9. Bulkheads Flashcards
Purposes of bulkheads
- Subdivide the vessel into compartments to separate different spaces. Can be watertight or non-watertight
- Divide the vessel into section to limit the amount of flooding that would occur if the shell plates were damaged.
- Act to reduce the spread of fire from one compartment to the next.
- Strength of a vessel is increase greatly to the installation of bulkheads.
- Act as pillars supporting the weight above as well as tying the decks to the bottom structure to reduce flexing
Categories of bulkheads
- Watertight
- Oil-tight (more stringent degree of watertight)
- Non-watertight (weather-tight)
Positions and number of watertight bulkheads
Necessity to ensure a certain degree of intact and positive stability in the event of damage to the hull.
Mandatory:
- Forward collision bulkhead
- Aft peak bulkhead
- Both ends of the machine space
- Adequate number to divide hull into smaller watertight compartments (provide evenly spaced spaces between them).
Rules governing fire protection and the spread of fire may result in the mandatory watertight bulkheads being used for fire protection and therefore a need for them to be fitted at specific location.
Bulkheads replace the frame at that location on the ship’s sides and are fitted intercostal to the decks.
Collision bulkhead
Heavily strengthened structure, watertight. Purpose is limiting the damage of a head on collision to the part of the bow forward to it.
Position laid down in SOLAS:
Cargo vessels: minimum 5%L and maximum 8%L from the forward perpendicular or 10 m, whichever is less. Maximize cargo capacity or storage volume.
Passenger vessels: within 5%L + 3 m
Could be slightly forward of 5%L if bulbous bow projection.
Aft peak bulkhead
Encloses the sterntube (propeller shaft) and sterntube gland. Prevents excessive flooding.
Needs only to extend to the first deck above the load water line (instead of freeboard deck) unless a passenger vessel.
Plating in the after peak bulkhead must be doubled or thickened around the stern tube to resist vibration and provide adequate support to the shaft assembly.
Watertight bulkheads
Must extend to the uppermost continuous watertight deck (freeboard deck, main deck, or strength deck).
Scantalings (plating thickness, stiffener size and spacing) determined by the anticipated load it will or might carry (deck load, tank pressure, flooding). Bulkheads that form boundaries for tank may be subjected to extremely high loading due to hydrostatic pressure.
Damage to the hull along with subsequent flooding of a space could accidentally occur at any time thus placing machinery space bulkheads under substantial load.
Additionally must consider corrosion through the life of the vessel and add a safety factor.
Watertight bulkheads - plate thickness
Minimum 5.5 mm
Deep tank boundary bulkheads: 6.5mm if length < 90m; 7.5 mm if length > 90 m
Collision bulkhead is approx 12% thicker than other watertight bulkheads because of possibility of involvement in collision damage.
Aft peak bulkhead plating often slightly increased to offset the effects of vibration in that area.
If non-watertight but structural, scantlings are the same as watertight bulkheads.
Partial bulkheads are non-structural and non-watertight but used to provide separation within living quarters and recreational areas of the ship - no requirements for plate thicknesses
Watertight bulkheads - Stiffeners
Fitted in order to resist buckling under load. Usually bulb bar or toe welded angle bars. May be continuously or intermittently welded.
Normally stiffeners are placed in a vertical orientation and are approx 760 mm apart. Spacing may be reduced depending on the bulkhead (oil-tight and collision are 610 mm).
Longitudinal bulkheads are sometimes stiffened with longitudinal stiffeners.
The size of stiffeners is dependent on length (height) and type of end connections (bracketed, lugged, or unattached). For longitudinal framed, and extra floor is fitted under the toe of the bracket - provide extra support for the bracket under the tank top. Large brackets are often flanged at their outer edge. The collision bulkhead and tank boundary stiffeners are bracketed at both top and bottom.
If a stiffener is cut, a horizontal stiffener (carling) is inserted to tie the cut stiffener to the adjacent stiffeners.
All is done to ensure that the opening does not take away from the required strength of the bulkhead.
Longitudinal bulkheads
Primary purpose is to subdivide the cargo area into smaller tanks. Provide double hull configuration. Provide significant longitudinal strength to the vessel.
If two longitudinal bulkheads are joined at their midsection, the piece is called a cross-tie.
Corrugated bulkheads
Corrugated by the use of heavy presses and does not require the fitting of stiffeners. More complicated to manufacture. The size of the corrugations determines the strength of the bulkhead. Reduce welding, less weight, less susceptible to corrosion.
Generally trapezoidal but could be rounded. Generally placed vertically except in longitudinal bulkheads where they are horizontal to add strength.
If need extra strength, add diaphragm plates to prevent the collapse of the corrugation under heavy loading.
Three types of vertical corrugated bulkheads
VC1 (without stools at inner bottom) : most common on smaller tankers including product tankers and chemical carriers
VC2 (with lower stools) : larger ships than VC1 and ships exceeding 40,000 dwt
VC3 (lower and upper stool) : largest tankers reaching aframax size
Testing bulkheads
Tested for integrity and water tightness, done by air pressure. Bulkhead is subjected to a prerequisite water pressure from a hose for a fixed period of time. The structural integrity of the bulkhead is inspected (buckling and other deformations). Leak tests are done by pressurizing air in compartment and checking for leakage.
Water testing : head of pressure to a height of 2.45 m above crown of the tank or max height in service
Air pressure : 0.21kg/cm3 (about 3 psi), then lowered and held at 0.14 kg/cm3 (about 2 psi).
Fore and aft peak bulkheads are tested at a head of pressure equivalent to the load water line level.
All other watertight bulkheads are subjected to an air pressure test of approx 3 psi / 20 kPa.
Passages through watertight bulkheads
Openings include doors, wires, pipes and propulsion shafts. Number and size should be kept to a minimum and must be made watertight.
Not normally permitted to have doors cut into the collision bulkhead.
Any hole cut into a bulkhead would weaken the structure so strength must be recuperated in order to maintain adequate structural and watertight integrity. This is done by fitting insert plates or doubler plates around the opening, rounding the corners of the opening and fitting stiffeners.
Watertight doors
Access for personnel to move from compartment to compartment. In general, the opening should be as small as possible while taking into account the frequency of use and who will be using it.
Hinged doors - not normally found below the waterline due to the possible opening and closing difficulty that may arise in flooded conditions. Considered weathertight, mostly used for access to superstructures and deckhouses. Sealed by means of a rubber gasket mounted on the door. A number of clips and wedges are used to force the door against the frame. Could have a central hand wheel instead of clips. Fitted with suitable bearing material and can be lubricated. Hinge pins made of gunmetal.
Sliding watertight doors
Fitted below the waterline, can close horizontally or vertically. Consists of a heavy frame mounted in a suitably strengthened opening in the watertight bulkhead. The frame can either be cast steel or fabricated from steel plates. A water-resistant, heat resistant and long-lasting gasket is fitted between the frame and the bulkhead OR welds hold the frame in plate. Guides are attached to the frame to provide a vertical or horizontal taper. The door is either cast or fabricated from steel and has the same taper. As the door slides closed, the taper causes it to mate firmly with the frame and provide a watertight seal. The sealing faces are either steel or brass. Horizontal sliding doors are mounted on sturdy wheels to allow easy movement.
Watertight door operating mechanisms (3 methods)
- Rack and pinion
- Gunmetal nut and threaded operating rod. The nut is firmly attached to the door while the threaded rod is attached to the frame. When the rod is turned, the nut moves along the length of the rod causing the door to open or close
- Hydraulic cylinder and ram assembly. The ram is fixed to the door and the cylinder is fixed to the frame or surrounding area. This is the most common and can easily be operated remotely.
All can be operated manually and most are fitted with a power operated device. Visual indication of the position of the watertight door is provided at each remote station. Limit switches fitted on the door and frame provide a means to stop the door when it is either fully open or fully closed. Mechanical doorstops are also provided in the event the limit switch malfunctions. Must be simultaneously operable from both sides of the door.
Master remote control stations are sometimes placed on the bridge. Local controls can override the remote station in order to open the door but the door will automatically return to the master location (closed or open) when the local control is released. Doors must be operable at 15 deg.
Wire Passages (3 methods)
- Wires may pass through individual watertight glands
- Numerous wires through a bulkhead may be placed within a common passageway (gland). The whole assembly is sealed (watertight and heat resistant).
- Synthetic packing pieces made to fit different sized wires. A compression piece is screwed down to tighten the joints around the wires.
Pipe Passages
- Welded pipe with flanges on both sides (and suitable gasket material)
- Machined bulkhead piece with gasket
- Short welded pipe with packing
