer your question. However, usually it is due to axial stressess (on side slopes or axial slopes, slopes that may be moving in general. In the one photo you saw a gouge and dent, it is common for C-SCC to develop in these types of features as well. Cracks will initiate under normal operating conditions, remember that hoop stress on axial cracks is not the initiating force (so you can have these cracks initiate at lower hoop stress or SMYS as well). The cracks will grow perpendicular to the stress. However, that being said, once the cracks initiate, they can grow from fatigue, so if you have a liquid line pay more attention. Also, once cracks initiate, they can grow at static loads much lower than 60% SMYS. Another way these cracks can develop is from residual stresses on a joint, so if a joint is misaligned, or fitted in at a tie-in point incorrectly for example you can have these cracks develop. I notice you have the 60% SMYS level (which is from DOT regs). SCC can certainly initiate at lower strsses, so if your lines operate above 40% SMYS you still might want to do an SCC assessment. If your lines are liquid, there is no lower boundry, I have seen SCC in liquid lines that operate at very low stress levels.

 

[09091960]

Hi Brimmer,

Thanks for educating on this very important topic.Industry experience to date has shown that SCC investigation program development can be highly dependent on the specific characteristics of particular pipeline systems, segments and geographical areas.Obviously this required experience from the operator to embark on an SCC investigation program.
Will the direct assessment method should be combined with a ILI tool to get best out of the SCC investigation program?

 

[brimmer]

09091960,

Can you please clarify your question for me, specifically what kind of ILI tool are you referring?

 

[09091960]

Brimmer

ILI tool which is capble of finding cracks in a gas pipe line. An Ultrasonic EMAT tool?

 

[brimmer]

Okay, generally you would do one program or the other, so either you would run a crack tool (an ultrasonic crack detection tool, or EMAT tool), or complete an SCCDA program without the pig. If you know your line has SCC and it is severe enough, I suggest to run a tool. If you don't know if your line has SCC or has no history of SCC, usually the cost of a tool is not justified, as these runs are expensive. You can develop an investigative dig prgram to select areas of higher SCC susceptibility and see what you find. The latter is an art form in itself, as you mention above specific to segments/areas. EMAT crack tools are only available in larger diameters at the moment, UTCD tools are down to 8 inch now, but they have to be run in liquid, so if your line is gas you will need to run a slug of water or deisel. Also the decision to run a crack tool will come down to your companies tolerance to risk, if you have a line running through population and are risk adverse, you might decide to go ahead with the tool.

 

[09091960]

Thanks Brimmer & all those who were involved in the discusion on SCC which was very helpfull to improve my knowledge in this area.