Subsea Tie-in Spools, Low design temperature and corrosion resistance

[newerbee]

Hi All,

We have a subsea well tie-in spool with minimum design temperature of -85degC (although the duration at these temperatures will be very short). The material has to be corrosion resistant such as 316L. However the required wall thickness of 316L material for pressure containment makes it impractical. 25%Cr would be accepted for its strength and corrosion resistance however it is limited to -46degC, so not suitable.

What material would you recommend for these applications? Any comments for their weld ability, H2S issues or CP compatibility would also be appreciated.

Is cladding of cryogenic materials such as UNS K81340 a normal practice?

Thanks in advance.

 

[SJones]

Setting -85 deg C as a minimum design temperature is going to give problems. Does the design code differentiate between minimum metal temperature and minimum design metal temperature - the two are not necessarily the same? If solid CRA does not fit the project requirement, then clad steel will be the only option, typically with N06625, but that may entail some fitness for service type approach to temperatures and stresses with associated controls to prevent pressurisation whilst colder than the minimum allowable temperature - see API 579-1, Part 3.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[cap1a79]

Newerbee,

Note that when applying cladding, the base metal (which is covered with clad) shall withstand against not only process pressure but also minimum temperature. As the query does not have detail information, I would give brief guideline as below.

1) Internal corrosion - Check whether the selected material is suitable against process conditions. If it is upstream crude oil, you need to check H2S partial pressure, temperature, chloride and so on based on NACE MR 0175.

2) External corrosion - If it is subsea application, the selected material shall be suitable against sea water environment. Generally, SS 316L could have pitting and crevice issue although it is already out of the option based on pressure.

3) Code compliance - It shall be in line with applicable codes. Again, note that the base metal (which is covered with clad) shall withstand against not only process pressure but also minimum temperature.



Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[newerbee]

Thank you for the replies

Lee, to answer yor comments

1- Alloy 625 will be used as the clad material for internal corrosion protection. the proposed base material is a low-carbon nickel alloy steel with nominal composition of 9Ni, which is particularly used for low-temperature or cryogenic pressure vessels and plants for processing, transportation, and storage of LNG down to -196°C [google]. I believe H2S pp chloride content of the fluid will not effect the base material due to the clad barrier.

2- CP will be the external corrosion protection method in addition to paint.

Nace MR 0175 is not applicable when Ni>1% for low alloy steels. However is sour service requirements still applicable to the base metal when you have clad?

 

[newerbee]

Steve,

I am not sure if I understand the difference between the minimum metal temperature and the minimum design metal temperature. The client has only one "minimum design temperature" which may only be seen during start-up due to Joule–Thomson effect.

 

[SJones]

Minimum design metal temperature is the lowest temperature at which the system will see design pressure. Minimum metal temperature has no pressure value associated with it. As for using 9Ni with 625 cladding subsea, good luck with that. It does not feature in any subsea materials selection menu.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[cap1a79]

Newerbee,

1 - As long as it can be demonstrated for long term integrity of the cladding, NACE MR 0175 for base metal can be waived. End user concurrence might be required.

(Quoted from NACE MR 0175-3, A.13)
"Unless the user can demonstrate and document the likely long-term in-service integrity of the cladding or
overlay as a protective layer, the base material, after application of the cladding or overlay, shall comply
with ANSI/NACE MR0175/ISO 15156-2 or this part of ANSI/NACE MR0175/ISO 15156, as applicable."

2 - Actually, if qualification test such as SSC testing is not supposed to be proceeded for the base metal, 9% Nickel is not allowed by NACE MR 0175, PART 2, A.2.1.2 & A.2.1.4. In case the demonstration as above No. 1 is not achievable, SSC testing shall be organized.



Lee SiHyoung,

 

[SJones]

Get smarter with the temperature derivation and control aspects as opposed to the materials.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[cap1a79]

Newwebee,

I think, as per the Steve's comment, it is worth reviewing API 579 in case the design allowable stress is below 25 ksi & stress ratio is below 0.3. At first, check the two of the values.

I have seen the application of LTCS based on stress analysis even for the low temperature which is below the common criteria of LTCS (ex., -48 C). From metallurgical aspect, there shall be a power (=stress) to make crack propagate. As it were, if the power is not sufficient, then cracking failure may not happen. For many cases, the minimum temperature is decided by "depressurizing" condition (unless it is extreme cold weather) and the power to propagate crack could not be enough unless there is heavy membrane stress (ex., nozzle load)

The below approach is one of the examples based on fig 3.7 of API 579 & clause 3.4.3.1, 3.4.3.4 and ASME B.31.3 Fig. 323.2.2B.

--> " Based on ASME B.31.3, if the stress ratio (Note 1) is below 0.3 and design allowable stress is between 25 and 20 ksi, the minimum "allowable" temperature can be set at -104 C."

Note 1 - The stress for the operating condition under consideration divided by the stress at the design minimum temperature.


Good luck!.



Lee SiHyoung,

 

[SJones]

@newerbee - any final thoughts to close the thread?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[newerbee]

Steve, no conclusions yet. This could turn into a lab work.

Regarding your comment "As for using 9Ni with 625 cladding subsea, good luck with that. It does not feature in any subsea materials selection menu", What do you think if it were low alloy steel with 3.5% Ni and 625 clad? This fits the definition of low alloy steel in iso21457 which also allows the 625 weld overlay for low alloy steels.

 

[SJones]

Did you rule out the use of solid 625, e.g ASTM B444 Grade 1 at 414 MPa SMYS? If you are looking at A333 Grade 3 then you are back to the issue that you highlighted in the start of the thread regarding excessive wall thickness: it is 240 MPa SMYS. Avoiding dissimilar material interfaces, as in overlay welding and girth welding, is probably best, particularly if PWHT becomes necessary, since hydrogen cracking from the CP could become an issue.



Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.