Free Surface Moment: The Most Overlooked Danger
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- Опубликовано: 1 июл 2024
- Without proper management, the shifting liquid in a ship’s tanks can create a lethal scenario. Free surface moment (FSM) is one of the most frequently misunderstood elements in ship operation. The deck officer that values their life wants to understand free surface moment. The physics behind it, and how it applies to ship operating limits.
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References
[1] G. Kerr, "The Last Cape Horners," RUclips, 02 Nov 2010. [Online]. Available: • Video . [Accessed 24 Nov 2020].
[2] Flow-3D, "Sloshing In LNG Tanks - Effects of Tank Motion | FLOW-3D," RUclips, 15 Jul 2017. [Online]. Available: • Sloshing in LNG tanks ... . [Accessed 23 Nov 2020].
[3] B. Parlatore, "Inspecting Fuel Tanks," Following Seas, 20 Jun 2018. [Online]. Available: www.followingseas.media/blog/.... [Accessed 24 Nov 2020].
[4] BumbleBeeAUV, "TRIVIATIME: Vessel More Unstall with HALF FILLED TANKS," BumbleBee, 22 Oct 2012. [Online]. Available: bumblebee.sg/blog/vessel-more.... [Accessed 24 Nov 2020].
[5] United States Coast Guard, "Free Surface Correction for Intact Stability Calculations," 46 CFR 170.285, Washington, D.C., Nov 24, 2020.
[6] B. Liogman, "Seawise University in Victoria Harbour (Hong Kong)," Wikimedia Commons, 1 July 1972. [Online]. Available: commons.wikimedia.org/wiki/Fi.... [Accessed 1 Dec 2020]. 
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Thanks for pointing out that Free Surface is not the same as sloshing. Before this, I don't think I ever considered there was a difference.
Hey, good to see you back again !
Thank you Nick I appreciate and enjoy your informative videos, keep up the good work!!
Thank you Mr. Nick for this crisp and concise explanation on the FSMs..
Love it Nick, I'm always interested to hear more from you...
Thank you
He’s back!
I always love these videos
Nick the naval architect. Well come as always we love your education.
Replay, Replay, Replay
I love that you are pushing fundamentals!
I’m an amateur naval historian and I have an idea for a video, which is to discuss damage control and survivability in modern merchant ships. For example, what would it take to build a hull that was resistant to under-keel torpedo detonations, where the gas bubble breaks the keel by lifting, then dropping the ship?
Nice video
Very informative and applies even to smaller craft. Sudden load-shifting is a thing all boaters should keep in mind. One thing about tanks- it seems that the shape of the tank can be a major factor. I thought long, wide, flatish, shallow tanks would be ideal but they might be much worse. Tall, narrow tanks would cause less trouble because the liquid just can't move as far from centerline, but if set far from centerline could still be used for adjusting trim.
Aircraft face similar problems. They usually have a center tank(s) very near the CG and tanks in the wings far from it. They can pump fuel between the wing tanks to maintain level. FSM isn't much of a problem because the liquid will bank with the plane, but maintaining level flight is.
Soooooo tall, narrow tanks placed far off CL would have minimum FSM but maximum effect on CG. How am I doing?
"A massive crack echoed from the holds", you got me there
As a high school student that is interested in naval architecture what would you recommend I do to prepare myself and better understand naval architecture?
Math and science will be critical skills. I intentionally leave out the math on this channel, but it is essential for all practicing naval architects. And then I recommend you find time on or around boats to get exposure to the maritime culture. ( Although, for all I know, you may be on a boat this very minute.). Volunteer with a vessel refit, ask to intern at a shipyard, help with traditional tall ships. You may need to get creative. Anything that gets you exposure on the water helps.
Hi Nick. That is a nice video to which I'd like to add one clarification. All is almost Ok except of the number of tanks to be taken into account in the simplified approach. We typically take one pair of tanks (two tanks, one in PS and the other one in SB) instead of one tank as you said. That is due to the need to adjust the transverse center of gravity after cargo loading since it is almost impossible to arrange it perfectly even from side to side. Then, during the voyage a pair of bunker tanks is slacken in order not to cause the list.
So, the presented idea is fine while you mistake one detail.
Interesting point. Which nation's regulations are you operating under? I didn't mention this in the video for brevity. But USCG rules also specify that when selecting the largest tank of each liquid, if the largest tanks happen to be a pair of port / starboard tanks, then we use both tanks. That situation happens frequently because designers also consider corrections for list when designing tank layouts. We like to create tools for that exact scenario of adjusting to get a perfectly level list. But it is not a universal guarantee and depends on the layout of each ship.
@@DatawaveMarineSolutions I am talking about Polish class requirements, however similar ones I have had on a Danish flag vessel built in Japan and some others. I believe you mean "the largest tank" as the tank numbered same, e.g. HFO DB 4 that typically means heavy fuel oil double bottom tank number 4 port side and starboard side. If that is the case we both speak about the same stuff, the pair of tanks may be slacked at one time, not a single tank. The latter would lead to a list, which is always undesirable.
Just for clarity in my own mind -- I assume you have some "complex" maths that find the worst case free moment for a give take volume and geometry. I assume there is no simple assumption of something like 75% full is the worst for all tanks. I also assume the calculation takes the given liquid into account due to density differences between sewage, saltwater, freshwater, and fuel. Also for a given tank do you take the location in the ship into account?
Excellent question. This is why we build stability models of the ship to consider these scenarios. In a stability model, we model the exact geometry, location, shape, and liquid of each tank. So it does change, depending on the density of the liquid. To find the worst case FSM, we use the mathematics to find the FSM at a single tank loading. And then we repeat these mathematics for every tank loading (start with tank 0% full, then 2% full, then 4% full, and so on.) With each loading, the stability model can calculate the exact waterplane inside the tank and determine resulting FSM. The model is also aware that if the tank is not level, the liquid all skews to one side. That changes the shape of the waterplane. This is where the complex maths comes in. Each individual step is not very bad. But there are several steps to determine FSM for a single tank load, and then you repeat that for dozens of tank loads and take the worst case from all those results. Oh, and then repeat that for every tank on the ship. This is why we let the computer do these tasks.
Actually, there is no "complex" math behind that. It is a pure geometry without dynamics considered, as it is done when liquid sloshing calculation. From the computational point of view looking for the worst case is fast and easy if you have a proper (simple!) software. The mentioned sloshing phenomenon is a completely different story.
so... with a single hulled vessel it would make sense to have several vertical tanks in a centerline from front to back of the ship... adding a pneumatic plunger at the top of each tank that pushes down on the liquid contents to prevent side to side movement and make it act as if a full tank at all times. lets say several of the tanks are connected so as to keep all of the tanks at the same volume of fluid at any given time.
if i were to be using a catamaran i would run the vertical tanks down the center line of both hulls and tie an equal number of tanks from each hull together and if waves were threatening to tip the ship sideways push more fluid to the rising side to shift center of gravity.
perhaps that is too complex to do or maybe too simple of an idea to counter free surface moments?
Just curious. would love feedback
looking into the concept of running two or three submersible hulls as a catamaran or trimaran, in order to just submerse the vessel for the duration of storms. thinking the hulls should be x bow hulls that transition to a more t shaped hull as you go further towards the back of the ship.
Typically, we don't have a plunger on the top of the tank to limit free surface moment (FSM). You are correct that it would work. But the practicalities of creating some type of movable watertight seal becomes too difficult. Generally, it is just easier to accept a certain level of FSM and divide your liquids into multiple tanks so the FSM stays manageable.
But the idea of pumping liquid between tanks is correct. Some ships will use that as a means to reduce rolling motion. They install a high speed pump and constantly move the liquid between two tanks dedicated to that purpose.
@@DatawaveMarineSolutions i wouldn't have any intention of moving the containers once they are built in place.
just have multiple fill and drain ports for various fluids. i figure a plunger shouldn't be that hard to keep sealed, just need good pneumatics or hydraulics to maintain an equal pressure across the containers. the idea is growing on me... i just doubt i will ever build anything that would be big enough to need it.
I think I just want a submersible house boat.... perhaps even one that can utilize ground effect when above the surface to move faster without actually having to break waves, except when recharging the compressors, hence why i was thinking Catamarans or trimarans... good distribution of mass, ballasts for keeping center of gravity low when not moving and light when i want to glide over the water...
probably just a pipe dream for now.... need better funding before i can look into actual construction.
I must be missing something. The references to sloshing were only fore/aft and FSM were port/starboard. What I don’t get is why baffles wouldn’t work if they’re placed properly. Unless you’re talking about a constant list to one side.
For FSM, yes, we assume that any heeling action is basically a constant list. Or at least slow enough that the tank liquids would pass completely through any baffles. Part of that is a carry over from earlier decades when we simplified the science to match the limited calculation capability at the time. But it's also about being conservative and safety conscious.
In the case of the baffles, you are right that they probably limit the liquids moving in a tank during normal ship motions, making the whole assessment of FSM super conservative. But "probably" is not safe enough, considering that the ship is the only thing keeping you alive at sea. Instead, we prefer the standard of "physically impossible." That is why we treat FSM the way we do. Give it a few more decades, and we may relax the safety standards a little more and start to give credit for baffles.
Anyone who has ever held a large tray of liquid and tries to walk with it will realise how unstable it is , once the water starts to shift about. {Edit....Maybe I just experienced ''Sloshing''!}
Curious where the number 66 came from.
Also curious.
How about vertical tanks?, circular on the x axis, a ten to one diameter to height ratio, baffled...as the tank level drops, the CG lowers as well.
Not sure if I understand the orientation correctly. But yes, circular tanks also have FSM, for all the normal ranges of fill level. (No tank has FSM, if it is completely full or completely empty.) Even if we consider the case of a horizontal cylinder for the tank, running lengthwise along the ship axis. In that case, the cross section of the tank contents doesn't change shape as the vessel heels. But the contents do change orientation related to the vessel. This still shifts the CG of the contents, which is how we generate FSM.
@@DatawaveMarineSolutions I was thinking a vertical cylinder, dozens in strategic locations. Now we all how liquids behave being sloshed side to side in a tall cylinder, but the movements are highly frequency dependent.
@@AndieBlack13 perhaps if there was a valve system the equalized the volume of liquid in each tank and used pneumatic plungers to keep the tanks at a "full status"... maybe even have the vales controlled to shift fluid to keep a unified center of gravity....?
The pictures about 5' into the video show center of gravity above the center of buoyancy. That's not how ships are designed. A ship with such characteristics would just capsize.
Sorry, but incorrect. In surface vessels, the center of gravity is above the center of buoyancy. The ship does not capsize because as the ship heels over, the shape of the immersed volume changes, shifting the center of buoyancy transversely.
In submarines, the center of gravity is designed below the center of buoyancy, because they have their entire volume submerged, with no change in the immersed shape as they heel.
It sounds counter-intuitive, but the center of gravity is above the buoyancy center. This is also why naval architects are very careful to control the center of gravity and match it against the volume and shape of the ship's hull.
For more information on the subject, you can refer to see "Principals of Naval Architecture", published by the Society of Naval Architects and Marine Engineers. Another reference would be "Basic Ship Theory" by Tupper.
@@DatawaveMarineSolutions Thank you for the prompt response, and thank you for clarifying. I stand corrected. By the way, I love the channel!
does that mean tall vertical tanks have less FSM?
Generally, yes. But if you compare a tall tank to a short one with the same horizontal shape and size, they would have the same FSM.
Sorry i am not a film maker to listen such stories ?!+
:)