Subsea Bolts, What is Typically Used for Corrosion Resistance

dengmech · Jul 2, 2013

I am specifying some bolting for a submersible application, the bolts will be submerged about 2 to 3 days then unsubmerged for a couple of weeks. I need 1.5" bolts with a proof strength of no less then 87 KSI. They will only be removed and installed when the equipment is serviced (on the order of every 1 to 3 years). Can someone tell me what is typically used for material corrosion resistance in submerged applications? 316 stainless steel, alloy steel with a PTFE coating? I'm concerned the PTFE coating is very thin and can easily be scratched off. The stainless steel is much better, but much more costly and I am getting into some real specialty fasteners to get the strength.

SJones · Jul 2, 2013

Is this cycle of immersion and drying continuous for one to three years? Will there be cathodic protection available during submersion (it doesn't have to be by deliberate application, it can be from connection to a protected structure)?

Steve Jones
Corrosion Management Consultant

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

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

dengmech · Jul 3, 2013

I'm not certain on the cathodic protection. The equipment is being mounted to a turret structure of an FPSO and we are just sub suppliers. I could ask my customer whether the turret structure is protected. Could you lend me some insight on the difference in bolt selection based on whether the structure was protected or not?

SJones · Jul 13, 2013

There will be two issues with CP:

1. It will protect alloys that might normally be subject to marine corrosion without it
2. The hydrogen generated on the protected surface may result in cracking and failure - particularly for high strength materials

In addition, dependent upon material choice, galling could be an issue that could prevent removal.

A193 B7 would probably be OK with the hardness restricted to a little lower than stated, usually HRC 34 max.

Steve Jones
Corrosion Management Consultant

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

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

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Subsea Bolts, What is Typically Used for Corrosion Resistance

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