+1 vote
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Sour Service considerations for system upgrade coming into contact with existing piping and equipment?

asked Jan 29, 2015 in Material by Normandy (21,000 points)
What should be done about existing process piping and equipment that is already in sour service?  The system was rated for 30 ppm H2S. Now it may see 230 ppm H2S.

Scenario to study and possible options to comply with H2S requirement: an existing maximum anticipated H2S concentration is 30 ppm in the reservoir.  The new system to be designed requires the maximum H2S design concentration is 230 ppm, even though the reservoir is not expected to become sour.  So, to which we are to design to apply conflicting H2S design criteria.

2 Answers

+1 vote
answered Jan 29, 2015 by Normandy (21,000 points)

All existing subsea piping & equipment are always designed for sour service, i.e. NACE compliant.  The only places, where NACE specifications must be met on existing topsides facilities, are those places where the increase to 230 ppm H2S would increase hydrogen sulfide partial pressure to more than 0.05 psia.  All material used in existing process piping and equipment should be NACE compliant (including all carbon steel and stainless steel). Therefore; the only NACE compliance issue is whether the welds in those areas are NACE compliant.

Welds in those areas need to be audited to “make sure they are NACE compliant.”  To do that, “Welding Procedures” and the “Weld Map” must be examined for each weld in those areas.

Two options exist for each weld found to be out of compliance:

  1. Field heat treat stress relieve, or
  2. Perform a weld audit; identify non-NACE compliant welds that were made in process systems that operate above 202 psig; cut those welds out; and re-weld in compliance with NACE.

(Note: Corrosion monitoring is NOT applicable because corrosion is not the issue. It is hydrogen induced cracking)

However, destructive testing required to confirm field heat treat stress relief will bring substantial direct costs, and prolonged production downtime probably eliminates Option 1, relative to Option 2.  The only other conceivable alternative to either option might be to change the design requirements.  Regardless, performance of the weld audit and choice of option to address non-compliant welds must be taken to address this potential failure. 

Consequences for not addressing this issue can include hydrogen induced cracking and catastrophic failure in materials in process systems, pressurized above 202 psig, that were not welded in compliance with NACE.

0 votes
answered Jan 29, 2015 by belmoreno (2,190 points)
Also for consideration is the remote potential for contamination exists if there is a gas buyback, in some design, from the export line is ever used for a black start of the riser base gas lift gas system.  A “black start” refers to a start when no wells at all are on-line to supply riser base gas lift.  Contamination potential would be by way of buyback gas from the export line being sour, though that gas is supposed to be sweet. That situation could add complication into the system upgrade.
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