Watertight Bulkheads

Bulkheads

• Vertical partitions in a ship arranged transversely or longitudinally are referred to as ‘bulkheads’
• Bulkheads that are of greatest importance are the main hull transverse and longitudinal bulkheads, dividing the ship into a number of watertight compartments.
• Lighter bulkheads, named ‘minor bulkheads’, which act as screens further subdividing compartments into small units of accommodation or stores, are of little structural importance.
• The bulkheads are classified mainly into following types:
  • Watertight
  • Non-watertight
  • Tank (Oil tight)

Watertight Bulkheads

• Watertight bulkheads are vertically designed watertight divisions/walls within the ship’s structure, starting from ship’s double bottom top until the upper main deck. The bulkheads avoid ingress of water in a compartment if the adjacent compartment is flooded due to damage in ship’s hull, structure etc.
• Each of the main watertight hold bulkheads may extend to the uppermost continuous deck, but in the case where the freeboard is measured from the second deck they need only be taken to that deck.

Advantages of Watertight Bulkheads

• Subdivision: Divides the vessel into small compartments so that in case of flooding after a collision or grounding etc. the ingress of water will be restricted to a particular compartment. Prevents progressive f looding and ultimate foundering of the vessel.
• Provides Strength: It helps in increasing the transverse strength of the ship which helps in reducing raking stresses, and are important in preserving the transverse shape of a vessel. These are important contributors to the strength of the hull girder. They resist the tendency to deform the shape of the shell. In the case of longitudinal bulkheads, they contribute to longitudinal strength of the hull.
• Hull Stress Distribution: Help to spread hull stresses over larger areas.
• Reduce FSE & Sloshing in tanks: In the case of liquid carriers, the bulkheads divide the vessel into tanks and reduce free surface effects on stability. This is especially so in the case of longitudinal bulkheads in tankers. Also, when the vessel is in motion in seaway, liquid cargoes slosh in the tanks. The bulkheads reduce the damage which could be caused due to sloshing of cargo in an open unrestricted space.
• Fire sub-division: In case of fire in a compartment, these bulkhead help to control the spread of fire to neighboring compartments. They are specially designed to withstand high temperatures for this purpose.

Number of watertight bulkheads — cargo ships

• The minimum number of transverse watertight bulkheads that must be fitted in a dry cargo ship are stipulated.
• A collision bulkhead must be fitted forward, an aft peak bulkhead must be fitted, and watertight bulkheads must be provided at either end of the machinery space.
• This implies that for a vessel with machinery amidships the minimum possible number of watertight bulkheads is four.
• With the machinery aft this minimum number may be reduced to three, the aft peak bulkhead being at the aft end of the machinery space.

Water-tight Bulkheads: General Spacing

• A minimum number of watertight bulkheads will only be found in smaller cargo ships.
• As the size increases the classification society will recommend additional bulkheads, partly to provide greater transverse strength, and also to increase the amount of subdivision.
• These should be spaced at uniform intervals, but the shipowner may require for a certain trade a longer hold, which is permitted if additional approved transverse stiffening is provided.
• It is possible to dispense with one watertight bulkhead altogether, with classification society’s approval, if adequate approved structural compensation is introduced.
• In container ships the spacing is arranged to suit the standard length of containers carried.

Water-tight Bulkheads Spacing: Bulk Carriers

• In the case of bulk carriers a further consideration may come into the spacing of the watertight bulkheads if a shipowner desires to obtain a reduced freeboard.
• It is possible with bulk carriers to obtain a reduced freeboard under The International Load Line Convention 1966, if it is possible to flood one or more compartments without loss of the vessel.
• Additionally, SOLAS amendments now require that bulk carriers constructed after 1 July 1999 and of 150 meters or more in length of single side skin construction and designed to carry solid bulk cargoes of 1000 kg/cubic meter or more when loaded to the summer load line must be able to withstand flooding of any one cargo hold.

Water-tight Bulkheads Spacing: Passenger Ships

• Spacing of watertight bulkheads—passenger ships Where a vessel requires a passenger certificate (carrying more than 12 passengers), it is necessary for that vessel to comply with the requirements of the International Convention on Safety of Life at Sea. Under this convention the subdivision of the passenger ship is strictly specified, and controlled by the authorities of the maritime countries who are signatories to the convention.

Water-tight Bulkheads Spacing: Cargo Ships

The applicable number and disposition of bulkheads are to be arranged to suit the requirements for subdivision, floodability and damage stability, and are to be in accordance with the requirements of National regulations.

• The number of openings in watertight bulkheads is to be kept to a minimum. Where penetrations of watertight bulkheads and internal decks are necessary for access, piping, ventilation, electrical cables, etc., arrangements are to be made to maintain the watertight integrity.

Water-tight bulkheads: collision bulkhead

• Of the watertight bulkheads, the most important is the collision bulkhead forward.
• It is a fact that the bow of at least one out of two ships involved in a collision will be damaged.
• For this reason a heavy bulkhead is specified and located so that it is not so far forward as to be damaged on impact. Neither should it be too far aft so that the compartment flooded forward causes excessive trim by the bow.

collision bulkhead – position

• A collision bulkhead is to be fitted on all ships and is to extend to the freeboard deck.
• It is to be located between 0.05LL or 10m, whichever is less, and 0.08LL aft of the reference point, where the LL is the load line length,
• Proposals for location of the collision bulkhead aft of 0.08LL will be specially considered.

Reference Point

• For ships without bulbous bows the reference point is to be taken where the forward end of LL coincides with the forward side of the stem, on the waterline which LL is measured.

• For ships with bulbous bows, it is to be measured from the forward end of LL a distance x forward; where x is to be taken as the lesser of the following:
  • half the distance, from the forward end of LL and the extreme forward end of the bulb extension (a/2 in diagram)
  • 0.015L
  • 3.0m.

• In general, the collision bulkhead is to be in one plane, however, the bulkhead may have steps or recesses provided they are in compliance with the limits prescribed above.
• As a rule this bulkhead is fitted at the minimum distance in order to gain the maximum length for cargo stowage.
• The collision bulkhead extends to the uppermost continuous deck.
• The collision bulkheads are 12% thicker than other watertight bulkhead and stiffener spacing is reduced to 600 mm to give extra strength to sustain collision.

The load line length is measured on waterline “d1” at 85 percent (0.85D) of the least molded depth “D.” The load line length is either:

• 96 percent of the total waterline length, or
• the waterline length from the FP (forward side of the stem) to the axis of the rudder stock, whichever is longer.

watertight bulkheads: Aft Peak bulkhead

• The aft peak bulkhead is intended to enclose the stern tubes in a watertight compartment, preventing any emergency from leakage where the propeller shafts pierce the hull.
• It is located well aft so that the peak when flooded would not cause excessive trim by the stern.
• Aft peak bulkhead may terminate at the first deck above the load waterline provided this is made watertight to the stern.

watertight bulkheads: machinery bulkheads

• Machinery bulkheads provide a self-contained compartment for engines and boilers, preventing damage to these vital components of the ship by flooding in an adjacent hold.
• They also localize any fire originating in these spaces.

Construction of watertight bulkheads

• The plating of a flat transverse bulkhead is generally welded in horizontal strakes.
• It has always been the practice to use horizontal strakes of plating since the plate thickness increases with depth below the top of the bulkhead.
• The reason for this is that the plate thickness is directly related to the pressure exerted by the head of water when a compartment on one side of the bulkhead is flooded.
• Apart from the depth, the plate thickness is also influenced by the supporting stiffener spacing.
• Vertical stiffeners are fitted to the transverse watertight bulkheads of a ship, the span being less in this direction and the stiffener therefore having less tendency to deflect under load.
• Stiffening is usually in the form of welded inverted ordinary angle bars, or offset bulb plates, the size of the stiffener being dependent on the unsupported length, stiffener spacing, and rigidity of the end connections.
• Rigidity of the end connections will depend on the form of end connection, stiffeners in holds being bracketed or simply directly welded to the tank top or underside of deck, whilst upper tween stiffeners need not have any connection at all.
• Vertical stiffeners may be supported by horizontal stringers, permitting a reduction in the stiffener scantling as a result of the reduced span.
• Horizontal stringers are mostly found on those bulkheads forming the boundaries of a tank space.
• It is not uncommon to find in present-day ships swedged and corrugated bulk- heads.
• The swedges like the troughs of a corrugated bulkhead being so designed and spaced as to provide sufficient rigidity to the plate bulkhead in order that conventional stiffeners may be dispensed with.
• Both swedges and corrugations are arranged in the vertical direction like the stiffeners on transverse and short longitudinal bulkheads.
• Since the plating is swedged or corrugated prior to its fabrication, the bulkhead will be plated vertically with a uniform thickness equivalent to that required at the base of the bulkhead.
• This implies that the actual plating will be somewhat heavier than that for a conventional bulkhead, and this will to a large extent offset any saving in weight gained by not fitting stiffeners.
• At the lower end of transverse hold bulkheads in bulk carriers a bulkhead stool is generally fitted at the lower end of the bulkhead.
• This provides a shedder surface for cargo removal rather than a tight corner at the bulkhead/tank top interface. Inclined shedder plates or gussets are also fitted between the corrugations directly above the stool.
• The addition of a vertical plate bracket under the inclined shedder plate at the midpoint of the corrugation stiffens and supports the corrugation against collapse under load.

• The boundaries of the bulkhead are double continuously fillet welded directly to the shell, decks, and tank top.
• A bulkhead may be erected in the vertical position prior to the fitting of decks during prefabrication on the berth or assembly into a building block.
• At the line of the tween decks a ‘shelf plate’ is fitted to the bulkhead and when erected the tween decks land on this plate, which extends 300–400 mm from the bulkhead.
• The deck is lap welded to the shelf plate with an overlap of about 25 mm.
• In the case of a corrugated bulkhead it becomes necessary to fit filling pieces between the troughs in way of the shelf plate.
• If possible the passage of piping and ventilation trunks through watertight bulk- heads is avoided.
• However, in a number of cases this is impossible and to maintain the integrity of the bulkhead the pipe is flanged at the bulkhead. Where a ventilation trunk passes through, a watertight shutter is provided.

PLANE WATERTIGHT BULKHEAD

CORRUGATED BULKHEAD

Bulk Carrier Bulkhead Stool

• Margin Line: A line drawn parallel to the bulkhead deck at side and at-least 76 millimeters below the upper surface of that deck. The term is used in connection with the method of subdividing merchant ships.
• Bulkhead Deck: The uppermost continuous deck to which all transverse watertight bulkheads are carried. The term is used in connection with the method of subdividing merchant ships.
• Watertight means capable of preventing the passage of water through the structure in any direction under a head of water for which the surrounding structure is designed.
• Swaged or Corrugated Bulkhead : Plate pressed to form “bumps” at spacing similar to traditional stiffeners, also called swedged or corrugated, or crimped

Advantages of Corrugated Bulkheads

• Reduce overall cost of ship design, construction, and life cycle maintenance
• Reduced part count
• Decreased welding
• Improved paint application
• Better paint adherence
• Better accessibility
• Testing watertight bulkheads

Testing watertight bulkheads (which form boundaries of tanks)

Structure to be tested	Type of testing	Structural test pressure	Remarks
Double bottom tanks	Structural testing	The greater of the following: > head of water up to the top of overflow > head of water up to the margin line	Tank boundaries tested from at least one side
Double side tanks Structural testing	Structural testing	The greater of the following: > head of water up to the top of overflow > 2.4 m head of water above highest point of tank	Tank boundaries tested from at least one side
deep tanks	Structural testing	The greater of the following: > head of water up to the top of overflow > 2.4 m head of water above highest point of tank	Tank boundaries tested from at least one side
Fuel oil bunkers	Structural testing	setting pressure of the safety relief valves, where relevant	Tank boundaries tested from at least one side
Ballast hold in bulk carriers	Structural testing	The greater of the following: > head of water up to the top of overflow > 0.90 m head of water above top of hatch
Forepeak and After peak when used as tank	Structural testing	The greater of the following: > head of water up to the top of overflow > 2.4 m head of water above highest point of tank	Test of the after peak carried after the stern tube has been fitted
Cofferdams	Structural testing	The greater of the following: > head of water up to the top of overflow > 2.4 m head of water above highest point of tank