Answer Construction Question 6

Question 6: (a) Considering the vessel as a compound beam define Bending moment shearing force. Which is the point of Maximum Bending Moment?

(b) Sketch and Describe Hatch coaming of a large bulk carrier.

Answer: (a) A ship may be regarded as non-uniform beam, carrying non-uniformly distributed weights and having varying degrees of support along its length.

(i) Still water bending:

Consider a loaded ship lying in still water. The upthrust at any one metre length of the ship depends upon the immersed cross-sectional area of the ship at that point. If the values of upthrust at different positions along the length of the ship are plotted on a base representing the ship's length, a buoyancy curve is formed. This curve increases from zero at each end to a maximum value in way of the parallel mid-ship portion. The area of this curve represents the total upthrust exerted by the water on the ship. The total weight of a ship consists of a number of independent weights concentrated over short lengths of the ship, such as cargo, machinery, accommodation, cargo handling gear, poop and forecastle, and a number of items which form continuous material over the length of the ship, such as decks, shell and tank top. The difference between the weight and buoyancy at any point is the load at that point. In some cases the load is an excess of weight over buoyancy and in other cases an excess of buoyancy over weight. A load diagram formed by these differences is shown in the figure. Since the total weight must be equal to the total buoyancy, the area of the load diagram above the base line must be equal to the area below the base line. Because of this unequal loading, however, shearing forces and bending moments are set up in the ship. The maximum bending moment occurs about midships.

Depending upon the direction in which the bending moment acts, the ship will hog or sag. If the buoyancy amidships exceeds the weight, the ship will hog, and may be likened to a beam supported at the centre and loaded at the ends.
When a ship hogs, the deck structure is in tension while the bottom plating is in compression. If the weight amidships exceeds the buoyancy, the ship will sag, and is equivalent to a beam supported at its ends and loaded at the centre.

When a ship sags, the bottom shell is in tension while the deck is in compression. Changes in bending moment occur in a ship due to different systems of loading. This is particularly true in the case of cargoes such as iron ore which are heavy compared with the volume they occupy. If such cargo is loaded in a tramp ship, care must be taken to ensure a suitable distribution throughout the ship. Much trouble has been found in ships having machinery space and deep tank/cargo hold amidships. There is a tendency in such ships, when loading heavy cargoes, to leave the deep tank empty. This results in an excess of buoyancy in way of the deep tank. Unfortunately there is also an excess of buoyancy in way of the engine room, since the machinery is light when compared with the volume it occupies. A ship in such a loaded condition would therefore hog, creating very high stresses in the deck and bottom shell. This may be so dangerous that if owners intend the ships to be loaded in this manner, additional deck material must be provided. The structure resisting longitudinal bending consists of all continuous longitudinal material, the portions farthest from the axis of bending (the neutral axis) being the most important, e.g., keel, bottom shell, centre girder, side girders, tank top, tank margin, side shell, sheer-strake, stringer plate, deck plating alongside hatches, and in the case of oil tankers, longitudinal bulkheads. Danger may occur where a point in the structure is the greatest distance from the neutral axis, such as the top of a sheer-strake, where a high stress point occurs. Such points are to be avoided as far as possible, since a crack in the plate may result. In many oil tankers the structure is improved by joining the sheer-strake and stringer plate to form a rounded gunwale.

(ii) Wave bending:

When a ship passes through waves, alterations in the distribution of buoyancy cause alterations in the bending moment. The greatest differences occur when a ship passes through waves whose lengths from crest to crest are equal to the length of the ship. When the wave crest is amidships, the buoyancy amidships is increased while at the ends it is reduced. This tends to cause the ship to hog. A few seconds later the wave trough lies amidships. The buoyancy amidships is reduced while at the ends it is increased, causing the vessel to sag. The effect of these waves is to cause fluctuations in stress, or, in extreme cases, complete reversals of stress every few seconds. Fortunately such reversals are not sufficiently numerous to cause fatigue, but will cause damage to any faulty part of the structure.

(b) Hatch coaming: Hatch coaming is vertical plating bounding a hatch for the purpose of stiffening the edges of the opening and resisting entry of water to ship hull or cargo space, also it forms a barrier to prevent accidental fall of personnel into hatches working nearby on deck.

Heights of coamings and cover closing arrangements in some instances depend on the hatch position. The positions differentiate between regions which are more exposed than others.
Position 1 indicates that the hatch is on the exposed freeboard deck, raised quarter deck, or superstructure decks within 25 per cent of the ship’s length from forward.
Position 2 indicates that the hatch is located on exposed superstructure decks abaft the forward 25 per cent of the ship’s length.
Hatches which are at Position 1 have coamings at least 600 mm high, and those at Position 2 have coamings at least 450 mm high, the height being measured above the sheathing. Provision is made for lowering these heights or omitting the coaming altogether if directly secured steel covers are fitted and it can be shown that the safety of the ship would not be impaired in any sea condition. Where the coaming height is 600 mm or more the upper edge is stiffened by a horizontal bulb flat and supporting stays to the deck are fitted. Coamings less than 600 mm high are stiffened by a cope or similar bar at their upper edge. The steel coamings extend down to the lower edge of the deck beams, which are then effectively attached to the coamings.