Pig Receiver Header 1

[rbashiz]

I am supposed to design a subsea pig receiver. There is a requirement to take the water and the MEG in front of the pig to the surface.

My question:

How should I size the header? By header I mean the pipe that the water from small branches on the big barrel flow into and will be connected to the surface.

I know it pretty much has to do with the pig speed but what is the rule of sizing it.

I appreciate your help.



Leni

 

[BigInch]

For the flowrate that the pipeline has when you are running the pig. I assume you are not talking about instrumented pigs, so you will want to be running these cleaning pigs as near to normal flow velocity as possible. The riser line size will depend on the lowest pressure you have at its inlet when running a pig and the remaining pressure drop driving flow to the surface. Additinally you probably don't want any velocity in the riser line over 8 ft/sec or so.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[rbashiz]

Thanks BigInch.

Even though your reply still aplies, I thing I better describe more. It is a subsea receiver for pre-commissioning purpose.

Each time a pig arrives to the receiver, it will push some water in front of it. This water will go to a 2" branch which is welded to the receiver and these 2" pipes all will be connected to a 4" or 6" header which in turn has to be connected to the surface boat. I need to size the header,(4", 6", etc...) so that the pressure build up behind the pigs do not exceed the capacity of the pump. I know it is safest to have a big pipe like 8" but it will make the receiver too heavy which is not favorable for installation purpuses,...I just want to optimize it. I understand this bit, I just know how to the details. Do you think I am on the right track at all?

Thanks again for your time.

Leni

 

[BigInch]

Same comment still applies. You will have some known or calculated pipeline outlet pressure on arrival at the 2" branch, depending on the maximum discharge pressure of your pump, flowrate when running the pig through that mainline. You should size the riser for that same flowrate and make sure you can do that flowrate with the pressure you have at the 2" branch and the pressure you have at the surface, and at the same time try to ensure that you have a velocity limit less than what will cause damage by water hammer, should your pig get stuck, or some other flowrate shutdown event occurs. Optimization for commissioning scenarios or for startup cases is not required, since the operation time at those conditions is extremely short. Optimization should only be necessary for normal operation scenario flowrates. You just need something that works long enough to get the line up and running for which operating costs for such a short time will be insignificant. Just be sure you have a technical solution that works long enough to clean the pipeline.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[rbashiz]

Thanks BigInch,

Not being very good with fluid mechanics, I just cannot get my head around the flowrate calculation. Ok, basically, Q=AV (V being the flow velocity=pig velocity =2m/s and A the inner area)and since Q is the same therefore the water velocity in the branches of receiver will be in a reverse relation with radius of main line and branch lines. This means the the velocity in branch lines will be very high, around 30 m/s. But since the receiver is temporary and there is water in the system, erosion is not an issue (or maybe I am wrong here). As far as the the pressure is concerned, the pressure in the branch lines will be the same as the head pressure (depending on the water depth). Should I add any thing else to the pressure due to velocity?

And there is the issue of selecting the right sizes for weldolets. And shall i follow PD 8010 when it comes to wall thickness calc. The more I think about this task the more complicated it sounds.

But I think i am trying to re-invent the wheel here. Unfortunately I cannot find any of these documented in our previous projects. Looks like people by default go and pick 2 or 3 inch branches all the time without documenting why.

 

[BigInch]

30 m/s is extremely high velocity.
Two disadvantages,
1) a large pressure drop, which you may be able to work with, or maybe not.
2) a high probability of water hammer shocks, if the pig sticks and flow comes to a halt.

Tell the manufacturer what design specification the weld-o-let must meet and the pressure, temperature and other system requirements and let them supply the fittings accordingly.

No matter what other people do, to follow in their footsteps you should first agree that they are running in the right direction. I have found BIG mistakes in large major oil company standards, specifications and procedures, so do not believe that anything holds the sacred words.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus