There are many forces acting on a ship. How they act is largely determined by the purpose the ship was built for. Forces on a tugboat will be different from the forces acting on a container ship. The types of forces that occur in waves are the same for every ship but the magnitudes and points of action depend on the shape of the ship below the waterline.
The pattern of forces on a ship is very complicated and largely depends on the following parameters:
- the weight of the empty ship
- the weight of the cargo, fuel, ballast, provisions, etc.
- ice
- hydrostatic pressure on the hull applied by the water
- hydrodynamic forces resulting from the movement of the ship in the waves
- vibrations caused by engines, propeller, pitching
- incident forces caused by docking, collisions
Longitudinal strength
In the structural design of the ships, a common practice is to express the design loads by means of the sagging and hogging bending moments and shear forces.
- The sagging and hogging bending moments and shear forces are hull girder loads.
- The hull girder loads are balanced by internal forces and moments affecting at a crosssection of the ship hull (stress resultants)
- The accurate prediction of the extreme wave loads is important in the ultimate strength assessment of the hull girder.
- For ships in a heavy sea, the sagging loads are larger than the hogging loads.
- The linear theories cannot predict the differences between sagging and hogging loads, e.g. strip theory
Shearing Force
When a ship is in calm water, the total upward force will equal the total weight of the ship. Locally this equilibrium will not be realized because the ship is not a rectangular homogeneous object. The local differences between upward pressure and the local weight give rise to shearing forces that lead to longitudinal tensions. The shearing force is the force that wants to shift the (athwart-ship) plane from one part of the ship to another. The submerged part of the ship clearly shows the difference in volume between the midships, the fore- and the aft ship; this is the reason for the difference in upward pressure.
Bending Stress
Uneven load distribution will produce a longitudinal bending moment.
- Buoyant force concentrates at bow and stern.
- Weight concentrates at middle of ship.
The longitudinal bending moment will create a significant stress in the structure called bending stress. A ship has similar bending moments, but the buoyancy and many loads are distributed over the entire hull instead of just one point. The upward force is buoyancy and the downward forces are weights. Most weight and buoyancy is concentrated in the middle of a ship, where the volume is greatest.
Hogging
- If the buoyancy amidships exceed the weight due to loading or when the wave crest is amidships, the ship will Hog, as a beam supported at mid length and loaded at the end.
- In this condition the overall weight is greatest near the bow and stern, with buoyancy being larger near midships. This has the effect of bending the structure in the other direction, placing the keel in compression and the deck in tension.
Sagging
- If the weight amidships exceed the buoyancy or when the wave trough amidships the ship will sag, as a beam supported at a ends and loaded at mid length.
- It is fairly evident, that the “sagging” longitudinal bending condition is creating significant stresses in the structure termed bending stresses. The bending direction is stretching the lower portion of the structure, hence tensile stresses are being created in the keel region. Conversely, the weather deck is being placed in compression because the bending direction is trying to shorten this part of the structure.
Wave Effects
In waves the sagging condition occurs if wave crests are at the bow and stern and hogging if a wave crest is at mid-ship.
• The sagging increases if the ship has large bow flare and the ship motions are large with respect to waves.
• The stern form of the ship can have the same effect if the ship has a flat bottom stern close to the waterline
Reducing the Effects of Bending Stress
Clearly, bending stress is a major cause of concern in establishing a safe ship structure. Unfortunately, because the ship is designed to go to sea where it will experience wave action, there is no method of removing the presence of bending moments.
However, an analysis as described above can allow the areas of the ship that will experience the greatest bending stresses to be determined.
Typically, bending moments are largest at the midship area of a ship. Also, because of the elastic flexure formula, it is clear that the keel and deck will experience the greatest magnitude of bending stress.
Consequently, it is important that these areas have sufficient strength to combat these stresses. Higher strength steels are common in these regions, and the cross sectional area of longitudinal structural elements increases the as you move further from the neutral axis.
Hull – Superstructure Interaction
Due to its distance from the neutral axis, bending stresses in the superstructures of ships can be very large. Unfortunately, it is often undesirable to use high strength materials or structural elements with large cross sections in the superstructure due to the problems this could create with stability. Consequently, other methods of reducing stress must be found.
One solution is the use of expansion joints. The primary reason for using expansion joints involves the shear between the deck and the superstructure. If a ship is hogging, then the deck is under tension. The deck also makes the bottom of the superstructure curve by pulling it outward, or placing it in tension. This outward pull, or shear load, between the superstructure and the hull is aggravated by the sharp corners where the hull and superstructure connect.
As a result, ships like those having Ticonderoga (CG 47) class and Oliver Hazard Perry (FFG 7) class hulls experience cracking in these areas. This is also a potential problem for the Arleigh Burke (DDG 51) class destroyers and YP-703.
Another solution is to break the superstructure up into short sections. However, this is often unsatisfactory in terms of space efficiency and ship habitability.
Source: www.worldmaritimeaffairs.com/
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