effect of ship motions on containing sand 3

[rahulbarve]

hi,
this is quite an interesting problem am involved in. we are designing a barge to contain some ores, properties similar to loose sand with angle of repose = 30 deg.
now, this barge will be having motions as it is in the marine environment. what we need to know is how the heap of the ore contained in the barge will behave. we are expected to design such that maximum ore is carried on the barge (thus leaving a slope of 30 deg at the top). this is important because if this whole sand is going to move, it will have great effect on the stability of the barge.
another problem is that the barge is uncovered and hence have to also understand the effect of rainfall on the ore motion..
thank you for the interest in this problem and will be glad if could get some help or references, etc
regds
rahul

 

[civilperson]

rahulbarve,
The marine environment is particulary unforgiving. Any amount of freeboard may be over topped with waves. Thus, hatch covers and closed compartments are standard. River traffic is less demanding with shorter water stretches for wave generation. The problem stated pre-supposes some unreliable assumptions. What loading and unloading facilities/methods do you have for the ore?

 

[aeoliantexan]

Your description of the material as loose sand with 30 degree angle of repose implies a cohesionless material. A cohesionless material won't stand at a slope steeper than its angle of repose, so if the barge is loaded at 30 degrees and then rolls a few degrees, the ore will slide. The slides may tend to be shallow when the material is dry, but after rainfall wets the top few feet, there will be apparent cohesion in the wet zone, and the slides will be deeper, at the base of the wet material or deeper. A sudden slide a few feet deep could have a really bad effect on the stability of the barge.

 

[concretemasonry]

Does the ore have any significantly different properties than sand? (Value, DENSITY, corrosion, expansion, etc?). There are thousands of large sand barges on the bigger rivers (Ohio, Mississippi, Missouri) and on the Great Lakes that function well. They are essentially flat scows that look like floating fields sized for the locks. They are not prone to a great deal of listing, so the shape and marine properties are well established. Generally, having extra width and/or length for access, unloading and cleaing is a big factor.

Are you in a ocean environment? There are numerous proven designs for ocean barges that are used in those environments also.

In the big pisture, it is important for the size and shape of the barge to be compatible with existing barges since a small number of barges (less than 6 or 9) is rare in a tow on many bodies of water.

 

[rahulbarve]

dear aeliantexan,concmasonry and civilperson
thanks for the response,
1)there is no scope for design for hatch covers, etc.. in fact i was thinking of using geotextiles.. but that too is over-ruled. hence have to just find out the dire consequences or limit the angle of slope(reduce capacity)

2) aeliantexan, is there any theory to calculate the effect of moisture content. how do i quantify the effect that you have mentioned (deeper slides)

3) this is basically a barge that will transport nearby ores to the container ship and is not itself a container ship...hence the ore(sand) will be above the maindeck of the barge..

regards
rahul barve

 

[aeoliantexan]

rahulbarve,
Apparent cohesion is caused by capillary tension pulling the grains together in a partially-saturated soil. A geotechnical lab should be able to measure it using triaxial compression tests on the ore at several different water contents. A geotechnical engineer can estimate the depth of wetting from a given amount of rainfall. Slope stabilty analysis using commercial software can give safety factors for different conditions and slopes and estimate the depth of the most critical failure surface. When failure occurs in this material, it will probably be sudden. I am not sure how to estimate the final position of the failed soil, which will govern the list of the barge. Of course, once the barge tilts and the sand slides, the barge will tilt some more, and the sand may slide again, etc. I'm no barge designer, but I wouldn't want to ride on that barge.

Would it be that expensive to raise the sidewalls a bit so the sand can be laid flatter? Ore is so much heavier than water that it would seem that the displacement of the barge will limit the capacity rather than the geometry of the load.