Subsea Bolts 3

[santome]

Have a client who has ask me to recommend subsea bolts for a 14", 20" 30" flat face flange. Could anyone please recommend bolting proper material that an standup to saltwater submergine. Flanges are critical as pipeline is used for oil.

 

[quadswift]

If it is critical, you should be use something like taper-lok flanges.
Welding Units UK will be able to advise you on flanges and bolting.

Have a look at

http://www.welding-units.co.uk

We have used them before and they do a good job.
Regards,
Quadswift

 

[apostol]

You can consider silicon-bronze material which is highly resistant to saltwater corrosion (also non magnetic and temperature conductive) with minimum tensile strength about 70000psi. I use it before (silicon-bronze threaded rods;bolts and washers)in permanently submerged pipeline with excellent performance during the first year of installation (inspections made during maintenance period)
Also it is relatively cheap.

I would like to read also other people comments on silicon-bronze material

 

[Mark62]

Typical bolting used for the making up of subsea flanges are "stud bolts according to ASTM A 193 high temp type B7" often with a PTFE coating. The flange will be corrosion protected generally by anodes on the structure to which is connect or you can add anodes to the metal work adjacent to the flange. High strength, high carbon content bolts should be avoided as these suffer form hydrogen embrittlement, caused by the action of the cathodic protection system.

 

[JGB]

Find someone who knows about and is an expert in loaded bolts and nuts in a salt water environment and about flanges. I question using a flat face flange.

TaperLok Knows flanges but not enough about loaded bolts in salt water.

This is not an easy cook book solution.

 

[TJAvery]

I know this is a bit late, but Mark62 brought up a good point about hydrogen embrittlement caused by the cathodic protection system. To avoid this issue, consider the hardness of the bolting/fastening materials. It's been my experience to limit the hardness to a maximum of 34HRC.

 

[MetalMickey]

The hydrogen embrittlement issue is somethings that really needs to be considered in your selection as Mark62 and TJAvery state.
A look through the Norsok standards will give good guidelines on bolt selection through:

http://www.standard.no/imaker.exe?id=244

This will irrespective of the coating the bolt has. An expensive solution, if the system will not part of the cathodic protection system (with no associated hydrogen embrittlement concerns however), is to use a Inconel 718 type. Pricey, but will probably outlast the flanges they connect!

 

[Galls]

I can't advise on the flange type, but regarding the bolts, there has been some good feedback already.

To add just a little, albeit late:

- Should stick to bolts of tensile strength less than 690 MPa if you are in an area covered under CP, so as to avoid hydrogen embrittlement. The same limit applies if the bolts may be covered in mud/sediment, as this condition can lead to the creation bacteria which can again lead to the production of hydrogen.
- If you can stay under 690MPa and are in a CP environment, suggest using B7M or L7M studs with 2HM nuts (all ASTM). Use uncoated, as long as you are sure you have a link to the CP system. If you use PTFE, removed some of it with a wire brush prior to installation, or ensure you have continuity with the CP system through measurements.
- If you can't stay under 690MPa, try using L7 or B7 (with 2H nuts) with a hardness less than 34HRC, but preferably 28HRC. Same coating comments as above.
- If all else fails, utilise MARINEL+ material, which is immune to hydrogen embrittlement, and sulphide stress corrosion cracking, but has a yield of 720MPa. No coating required.
- Outside of a CP system, use Inconel 625 to avoid issues (max 35HRC), against ASTM B446-93.

 

[AndrewLindsay]

I concur with the suggestions above regarding B7M studs etc. I would like to add that I have had good experience with using washer faced nuts to ensure continuity across bolted connections.

Regards

Andrew

 

[SJones]

EEMUA 194 indicates that B7 be limited to HRC 34 max as per ISO 13628-1 but recommends a maximum HRC 28 or HV 300 and discourages the use of PTFE coatings because of the difficulty of ensuring continuity and (my addition) coating disbondment shielding the CP. Shell "bans" the use of Marinel for subsea bolting owing to fears regarding hydrogen embrittlement although EEMUA 194 claims that "only precipitation hardened cupro-nickel and precipitation hardened austenitic alloys have been found to be totally immune to hydrogen embrittlement" (note: do not confuse curo-nickel with nickel copper, e.g. K500)and goes on to promote a certain 20 Ni type copper alloy. As per the hydrogen embrittlement warnings, if you end up with zinc plated fasteners, make sure they have had a post plate hydrogen release heat treatment.

Steve Jones
Materials & Corrosion Engineer

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