Name: ESTRERA JR. RENATO M.
DATE: APRIL 23, 2022
SECTION: MTJ2-B6
Research assignment on the intact stability
requirements
Understanding Ships' Intact and Damaged Stability
When a vessel is in dry dock or before being put into the sea, it can only be assumed to be motionless
and upright. Once at sea, the ship must contend with a variety of weather conditions as well as the
consequences of external and internal changes.
Several forces act on a ship at all times, including saltwater, wind, internal mass weight, free surface
effect, and so on. As a result, a vessel’s ability to remain stable and afloat in all conditions is critical.
Understanding the capabilities of a surface ship
Stability is classified into two categories. First and foremost, there is intact stability. This branch of
research looks at the stability of a surface ship when the hull is intact and no compartments or
watertight tanks are destroyed or inundated by seawater.
Surface Ships' Intact Stability:
Equilibrium is the underlying notion behind the knowledge of a floating body's intact stability. For a
floating ship, based on the relationship between the positions of the center of gravity and the center of
buoyancy, there are three sorts of equilibrium circumstances that might arise.
1. Equilibrium in a Stable State:
Examine the diagram below. When the vertical position of G is lower than the location of the transverse
metacenter, a stable equilibrium is attained (M). The center of buoyancy (B) now transfers to B1 as the
ship heels to an angle (say theta-Ɵ). In this situation, the lateral distance or lever between the weight
and buoyancy produces a moment that returns the ship to its original upright position.
The Righting Moment is the force that causes the ship to return to its original orientation. The spacing
between the vertical lines flowing through G and B1 is the lever that causes a ship to correct itself. The
Righting Lever (abbreviated as GZ) is what it’s named (refer to the figure above).
2. NEUTRAL EQUILIBRIUM:
This is the deadliest scenario imaginable, since any surface vessel, and all safety procedures must be
used. Taken in order to avert it When the CG’s vertical location coincides with the transverse
metacentre, this occurs (M). In this situation, no righting lever is generated at any angle of heel, as
indicated in the diagram below. As a result, any heeling moment would not cause a righting moment,
and the ship would stay heeled as long as neutral stability prevailed. The danger here is that, in a
neutrally stable shift with a wider angle of heel, an unwanted weight shift caused by cargo movement
could result in an unstable equilibrium position.
3. Unstable Equilibrium:
Is a term used to describe a state of being in a when the vertical position of G is higher than the location
of the transverse metacenter, an unstable equilibrium is created (M). The center of buoyancy (B) now
transfers to B1 as the ship heels to an angle (say theta-Ɵ). However, the righting lever is now negative,
which means that the moment created will result in more heel until a stable equilibrium is attained. The
ship is considered to capsize if the condition of stable balance is not reached by the time the deck is not
submerged.
Type of External Heeling Moments intact of ship:
1. Beam Winds:
The ship’s part above the waterline is affected by beam winds. On the underwater part of the hull, the
resistance works as an opposing force. In this example, two sets of force couples and accompanying
moments are formed. In the diagram below, note the forces acting on the ship.
The heeling moment is formed by the wind force and water pressure in a clockwise direction, while the
righting moment is created by the weight and buoyancy coupling in an anti-clockwise direction. As a
result, when a ship encounters beam winds, it will till up to the angle where the righting moment
generated cancels out the heeling moment.
2. Lifting of Weight by the Sides:
When the deck top crane is used to load or unload weights, the sides of the ship are normally loaded or
unloaded. A movement in the center of gravity causes a heeling moment in this situation.
To learn more, the key concept to grasp is that when a weight is hoisted by a crane, its weight acts on
the fulcrum, which is the end of the crane's derrick, regardless of the weight's height above the ground.
This also means that when a weight (say, a container) is raised from the berth, the container's weight
acts through the end of the derrick (which is a stationary point in relation to the ship).
3. High-Speed Turning Manoeuvres:
When a ship turns, a centrifugal force acts horizontally on the ship's center of gravity in the opposite
direction of the turn. The hydrodynamic pressure acting in the opposite direction on the submerged part
of the hull counteracts this force. The ship heels in the opposite direction of the turn until the righting
moment provided by the weight and buoyancy couple equalizes the heeling moment generated by a
couple of centrifugal force and hydrodynamic pressure, as shown in the diagram below. The more the
centrifugal force generated by a steeper turn, the greater the angle of heel.
4. Grounding of ship:
The upward response force at the point of contact between the hull and the seabed causes heeling
when a ship grounds in such a way that just one side of the underwater hull is impacted. The upward
response force (R) absorbs some of the energy of the ship's forward motion, causing the ship to rise up
to some extent at first. As the tide recedes, the ship sinks deeper into the rock, increasing the
magnitude of the reaction force. The buoyancy decreases in this situation because the ship's weight (w)
is now supported by a mix of the reaction force (R) and remaining buoyancy force (w-R), as indicated in
the diagram below.
5. Tension on mooring lines:
While berthed at a port, ships are moored to bollards, and when loading oil from offshore loading
facilities, they are tethered to guyed buoys. Increased tension on mooring lines causes the ship to heel if
they are over-tightened or if the ship wanders away from the moored position. This, however, can be
easily avoided by employing proper mooring practices.