Rate of pressurisation 1

[olef]

I have been asked if it is ok to increase the rate of pressurisation of a subsea pipeline. Today the pressurisation of a 15 km long pipeline takes 30 hrs to app. 70 barg. I feel that this is somewhat conservative.

Can anyone provide me with some rule of thumb (if any) for pressurisation rate for pipelines? (Temperature concerns are of course taken care of.)

Regards
OleF

 

[NMcC]

OleF,
Unfortunately, there is no particular rule of thumb for the maximum rate of pressurisation of a subsea pipeline. Find out who specified the pressurisation rate and what the reason is behind it.
Important: Do not ignore this limitation if you haven't got a clue as to why it exists!
In starting up a gas pipeline (i'm assuming here) there are two main considerations that affect the pressurisation rate:
1) The Closed in Tubing Head Pressure of the well, the pressure in pipeline before startup and the choke size. The maximum choke opening may have a sufficiently small Cv that it restricts the flow through it at these conditions. There would no way to resolve this without changing out the choke for a larger one.
And for the bad news:
2) I know you mentioned temperature issues were taken care of, but the other big thing restricing your pressurisation rate is joule-thomson cooling of the gas. With a large upstream pressure and a low downstream pressure, the gas expansion across the valve results in some very severe sub-zero gas temperatures inside your pipeline (in the magnitude of -60ºC for a natural gas, depending on your max dP across the choke). The minimum design temperature of your pipeline can easily be breached resulting in some rather unpleasant consequences for your expensive subsea equipment as well as yourself, i would imagine.
Solution: restrict the flowrate through the valve until the dP across it no longer results in dangerous gas temperatures. Usually involves setting the choke at minimum opening and playing a waiting game (possibly as long as the 30 hrs you mentioned). AND inject as much methanol as you can upstream the choke (you will be doing that anyway to prevent hydrate formation). This will increase the average heat capacity of the fluid and reduce the severity of the J-T effect. The maximum gas rate through the choke depends on balancing the heat lost from the pipeline wall to the gas and the heat gained by the pipeline wall from the surrounding seawater (one of the few times pipeline insulation is your enemy). When you find the gas-methanol rate that doesn't lower the wall temperature below your minimum (at any point in the pipeline) you found your start up rate.

Hope this helps
NMcC

 

[olef]

Thanks for your very wise considerations regarding pipeline pressurization. I could not agree more. You wouldn't happen to have a good formula for the heat balance downstream the choke valve ?

In my first posting I forgot to mention that I was talking of pressurization through an orfice from the gas injection compressor to the pipeline to avoid starting subsea wells against an empty pipeline.

As for the reason for the rate limit; people often tend to stick strictly to codes, and one particular code states that one should not have a gas velocity higher than 60 m/s, which is applicable for the 4" pressurising pipe downstream the orifice. However, this is duplex pipe (2500 #), and pressurisation happens quite seldom, so I'm willing to increase velocity at zero backpressure considerably, to say M=0,6. Once the backpressure reaches 10 barg, the velocity will be low anyway.

What I really meant to ask for, was if there are any other aspects to consider, like time for elongation of the pipeline due to pressure, etc.

Regards
OleF