pressure testing subsea pipes

[WEBALL]

Hello, My client wishes to Hydro pressure test a subsea pipeline at 700 mtr deep, he wants to connect a rubber flexible riser (flooded) to the pipeline. he wants to test at 70 bar.
the rubber hose is rated at 85 bar max W.P. with 2:1 safety
what pressure needs to be injected at the surface to test the subsea pipe to 70 bar ?

 

[LittleInch]

More or less 70 bar. If you're using sea water as the gudrun test water then static head cancels out. out

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WEBALL]

Hi Littleinch, Thank you, I ask the question to get unbiased reply, I think if it is a steel pipe going down to the seabed, I agree 70 bar, but with rubber hose i cannot see the theory that you put 10 bar in at the surface it equates to 80 bar at seabed.
I think the hose would expand and stretch until you equal the input point (surface) pressure to equal the pressure at depth. As you cannot compress a solid, any liquid pumped in at the top cannot force greater pressure at the bottom, until it passes the Equilibrium point. (so I was thinking 140 bar required).

Like pumping water up a hill, there is more pressure at the bottom and at the top would just spill out at 1 bar, but the pressure at the bottom is generated by the weight of water. I am hoping you can explain to me as the next point involves calculating the weight of the hose in the water.
The hose is 7.75 kg per mtr, Capacity of a 4" hose is 8.11 ltr per mtr, out side dimension of the hose is 5".
displacement weight of water difference between 4" and 5" is 3.19kg per mtr (the hose in water is heavier by 3.19 kg per mtr)so 700 mtr would be 2,233 kg or is this reduced by pressure at depth. ?

appreciate you time -knowledge, thank you.

 

[Compositepro]

WEBALL, LittleInch is correct. Since your logic is wrong, I cannot follow it. If the water inside and outside the tube is the same density, then gravity will not create any difference in pressure between the inside and outside of the tube. Just think about it. Imagine a tube that is open at the bottom and compare that to one that is closed at the bottom.

 

[LittleInch]

weball,

Your OP stated the hose was good for 85 bar.

the 80 bar at the bottom of your 700m hose with 10 bar inlet is only valid when compared to atmospheric pressure. I.e. imagine a vessel at atmospheric pressure lowered to the seabed and connected to your pipe a pressure guage would read 80 barg.

When compared to local differential pressure at 700m depth though, the pressure from 700m of water is approx. 70 bar and basically cancels out 70 bar in the hose for the item you are pressurizing which is exposed to the seawater. Thus it only sees a differential pressure of 10 bar which you applying at the surface.

Weight of hose isn't impacted by depth so your 2233 kg is correct.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WEBALL]

Thank you Little inch, I think I am getting there. I do not wish to offer this hose for the pressure test unless it is safe to do so.
They will be putting pressure in at the surface of 70 bar, within the maximum working pressure of the hose. I am just struggling with the concept that down at the bottom it will be putting anything in the pipeline. is there a way I can test this at the surface to Proof the safety of the test. ?
thank you for the explanations.I am more used to working at the surface.

I have another topic, quite controversial on Roof Tank Drain hoses we manufacture. Is this better to start on a new thread. ?
thanks
Peter.

 

[Compositepro]

"I am just struggling with the concept that down at the bottom it will be putting anything in the pipeline."

That sentence just does not make sense.

 

[LittleInch]

What you need to remember is this:

By pressure, what is meant in this context is actually the differential pressure between INTERNAL pressure and EXTERNAL pressure.

Above sea level EXTERNAL pressure remains essentially fixed (atmospheric pressure) at least once INTERNAL pressure goes above 10 bar.

Below Sea Level EXTERNAL pressure varies with water depth.

It is approximately 1 bar for every 10m.
So in your hose the pressure at the surface is 70 bar relative to atmosphere.
At the bottom of your 700m deep hose the external pressure is approx. 70 bar.
The internal pressure is your 70 bar surface pressure plus approx. 70 bar due to the depth.

Hence the differential pressure is 70 bar + ~70 bar minus ~70 bar.

Testing it?
Just lower it 700 m down, close the end and pressure it at the surface to 70 bar.
The biggest issue I see is static axial load on the hose. I assume when filled with water the hose sinks and therefore 700m hanging off the boat will end up with a significant axial load??

Start a new thread probably in the storage tank forum? for your other issue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WEBALL]

Thank you Littleinch,
I have now got this, the axial load is not a problem and not that much to us, we are used to bigger loads.
the internal pressure equates to far more load than the static load.
I can tell my client the hose is capable of doing this job.
Weight and pressure at the surface we are much more used to.

thank you for the help I think this thread can be closed.
I will open a new one for the other issue.

 

[Compositepro]

It is nice that your equipment can handle the weight of the hose, but that is not the concern. Can the hose handle the weight of 700 m of hose hanging from one end. Fortunately, it is in water and the buoyancy forces will reduce this weigh considerably.

 

[LittleInch]

OK, hope it works out.

My point as composite pro points out is not the load per se, but whether your flexible rubber riser has sufficient axial strength capacity.

It is not an uncommon service, but there are "risers " and "deep water risers" and hoses.

Not all the same thing.

Also see what happens to the riser when you add 70 bar pressure in terms of shrinkage. You don't want this force to add to the weight of the hose. Most flexibles have an effective Poissons ratio much higher than steel and over 700m the reduction in length can be considerable.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WEBALL]

Thank you both, the calcs on the weight are higher in the thread. as has been stated a hose full is better than a dry pull. WE have burst many hoses on tests both dry and wet and burst by pressure, end tensile is normally quite close to the equevilent to tensile at burst in the hose we use.
Spiral wound or braided will hold the load with a good ratio of safety.
I know some riser are made to stretch to compensate for ship heave and break-aways are always fitted.
thank you.