Abstract: The article presents an analysis of the commonly used method for accounting for the effect of liquid sloshing in partially filled tanks/holds on ship stability, in the form of a ∆GM correction. The limitations of the method are described. A comparison is presented between the actual heeling moments caused by liquid sloshing and the values accounted for by ∆GM. Situations are presented where the use of the ∆GM correction may lead to an erroneous assessment of the ship’s stability condition after flooding of the hull interior.1. INTRODUCTION
A properly performed assessment of a ship’s stability is one of the fundamental conditions guaranteeing its safe operation in an intact state and a necessary condition for making correct decisions in emergency situations, e.g., flooding of the interior. One of the most important elements of assessing a ship’s stability is accounting for the movement of liquid in partially filled tanks/holds during ship heeling (e.g., heeling due to wind, rolling in waves). This phenomenon is referred to as the free surface effect of liquids. A commonly accepted method for accounting for liquid sloshing is the apparent raising of the ship’s center of gravity by a value ∆GM, called the free surface correction.
However, the ∆GM correction is only accurate in calculations concerning small angles of heel. At large angles, it provides overestimated values, but since this results in an additional stability margin for the ship, it is said that the calculations are on the safe side, and this approach is accepted by the regulations of all classification societies supervising the construction and operation of ships.
In the case of a ship in an intact state, the only negative consequence of applying a ∆GM correction that overestimates the effect of free liquid surfaces may be a limitation on the mass of the cargo carried.
The situation changes fundamentally when analyzing the stability of a ship in an emergency state, during the flooding of the hull interior. If the flooding of the interior leads to a loss of ship stability, the primary cause is precisely the effect of water sloshing in partially flooded compartments. Therefore, it is important that emergency stability calculations are performed as accurately as possible. (…)
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Source: Logistyka Magazine
Assessment of damaged ship stability using the added mass method – PDF file for download