# Pressure variation

1. Sep 25, 2015

### jamese

If we have a water filled horizontal pipe on seabed with 200 bar in it.

And boat with a flexible downline is connected to the pipeline in 100m of water depth.

a) If the vessel bobs up and down by 5m I think that the pressure in the pipeline read at vessel changes by 0.5 bar each way, due to the vessel moving up and down on a virtual 2000m long hydraulic grade line?

b) But subsea the pressure in the pipeline does not change, since the pipeline position does not change on the virtual hydraulic grade line?

Are a) and b) correct?

Thanks

2. Sep 25, 2015

### Bystander

... implies +/- 2.5 m?
... implies +/- "?"

3. Sep 25, 2015

### jamese

Hello,

I mean a total variation of 10m.

5m up and 5m down,

Thanks

4. Oct 5, 2015

### jamese

To phrase the question another way;

Imagine we have a vertically orientated closed liquid filled pressurized pipe.

The pipe is shaped like a spring; such that the vertical distance between the two ends can be varied whilst keeping it liquid filled.

If we vary the vertical distance between the two ends, does the pressure vary at just one end or at both ends?

5. Oct 5, 2015

### Bystander

"The pressure difference between both ends" varies with vertical distance. Pressure at a single end is not defined by the statement given.

6. Oct 5, 2015

### jamese

OK agreed on the pressure difference between both ends varying in accordance with vertical distance.

With respect to whether pressure varies at both ends or just one end;

I am tempted to believe that pressure just varies at the lower end, in accordance with rho x g x h.
And the pressure at the upper end remains unchanged.

Is the above correct?

Thanks

7. Oct 19, 2015

### jamese

(assuming no change in system volume)

I clarified this further with my colleague (PHD in physics and works with fluid dynamics).

Weirdly (to me anyway) apparently the pressure will change at the top and bottom of the pipe.

The difference between top and bottom varies in accordance with rho x g x h, however the average pressure (the pressure at the vertical midpoint) always remains the same in a closed system.

He made an OLGA model to demonstrate this.