Material Selection - Salinity problems. 2

[KlausTruelsen]

Good day.

I’m a newly graduated engineer and I work in the Oil & Gas industry in Denmark. I have been tasked with performing a material selection for an offshore oil facility in the North Sea. As part of this assignment I have access to the predicted flow parameters and I must admit that the salt content predicted have me stumped somewhat.

My question is; Are the materials I have specified as options acceptable for the service?

My two material options are.

- Inconel 625 cladded Carbon Steel

- Super Duplex stainless steel

The process parameters are;

Pressure: 780 Bar (Before PSV, 150 Bar after)
Temperature: 160 deg C
mol%CO2: 0.1-0.2
ppm H2S: <5

Total Dissolved Solids: 106868 mg/L
Chlorite content: 64830 mg/L

Density of formation water is about 1077.2 g/cm3

I look forward to hearing any thoughts on the subject.

Kind regards.

- Klaus Truelsen

 

[TVP]

NORSOK standards

http://www.standard.no/no/Fagomrader/Petroleum/NORSOK-Standard-Categories/M-Material/

DNV standards and recommended practices

http://webshop.dnv.com/global/category.asp?c0=2624#2612

 

[SJones]

Take us through how you arrived at the two options.

Steve Jones
Materials & Corrosion Engineer

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

 

[KlausTruelsen]

I may have been somwhat unclear about the application of the material.

The materials in question is to be used for process piping in the meantioned process parameters.

I have performed a CO2 corrosino rate prediction based on the C. de Waard model and im predicting 4-4,5 mm/year corrosion based on this. This is unacceptable for the use of carbon steel.

From here on the two material options are primarily based on the temperature and the salinity of the FEED.

I hope this cleares up my situation.

- Klaus Truelsen

 

[strider6]

the corrosion rate calculated with different models based on DeWaard usually are slightly different but at that temp and pressure it's likely that a carbon steel is not suitable.

In my opinion as a first impression the two alternatives 625+CS or Super duplex are acceptable.

there are some guidelines that you can use to select your materials:

A Guideline To The Successful Use Of Duplex Stainless Steels For Flowlines

http://www.intetech.eu/images/stories/papers/Paper51.pdf?phpMyAdmin=da2b7f4483f7d9630453667795e437cf

Selection guidelines for corrosion resistant alloys in the oil and ...

http://www.stainless-steel-world.net/pdf/10073.pdf

and also you can use the NACE / ISO.

other factors that you've to consider and that you've to exclude are the presence of elemental sulphur, but i think taht with your H2S pression in shloud not be present, and also the presence oxygen.

S




Corrosion Prevention & Corrosion Control

 

[KlausTruelsen]

thank you strider6.

I'll be reading you'r references as well as taking a look at NORSOK M-001 again. From the process information i have available elemental sulphur will not be a problem, neither will the H2S partial pressure (even before the PSV)

- Klaus Truelsen

 

[SJones]

For PSV, do you mean choke valve? At 5 ppm, the partial pressure of H2S in the API 15000 rated section exceeds the limits of ISO 15156 and you will have to use materials that are compliant. Work on design pressure for the calculation and don't believe the reservoir engineers who say less than 5 ppm - check whether there will be water injection for example. Take a look at the dimensions of API 15000 rated piping and valves and see whether a solid CRA is cost effective. The PN 150+ piping is straightforward and a 25Cr super duplex will work just fine but watch out for external chloride SCC issues at 160 deg C or CUI if the line is insulated.

Steve Jones
Materials & Corrosion Engineer

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

 

[KlausTruelsen]

Yes saying PSV i meen the choke valve. sorry for the confusion.

I haev rechecket the ISO 15156/NACE H2S limitations and the 5 ppm H2S will result in sour service at 760 Bar but within the allowable limitations for Super Duplex SS.

I have specifyed NORSOK M-501 paint codes as to avoid SCC problems and as of now no insulation will be used.

Based on "

http://www.stainless-steel-world.net/pdf/10073.pdf"

Duples steel will be acceptable in reguards to the salinity contenth of the streem.

right now my primary worry is that the mixture of salinity and potential sour service at a later time will be problematic.

Thanks for you'r help to all of you.

- Klaus Truelsen