Pressure testing of watertight bulkheads
After installation of the bulkheads, they are to be tested for their integrity and water tightness. Since it is not feasible to fill all the cargo holds or compartments with water for this purpose, the test is done by a pressure hose. In this process, the bulkhead is subjected to a prerequisite water pressure from a hose for a fixed period of time, after which, the structural integrity of the bulkhead is inspected (checks are done for buckling and other deformations). Leak tests can also be done by pressurising the air in a compartment and checking for leakage of air to the other compartment.
Watertight Doors
Intactness is the primary purpose to be maintained by a watertight bulkhead. But in most ships, there are situations unavoidable where access from one compartment to another is a necessity. For example, an under-deck access from one cargo hold to another or from one compartment to another in case of passenger ships. In most ships, access to the shaft tunnel is necessary especially to monitor shaft oil temperatures or for repairs in the region. This purpose is solved by the use of watertight doors.
The bulkhead panel is usually cut out in a rectangular shape accommodating a watertight door. However, special incorporations are made in the structural design of the region around the door opening :
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The dimensions of the opening are kept to the minimum.
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An opening results in a major structural discontinuity, resulting in stress concentration around the opening. To maintain stress levels below safety limits, the opening is strengthened by doubler plates to increase the thickness of the bulkhead plate around the opening.
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If a vertical bulkhead stiffener comes in the way of the opening, it is terminated at the upper and lower edges of the opening. However, designers might choose to increase the stiffener spacing to avoid this. In that case, the scantling of the stiffeners adjacent to the opening are increased from the remaining stiffeners.
Watertight doors are usually hydraulically or electrically operated, and are either horizontally or vertically sliding. The reason why swinging doors are not provided in watertight bulkheads is because it would be impossible to close a swinging door in case of flooding. It must be easily operable even when the ship has listed to 15 degrees to either side, and the control system should be so designed that the door can be operated from the vicinity as well as remotely, i.e. from a position above the bulkhead deck. In all ships, visual indicators are provided at the remote control location to denote whether the door is open or closed.
Watertight doors are also subjected to pressure tests after installation to check for their structural integrity at design hydrostatic pressure in case of complete flooding up to the bulkhead deck.
SOLAS Rules Pertaining to Watertight Bulkheads
One of the most important regulations to be complied with during the design of watertight bulkheads and doors are that of SOLAS, and some important ones are discussed below :
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The number of openings for pipes and access should be kept to minimum in order to retain the strength of the bulkhead. In case such openings are provided, proper reinforcement must be provided so as to prevent stress concentration, and retain water tightness of the structure. Proper flanging must be incorporated in openings for pipelines and cables.
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Not more than one watertight door is allowed per watertight bulkhead. However, in case of ships having twin shafts, there may be two watertight doors, each providing access to the two shaft tunnels on either side. The mechanical gears required for manual operation of these doors must be located outside the machinery spaces.
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The time required to close or open any watertight door when triggered from the control room or navigation deck should not exceed 60 seconds when the ship is in upright condition.
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The transverse location of the watertight doors should be such that they must be easily operable even when the damage to the ship is within one fifth of the ship’s breadth from its side shell.
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Every watertight door should be equipped with an audible alarm distinct from all other alarms in the area. In case the door is being operated remotely, the alarm should start sounding at least 5 seconds before the door begins to slide either way, and must continue till it has completely opened or closed. However, if operated in situ, the alarm must sound only when the door is sliding. In case of passenger ships, the audible alarm must be accompanied by a visual alarm.
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All watertight doors that are accessible during voyage must be locked via an authorised unlocking system.
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Access doors and hatches on watertight bulkheads must remain closed when the ship is at sea. Visual indicators must be provided for every access hatch to indicate their status at the location and the navigation bridge.
Fire Class of Bulkheads
In order to prevent the propagation of fire from one compartment to another, all watertight bulkheads are also provided with fire-resistant paneling. However, depending on the extent to which bulkheads can retain the fire and smoke to the affected side, they are classified into three categories :
Class-A Panel : All watertight bulkheads are Class-A type. Bulkheads of Class A must be constructed of steel or equivalent material and should pass the standard fire test, preventing the passage of fire or smoke to the unaffected side for at least one hour. With Class A bulkheads in use, the average temperature on the unaffected side must not exceed 120 degree Celsius. Added to that, there are three categories of Class A panels depending on the time up to which the temperature at any point on the bulkhead must not rise above 160 degree Celsius :
A-60 Panel: 60 minutes.
A-30 Panel: 30 minutes.
A-15 Panel: 15 minutes.
A-0 panel: 0 minutes.
Class-B Panel : Bulkheads of Class B are constructed of materials that are approved by SOLAS and classification societies as incombustible materials. And should pass the standard fire test, preventing the passage of fire or smoke to the unaffected side for at least thirty minutes. With Class B bulkheads in use, the average temperature on the unaffected side must not exceed 120 degree Celsius. There are two types of Class B panels depending on the time up to which the temperature at any point on the bulkhead must not rise above 206 degree Celsius :
B-15 Panel: 15 minutes.
B-0 panel: 0 minutes.
Class-C Panel : Class C bulkheads and decks are constructed of materials that are approved by SOLAS and classification societies as incombustible, but they are not required to meet any requirements related to rise in temperature or passage of smoke and flame to the unaffected side.
Class A and B panels are used adjacent to most of the enclosed spaces within the ship, for example: cargo holds, control stations, stairways, lifeboat embarkation stations, galleys, machinery spaces, tanks, public spaces and accommodation areas. Class C panels are mostly used in open decks and promenades, where requirement of fire safety is minimum. They can also be used between two similar spaces if they are not separated by a watertight bulkhead, in which case a Class A panel is mandatory.
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