High Partial Pressure H2S Pipelines 2

[mcmidkiff]

We are constructing a pipeline which will operate at around 1000 psig. The gas will contain 1000 ppm to 4000 ppm H2S in natural gas. I have a chart that indicates the line should be constructed to NACE Plus. What is NACE Plus? I have done plant construction to NACE. However, I have not done pipeline construction to the extent of stress relieving welds. Is stress relieving typical for pipelines. Can X-42 pipe conform to NACE MR-0175?

 

[DrillerNic]

Basically at that PP of H2S, as well as suphide stress corrosion cracking (SSCC) which is what NACE MR 0175 is designed to prevent, you also get Hydrogen Induced Cracking (HIC, also known as hydrogen embrittlement or step wise cracking), which NACE MR0175 expressly does not consider.

SSCC is prevented by reducing the residual stress in the steel; HIC is prevented by altering the steel chemistry (low levels of manganese inclusions, micro alloying additions to control the shape of these manganese inclusions) and by controlling the steel's processing. Talk to a steel mill for more information and advice.

PWHT is often done on pipeline construction, especially offshore where they tend to use higher strength steels and automatic welding machines. Onshore it can be done, but it's a pain...

 

[SJones]

What is the "chart" you refer to? Is it part of the construction specification package? As for "NACE Plus", I've never heard of it (particularly as it is ISO 15156 outside of the US now). ISO 15156-2, A.2.1.4 states "Tubular products with an SMYS not exceeding 360 MPa.........are acceptable in the as welded condition". Of course, if the pipeline owner is sensible, they will still specify that welding procedure qualification shall include hardness testing. Only if the welds can't get through that testing should PWHT rear its ugly head.

Steve Jones
Materials & Corrosion Engineer

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

 

[MortenA]

As far as i know strictly speaking NACE is only for plant piping and equipment not for pipelines. I forward link to the NORSOK standards that includes a special spreadsheet for calculation H2S and CO2 corrosion including the effect of operating conditions and inhibitors etc.

http://www.standard.no/imaker.exe?id=1369

Best regards

Morten Andersen

 

[mcmidkiff]

I was mistaken in thinking the graph (chart) was part of MR-0175. Looks like it was attached to my hard copy of the Standard and not part of the standard. The chart plots H2S (ppm) against System Pressure (psia) and has lines for 0.05 psia H2S partial pressure and 5.0 psia H2S partial pressure. The NACE Plus Region is for partial pressures in excess of 5 psia.

 

[SJones]

I still can't see what specific technical requirements "NACE Plus" might entail. Perhaps some other contributor may be able to elaborate.

Steve Jones
Materials & Corrosion Engineer

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

 

[SJones]

Morten,

Table 1 of ISO 15156-2 states that it is applicable to "flow-lines, gathering lines, transportation pipelines for liquids, gases, and multiphase fluids."

Steve Jones
Materials & Corrosion Engineer

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

 

[DrillerNic]

NACE MR 0175 covers "Oilfield Equipment" and in the introduction says that this may include flowlines etc. Also in it's introduction it states that it does not cover other failure mechanisms as well as SSCC associated with H2S (ie HIC), nor does it cover Chloride Stress Corrosion Cracking or combinations of H2S and CO2 corrosion.

I was working on a Shell pipeline project several years ago and they had a specification for Line Pipe conforming to NACE MR0175 and a separate Sour Service Line Pipe specification. As I remember, the difference between the two was the sour service line pipe had much tighter limits on the steel chemistry and allowable processing (ie achieving strength by controlling the cooling rate was preferred to cold working the steel by rolling it and so on). MnS inclusions are very ductile and can form large'pancake' shaped inclusions in a rolled steel, which then act as nucelation site for atomic hydrogen fromteh H2S to form hydrogen molecules inside the steel matrix, leading to hydrogen blistering, step wise cracking etc.

My guess is that "NACE Plus" isn't an official specification and is proabaly a client specific shorthand for restricting the hardness as per NACE MR 0175 (to pretect against SSCS) and also reducing the amount of Mn & S and adding micro alloying elements to the steel chemistry (to protect against the other H2S failure mechanisms). Changing the steel chemistry radically means you might have to move to Pce rather than CE for your weldability considerations, too.

 

[SJones]

DN,

The usual Shell specification for linepipe kills the SSC/HIC birds with the same stone - seems strange to have 2 specs. ISO 3183-3 does a better job than API 5L in this respect too.

Personally, I hope never to have to refer to NACE MR0175 again as, to me, it is ISO 15156. ISO 15156-2 also deals with HIC, SOHIC, SZC, and SWC although it does not give the chemistry restrictions that have become the norm.

Steve Jones
Materials & Corrosion Engineer

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

 

[mcmidkiff]

So should I be constructing to ISO 15156 rather that MR0175? What is the difference between 15156 and 15156-2? I purchased 15156-2, assuming it was the latest, but I was a little unclear on how to specify to it, other than a blanket statement that the material shall conform. It looked like I could pick and choose whether I wanted strict conformance for all conditions or if I want conformance at a unique set of conditions. I also read the document to say that I could choose whether or not to require resistance to HIC, SOHIC, SZC, and SWC. Does 15156 read differently in this regard?

 

[SJones]

It looks like we are now moving into a thread that should come under the Engineering Codes & Standards forum. ISO 15156 is in three parts: Part 1 is a general part that outlines the principles behind the other two parts. Part 2 deals with carbon steels, low alloy steels, and cast iron. It would be beneficial to use Parts 1 and 2 together in order to achieve correct application. As to whether you pick and choose to select the "prequalified route" (Part 2 Annex A) or the "qualification by testing route" (Part 2 Annex B), and the HIC resistance requirements of Part 2 Annex B, B.4, to identify suitable materials cannot be advised here as it will be dependent upon the specific technical, legal, and contractual conditions pertaining to the contract.

My understanding is that ISO 15156-1,2,3 are supposed to be identical to NACE MR0175 and that the latter designation is only maintained because US laws explicitly refer to MR0175. I have not seen a copy of the "equivalent" version of MR0175 but am concerned when people refer to clause numbers in it that do not appear to match with ISO 15156 which suggests that, editorially, they may not be identical.

Steve Jones
Materials & Corrosion Engineer

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

 

[MortenA]

Steve

Thanks.

Best regards

Morten