Free Surface Effect
Encyclopedia
The free surface effect is one of several mechanisms which can cause a craft to become unstable and roll over (capsize
). It refers to the tendency of liquid
s — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to slosh about: to move in response to changes in the attitude
of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea state
s caused by waves & wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not completely full is described as a "slack tank", while a full tank is "pressed up".
In a normally laden vessel any rolling from perpendicular is countered by a righting moment generated from the increased volume of water displaced by the hull on the lowered side. This assumes the center of gravity of the vessel is relatively constant. If a moving mass inside the vessel moves in the direction of the roll, this counters the righting effect by moving the center of gravity towards the lowered side. The free surface effect can become a problem in a craft with large partially-full bulk cargo compartments, fuel or water tanks, (especially if they are spanning the full breadth of the ship), or from accidental flooding, such as has occurred in several accidents involving RORO
type ferries. If a compartment or tank is either empty or full, there is no change in the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). However, if the compartment is only partially full, the liquid in the compartment will respond to the vessel's heave, pitch, roll, surge, sway or yaw
. For example, as a vessel rolls to the left, liquid will displace to the left side of a compartment, and this will move the vessel's center of mass towards the left. This has the effect of slowing the vessel's return to vertical. Also, the momentum
of large volumes of moving liquids cause significant dynamic
forces, which act against the righting effect. When the vessel returns to vertical the roll continues and the effect is repeated on the opposite side. In heavy sea states, this can become a positive feedback
loop, causing each roll to become more and more extreme, eventually overcoming the righting effect leading to a capsize. While repeated oscillations of increasing magnitude are commonly associated with the free surface effect, they are not a necessary condition. For example, in the cases of both the SS Normandie
and MS al-Salam Boccaccio 98, gradual buildup of water from fire-fighting caused capsizing in a single continuous roll.
To mitigate this hazard, cargo vessels use multiple smaller bulk compartments or liquid tanks, instead of fewer larger ones, and possibly baffles within bulk compartments or liquid tanks to minimize the free surface effects on the craft as a whole. Keeping individual bulk compartments or liquid tanks either relatively empty or full is another way to minimize the effect and its attendant problems. Tanks or compartments that do not straddle the vessel's centerline are somewhat less prone to destabilising oscillations. Similarly, narrow compartments (aligned bow to stern) and compartments at the extremes away from the centerline, are less prone to cause instability.
Flooding, liquid cargo leakage, or unintended water (from precipitation or waves) in any compartments or on any decks of watercraft, and the resulting free surface effect are often a contributing cause of accidents, capsizes, and casualties e.g. the loss of TEV Wahine
(Wellington, New Zealand, April 1968), the MS Herald of Free Enterprise (Zeebrugge, Belgium, March 1987), and MS Estonia (Baltic Sea, September 1994). In the case of the RORO ferry MS al-Salam Boccaccio 98 (Red Sea, February 2006) improper fire-fighting procedures caused flooding leading directly to instability and capsize, which resulted in over a thousand fatalities (388 rescued from 1312 passengers + 96 crew).
Free surface effect can affect any kind of craft: ranging from watercraft (where it is most common), to bulk cargo or liquid tanker semi-trailers and trucks (causing either jackknifing
or roll-overs), or aircraft (especially fire-fighting water-drop and refueling tankers where baffles mitigate but do not eliminate the effects). The term "free surface effect" implies a liquid under the influence of gravity. Slosh dynamics is the overarching field which covers both free surface effects and situations such as space vehicles, where gravity is inconsequential, but inertia and momentum interact with complex fluid mechanics to cause vehicle instability.
Capsize
Capsizing is an act of tipping over a boat or ship to disable it. The act of reversing a capsized vessel is called righting.If a capsized vessel has sufficient flotation to prevent sinking, it may recover on its own if the stability is such that it is not stable inverted...
). It refers to the tendency of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to slosh about: to move in response to changes in the attitude
Attitude dynamics and control
Spacecraft flight dynamics is the science of space vehicle performance, stability, and control. It requires analysis of the six degrees of freedom of the vehicle's flight, which are similar to those of aircraft: translation in three dimensional axes; and its orientation about the vehicle's center...
of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea state
Sea state
In oceanography, a sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with...
s caused by waves & wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not completely full is described as a "slack tank", while a full tank is "pressed up".
In a normally laden vessel any rolling from perpendicular is countered by a righting moment generated from the increased volume of water displaced by the hull on the lowered side. This assumes the center of gravity of the vessel is relatively constant. If a moving mass inside the vessel moves in the direction of the roll, this counters the righting effect by moving the center of gravity towards the lowered side. The free surface effect can become a problem in a craft with large partially-full bulk cargo compartments, fuel or water tanks, (especially if they are spanning the full breadth of the ship), or from accidental flooding, such as has occurred in several accidents involving RORO
RORO
Roll-on/roll-off ships are vessels designed to carry wheeled cargo such as automobiles, trucks, semi-trailer trucks, trailers or railroad cars that are driven on and off the ship on their own wheels...
type ferries. If a compartment or tank is either empty or full, there is no change in the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). However, if the compartment is only partially full, the liquid in the compartment will respond to the vessel's heave, pitch, roll, surge, sway or yaw
Ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.- Translation :HeaveSwaySurge-Vertical axis:Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes...
. For example, as a vessel rolls to the left, liquid will displace to the left side of a compartment, and this will move the vessel's center of mass towards the left. This has the effect of slowing the vessel's return to vertical. Also, the momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of large volumes of moving liquids cause significant dynamic
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
forces, which act against the righting effect. When the vessel returns to vertical the roll continues and the effect is repeated on the opposite side. In heavy sea states, this can become a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
loop, causing each roll to become more and more extreme, eventually overcoming the righting effect leading to a capsize. While repeated oscillations of increasing magnitude are commonly associated with the free surface effect, they are not a necessary condition. For example, in the cases of both the SS Normandie
SS Normandie
SS Normandie was an ocean liner built in Saint-Nazaire, France for the French Line Compagnie Générale Transatlantique. She entered service in 1935 as the largest and fastest passenger ship afloat; she is still the most powerful steam turbo-electric-propelled passenger ship ever built.Her novel...
and MS al-Salam Boccaccio 98, gradual buildup of water from fire-fighting caused capsizing in a single continuous roll.
To mitigate this hazard, cargo vessels use multiple smaller bulk compartments or liquid tanks, instead of fewer larger ones, and possibly baffles within bulk compartments or liquid tanks to minimize the free surface effects on the craft as a whole. Keeping individual bulk compartments or liquid tanks either relatively empty or full is another way to minimize the effect and its attendant problems. Tanks or compartments that do not straddle the vessel's centerline are somewhat less prone to destabilising oscillations. Similarly, narrow compartments (aligned bow to stern) and compartments at the extremes away from the centerline, are less prone to cause instability.
Flooding, liquid cargo leakage, or unintended water (from precipitation or waves) in any compartments or on any decks of watercraft, and the resulting free surface effect are often a contributing cause of accidents, capsizes, and casualties e.g. the loss of TEV Wahine
Wahine disaster
The Wahine disaster occurred on 10 April 1968 when the TEV Wahine, a New Zealand inter-island ferry of the Union Company, foundered on Barrett Reef at the entrance to Wellington Harbour and capsized near Steeple Rock...
(Wellington, New Zealand, April 1968), the MS Herald of Free Enterprise (Zeebrugge, Belgium, March 1987), and MS Estonia (Baltic Sea, September 1994). In the case of the RORO ferry MS al-Salam Boccaccio 98 (Red Sea, February 2006) improper fire-fighting procedures caused flooding leading directly to instability and capsize, which resulted in over a thousand fatalities (388 rescued from 1312 passengers + 96 crew).
Free surface effect can affect any kind of craft: ranging from watercraft (where it is most common), to bulk cargo or liquid tanker semi-trailers and trucks (causing either jackknifing
Jackknifing
Jackknifing means the folding of an articulated vehicle such that it resembles the acute angle of a folding pocket knife. If a vehicle towing a trailer skids, the trailer can push it from behind until it spins round and faces backwards. This may be caused by equipment failure, improper braking, or...
or roll-overs), or aircraft (especially fire-fighting water-drop and refueling tankers where baffles mitigate but do not eliminate the effects). The term "free surface effect" implies a liquid under the influence of gravity. Slosh dynamics is the overarching field which covers both free surface effects and situations such as space vehicles, where gravity is inconsequential, but inertia and momentum interact with complex fluid mechanics to cause vehicle instability.
External links
- Discussion of the free surface effect on the Transport Canada website. Link updated 2011-02-15.
- Possible causes of ferry disaster
- Ship inspection maritime guide.
- Marine Engineer World - animations illustrating the effect of compartment geometry on the free surface effect