The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize. It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior approximates that of liquids — 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. When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".
Stability and equilibrium
In a normally loaded 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 tanks, or water tanks, or from accidental flooding, such as has occurred in several accidents involving roll-on/roll-off 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. 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 port, liquid will displace to the port side of a compartment, and this will move the vessel's center of mass to port. This has the effect of slowing the vessel's return to vertical. 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 seas, 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 and, gradual buildup of water from fire-fighting caused capsizing in a single continuous roll.
Mitigation
To mitigate this hazard, cargo vessels use multiple smaller bulk compartments or liquid tanks, instead of fewer larger ones, and possibly s 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. Hydraulic tankers use water to displace lighter oil to keep the tank full at all times. Tanks or compartments that do not straddle the vessel's centerline are somewhat less prone to destabilising oscillations. Similarly, narrow compartments and compartments at the extremes away from the centerline, are less prone to cause instability.
Historical examples
Flooding, liquid cargo leakage, or unintended water 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 , , and . In the case of the RORO ferryal-Salam Boccaccio 98, improper fire-fighting procedures caused flooding leading directly to instability and capsize. In both the cases of the al-Salam Boccaccio 98 and, severe listing followed immediately after the ship had undergone a hard turn, causing unstable volumes of water to surge from one side of the ship to the other.
The free surface effect can affect any kind of craft, including watercraft, bulk cargo or liquid tanker semi-trailers and trucks, and aircraft. 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.
