Racking can be defined as a motion-induced shear stress deformation formed on the hull structure due to the external wave loads acting on the vessel. When the vessel is subjected to a rotational roll motion about its axis, the transversal sections tend to fail due to the deformations at the corners due to shear-induced forces.
Let us consider the example of an empty cardboard box, if we provide a transversal force along the length of the box the shape gets disoriented and takes up the shape of a parallelogram, now a similar happens to a vessel when the acceleration from the roll motion imparts a transversal force on the vessels surface a similar behavior can be observed.
During this phenomenon, the upper part of the vessel tends to move in the athwart direction with respect to the lower sections. Racking stresses tend to concentrate at the corners of a vessel which can eventually build up due to dynamic loading and ultimately lead to failure.
How Are Racking Forces Computed?
To maximize storage every ship owner prefers a design having huge storage spaces which means reducing the number of transversal bulkheads. Now by doing so the lower part of the hull will be stiffer compared to the upper sections and correspondingly the values of stress.
Numerical computations are performed with the help of FE (Finite Element) analysis which computes the loads and the factor of safety of all the sections which are subjected to racking.
There may be a various number of boundary conditions applied and this can be a complex process based on which section for which we are computing the forces. There are also other computational methods such as empirical formulas provided by the classification societies which are used by designers.
The structural members which are designed to resist these forces are mainly the transverse bulkheads, web frames, frames, brackets, and beam knees. It should also be noted that the stiffening of the forward and aftward sections of the vessel also plays a huge role in resisting racking forces.
Racking forces are a cause of the dynamic motion of the vessel and are zero during static conditions hence racking forces correspond to fatigue loading on a vessel.
The above calculations give us values of deflection and stresses which can lead to further cases of failure. Racking forces are a primary cause of transversal forces of both roll and sway and a combination of both. The forces for vertical motion such as heave and pitch are lesser when compared to transversal forces, but can increase with loading conditions.
What Are The Differences Between Racking And Pounding Ships?
As described earlier racking is the motion-induced shear stress due to the rotational motion of the vessel about its centerline towards the port and starboard sides. The roll motion is the highest for bow quartering seas and lowest for stern quartering seas.
Pounding can be defined as the force acting on the bow and aft ends of the vessel due to the rotational motion of the vessel’s midship towards the forward and aftward sides.
Pitching, heaving, and a combination of the two is the primary cause of pounding and can give rise to slamming effects which are highest for head sea conditions.
Methods To Reduce Racking
- Motion Stabilization
Since racking is a motion-induced phenomenon, we can consider motion stabilization which is the reduction of the transversal motions with help of active devices such as fin stabilizers, and gyroscopes and also with the help of passive stabilizers such as bilge keels, ballasting, and so on.
- Structural design and Additional transversal support
During the structural design stage, sufficient factors of safety should be allotted for the primary members subjected to racking force such that the vessels ply safely throughout their design life. Also, Additional support structures can be employed to stiffen the upper sections of the vessel so that it can have a higher resistance against racking.
Conclusion
Racking is associated with global stress concentrations on a vessel and is caused due to dynamic motions which directly relate to cyclic loadings. Proper measures should be adopted to ensure the vessel is structurally stable with periodic surveys and also adequate measures should be adopted to reduce the motions.
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