5 replies to this topic

[racerX]

Hi

Is there any standard acceptability criteria for vessel failure in offshore installations. The situation is that we are analysing how long the vessels might survive in case of a fire scenario. So, what should the criteria be for blowdown time or PFP. API 521 states blowdown rate should be to 100 psig in 15 mins,and the time for vessel survival according to the acceptance criteria for the specific facility.

The qualitaitve fire risk assessment has given an exceedence curve, (which is a plot of cummulative freequency of occurence of a fire event vs the fire duration), as the frequency increases the fire duration decreases. The problem is that we are not able to justify the selection of a particular frequency to determine the corresponding survival time

If anyone has come across any information to this, please do let me know.
Also, how close can the stress on the vessel go to the UTS, to ensure vessel survivability, (margin of safety),

Any reference to a standard or best practice will be appreciated

[Zauberberg]

According to API 14G, "Offshore Fire Prevention and Control", API 520/521 are equally applicable for designing platform blowdown systems. The API 14E referenced in the material attached below is related to sizing of offshore piping systems.

Some items to be specially considered in design of offshore blowdown systems are listed as well.

Attached Files

•  API 14G.JPG   207.66KB   44 downloads

Edited by Zauberberg, 05 September 2010 - 11:48 PM.

[racerX]

@chemsac2: thanks, the document is quite detailed, however it does not conatin the info I was looking for, It does not mention any acceptance criteria for vessel failure or specific failure mode to be followed,

@zauberberg: thanks,what I was specifically looking for was, what is the industry accepted times for vessel failures, ( if there are ), and when is the vessel supposed to have failed,i.e. theoretically it is when calculated stress equals UTS, but certainly it cannot be allowed to reach almost the UTS, so how close can the calculated stress actually reach the UTS,

Kindly do no not mind if I have repeated my questions, I do not have much knowledge regarding the strength of materials and related field !

[chemsac2]

Attached document recommends that the UTS be reduced by 20% or more, depending on the reliability of the UTS data for safety margin.

As for industry accepted times for vessel failures, I have not heard of any such criteria.

API 521 asks to consider emergency to be about 30 minutes duration, but asks for passive fire protection to withstand fire for 2 hours.

I would plot applied stress versus time and vessel strength versus time. This would give an idea of time for which vessel can withstand increasing internal pressure and decreasing strength. If this time is less than 30 minutes, passive fire protection would be required. If it is more than 30 minutes, PFP need not be applied.

Regards,

Sachin

Attached Files

•  106. Fire_Salater_PSV_Calcs.pdf   78.2KB   68 downloads

[Zauberberg]

Sachin,

That was an excellent post, and very interesting article. A well deserved star for you.

Best regards,

[kkala]

To the interesting posts of this thread, let me add another view, at least for land (offshore or onshore?) installations: design of fire fighting system aims at keeping the vessel intact for indefinite time (in theory). This cannot be always achieved in practice, for there can be water shortage, or fire stronger / wider than what predicted in regulations, or other unpredicted factors. This may be due to a weak action plan, usually corrected after the fire.
Regulations demand about 2 hour water hold up just to face the first events of fire, until external fire fighting is activated by fire fighting trucks or other means. And half an hour foam supply, until fresh foam comes with mobile means. Passive protection may not be feasible to last more than two hours, nevertheless (α) fire water can then be used to cool structures (β) even if pipe racks & pipes fail, destruction is small compared to a vessel failure.
I remember the fire fighting design of a propylene sphere in a refinery(1990). A mechanical isolation valve blocked the inlet/outlet automatically on high temperature (emergency isolation) and a deluge system was activated to cover the sphere with water. "Belly" and supporting legs of the sphere were water sprayed by another system, irrespectively of any passive protection. I do not remember any special protection when all liquid propylene would have boiled off through PRVs, but nowadays this would be also faced (PRV for gas expansion, depressuring, etc). And at least in one refinery here, sea water is transferred and used for fire fighting when the mild water is exhausted (after 2-3 hr).
Generally a fire threatening a vessel is a major accident, so all the external area moves for fire fighting aid according to an existing emergency plan (at least it is said so). A vessel failure may cause victims around it, so "calculated" failure after some time can be hardly acceptable.
Any comments on the above would be welcomed.

Edited by kkala, 08 September 2010 - 11:40 PM.