Bulkheads can be classified under two heads: transverse subdivision water tight bulkheads and non water tight bulkheads as depicted in Fig.5.5.
These transverse sub division water tight bulkheads subdivide the ship in several water tight compartments depending onfloodable length and strength requirement. The concept offloodable length is the total length of a ship that can be flooded leading to heeling and trimming of the vessel however the deck will not get immersed. In fact the water surface, i.e. the water line will remain tangential to an imaginary line referred to as margin line that is 75 mm below the deck at side.
Longitudinal bulkheads in the cargo holds are widely used in case of oil tankers. The transverse sub division bulkheads divide the ship in several water tight com- partments. Whereas the longitudinal bulkhead sub divides a vessel longitudinally along the transverse plane. The purpose is to divide the cargo space in longitudinal compartments. Depending on the breadth of the vessel, there can be one central line longitudinal bulkhead or there can be more number of longitudinal bulkheads port
Fig. 5.4 Typical curved subassemblies
Fig. 5.5 Classification of bulkheads
and starboard. This longitudinal division is done to reduce the effect of free surface. In oil tankers and liquid cargo carriers, the free surface of the liquid in the cargo hold gives a negative effect on the vessel’s stability. The reduction in the meta- centric height is more if the free surface area is more. Hence longitudinal bulkheads are used to reduce this free surface area thus reducing the negative effect on stability.
The non-water tight bulkheads are generally the bulkheads in the accommoda- tion region referred to as accommodation bulkheads. Also non water tight bulkhead is there in the fore end construction. It is referred to as wash bulkhead and provides additional strength to the fore end construction against slamming and pounding forces. This wash bulkhead is generally placed at the centre line of the ship, hence in essence it can be termed as longitudinal bulkhead.
5.3.1
Transverse Water Tight Bulkhead
Transverse subdivision bulkheads can be offlat stiffened plate construction or of corrugated construction. The basic functions of these bulkheads are:
(i) They divide the ship into several watertight compartments.
(ii) If by accident any compartment gets flooded, the flooding is kept confined within that compartment by these bulkheads. These are designed to take the hydrostatic load in case offlooding.
(iii) These bulkheads provide support to the longitudinals. These longitudinals run continuously piercing through the bulkheads. They are welded to bulk- heads and the opening is thoroughly sealed to make them water tight. Thus it provides support to the longitudinals.
(iv) These bulkheads are one of the major members providing transverse strength to the hull structure. It prevents racking of the hull.
(v) Should anyfire break out in any cargo hold, these bulkheads should also be able to confine the fire within the hold. This talks about the material of the bulkhead, it cannot be made of any easily combustible or low melting material.
5.3.1.1 Flat Stiffened Bulkhead
The bulkheads provide transverse strength. However in the event of some hull damage leading to water ingress in the compartment, the hold bulkheads will be subjected to hydrostatic loading as shown in Fig.5.6. As it is well known that the hydrostatic loading is directly proportional to depth of water within the hold, the loading on the bulkheads will be maximum at the bottom of the bulkhead.
Because of this the plating arrangement of these bulkheads are horizontal streaks of plates with reducing thickness from bottom to the top of the bulkhead as shown
in Fig.5.7. In this case the plates are arranged breadth wise along the height of the bulkhead. Thus with reducing thickness along the height, the net weight of the bulkhead is reduced.
Stiffening offlat plate bulkheads can be either vertical or horizontal. The stiffener orientation should be such that results in minimum free span of the stiffeners. That means minimum length between the support points. With increase in the span, for the same loading condition, the bending moment increases proportional to the length squared. This will require stiffener of higher section modulus to keep the stress level within the permissible limits. This will call for stiffener of higher scantlings. Hence it is important to have the stiffening arrangement such that it provides for minimum free span.
In case of flat plate subdivision bulkheads of general cargo carrier vertical stiffeners are used. Since such vessels will generally have at least one lower deck and tank top, therefore the vertical stiffeners of the bulkhead will get a natural support at these intermediate points. This will reduce their free span and thus the resulting bending moment for the same load will be highly reduced. Hence stiff- eners of lower scantlings can be used to attain same amount of strength. A typical section is shown in Fig.5.8.
In case of oil tankers or bulk liquid cargo carriers, there will not be any tween deck instead it is very likely that there will be longitudinal bulkhead for reducing
Fig. 5.6 Hydrostatic loading on the bulkheads of aflooded cargo hold
Fig. 5.7 Plating arrangement of a typicalflat plate bulkhead
free surface effect. Hence in such a condition the stiffeners of transverse subdivision bulkheads will be laid horizontally, having support at the side shell and the lon- gitudinal bulkheads. Thus the free span Shin this configuration will be less com-
pared to having it in vertical orientation, Svas can be seen in Fig.5.9.
Fig. 5.8 Bulkhead stiffening arrangement in general cargo ship
Fig. 5.9 Comparison of free span of stiffener orientation in oil tanker
5.3.1.2 Corrugated Bulkhead
The stiffness is achieved by providing corrugations to the plate. Here the geometry of the corrugation is decided based on the section modulus requirement, as in case of stiffened bulkhead, the section modulus of stiffener is decided. The section modulus depends on the depth and width of the corrugations as shown in Fig.5.10. From fabrication point of view, it is advantageous to use corrugated bulkhead, provided, the shipyard has adequate facility in terms of hydraulic press and nec- essary die for fabricating the corrugated units. Once the corrugation parameters are worked out depending on the section modulus requirement as per the classification rules, these individual corrugated units are fabricated. These are produced by a single stroke in a hydraulic press using suitable male and female die. A typical V-type male female die is shown in Fig.5.11.
Each plate is thus individually corrugated and thefinal bulkhead is constructed by butt welding these individual units. The involvement of welding is much less here compared to that offlat plate bulkheads with stiffeners. The corrugations are given along the plate length. In general where ever corrugated bulkheads are used, the corrugations are kept along the vertical. Therefore the advantage of reducing plate thickness as obtained inflat plate bulkheads is not there in case of corrugated bulkheads. These type of bulkheads are generally used in bulk carriers and oil tankers.
5.3.2
Non Water Tight Bulkheads
The partition bulkheads in the accommodation region and the wash bulkhead in the fore end construction are non water tight bulkheads. The wash bulkheads even have fairly large openings to make the structure lighter. These openings are referred to as lightening holes. These bulkheads can be offlat stiffened plate construction or of corrugated construction.
Fig. 5.10 A typical geometry of a corrugated bulkhead