Pump Barge Bouyancy Factor of Safety

[rlewistx]

I hate to double post, and I think it is against the rules, but I posted this first in the ship building forum and the is very little activity there. I'm hoping maybe someone in the general structural forum may be able to help.

I've been asked to check the buoyancy of a pump barge on a lake. I can calculate the theoretical numbers ok. What I want to know is what is the industry standard of the factor of safety against submerging? What should the minimum ratio be of the max. potential buoyant force divided by the actual weight be? Can anyone shed some light on this?

Thanks.

Rich

 

[dhengr]

Rlewistx:
Min./max. freeboard, current, wave and wind action expected, means of mooring, etc. might be the bigger issues. Then, the loads may not all be centered on the barge. What you actually calc. is the buoyancy affect, against D.L’s (vessel weight) and Total loads, and what this means to the deck level above the water line. I’m no barge or ship building expert, but I wouldn’t expect some std. FoS w.r.t. submerging. Sometimes they pump a barge partly full to ground it, or to lower it enough so they can move it under a load; then they pump water out of the various cells within the barge to raise it and the load. Maybe your best bet would be to talk with a couple barge operators for what they would allow or could work with, and what their equip. will tolerate. Then lateral loads from current, wave and wind action lead to mooring forces.

 

[KootK]

I don't know the answer but I know where I might look for it: Link

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TLHS]

Retain the services of a naval architect. You may be able to rule of thumb things, but you're going to be a territory where you straight up don't know the things that you don't know. That's the worst situation to be in.

You aren't just going to be looking at buoyancy. There are also stability concerns that tie into the amount of bouyancy that's necessary.

 

[BUGGAR]

You also have to check the metacentric height - a measure of the "tippiness" of the vessel. I've designed a few barges and floats but don't have time right now to help (see that thread about being overworked)

Bob

 

[chicopee]

I believe, if I remember correctly that OSHA 1926 requires a 2' freeboard.

 

[BUGGAR]

O.K. the boss isn't looking.

Is the pump a dredge pump? How much water does it move? Will the water added to the pump and lines create off-center weights on the barge? Is the pump located off-center? I see stability calculations in your future. Barge live loads? Spare pump?

Bob

 

[cvg]

barge pump or barge for dredging with a pump ? Not sure we know what you are describing.

http://www.hytran.com/bargepump.htm

http://www.dredgebrokers.com/

 

[LittleInch]

So long as it's greater than 1 then the vessel shouldn't sink, but that is not your real issue, it's about how much freeboard you have, stability of the vessel, waves, wind, current it has to be designed for all the things dhengr says.

Problem is if it's too high then vessel might capsize in a small wave / wind/ tow situation, too low and it might still capsize, get swamped by waves, heel, pitch, roll due to various actions and design requirements

So if you actually respond to any of these posts I think you'll have to look at a lot of other things not just "will it float". Of course a few more details (length, width, depth, weight, a picture / sketch / drawing) will make it all a bit more "real" and might get you closer to some real information or comment on the design.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rlewistx]

Let me give a little more information.

The pump barge is located at a strip mine. It is on a pond/lake. Only wave action will be wind blown waves on lake.
It is approximately 14 feet x 18 feet and 2.5 feet height. it's basically a metal box with some tapered sides.
It has an intake and out-feed line.
It currently sits low in the water.

 

[LittleInch]

why not give us all the information rather than little drip feeding? It's not a national secret.

How low is "low in the water"?

Is this to allow the pump to pump lake water to the mine? Where is the C of G relative to the vessel / waterline? does the pump sit in the bottom or on a platform on top?

If it's only 2.5 ft high when out of the water it seems too low to me to resist much in the way of wave action, especially if it is moored in a fixed direction.

Be an engineer and draw a drawing and add some data (e.g. weights of pump, vessel, height of waves, amount of freeboard)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BUGGAR]

Tapered sides can bite you when designing for buoyancy.

No further comments pending more data.

B.

 

[rlewistx]

LittleInch,
Thank you for your information. I can provide some additional information. I do need to provide some confidentiality for my client so that is why I do not just post everything.
I have attached a sketch of the barge to give you an idea of the configuration. The pipe around the outside is a bumper guard to allow it to be pushed around while on land.

The total weight of the barge and pump is about 20,000 pounds. The framing is all steel. Pump itself is about 6,500 pounds. It is a diesel pump, above deck. I don't have a center of gravity of the barge but it is not far off center. The buoyant force of the barge volume is about 26,000 pounds. The lowest point is where the little jib crane picks up the hose line. It has about 6 inches of freeboard. It is moored out in the lake and sucks water from an intake line and sends it out on an outflow line.

 

[cvg]

so this is a barge pump as in my first link (hytran or custom) most of these are designed by a manufacturer which is usually cheaper than custom design and fabrication. so in the past, I have not calculated a FS for flotation.

the barge pump will be tethered by an anchor as well as by the pump discharge hose. I would expect that unless this is a fairly large, deep pit lake, the water depth will be very shallow and wave action would be minimal. either way, the center of gravity will be low and the floats will act like outriggers. tipping over is highly unlikely. the floats should be sealed, so you just need to keep the pump shaft coupling, pulleys and belts above the water level and the pump intake below water / above the bottom. 1 foot freeboard is approximately typical. as long as your factor of safety is above 1 and the floats don't leak, it wont sink. OSHA does not govern here, it would be MSHA which does not have a requirement for FS.

 

[LittleInch]

Six inches of freeboard doesn't look like a lot to me. Remember as soon as you get any water in there (rain?) unless this thing has multiple bulkheads we can't see, it will all move to the lowest point (seemingly at the jib) and rapidly start to list, reducing the free board even further at that point then sink. Your bumper bar might add some buoyancy but not enough once you get an inch or two or water inside it. Look up Herald of Free Enterprise to see what happens to much larger vessels once you get even small amounts of water slopping around inside a flat bottomed vessel.

as said before, it's not really a FOS that's important, but freeboard and how to stop water pooling and rapidly moving about a flat surface inside the vessel.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

this is not a vessel. it is a pump mounted on floats. the floats are sealed. unless the floats are damaged and leak, it will not take on water.

 

[LittleInch]

Is it a sealed float? I've re-read the posts and the OP hasn't said so and the drawing is unclear (perhaps the title confused me - a barge to me is an open box . If it is sealed then cvg is correct, but 6 inches still sounds low to me, but if the waves and wind are really very low then it might be Ok.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rlewistx]

it is a 'sealed' barge, in that there is a metal plate deck. I couldn't tell you offhand if it leaks. The pipe around the perimeter is punctured so it is just dead weight.

 

[LittleInch]

Kind of crucial. maybe that's why there isn't much free board....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

every barge pump I have ever seen had sealed floats, never an open vessel. A typical design for a large barge pump is a steel deck mounted on a pair of pontoons. Smaller ones often have several polypropylene floats like the Hytran design I posted.

sealing the pontoons or floats against leakage is absolutely critical. it appears that the "pipe around the perimeter that is punctured" might be some sort of guard rail, but since it appears to mount directly to the float at numerous locations, there could be leakage at the connections.