FRAMING SYSTEM & BULKHEAD PHASE 1

 

Framing system

• In transverse framing system, frames are attached at right angles to the keel, spaced between 610 and 910 mm. apart. The frames are secured at the lower ends to the keel and at the upper ends to the deck beams. 
• Longitudinal strength comes from the keel, longitudinals, tank top and decks to resist stress caused by the rolling forces of the sea. The side plating also takes much of this stress. This method allows for simplicity in construction and transverse strength in short vessels. However, as ships grew in length, longitudinal strength became increasingly important. 
• In longitudinal framing, very heavy transverse frames are spaced about 3 to 5 m. apart. A large number of longitudinal frames are then attached to hold the shell plating. The longitudinal frames at the sides fit into notches cut into the transverse frames, while the ones near the bottom of the ship are sometimes made continuous between transverse bulkheads. 
• Under the tank top, except for notches cut for the bottom and tank top longitudinals, the transverses are much like ordinary floor plates. The deck longitudinals furnish ample strength, even when large hatch openings must be accommodated. 
• Where a ship’s length exceeds 120m. it is considered desirable to adopt longitudinal framing. Tankers greater than 200m. in length must be framed longitudinally.

Transverse framing system

Advantages:

• It resist Hydrostatic / local loading in bow, engine room and cargo hold of bulk carrier.
• It absorbs the vibration in engine room and accommodation.
• It is adopted for general cargo ships where maximum bale capacity is required.

Disadvantages:

• It is heavier than longitudinal framing system.
• Much deformation occurs during block fabrication.
• It is not  easily accessible for welding and non-destructive testing.
• Scantlings of the main transverse frames are primarily dependent on their position, spacing and depth, and to some extent on the rigidity of the end connections.

Longitudinal framing system

Advantages :

• It is adopted in large container ships and bulk carriers.
• Compared to transverse framing system it is more resistant to buckling especially in upper deck. 
• It resists compressible stresses of the deck or bottom in a seaway. 
• It supports side shell and bulkhead plating which are subjected to variable pressures from the sea or from liquid cargo. 
• The hydrostatic pressure on each successive longitudinal and its associated plating increases with its depth below the waterline or below the surface of the liquid in the tank. To withstand this pressure each longitudinal can be sized to withstand the maximum pressure associated with its depth in the ship, thus achieving an efficient use of structural material. This cannot be achieved effectively with transverse frames.

Disadvantages :

• Deep transverse web frames intrude into prime cargo spaces for general cargo ships carrying packaged cargo. 
• As the hull narrows towards the bow and stem, the girth necessarily reduces and the longitudinals come closer together. This creates difficulties in construction when they converge so close that some longitudinals have to be eliminated.

Bulkheads

Learning Objectives

After completing this topic you will be able to:

• Be familiar with the Regulations governing the number and position of bulkheads.
• Define the various types of bulkhead
• Sketch and describe the construction of a bulkhead along with its attachments and systems of passing lines without compromising the integrity of the bulkhead
• Explain the uses of wash bulkheads, cross ties, procedures for testing bulkheads
• Explain how transverse bulkheads are effective in withstanding racking stresses.
• Compare a cofferdam/ flat plate and corrugated bulkhead.

Man learns through his mistakes both human and technological. These mistakes and their horrendous consequences leads to a thought process that aims to put in places procedures and processes which are intended to avoid such incidents in the future. The IMO with SOLAS as its main safety convention are the vehicles through which such incidents are routed and result in improved safety measures/ regulatory regime. These are then considered by IACS and the member societies bring out their amended construction rules to be followed when maritime structures are built.

When a compartments watertight integrity is breached, we term it “Bilging”. This results in the loss of buoyancy of that watertight compartment. In the case of a ship without subdivision this would mean rapid sinking since the bilging would have resulted in the total loss of reserve buoyancy.

The two disasters which lead to research and development of maritime structures and a substantial positive impact on safety was brought on by the loss of the Titanic (1912) and the Herald of Free Enterprise (1987). At this juncture we will only consider the impact the loss of the Titanic made with respect to subdivision.

The Titanic had 15 “watertight” bulkheads which divided the hull into 16 compartments and lead to its builders “Harland & Wolfe” to term the ship “unsinkable”. The number of bulkheads far exceeded the requirements of the then regulatory regime. As we all know events proved otherwise, with post sinking investigations revealing that the watertight bulkheads were far from being watertight.

Bulkheads

Bulkhead definitions

Class A

Class A divisions are forming bulkheads and decks that;

• Constructed of steel or equivalent
• Suitably stiffened
• Prevent passage of smoke and flame to the end of one hour standard fire test
• Insulated using non-combustible material so that average temperature on exposed side does not rise above  and point temperature above . The time the bulkhead complies with this governs its class A-60 60min
  A-30 30Min 
  A-15 15Min 
  A-0 0Min

CLASS  B

• These are divisions formed by bulkheads, decks, ceilings and lining Prevent passage of flame for first half hour of standard fire test Insulated so average exposed side temperature does not rise more than  above original and no single point rises more than  above original.
• The time the bulkhead complies with this governs its class:
  B-15 15 Min
  B-0 0 Min

Constructed of non-combustible material and all materials entering the construction are similarly non-combustible except where permitted

CLASS  C

• These are divisions constructed of approved non-combustible materials. Combustible veneers are allowed were they meet other criteria.
• Main vertical zones Divided by Class A bulkheads and not exceeding 40m in length

Bulkheads

There are three basic types of bulkhead, watertight, non watertight and tank.

Different types of bulkheads are designed to carry out different functions.

The watertight bulkhead several important ones;

1. It divides the ship into watertight compartments giving a buoyancy reserve in the event of hull being breached. The number of compartments is governed by regulation and type of vessel
2. Cargo separation
3. They restrict the passage of flame
4. Increased transverse strength, in effect they act like ends of a box
5. Longitudinal deck girders and deck longitudinal are supported by transverse watertight bulkheads which act as pillars
   

 BULKHEADS

Types of Bulk Head:

There are three basic types of bulkhead, watertight, non watertight and tank. Different types of bulkheads are designed to carry out different functions. The watertight bulkhead several important ones;

It divides the ship into watertight compartments giving a buoyancy reserve in the event of hull being breached. The number of compartments is governed by regulation and type of vessel:

• Cargo separation.
• They restrict the passage of flame
• Increased transverse strength, in effect they act like ends of a box
• Longitudinal deck girders and deck longitudinal are supported by transverse watertight bulkheads which act as pillars

Test of water tight bulkheads:

• Watertight bulkheads must be tested with a hose at a pressure of 200 Kn/. The test being carried out from the side on which the stiffeners are fitted and the bulkhead must remain watertight.
• Water tight bulkheads which are penetrated by pipes, cables etc. must be provided with suitable glands which prevent the passage of water.

 

Longitudinal Bulk Heads

  Tank Side Brackets

• The lower end of the frame may be connected to the tank top or hopper side tank by means of a flanged or edge stiffened tank side bracket as illustrated in Figure above.

Regulations of Classification Societies regarding Bulkheads

Regulations governing the number and position of watertight bulkheads

(Unless specially mentioned the IACS Rules and Regulations used in this book are for Steel Bulk Carriers Classed with NK/ LRS. Students who are interested in further advancing their knowledge are guided to the Rules and Regulations published by these Societies as well as other members of IACS eg IRS)

Watertight subdivision limits the loss of buoyancy. SOLAS lays down subdivision criteria for all ships including passenger ships thereby laying down mandatory requirements for watertight transverse bulkheads.

SOLAS 2014 Chapter II – I, Regulation 6 and 7 lays down the method of calculating the subdivision index which forms the basis in arriving at the required subdivision of a particular ship. The separation between two watertight bulkheads is also linked to the vessels floodable length curves. This will be dealt with in the module on ship stability.

Subdivision of Passenger ships are subject to standards which are far in excess of cargo ship rules.

Number & Arrangement of Transverse Watertight Bulkheads

There are three types of bulkheads

• Watertight
• Oil tight
• Non Watertight

All ships, in addition to complying with the requirements of Table 1, are to have at least the following transverse watertight bulkheads:

• One collision bulkhead 
• One after peak bulkhead 
• Two bulkheads forming the boundaries of the machinery space in ships with machinery amidships, and a bulkhead forward of the machinery space in ships with machinery aft (Aft Peak bulkhead). In the case of ships with an electrical propulsion plant, both the generator room and the engine room are to be enclosed by watertight bulkheads.

Additional bulkheads

For ships not required to comply with subdivision requirements, transverse bulkheads adequately spaced, and not less in number than indicated in Table 1, are to be fitted. This means that all ships have to have to comply with Table 1 and the additional bulkheads as stated in the preceeding paragraph.

Regulations of SOLAS Convention, 2020 regarding Bulkhead

Chapter II-1 of SOLAS Convention

Reg. 10 – Construction of watertight bulkheads

• Watertight means having scantlings and arrangements capable of preventing passage of water in any direction under the head of water likely to occur in intact and damaged conditions.
• Watertight subdivision bulkheads shall be capable of supporting at least the pressure due to a head of water up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• Bulkhead deck of a passenger ship means the uppermost deck to which the main bulkheads and ship’s shell are carried watertight.
• Freeboard deck normally means the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings in the weather part thereof, and below which all openings in the sides of the ship are fitted with permanent means of watertight closing.

Reg. 11 – Initial testing of watertight bulkheads, etc.

• Watertight spaces which are not intended for holding liquids shall be subjected to hose test or, if it is not practicable, to following inspections :

• Thorough inspection of the watertight bulkhead, and
• Visual examination of welded connections, including dye penetration test or ultrasonic test or leak test.

• Tanks intended to hold liquids shall be tested for watertightness and strength by filling with water to a head given in Reg. 10, or to a head to the top of the air-pipe or to a head of 2.4m. above the top of the tank, whichever is greater.

Reg. 12 – Peak and machinery space bulkheads, shaft tunnels, etc. 

• Collision bulkhead shall be located at a distance from the forward perpendicular of not less than 0.05L or 10m., whichever is less. Flag State may permit the distance to be changed to 0.08l or 0.05L + 3m., whichever is greater.
• This bulkhead shall be watertight up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• Where a long forward superstructure is fitted, then the collision bulkhead shall extend weathertight to the deck above the respective decks stated above.
• If the  ship is fitted with an extension forward of the forward perpendicular e.g. bulbous bow, then the above distance shall be measured from a point at mid-length of the extension or 0.015L forward of the forward perpendicular or 3m. forward of the forward perpendicular, whichever gives the smallest measurement.
• Doors, manholes, access openings, ventilation ducts, etc. shall not be fitted in the collision bulkhead below the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• The bulkhead may be pierced by maximum one pipe below the respective decks, for dealing with fluid in the forepeak tank, and fitted with screw-down valve located inside or outside the fore-peak tank, as decided by the Flag State. The valve shall be of approved material.
• If the forepeak tank is divided into two tanks for different liquids, then two pipes may pierce the collision bulkhead as stated above.
• Bulkheads shall be fitted to separate the machinery space from the cargo and accommodation spaces forward and aft, and shall be watertight up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• An afterpeak bulkhead shall be fitted and made watertight as stated above.

Reg. 13 – Openings in watertight bulkheads below the bulkhead deck in passenger ships

• Minimum number of openings shall be provided, compatible with proper working of the ship and with satisfactory means of closing.
• Where pipes, scuppers, electric cables, etc., pass through the bulkhead, its watertight integrity shall be ensured.
• Lead or other heat sensitive materials shall not be used in systems, which penetrate the bulkheads.
• Doors, manholes or access openings are not permitted in transverse bulkheads between two cargo spaces.
• Only one door is permitted for the bulkhead within machinery spaces, other than the door to shaft tunnel.
• If there are two or more shaft tunnels, then these shall be connected by intercommunicating passage.

Watertight sliding door

• Only one door is permitted between the machinery space and shaft tunnel.
• It shall be provided with high sills.
• It shall operate vertically or horizontally.
• Maximum clear opening shall be 1.2m. 
• It shall be capable of being closed by hand or power with the ship listed 15° either way. 
• It shall be so located that if the ship sustains damage within one-fifth of the breadth of the ship, its operation shall not be impaired.
• Indicators for showing whether the door is open or closed shall be provided at the Bridge and at position where the hand gear is located.

Hand gear

• It shall be operated by hand gear from both sides of the door and also from outside the machinery space above the bulkhead deck to close the door in maximum 90 sec. with the ship upright.
• Audible alarm shall sound when the door is moving, and intermittent visual signal may be given at the door. 

Power operation

• Electric power from emergency switch board, hydraulic or any other form of power shall close the door from both sides in 20 to 40 sec.
• Power shall be capable of closing all doors simultaneously from the Bridge in not more than 60 sec. with the ship upright, but not for opening the door.
• Audible alarm shall sound when the door is remotely closed, for 5 to 10 sec. before it starts closing, and continue to sound till it is closed.
• Intermittent visual signal may be given at the door. 

Reg. 13-1 - Openings in watertight bulkheads and internal decks in cargo ships

• Minimum number of openings shall be provided, compatible with proper working of the ship and with satisfactory means of closing.
• Where pipes, scuppers, electric cables, etc., pass through the bulkhead, its watertight integrity shall be ensured.
• Doors and hatch covers, which provide watertight integrity to internal openings at sea shall be ---

• Sliding type,
• Operated by power and hand gear,
• Closed remotely from the Bridge,
• Operated from both sides of the bulkhead, and
• Provided with---

• Indicators to show whether the door is open or closed;
• Audible alarm when the door is closed;
• Hand gear on both sides of the door.

Reg. 16 – Construction and initial tests of watertight closures

• Doors and hatches shall be tested by water pressure to the maximum head of water it may sustain in a final or intermediate stage of flooding.
• In lieu of above, prototype pressure testing of each type and size of door before fitting, corresponding to the head of water stated above, may be carried out.

Regulations of SOLAS Convention, 2020 regarding Bulkhead

Chapter II-1 of SOLAS Convention

Reg. 10 – Construction of watertight bulkheads

• Watertight means having scantlings and arrangements capable of preventing passage of water in any direction under the head of water likely to occur in intact and damaged conditions.
• Watertight subdivision bulkheads shall be capable of supporting at least the pressure due to a head of water up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• Bulkhead deck of a passenger ship means the uppermost deck to which the main bulkheads and ship’s shell are carried watertight.
• Freeboard deck normally means the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings in the weather part thereof, and below which all openings in the sides of the ship are fitted with permanent means of watertight closing.

Reg. 11 – Initial testing of watertight bulkheads, etc.

• Watertight spaces which are not intended for holding liquids shall be subjected to hose test or, if it is not practicable, to following inspections :

• Thorough inspection of the watertight bulkhead, and
• Visual examination of welded connections, including dye penetration test or ultrasonic test or leak test.

• Tanks intended to hold liquids shall be tested for watertightness and strength by filling with water to a head given in Reg. 10, or to a head to the top of the air-pipe or to a head of 2.4m. above the top of the tank, whichever is greater.

Reg. 12 – Peak and machinery space bulkheads, shaft tunnels, etc. 

• Collision bulkhead shall be located at a distance from the forward perpendicular of not less than 0.05L or 10m., whichever is less. Flag State may permit the distance to be changed to 0.08l or 0.05L + 3m., whichever is greater.
• This bulkhead shall be watertight up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• Where a long forward superstructure is fitted, then the collision bulkhead shall extend weathertight to the deck above the respective decks stated above.
• If the  ship is fitted with an extension forward of the forward perpendicular e.g. bulbous bow, then the above distance shall be measured from a point at mid-length of the extension or 0.015L forward of the forward perpendicular or 3m. forward of the forward perpendicular, whichever gives the smallest measurement.
• Doors, manholes, access openings, ventilation ducts, etc. shall not be fitted in the collision bulkhead below the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• The bulkhead may be pierced by maximum one pipe below the respective decks, for dealing with fluid in the forepeak tank, and fitted with screw-down valve located inside or outside the fore-peak tank, as decided by the Flag State. The valve shall be of approved material.
• If the forepeak tank is divided into two tanks for different liquids, then two pipes may pierce the collision bulkhead as stated above.
• Bulkheads shall be fitted to separate the machinery space from the cargo and accommodation spaces forward and aft, and shall be watertight up to the bulkhead deck of passenger ships and the freeboard deck of cargo ships.
• An afterpeak bulkhead shall be fitted and made watertight as stated above.

Reg. 13 – Openings in watertight bulkheads below the bulkhead deck in passenger ships

• Minimum number of openings shall be provided, compatible with proper working of the ship and with satisfactory means of closing.
• Where pipes, scuppers, electric cables, etc., pass through the bulkhead, its watertight integrity shall be ensured.
• Lead or other heat sensitive materials shall not be used in systems, which penetrate the bulkheads.
• Doors, manholes or access openings are not permitted in transverse bulkheads between two cargo spaces.
• Only one door is permitted for the bulkhead within machinery spaces, other than the door to shaft tunnel.
• If there are two or more shaft tunnels, then these shall be connected by intercommunicating passage.

Watertight sliding door

• Only one door is permitted between the machinery space and shaft tunnel.
• It shall be provided with high sills.
• It shall operate vertically or horizontally.
• Maximum clear opening shall be 1.2m. 
• It shall be capable of being closed by hand or power with the ship listed 15° either way. 
• It shall be so located that if the ship sustains damage within one-fifth of the breadth of the ship, its operation shall not be impaired.
• Indicators for showing whether the door is open or closed shall be provided at the Bridge and at position where the hand gear is located.

Hand gear

• It shall be operated by hand gear from both sides of the door and also from outside the machinery space above the bulkhead deck to close the door in maximum 90 sec. with the ship upright.
• Audible alarm shall sound when the door is moving, and intermittent visual signal may be given at the door. 

Power operation

• Electric power from emergency switch board, hydraulic or any other form of power shall close the door from both sides in 20 to 40 sec.
• Power shall be capable of closing all doors simultaneously from the Bridge in not more than 60 sec. with the ship upright, but not for opening the door.
• Audible alarm shall sound when the door is remotely closed, for 5 to 10 sec. before it starts closing, and continue to sound till it is closed.
• Intermittent visual signal may be given at the door. 

Reg. 13-1 - Openings in watertight bulkheads and internal decks in cargo ships

• Minimum number of openings shall be provided, compatible with proper working of the ship and with satisfactory means of closing.
• Where pipes, scuppers, electric cables, etc., pass through the bulkhead, its watertight integrity shall be ensured.
• Doors and hatch covers, which provide watertight integrity to internal openings at sea shall be ---

• Sliding type,
• Operated by power and hand gear,
• Closed remotely from the Bridge,
• Operated from both sides of the bulkhead, and
• Provided with---

• Indicators to show whether the door is open or closed;
• Audible alarm when the door is closed;
• Hand gear on both sides of the door.

Reg. 16 – Construction and initial tests of watertight closures

• Doors and hatches shall be tested by water pressure to the maximum head of water it may sustain in a final or intermediate stage of flooding.
• In lieu of above, prototype pressure testing of each type and size of door before fitting, corresponding to the head of water stated above, may be carried out.              
•  

• Watertight Bulkhead

  Standard Watertight Bulkhead

  The total number of watertight bulkheads is determined by Table 1. The watertight bulkhead is a vertically designed transverse bulkhead of steel construction. The thickness of metal at the lower end is more than the upper end to tolerate the increase in water pressure loading as the depth of water in the bilged compartment increases with flooding. Plating is horizontal.

  
  

  Strength is provided by bulb angle or angle bar stiffeners not more than 760 mm apart. The ends of the stiffeners are bracketed to the deck beams and tank top brackets.

  
  

  In addition to creating reserve buoyancy these bulkheads:

  • Add to transverse strength by resisting racking and torsional forces 
  • Resists the spread of flame 

  
  

  

  
  

  Collision Bulkhead

  Watertight up to the freeboard deck, and fitted within 0.05L or 10m whichever is less from the FPP. Where any part of the ship below the waterline extends forward of the forward perpendicular, e.g. a bulbous bow, the distances, in metres, as mentioned above are to be measured from a point either:

  • At the mid-length of such extension, or 
  • At a distance 1.5% of the length L of the ship forward of the forward perpendicular, or 
  • At a distance 3 m forward of the forward perpendicular

  
  

  No door, manhole or any opening is allowed in this bulkhead. The steel plate is around 12% thicker than other watertight bulkheads with the stiffeners 610 mm apart.

  
  

  
  

  
  

  Aft Peak Bulkhead

  An aft peak bulkhead that forms a watertight compartment for the stern tube, and rudder trunk has to be fitted. It may terminate at the first deck above the summer load line provided the deck is made watertight to the stern, thereby creating a watertight compartment along with the shell plating and the aft peak bulkhead.

  
  

  
  

  
  

   
   

  Watertight Bulkheads

  Bulkheads
  Bulkheads
   are vertical partitions on a ship which are fitted transversely or 
  longitudinally. Watertight bulkheads divide the ship into watertight 
  compartments and thus restrict the amount of water which enters the 
  ship. Transverse bulkheads reduce the distortion of the side shell. This
   reduces the racking stresses considerably.
  
  Water Tight Bulkheads on Ships: Construction and Arrangement
  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 and instability problem. If 
  the water ingress is not constrained in time, the ship will capsize and 
  sink. For this reason, water tight bulkheads are fitted on ships to 
  avoid such situations.
  
  What are 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.
  
  
  Advantages of Watertight Bulkheads
  They help in subdividing the ship into number of watertight compartments, thus increasing watertight integrity of the ship
  It helps in increasing the transverse strength of the ship which helps in reducing raking
  In case of fire bulkheads restrict the spread of the same to other compartments.
  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 bulkhead incorporated with a number of stakes 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 at the bottom as with 
  increase in depth the pressure of the water increase. The horizontal 
  plating 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 bulkhead 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-
  Collision bulkhead placed at forward of the ship at 1/20 L (L is length of the ship) and it should be continuous to upper deck.
  
  
  One an aft peak bulkhead which protects and encloses the stern tube and rudder trunk of the ship
  Two bulkheads enclosing the engine room from 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.

   
   

  Weather tight bulkheads

  Weather tight bulkheads

  The term weather tight is closely associated with closing devices which restrict the ingress of sea water. I.e. they may be termed watertight in one direction but not in the reverse direction. LRS defines weather tight as “A closing appliance is considered weather tight if it is designed to prevent the passage of water into the ship in any sea conditions”.

  
  

  A bulkhead which fits this description is termed a weather tight bulkhead.
  
  
  
  

  Oil tight Bulkhead

  Oil tight Bulkhead

  These bulkheads have the same construction requirements as their watertight brothers. They need to be significantly stronger, and as such they have larger scantlings. The testing regimen however is more stringent.

   

  Corrugated Bulkhead

  Instead of stiffeners the bulkhead may be corrugated or swedged. The troughs are vertical in the case of transverse and horizontal in the case of longitudinal bulkheads. The bulkhead may end in a stool construction at its lower end. The corrugations are centered over the double bottom girders, and as with all watertight bulkheads, they are placed over a watertight DB tank floor. On high bulkheads horizontal diaphragm plate may be used in the troughs to strengthen the bulkhead.

  
  

  Corrugated bulkheads afford a reduction in weight and also offer a smoother surface, which is important when it comes to cleaning cargo residues from bulkhead surface. The reduction in weight is somewhat compromised by the fact that it is economically unviable to construct corrugated bulkheads that are thicker at the bottom, and hence the thickness for the bottom sections loading is carried throughout the bulkhead leading to a heavier section. These bulkheads are also more capable of withstanding bending moments and pillar loads.

  
  

  

  
  

  Corrugated  watertight bulkhead

  
  

  

  
  

  Bulk carrier—bulkhead stool

  
  

  
  

  
  

  
  

  
  

  
  

  Corrugated bulkhead with additional stiffening

   

   

  Openings in watertight Bulkheads

  Opening in watertight bulkheads

  Opening in watertight bulkheads may be classed into two ie

  • Penetrations for access, piping, ventilation, electrical cables 
  • Opening in the watertight bulkhead below the freeboard deck which are used for the movement of man and material. 

  We will now consider only the former:

  • At the design stage the number of penetrations should be kept to a minimum.
  • Lead or other heat sensitive materials must not be used in a system that penetrates a main transverse watertight bulkhead
  • A main transverse watertight bulkhead must not be penetrated by valves or cocks unless they are a part of a piping system
  • The penetration must be watertight and not compromise the strength of the bulkhead. A ventilator through the bulkhead will require a watertight flap.

  
  

  
  

  
  

  

   

   

  Purpose of wash bulkheads

  Wash bulkheads

  Non water tight bulkheads, usually longitudinal, used to reduce the transverse movement of liquid. They are found in Peak tanks and Cargo deep tanks. They also sometimes have a supportive role and act like pillars.

  
  

  
  

  Use of cross ties in tankers

  Cross Ties

  Horizontal cross ties are introduced in the wing tanks to connect the vertical transverse webs at the ship's side and longitudinal bulkhead. The cross ties are designed to stiffen the tank side boundary bulkhead structure against transverse distortion under liquid pressure, and are bracketed at each end.

  
  

  They are constructed like face plates, and can be horizontally or vertically stiffened. Two or three horizontal cross ties are provided depending on the vessel's depth, diagonal cross ties may also be fitted depending on the stress profile of the tank section.

   

   

   

  
  

  

  

   

  Procedure for testing of bulkheads

  Testing of Bulkheads

  
  

  Type of Bulkhead	Test Required	Remarks
  Watertight bulkhead including penetrations	Unless a specific test is indicated, this may be a Hydrostatic, hydro pneumatic test, air or other medium test.	A hose test may be considered
  Watertight bulkhead of a ballast hold	Head of water up to top of cargo hatch coaming	Air vents if fitted below this level require to be blanked off
  Peak tank bulkheads	The greater of: head of water up to the top of the overflow head of water 2,4 m above top of tank.
  Oil tight bulkhead	The greater of: head of water up to the top of the overflow head of water 2,4 m above top of tank. head of water up to top of tank,  plussetting of fitted pressure-relief valve

  
  

  By the end of this presentation you will understand the mandated procedures for the testing and certification of bulkheads. Watertight bulkheads that do not form the boundaries of a liquid tank are not required to be tested by filling them with liquid upto the top of the overflow or air pipe. They may be hose tested. A hose test is carried out using a nozzle whose diameter is not less than 12mm. The nozzle should be no more than 1.5 m from the testing point and the pressure in the hose being at least 2 x 105 Pa. The test is to be carried out from any one side of the bulkhead. Where hose testing is not possible then it may be replaced by a careful visual inspection supplemented by dye penetrant and ultrasonic testing. On ships where the cargo hold is allowed to be ballasted, hose testing is not allowed. The tank should be filled up with water upto the top of the overflow or air pipe. Or 0.9 mtrs above the top of the hatch cover, whichever is greater. In the case of cargo tanks, fore peak, aft peak and coffer dams,  it is the greater of the top of the overflow or air pipe or 2.4 mtrs above the highest point of the tank. In the case of pneumatic testing the setting pressure of the safety relief valves. It is important to note that hose testing is allowed for cargo tanks.

   

  Cofferdam

  Coffer dams

  A cellular void space within a ships structure is called a coffer dam. Like a bulkhead separates two spaces or divides a space into two, a coffer dam does the same with a larger degree of integrity since it incorporates a void which would contain any breach of either of the dam boundaries. This would contain the leakage and prevent it spreading into other areas.

  
  

  A bulkhead on the other hand would allow the spaces it divides to comingle if there is any breach in the bulkhead itself.

  
  

  
  

  
  

  Interior of a coffer dam

   

  Racking stresses and transverse bulkheads

  Racking stresses and Transverse bulkheads.

  Uneven water pressure on the ships hull during the rolling of a ship and the associated acceleration forces subject the hull girder to transverse stress called Racking Stress. The deck tends to move laterally with respect to the bottom shell and DB structure. Racking is maximum at the corners and resistance to racking is provided by the shear stress of the girder and transverse bulkheads and framing.

  
  

  The following media explains about the racking stresses and transverse bulkheads