3 replies to this topic

[neel_avi]

Dear Experts,

 

The query is related to cold vent booms on platforms for a particular offshore facility. All facilities are exactly similar. I will be discussing on a typical vent boom on one of the platforms. 

 

The constant source of gas through the vent booms are

1. Purge gas to the vent boom 200-300 sm3/hr. 

2. Motive gas discharge from the pneumatically operated Chemical injection pumps (500-600 sm3/hr)

3. Intermittent release of gas from the pneumatically operated closed drain drum pump which operates on the level of the closed drain drum - 2500 sm3/hr

 

The vent boom has been designed for 4200 sm3/hr of peak gas load from one of the relief devices. Vent boom has a flash back arrangement and is located on the main deck (thus below the helideck) at an inclination of 15 Deg from the horizontal plane and at a horizontal distance of 40 ft from the nearest structure on a platform. 

 

Originally there was no trip envisaged on ignition of the vent boom. Recently, during a bad weather window, the vent boom got ignited due to lightening. The flame was insignificant and it was put off immediately after stoppage of the chemical injection pumps (after operator intervention on the next day). However, in the public interest (which I do not know what it is) it is now being decided to trip the platform on vent boom ignition.

 

The platform will now trip on activation of all of the following:

1. Melting of Fusible loop at the vent boom tip which will initially trip the injection pump.

2. Melting of fusible loop at a distance of 1 metre from the vent boom and towards the platform. 

3. HIgh pressure (5 Kpag) sensing in the vent KOD which will be equivalent to around 3500 sm3/hr. 

 

 

Some points which may be considered

1. The platform has a total shutdown logic on platform fire 

2. Vent boom design was checked on the basis of the radiation calculations as per API. 

3. Vent boom tip material of construction is not known and for keeping sanctity of the design, I am assuming that it is designed for ignition case.

4. On vent boom fire and activation of high pressure alarm for the vent KOD, strobe lights on heli deck will be activated to prevent landing 

 

I am not convinced about the tripping, unless the MOC of the vent boom is not suitable (in that case platform should trip on activation of the first fusible loop itself).

 

Wished to receive suggestions / thoughts on the above to help me out. 

 

In case of requirement of any details please do let me know.

 

Regards

 

Avijit

 

 

[cuchuoi2005]

Hi Avijit

Several comments as following:

If you check the radiation, if it pass what will you do? install a flaretip to ensure gas is 100% burned safely or you still want to maintain the cold vent concept?

If you apply the shutdown logic, how do you detect the fire? Fire detector or CCTV? Are they reliable enough for the such nagnitude of incident?

I don't think the material of vent tip is a problem, possibly the design pressure of flare scrubber is a problem, please check if it can withstand to the internal detonation due to flash back.

Normally, we design the cold vent with snuffing unit? Do you consider it yet?

The fire you experienced is due to bad whether, please check on the static electric source also, if there is any other mitigation way?

Regards
Cuchuoi

[shan]

The problem is that there is no flame arrestor on your vent tip.  When it is ignited, the flame may burn back to the undesired points along the vent line.  Therefore, it is necessary to trip the facilities to avoid the possible disaster in the case vent tip is ignited accidentally.

[cuchuoi2005]

According to API 14J, flame arrestor is not recommeded because it's easy to get blocked leading to another block outlet.