Energy Calculation in a Pressurized Pipeline

[JWG0824]

I am venturing into an area where I have little experience. I am trying to calculate the stored energy in a subsea riser. The depth of the riser is 6000 fsw. The ID is 11 inch nominal and the length is 14,000 ft. The pressure during test is 15,000 psi. I am doing this to help with some safety issues so I wnat to be sure that I am doing this correctly. Thanks for any help or advice.

 

[CRG]

Try doing a search on this site using the key words "Joule Thomson"

 

[mgp]

Try this thread:
Thread794-26767

Regards
Mogens

 

[sailoday28]

CRG (Mechanical)Why Joule Thompson?

If one is after the specific energy,e, of a static column of water, then measuring z, the elevation from an arbiturary datum

e=u+z where u refers to the internal energy as a function of 2 variables such as P and T.

The total energy is therefore a summation of edm (m being the mass at a specific height) over the total length.

 

[israelkk]

Assuming the pressurise medium is gas the energy equal P*V where:

P - pressure of the gas
V - volume of the gas (volume inside the pipe)

 

[CRG]

I would expect a substantial change in the temperature of the gas as it expands. And as such, if you wanted an accurate calculation set you would need to account for the drop in temperature of the discharged gas. Otherwise, the assumption that it is an ideal gas would yield a less accurate calculation set.

 

[sailoday28]

What is the fluid in the riser? What is the temp of the surrounding medium?

 

[JWG0824]

This is a riser in the Gulf of Mexico. We may have divers in the area but I want to be sure. I have a safe pressure for humans but I need to calculate the energy released in case the line fails during the test. The line is 11 inch ID, 14,000 ft long, water inside. The divers will be a depth of 90 feet. I know that only a small portion of the total energy in the line will be released in the event of a failure. I am trying to find some basis for making an assumption as to how much energy would be released. I am thinking that I am getting into the area of a hypersonic shock wave. I hope I am not making this more complicated than it is. Thanks so much for all of the help you have give me so far. Any other help is greatly appreciated!

 

[spayette]

The NASA Glenn Research Center has a calculation method for pneumatic testing which estimates the energy of the vapor and gives a recommended safe distance from the test area. The link is as below. Also, Lawrence Livermore Labs has a similar method which also includes liquids. I don't have their link but I'm sure that you can find it by doing a web search.

http://smad-ext.grc.nasa.gov/gso/manual/chapter_07.pdf

 

[25362]

The stored energy would the sum of the pipe's spring energy and the compressed fluid (test water) energy.
Taken from the literature for vessels:

The spring energy could be estimated from:
V.P.S (1.25 - 2[σ] + 1.25[σ] ²)

The pressure energy:
([½] V.P²/ [β]) [1 + 2S(1.25 - [σ])/E]

where:

V: volume of the pipe
P: internal pressure
S: hoop stress = PD/2t
[σ]: Poisson's ratio for the pipe's material
E: Modulus of elasticity of the pipe's material
[β]: modulus of elasticity of the water used for testing
D: pipe diameter
t: pipe wall thickness

 

[JWG0824]

Thanks for these inputs! I am reviewing the information from NASA and Lawrence Livermore. One this I am still not finding is how much of the total energy of the pipeline is released if a failure occurs during testing. If I can get some solid information for that I can calculate the equivalent pounds of TNT and from there come up with a safe distance for the divers. Thanks again to everyone for their help with this.

 

[jerin]

Are you sure you want to create safety calculations this way? As hinted by 25362, a substantial - if not the major - consideration is the failure (or lack thereof) of the pressure vessel (pipe,tank, riser...whatever you want to call it) itself. And once failure has occurred, the failure mode is important. Even if the material is the same, different vessels are manufactured differently and will fail differently. And I don't agree that only a "small portion" of the riser's stored energy will be released in a failure - once containment is lost, all energy will be released until the gradient is eliminated (i.e., pressure inside = pressure outside). After failure, it is the energy release over time that is important (e.g., the difference between a balloon popping versus a slow release), and you apparently believe that the differential energy release from the failed riser will be slow. Why? Do you have data on the riser construction's likely failure mode?

This is likely not the first undersea riser to be tested. Look for literature on similar projects, find the engineers that worked on them, and ask them how they determined a safety zone with respect to riser failure during a test.

 

[mrtngould]

if it is any help this handy calculator can do the workings for you just type in the spaces what is required and click calculate(click the link

http://pneumaticsonline.com/imges/PneumaticsOnlineCalculator.exe

 

[JWG0824]

The behind the scenes issue here is that there are two sides battling in this argument. One side does not want any divers in the area during the test, another side believes that it is safe. While I agree that the energy will be released until there is an energy balance, it is the large release at the instant of a catastrophic failure that will, I believe, harm the divers in the area the most. It's getting some sound reasoning behind whatever conclusions will be made that is proving allusive.