Load shifting
editLoad shifting occurs, when a load that is stable becomes unstable, and shifts. This changes the COM(center of mass) of a vessel or aircraft making them possibly unstable. [1]This is dangerous, because if the load shifts enough to change the center of mass of the vessel or aircraft, the craft itself becomes unstable and can roll or pitch uncontrollably sometimes with disastrous results. It may seem like a simple problem, but many incidents occur each year where improperly loaded or contained loads shift enough to cause problems. There are several guidelines that have been in place by multiple governments regarding load shifting.
Boats are used to transport a majority of today’s goods()(approximately 90% of all non bulk cargo). Handling loads such as what cargo creates, is part boat design and part operation.
Design
editThere are many types of loads that vessels carry that can shift, including container, bulk, liquids, and fluids that leak into bilges
To design a boat to deal with loads such that they do not shift in the modern world is rather simple. Most loads are in containers measuring 1/2, 1 or 2 TEUs, which are locked to each other and the deck with twist-locks, and occasionally reinforce the structures with steel cables. When containers become an issue with the stability of the boat, for example all the containers broke free and are hanging over the side shifting the center of mass, most ships will cut the containers loose and add extra ballast water to compensate [1]. If objects fall around in containers, it isn’t a huge deal unless they knock the container free, as container ships carry thousands of containers so having a couple with shifting loads, isn’t a big deal. It also makes it easier if loads are shifting inside because the container itself will contain loads and make it so they can only shift a few feet.
Just like fluids, bulk cargo can and will shift if a ship rolls enough to permit. Shifting loads of bulk cargo can be very dangerous. In order to eliminate this threat, on most ships that carry bulk, they tend to be lower in the water and carry cargo up to the deck and as due to the nature of bulk not a one it. If it becomes a dangerous passage, sometimes a honeycomb like structure will be added to prevent bulk from shifting enough to endanger the vessel and its crew.
[2][3]
Tank
editFluids are the easiest thing to shift, and the most dangerous due to the free surface effect. As a boat rolls, liquids tend to shift towards the lowest part of the vessel. This is the free surface effect. When this happens more weight is now being thrown towards the low side and will cause a more severe roll or potential capsize. To minimize/reduce this risk, tanks are built in sections split up by perforated panels. These panels prevent sloshing, but still allow liquid to transfer keeping the levels amount the tank equal.
[2]
Similar to tanks, bilges have ridges, but they also tend to have slopes towards pockets, where water gathers at the suction for pumps, to then be treated and sent overboard. The major problem with bilges is that water can move large distances and drastically shift the center of mass[1]
Guidelines.
editMany nations have guidelines as to how kids should be handled and stored. For the naval world it is often determined by SOLAS, IMO, and others.
http://www.imo.org/en/MediaCentre/HotTopics/container/Pages/default.aspx
This site has links to all necessary information.
Design
editCargo planes are designed to carry large loads long distances at speed. In order to stay relatively stable they must prevent loads from breaking free or shifting, because unbalanced kids on an aircraft can lead to major catastrophe. The reason for this, is if the center of mass leaves an invisible box, called the flight zone!, the plane is no longer stable and the pilots have to fight the aircraft to stay aloft.
[4]
Loading
editSimilar to shipping many countries have rules for loading aircraft. There is no world wide regulations but many are similar. The US transportation administration uses these guidelines[4]
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Incidents caused by shifting loads
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- ^ a b c Barrass, B., & Derrett, C. (2006). Ship Stability for Masters and Mates (6th ed.). Elsevier Science.
- ^ a b Tupper, E. (2004). Introduction to Naval Architecture: Formerly Muckle's Naval Architecture for Marine Engineers (4th ed.). Elsevier Science.
- ^ u, Xue, Vassalos, Liu, & Ni. (2017). Numerical investigation of rolling response of a 2D rectangular hold, partially filled with moist bulk cargo. Ocean Engineering, 142, 348-362. Retrieved from https://csum-primo.hosted.exlibrisgroup.com/primo-explore/fulldisplay?docid=TN_sciversesciencedirect_elsevierS0377-2217(18)30618-0&context=PC&vid=01CALS_MAL&search_scope=01CALS_MAL&tab=everything&lang=en_US
- ^ a b US Department of Transportation. (2016). Weight and Balance Handbook. Retrieved from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf