pipeline internal corrosion 3

[jfduroch]

A pipeline has been partially filled with seawater following its accidental rupture it is simple Carbon Steel X65. Let admit that there is 1mm corrosion allowance. How long would it take for seawater to degrade the line internally knowing that the line has been sealed plugged just after its rupture.

 

[gr2vessels]

jfduroch,
The corrosion process will continue until the oxygen dissolved in the seawater will be used up. Then it will stop. Perhaps not more than say, 0.1 mm. The presence of chlorides in the seawater will generate pitting locations of concentrated corrosion. If you drain, flush with clean treated water and dry the pipe, nothing much will happen, apart of that rusty colour inside the pipe. However, bring some fresh water with dissolved oxygen, the points of pitting will degenerate quite quickly in advanced corrosion points, most likely to perforate the pipe at a rate higher than in other places, perhaps at some 0.1 - 0.5 mm per year, depending also on temperature, fluid flow velocity, etc..There is a wealth of information on the net, have a go at it.
cheers, gr2vessels

 

[strider6]

what's the fluid inside the pipeline? gas? oil? a mixture? h2s?
to give a reply is necessary to have an idea of the type of service and chemical composition of the fluid.

S.

http://www.corrosionist.com

 

[SJones]

Depends what you mean by 'degrade'. The steel is constantly 'degrading' in air or seawater. If you mean how long before the corrosion allowance is consumed - difficult to say. The best advice is get the water out PDQ - if the oxygen and chlorides don't start something, the microbes might!

Steve Jones
Materials & Corrosion Engineer

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

 

[brimmer]

A good response from gr2vessels. You will not want to leave the sea water in the pipeline, eventually you will have to clean it and dry it. As far as a specific time period, difficult to predict beacuase you would be dealing with localized pitting corrosion which would most likely cause the failure. Corrosion rates for oxygenated sea water on carbon steel are around 60 mpy, this is a general rate, the pitting rate would potentially be much higher, but predicting how much higher is very difficult. It depends on your local conditions. Something to think about is if there is already pitting or corrosion in this line, you may not be starting from full wall thickness, but from existing corrosion so your corrosion allowance may already be gone or reduced.

 

[jfduroch]

Thank you all for your response which I did find absolutely satisfactory. We are definitely in the situation that gr2vessels describes. Repair is going to take place we are taking about a WI injection line the line will be flushed pigged and tested with treated seawater as it is plugged for the time beeing no fresh oxygenated water is circulating at the contained oxygene should have been used up.

 

[stanweld]

I hope that you added corrosion inhibitor for extended time periods in seawater.

 

[SJones]

A review of the following paper may be of some use:

Romero JM, et al, Localised Corrosion Of API X52 And API X65 Pipeline Steels In The Presence Of Treated Seawater Used In The Oil Industry, Corrosion, Vol 60, (7), 2004, pp689-696

Steve Jones
Materials & Corrosion Engineer

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

 

[strider6]

if your pipeline contains oil (h2S..) you can then have other problems other than simply O2 corrosion. Elemental sulphur deposition, MIC...

S.

http://www.corrosionist.com

 

[MortenA]

In WI pipeline corrosion caused by micro organisms (bacteria) is common causeing pit corrosion - maybe added by erosion since its a bend?

Maybe ruinning at batch of biocide would help - but that could cause leakage of biocide to the sea. Dont know how serious your local authorities is about this in your ares (dont know your area either

Best regards

Morten

 

[shelmikedmu]

I have experienced what you are asking and I can tell you that the time to deplete the oxygen is never really reached inside the pipe, due to dissimilar metal corrosion and areas of the pipe wall covered with or without oils, gells, or solids. The salt water is a great electrolyte and facilitates this corrosion - let alone the bacterial issues mentioned by others. Both will happen in the line, and simply waiting and not doing something proactive and positive to stop corrosion can be very costly for offshore lines. I would recommend sweeping out the water, following with a pig pushing an inhibitor matched to the corrosive agents in the water from samples. You can never tell what might be there from production. M. Ames -