Under water crush depth.
Post Reply   Forum
Posted by: Mylo42

08/23/2005, 10:50:12

Author Profile eMail author Edit

I am building a submersible in which the pressure hull is made of 28" steel pipe with a wall thickness of 1/2" (140lb/ft weight of pipe). Assuming that the flat end caps are as integral as the pipe due to reinforcement. How can I calculate the crush depth ? This # would obviously be halved to obtain a safe operating max. depth.

Thanks for the help.

Mylo42








Post Reply | Recommend | Alert Administrator View All   | Next |

Replies to this message


Re: Under water crush depth.
Re: Under water crush depth. -- Mylo42 Post Reply Top of thread Forum
Posted by: swearingen

08/23/2005, 18:34:18

Author Profile eMail author Edit

Using Roark's and plate theory, I get that the end plate should begin to yield at about 85' of depth (37psi). I thought that maybe it isn't that deep until you calculate the total load on the plate - 22,700lb - over 11 tons! It'll hold up a little deeper until it passes the hump on the stress-strain curve, then it's a goner. This does not take into account the increase in pressure on the inside from the decreasing volume, so that'll add a few feet of depth. To be on the safe side, I wouldn't take it down deeper than 40' or so.

If you dome the ends you can get much deeper...








Post Reply | Recommend | Alert Administrator Where am I? Original Top of thread | |
Re: Under water crush depth.
Re: Re: Under water crush depth. -- swearingen Post Reply Top of thread Forum
Posted by: dino snider

01/23/2006, 23:17:38

Author Profile eMail author Edit

Yes, end domes would improve the crush depth.
However, see http://www.mech.port.ac.uk/CTFR/Silsub3.html.
Strangely, concave vs convex end domes make sense.

Go to pbsubs.org.
Their .xls program will do what you want.
Anybody care to run said calc for me? I don't have Office.

I'm writing scifi story using seacrete concrete domes. Compressive strength is great.








Post Reply | Recommend | Alert Administrator Where am I? Original Top of thread
Re: Under water crush depth.
Re: Re: Under water crush depth. -- swearingen Post Reply Top of thread Forum
Posted by: Mylo42

08/24/2005, 01:12:19

Author Profile eMail author Edit

Swearingen,

Thanks. What is the formula for Roark's plate theory ? Incidently, as a result in revising my hull structure plan to occomodate for better bouyancy, my pressure hull is a 16' aluminum pipe 30" in diameter, 3/8" wall thickness, bulkeads at 7' from one end, and 3' from the other with the end caps and bulkheads at 1/2" thickness. I need to find out where I can find hemispherical aluminum end caps of the size I need. I do realize the added strength this will provide. I am looking to obtain a safe operational depth of 30M (Just shy of 3 atmospheres. 99'), which means, the hull must retain it's integrety to twice that depth....to be safe. I knew I should have studies hydrodynamical engineering instead of goofing around.

Thanks again,

Myles.








Post Reply | Recommend | Alert Administrator Where am I? Original Top of thread | |
Re: Under water crush depth.
Re: Re: Under water crush depth. -- Mylo42 Post Reply Top of thread Forum
Posted by: swearingen

08/24/2005, 16:43:37

Author Profile eMail author Edit

I worked with one of our mechanical engineers and modeled this one on a code calculating pressure vessel program. It appears that for 6061 aluminum, the minimum required wall thickness would be 5/8". This will get you almost to 200' depth. The problem is your caps need to be a good bit thicker than 1/2" unless they're domed.

The Roark's plate stress equations are pretty involved. Find someone that has a copy of Roark's Formulas for Stress & Strain and look them up. It's based on a closed form solution, so it's straightforward plug-and-chug, but you have to watch your units. That will give you moments which you have to compare to a bending stress calc.








Post Reply | Recommend | Alert Administrator Where am I? Original Top of thread

Powered by Engineers Edge

© Copyright 2000 - 2019, by Engineers Edge, LLC All rights reserved.  Disclaimer