I have 100 km 32" offshore pipeline. The pipeline has two blowdown lines in its initial and final point. I want to calculate blowdown rate using depressuring utility in HYSYS 2.4.1. I calculate the volume from the pipeline volume (32" ID and 100 km length--and split into two) and use final pressure 40 psig (from initial pressure 1150 psig--the design pressure), and the time required for blowdown is 15 minutes (as per API 521).
The gas is 280 MMSCFD (sales gas quality).

And I have some problems regarding this calculation :

1. When I see the result I think the blowdown rate is too high (5500 MMSCFD). Is there any suggestion about this result because the flow itself only 280 MMSCFD ? Should I change the volume?
2. Which priority should be used: The final pressure or the blowdown time if the time is reached but the final pressure have not reached the desired value?
3. Is there any special consideration for offshore pipeline blowdown?
4. Should I flare or just vent the blowdown gas?

Thx.

Raxaz,

Depressurizing a 100 km 32" offshore pipeline as per guidelines of API RP 521, section 3.19.1 is not realistic!! You would require a very very large flare for this... Intention of these API RP 521 guidelines is that you reduce internal pressure of a section so that in case of an external fire no hazardous rupture occurs. Note that the API 521 will not be adequate e.g. in case of jet fires (see guideline that can be obtained from scandpower.com, but this is a different discussion.

An offshore pipeline can only be exposed to an external fire at the inlet and outlet parts e.g. just below the riser isolation valves. In case you have subsea isolation valves, this is not even true. The parts that can be exposed to external fire are usually protected by Passive Fire Protection, so that pipeline will not rupture, even in case a jet fire is impinging on it. You should also reduce likelihood of such event happening, by choosing good location of riser isolation valves, reduce number of flanges, corrosion monitoring program etc.

Conclusion is that your pipeline should not be depressurized as per API RP 521. Only low rate depressurizing is required for in case you want to do maintenance on the pipeline etc.

I assume question whether you should flare or vent depends on environmental regulations.

Another reference to the Hydro/Scandpower approach to depressurizing to protect against rupture because of external fire (including jet fires) is following article in Chemical Engineering Progress.

There is also a recent book (2003) on the subject: Guidelines for the Design and protection of Pressure Systems to Withstand Severe Fires.

Again, this is not applicable for subsea pipelines since it would result in excessive flowrates to flare.

Conclusion is that your pipeline should not be depressurized as per API RP 521. Only low rate depressurizing is required for in case you want to do maintenance on the pipeline etc.

I understand this, thx. But for maintenance what's the basis for flowrate calculation e.g. only x% from the total pipeline volume? And i think i would flare the gas so I need the maximum flowrate to size the flare system (piping, knock out drums, flare stack size etc.)

Raxza,

I am not sure about depressurizing rate for maintenance purposes, but common sense tells me that you have to find the economic optimum solution. Normally, size of your flare system would be determined by other requirements (e.g. blowdown of part of the production facility as per API RP 520). You would then depressurize the pipeline for maintenance with such a rate that the flare system will not be overloaded. Depressurizing the pipeline would be a rare event and so it would not really matter whether it takes 12 hours or 24 hours. However, you should do a check to see whether the time required would be reasonable (e.g. not longer than 24 hours).

From your posts I get the impression that you are designing a flare system, that is used only for depressurization of the pipeline. This seems strange to me, since I would expect a flare system to be present on either side of the pipeline. Please explain.

I do not think there are guidelines for sizing a flare system for maintenance depressurization only. What I would do is select a reasonable time for depressurization (e.g. 12 hours or 24 hours) and then check whether I get a reasonable sized flare system.

Basically, you would have to find an economic optimum. The higher the depressurizing rate the shorter the "wait time" would be, but also the larger your flare system will be. Where the optimum will be depends among other things on how often you expect you would have to depressurize the pipeline...

I just spoke with a collegue who has more experience with operating offshore pipelines. He told that it is common to first reduce pressure in the pipeline as much as possible without flaring, so that the valuable sales gas is not lost. Only when pressure has dropped so far that you cannot continue producing from the pipeline, you start to flare the remaining contents. It is not unusual that this takes 24 hours or so, but again, we are talking about a rare event.

Raxza, I don't want to be offensive, but isn't there anyone in your company you can ask these questions? It seems a bit odd to me that you are asked to design a flare system but clearly do not have sufficient knowledge to do so...