BULKHEADS - NAVAL ARCHITECT (STABILITY) CHIEF MATES PHASE 1

 

CHIEF MATES 
PHASE 1 

SUBJECT : NAVAL ARCHITECT (STABILITY) / 

                  : SHIP CONSTRUCTION

TOPIC :BULKHEADS 

The Safety of any ship highly depends on its water tight integrity and structural strength. That is why ships are so heavily stiffened and strengthened to survive rough weather, accidental grounding or collision.

However, the structure of a ship can still get damaged allowing water ingress, resulting in instability which, if is not controlled in time, can cause the ship to capsize and sink.

For this reason, water-tight bulkheads are fitted on ships to avoid such situations.

Bulkheads in ships are similar to internal walls dividing a building into separate rooms.  Vertical partitions arranged either transversely or longitudinally in ships are known as transverse bulkheads and longitudinal bulkheads respectively.

Bulkheads are either watertight or non-watertight although such terms as oil tight and gas tight bulkheads have been used.

What are Watertight / non-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.

Transverse watertight bulkheads divide the main hull into many different watertight compartments. Watertight bulkheads are attached to the shell, the deck, and the bottom or tank top by welding.

Non watertight bulkheads are any other types of bulkhead which are non- watertight such as centerline wash bulkhead in the peak tanks, partial bulkheads in the accommodation spaces, stores and cargo holds.

Types of Bulkheads

The three basic types of bulkheads found on most ships are:

Ø  Watertight bulkheads

Ø  Non-watertight bulkheads and

Ø  Oil tight or tank bulkheads

 

Watertight Bulkheads

“Watertight” means that water (or any other liquid) is not able to pass through the structure into or out of any of the watertight compartments. In other words, “watertight” means prevention from the passage of water in any direction.

The vessel’s hull, main deck and bulkheads between compartments must be watertight. Watertight bulkheads must be watertight at least up to the main deck. Any openings on such bulkheads must be equipped with watertight closing devices.

Weathertight Bulkheads

“Weathertight” means that in any sea condition water cannot penetrate into the vessel. In other words, weathertight means prevention from the passage of water in one direction only.

Hatches, windows and portholes must be equipped with weather tight closing devices. The same applies for doors and other openings on enclosed superstructures.

Oil Tight Bulkheads

“Oil tight” bulkheads form boundaries of tanks used for the carriage of liquid cargoes or fuel.

Cofferdam

In cargo ships where various liquid cargoes are carried, Cofferdams are void spaces between tanks holding liquids or fuel to arrest contamination of liquid of different density. Cofferdams are also fitted between tanks carrying fresh water and oil.  Pump rooms and ballast tanks can be designed to take the place of a cofferdam.

TRANSVERSE WATER-TIGHT BULK HEADS

There are two types of bulkhead construction:

i)       Plain bulkhead

ii)      Corrugated bulkhead

 

Plain Bulkhead

Plain bulkheads consist of plates stiffened by rolled sections such as bulb plates and angles spaced approximately 760mm apart. The thickness of the plates is generally thickest at the bottom because of the maximum hydrostatic pressure experienced there, and thinnest at the top and generally not less than 6.5 mm thick.  The plates of the bulkhead are laid in a horizontal direction.

Where the depth of the bulkhead is great, horizontal stringers or girders are fitted as well as vertical girders with face plate and tripping brackets.

 

Corrugated bulkheads

A corrugated plate is stronger than a flat plate if subject to a bending moment or pillar load along the corrugations. Corrugations are formed on a corrugated bulkhead to eliminate the need to fit the vertical stiffener, as in those of the plain bulkhead.

A corrugated plate is stronger than a flat plate without stiffening if subject to bending moment or a pillar load along the corrugations. The elimination of vertical stiffeners also results in saving in steel weight and cost of stiffeners. The angle of corrugation is normally about 45 degrees.

The troughs are vertical on transverse bulkheads but must be horizontal on continuous longitudinal bulkheads, which form part of the longitudinal strength of the ship. Diaphragm plates or horizontal stringers are fitted on the bulkhead to keep the corrugation in place.

 

 

Advantages or Functions of Watertight Bulkheads

1.   Subdivision: Divides the vessel into small compartments so that in case of flooding after a collision/grounding etc, the ingress of water will be restricted to a particular compartment, prevent progressive flooding and ultimate foundering of the vessel.

2.   It helps in increasing the transverse strength of the ship which helps in reducing racking stresses and are important in preserving the transverse shape of a vessel.

3.   These are important contributors to the strength of the hull girder.

4.   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.

5.   Help to spread hull stresses over larger areas.

6.   In the case of liquid carriers, the bulkheads divide the vessel into tanks and reduce free surface effects on stability.

7.   This is especially so in the case of longitudinal bulkheads in tankers.

8.   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.

9.   Fire 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.

In order to fulfill these functions, watertight bulkheads must extend from the bottom plating right up to the bulkhead deck (uppermost continuous deck).

Construction and Design of Watertight Bulkhead

The watertight bulkhead is made stronger and thicker than other bulkheads to sustain the water pressure in case of water ingress. They are large area bulkheads, incorporated with a number of strakes of plating, which are welded to the ship’s structure at side shell, tank top and deck.

The plating is arranged horizontally and stiffening is done vertically.

The thickness of water tight bulkhead increases towards the bottom as with increase in depth the pressure of the water increase. The horizontal plating’s (strake’s) thickness is gradually increased towards the bottom of the bulkhead.

Strengthening is increased by vertical plate bulb stiffeners or toe angle bar, welded and spaced about 760 mm apart. The ends of bulkhead stiffeners are bracketed to the deck beams and tank top.

The collision bulkheads are 12% thicker than other watertight bulkheads and stiffener spacing is reduced to 600 mm to give extra strength to sustain collision.

Arrangement of Watertight Bulkheads on Ships

The minimum number of water tight bulkhead depends upon the length of the ship and the location of its machinery space. All sea going merchant ships must have at least following bulkheads:

Ø  Collision bulkhead placed at forward of the ship at 0.05L (ships’ length) and it should be continuous to upper deck.

Ø  An aft peak bulkhead which protects and encloses the stern tube and rudder trunk of the ship.

Ø  Bulkheads enclosing the engine room at each end (fore and aft) if the location of the engine room is at mid ship. If the engine room is located at the aft of the ship, the aft peak bulkhead forms the part of aft bulkhead of engine room. Hence only one bulkhead is required to be placed at fore part of the engine room, separating it from the cargo space.

 

 

 

 

Definitions:

Ø  Permeability of a space is the proportion of the immersed volume of that space which can be occupied by water.

Ø  Weather-tight means that in any sea conditions water will not penetrate into the ship.

Ø  Water-tight means having scantlings and arrangements capable of preventing the passage of water in any direction under the head of water likely to occur in intact and damaged conditions.

Ø  Bulkhead deck in a passenger ship means the uppermost deck at any point in the subdivision length to which the main bulkheads and the ship's shell are carried watertight and the lowermost deck from which passenger and crew evacuation will not be impeded by water in any stage of flooding. The bulkhead deck may be a stepped deck. In a cargo ship the freeboard deck may be taken as the bulkhead deck.

Collision bulkhead :

1. A collision bulkhead shall be fitted which shall be watertight up to the bulkhead deck.

This bulkhead shall be located at a distance from the forward perpendicular of not less than 0.05L or 10 m, whichever is the less, and except as may be permitted by the Administration, not more than 0.08L or 0.05L + 3 m, whichever is the greater.

2. Where any part of the ship below the waterline extends forward of the forward perpendicular, e.g., a bulbous bow, the distances shall be measured from a point either:

.1 at the mid-length of such extension;

.2 at a distance 0.015L forward of the forward perpendicular; or

.3 at a distance 3 m forward of the forward perpendicular, whichever gives the smallest measurement.

3. The bulkhead may have steps or recesses provided they are within the limits prescribed in paragraph 1 or 2 above.

4. No doors, manholes, access openings, ventilation ducts or any other openings shall be fitted in the collision bulkhead below the bulkhead deck.

5.1 Except as provided in paragraph 5.2 below, the collision bulkhead may be pierced below the bulkhead deck by not more than one pipe for dealing with fluid in the forepeak tank, provided that the pipe is fitted with a screw-down valve capable of being operated from above the bulkhead deck, the valve chest being secured inside the forepeak to the collision bulkhead.

The Administration may, however, authorize the fitting of this valve on the after side of the collision bulkhead provided that the valve is readily accessible under all service conditions and the space in which it is located is not a cargo space.

5.2 If the forepeak is divided to hold two different kinds of liquids, the Administration may allow the collision bulkhead to be pierced below the bulkhead deck by two pipes, each of which is fitted as required by paragraph 5.1, provided the Administration is satisfied that there is no practical alternative to the fitting of such a second pipe and that, having regard to the additional subdivision provided in the forepeak, the safety of the ship is maintained.

All pipes passing through a watertight bulkhead must be flanged to the bulkhead and do not pass directly through it. 

6. Where a long forward superstructure is fitted, the collision bulkhead shall be extended weather-tight to the deck next above the bulkhead deck.

The extension need not be fitted directly above the bulkhead below provided it is located within the limits prescribed in paragraph 1 or 2 with the exception permitted by paragraph 7 and that the part of the deck which forms the step is made effectively weather-tight.

The extension shall be so arranged as to prevent the possibility of the bow door causing damage to it in the case of damage to, or detachment of, a bow door.

7. Where bow doors are fitted and a sloping loading ramp forms part of the extension of the collision bulkhead above the bulkhead deck, the ramp shall be weather-tight over its complete length.

In cargo ships the part of the ramp which is more than 2.3 m above the bulkhead deck may extend forward of the limit specified in paragraph 1 or 2. Ramps not meeting the above requirements shall be disregarded as an extension of the collision bulkhead.

8. The number of openings in the extension of the collision bulkhead above the freeboard deck shall be restricted to the minimum compatible with the design and normal operation of the ship. All such openings shall be capable of being closed weather-tight.

9. Bulkheads shall be fitted separating the machinery space from cargo and accommodation spaces forward and aft and made watertight up to the bulkhead deck.

In passenger ships an after peak bulkhead shall also be fitted and made watertight up to the bulkhead deck.

The after peak bulkhead may, however, be stepped below the bulkhead deck, provided the degree of safety of the ship as regards subdivision is not thereby diminished.

10. In all cases, stern tubes shall be enclosed in watertight spaces of moderate volume.

In passenger ships the stern gland shall be situated in a watertight shaft tunnel or other watertight space separate from the stern tube compartment and of such volume that, if flooded by leakage through the stern gland, the bulkhead deck will not be immersed.

In cargo ships, other measures to minimize the danger of water penetrating into the ship in case of damage to stern tube arrangements may be taken at the discretion of the Administration.

Construction of watertight bulkheads

1. Each watertight subdivision bulkhead, whether transverse or longitudinal, shall be constructed having scantlings as specified in the regulations.

In all cases, watertight subdivision bulkheads shall be capable of supporting at least the pressure due to a head of water up to the bulkhead deck.

2. Steps and recesses in watertight bulkheads shall be as strong as the bulkhead at the place where each occurs.

Initial testing of watertight bulkheads

1. Testing watertight spaces not intended to hold liquids and cargo holds intended to hold ballast by filling them with water is not compulsory.

When testing by filling with water is not carried out, a hose test shall be carried out where practicable. This test shall be carried out in the most advanced stage of the fitting out of the ship.

Where a hose test is not practicable because of possible damage to machinery, electrical equipment insulation or outfitting items, it may be replaced by a careful visual examination of welded connections, supported where deemed necessary by means such as a dye penetrant test or an ultrasonic leak test or an equivalent test.

In any case a thorough inspection of the watertight bulkheads shall be carried out.

2. The forepeak, double bottom (including duct keels) and inner skins shall be tested with water to a head corresponding to its design pressure.

3. Tanks which are intended to hold liquids, and which form part of the watertight subdivision of the ship, shall be tested for tightness and structural strength with water to a head corresponding to its design pressure.

The water head is in no case to be less than the top of the air pipes or to a level of 2.4 m above the top of the tank, whichever is the greater.

4. The tests referred to in paragraphs 2 and 3 are for the purpose of ensuring that the subdivision structural arrangements are watertight and are not to be regarded as a test of the fitness of any compartment for the storage of oil fuel or for other special purposes for which a test of a superior character may be required, depending on the height to which the liquid has access in the tank or its connections.

Strengthening of openings in W/T bulkheads

The strength of a bulkhead, in way of an opening, is maintained by the following method:

1. Insert plate around the opening

2. Face bar to be fitted on the opening for class 1 W/T door and brass facing strip for class 2 & 3. the corners op the opening are radiused.

3. Web frames fitted beside the class 1 W/T door opening to strengthen the bulkhead.

MIDSHIP SECTION – LARGE TANKER

 

MIDSHIP SECTION – GENERAL CARGO

 

 

COMPOSITELY FRAMED BULK CARRIER