# Homework Help: Hydrstatic head and time

1. Jun 24, 2013

### arjal

1. The problem statement, all variables and given/known data

Hi all I have an object (which has valves attached) underwater which rotates from a starting point of 17 degrees to 0 , as the object rotates the hydrostatic head on the valves increases over time.

how can I calculate the head increase over a time period (per 10 seconds) until I reach 0 degree final angle ?

initial angle = 17 deg
final angle = 0 deg
initial flow rate = 2.7 m3/s per 10 seconds ( at 100m head)

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 24, 2013

### CWatters

Not my field but I think we are going to need more info and probably a diagram.

For example if the object is very small then turning it by any angle won't increase the depth of any part relative to any other by very much. So the pressure on any part isn't going to change much.

3. Jun 24, 2013

### arjal

hey this is an underwater offshore jacket which will be rotated to a vertical position so a heavy item !!

the jacket legs are ballasted with water so as its ballasted the jacket rotates, hence the head is changing as it gets to a vertical position.

As its being ballasted the valves move towards the bottom hence a greater hydrostatic head.

4. Jun 24, 2013

### SteamKing

Staff Emeritus
Like CW said, a diagram would be quite helpful to us in visualizing your problem. A jacket is a complicated structure.

What are the references for your angles? Is an angle of zero degrees horizontal? Vertical?
What are the dimensions of your jacket? I think that would affect the answer.
What water depth is the jacket designed to be installed in?
Where on the jacket are your valves installed?

You can see that the information provided is insufficient for us to help you.

5. Jun 26, 2013

### arjal

ok unfortunately I dont have a drawing atm, but I can simplify the problem.

If you have an object lying at sea at 90 degrees horizontal this then rotates until it reaches vertical i.e. 180 degrees

I need to calculate how long it takes to get to 180 degrees say incrementally every 10 seconds. The object is 140m long and is ballasted at a rate of 2.3 m3/s (per 10 seconds).

Thanks

6. Jun 26, 2013

### CWatters

That's a much harder problem (too hard for me). The speed at which it rotates will depend on things like the drag forces as it moves through the water.

7. Jun 26, 2013

### SteamKing

Staff Emeritus
A lot more information is needed. Dimensions of the jacket, the weight of the jacket (empty), location of the c.g. of the jacket (empty), sizes and locations of the ballast tanks, ...

I don't know if this is some school project or if someone wants to determine the response of a real structure, but the ballast rate is not enough information by far to even estimate how long it would take to upend the jacket.

I would recommend that you consult a naval architect who is experienced in working with offshore structures.
Be prepared to furnish a lot of information for him to review.