Bulkheads in ships can be defined as the vertical separations on a vessel that run both transversely and longitudinally. Their primary function is to provide structural rigidity for a vessel against forces in both transversal and longitudinal directions and also act as a sub-division for separating the vessel into a number of watertight compartments.
When designing bulkheads, considerations are made on the overall structural integrity with respect to the loads carried by the bulkheads. Mostly transverse bulkheads carry both vertical and transversal loads and provide structural rigidity against torsional forces.
Most of the primary bulkheads used for structural reinforcements are made water-tight so as to prevent water from occupying the entire volume of the vessel during undesired events like flooding.
They also act as containing the fire within compartments. There are also secondary bulkheads that are used for separating interior spaces such as accommodation spaces which don’t have any structural importance.
The design of bulkheads in ships is followed as per classification society rules and was made very strict after one of the major accidents in the history of cruise vessels, The Titanic. The titanic sank due to the absence of bulkheads extending vertically to the main deck which led to the overall flooding of the vessel.
Rules have been made so as to ensure further events like the same don’t happen in the future. The class society put forwards a rule where a vessel of a particular length should have a minimum number of bulkheads with primary bulkheads such as collision bulkheads, aft peak bulkheads, and watertight bulkheads.
Different Types Of Bulkheads in Ships
Different bulkheads are provided for different functions and they vary as per the locations. The following are the most common types of bulkheads found in most vessels.
- Collision Bulkhead
It is the forward most bulkhead, regardless of the type of the vessel. The primary function of this bulkhead is to contain flooding throughout the vessel by acting like a solid wall that is located aft of the bow, such that even if the bow gets damaged during a collision, the bulkhead will prevent the water from flooding into any other compartments.
The structural design of collision bulkheads in ships is also done with this regard and normally a stronger bulkhead design is adopted and positioned in such a way that it is at an optimum location from the bow, not too aft or not too close with respect to the bow.
A classification society also gives specific rules regarding the location of the collision bulkheads relating to the overall length of the vessel and also factors that depend on the geometry of the vessel’s bow design.
- Aft-Peak bulkhead
The aft peak bulkhead is designed to house the stern tubes of a vessel in a contained environment such that it acts like a flood barrier during undesired events. The location of the aft peak bulkheads is determined while keeping in mind that the vessel should not trim excessively by the stern during flooding.
- Watertight bulkheads
The sub-division of any vessel mostly comprises water-tight bulkheads. They are designed in such a way so as to contain flooding within a certain localized region of the vessel during flooding, which thereby gives the vessel better reserve buoyancy. Reserve buoyancy is the key in terms of vessel stability for safely returning to port without having catastrophic damage.
Watertight bulkheads in ships segregate the machinery spaces, cargo holds, and so on. For machinery spaces, they are also designed to localize fire within the specified spaces such that the fire doesn’t spread throughout the vessel.
How many bulkheads does my vessel need?
A minimum number of bulkheads in ships and their functions are determined by the classification society normally based on the length of a vessel. As the length increases more bulkheads are needed so as to not only subdivide the vessel but also provide transversal strength.
For passenger vessels, the classification society makes sure that maximum safety is provided such that no danger to human life happens at the sea. It is necessary that all passenger vessels carrying more than 12 people should adhere to the SOLAS (Safety of Life at Sea) rules.
The rules specify the design and construction of bulkheads so that even if undesired events such as flooding, the water is confined in a specific compartment and the vessel doesn’t lose its stability due to excessive trim and sinkage.
Design and Construction of Bulkheads.
Bulkheads are designed to calculate the global loads acting on them during a vessel’s lifecycle. Naval Architects use computational tools such as FEA (Finite element analysis) to compute the loads acting on the bulkheads and come up with an efficient design solution that gives the builder the details about the material and its geometric properties.
On smaller-sized vessels, bulkheads are usually constructed with a single plate that is welded onto the side and bottom plates. In the case of larger vessels, they usually have two or more sections welded together to form a single unit. Also, the thickness of these units increases with depth such that they are structurally stable during flooding, when the entire compartment gets flooded the lower sections will be subjected to the maximum hydrostatic forces.
In addition, the bulkheads are further stiffened with the help of transversal or vertical stiffeners which normally have flat, angled, or bulb sections.
In most of the cases, we see vertical stiffeners, as transversal stiffeners would need to cover a longer span for vessels with a higher beam, whereas in the case of vertical stiffeners we can keep the overall scantlings of the vessel low, by addition of stringers at the midplane of the bulkheads which can reduce the overall span of the vertical stiffeners.
For efficient load transfer from the top and bottom plating of the bulkheads in ships, it is necessary for the stiffeners to end at the plating in a well-defined manner. At the upper and lower plating of the bulkheads, end brackets are provided at the intersection points which connect the stiffeners to the plating which corresponds to a hinged boundary condition that accounts for an efficient load transfer from the plates to the stiffeners.
What Are Corrugated Bulkheads?
Corrugated bulkheads in ships are those whose plates are shaped into alternative grooves and ridges, i.e., corrugations and ridges prior to their fabrication which allows the elimination of structural stiffeners on bulkhead plates.
These corrugations can either be vertical or transversal in direction. They are easy to fabricate, as they reduce the number of welded sections, and make installation easier. Also, the corrugations provide ease of repair and maintenance.
On the downside, corrugated bulkheads are made up of plates having a uniform thickness which can increase the overall weight when compared to bulkheads having stiffeners. They are most commonly used in average-sized chemical or product tankers and are positioned as a boundary between each individual cargo tank. They are also treated with special compounds such that the chemical doesn’t corrode the bulkheads easily.
Testing And Safety For Bulkheads
Bulkheads and their boundaries are tested so as to make sure they are watertight. Bulkheads forming tank boundaries are usually tested by pressure testing the tanks. Both forepeak and aft peak bulkheads are tested by filling water till the load water line and ensuring that there are no leakages.
For all other bulkheads, they are normally hose tested for about 200 kn/m2 against the side with stiffeners whereas in the case of corrugated bulkheads any side may be chosen.
In terms of insulation for fire protection, it is made sure that for those bulkheads between the boundaries of machinery spaces the average exposed side temperature that is the walls facing towards the machinery space does not rise above 139 degrees Celsius and the temperature at a single concentrated point should not rise above 225 degree Celsius.
Bulkheads in ships are an essential part of a marine vessel in terms of providing both structural integrity, subdivision, and safety for the vessel during undesired events such as flooding.
Bulkheads help in containing the spread of water in localized spaces during flooding such that the overall reserve buoyancy is maintained with which the vessel can reach safely back to port without any catastrophic damages.