Flexible flowline pressurisation/depressurisation concern during operations

[cacaboose]

Hi all,

Background:
Flexible piping has a number of possible failure mechanisms, one of which is caused by rapid depressurisation. i.e. polymer blistering, or in some cases, carcass collapse due to permeated gas expansion in multi-layered PVDF flexibles. API RP 17B provide general pressurisation/depressurisation limits for strength/leak testing (note: these limits are ultimately to the discretion of suppliers due to specific design and test conditions).

Question:
Is there any knowledge of using pressurisation/depressurisation limits during operations (e.g. well shutdowns)? My pipe in question is a subsea 12" unbonded oil production line with a max operating pressure of 50 barg. With an operating pressure much lower than design (210 barg), is there any chance of possible depressurisation failures or is this only seen in strength/leak testing, i.e exposing the line to 210+5% barg. Maybe there is a study that has related initial pressures and depressursiation rate to failures?

I believe there is no concern during operations as both standard and supplier do not disclose any operation rates but I would like a second opinion.

Thank you in advance.
J

 

[BigInch]

I think there is still insufficient experience in the use of this material and that you should try to talk to the mfgr about it. It seems that your concerns would apply.

 

[cacaboose]

Cheers BigInch, you are correct. This is a relatively new failure mechanism for a complex selection of materials in harsh conditions. They (& research papers) are my next port of call. Ill reply to the thread once I have an answer for future investigators.

 

[SJones]

Oil & Gas UK Guidance Note On Monitoring Methods And Integrity Assurance For Unbonded Flexible Pipes:

"Rapid Depressurisation (Multi-layer PVDF pressure sheath

...collapse the internal carcass.....This initial failure does not tend to be catastrophic, however, in a number of cases it has lead (sic) to a failure of the flexible.

So far, this problem has only been experienced on 3 layer PVDF products. It is important that manufacturer specified decompression limits are adhered to."

Blistering is also discussed as a separate "degradadation and failure driver" but with no mention of any impacts - irrespective of whether a failure is not considered as catastrophic!


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.

 

[cacaboose]

Hi Steve,
Thank you for your input. I am aware of the SureFlex JIP that this study originated from. I will look through it to gain some understanding before contacting manufacturers.
Best rgds, J