Bulkheads

Vertical partitions in a ship arranged transversely or fore and aft are referred to as 'bulkheads'. Those bulkheads which are of greatest importance are the main hull transverse and longitudinal bulkheads dividing the ship into a number of watertight compartments. Other 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 main hull bulkheads of sufficient strength are made watertight in order that they may contain any flooding in the event of a compartment on one side of the bulkhead being bilged. Further they serve as a hull strength member not only carrying some of the ship's vertical loading but also resisting any tendency for transverse deformation of the ship. As a rule the strength of the transverse watertight bulkheads is maintained to the strength deck which may be above the freeboard deck. Finally each of the main hull bulkheads has often proved a very effective barrier to the spread of a hold or machinery space fire.

SPACING OF WATERTIGHT BULKHEADS — CARGO SHIPS The minimum number of transverse watertight bulkheads which 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.

Of these bulkheads perhaps 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. Lloyd's Register gives the location for ships whose length does not exceed 200 m as not less than 5 and not greater than 8 per cent of the ship's length (Lloyd's Length) from the fore end of the load waterline. As a rule this bulkhead is fitted at the minimum distance in order to gain the maximum length for cargo stowage. 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. 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.

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. Table 18.1 indicates the number of watertight bulkheads recommended by Lloyd's Register for any cargo ship. 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 Lloyd's Register approval, if adequate approved

Table 18.1 Bulkheads for Cargo Ships

Above

Length of ship (metres) >ve Not exceeding

Total number of bulkheads Machinery midships Machinery aft

65 85 105 115 125 145 165

65 85 105 115 125 145 165 190

190 To be considered individually structural compensation is introduced. In container ships the spacing is arranged to suit the standard length of containers carried.

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. The collision bulkhead extends to the uppermost continuous deck and the aft peak bulkhead may terminate at the first deck above the load waterline provided this is made watertight to the stern, or to a watertight transom floor.

In the case of bulk carriers a further consideration may come into the spacing of the watertight bulkheads where 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 (see Chapter 31) if it is possible to flood one or more compartments without loss of the vessel. For obvious reasons many shipowners will wish to obtain the maximum permissible draft for this type of vessel and the bulkhead spacing will be critical.

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, 1974 (see Chapter 29). 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. In the United Kingdom the controlling authority is the Marine and Coastguard Agency.

The calculations involved in passenger ship subdivision are dealt with in detail in the theoretical text-books on naval architecture. However the basic principle is that the watertight bulkheads should be so spaced that when the vessel receives reasonable damage, flooding is confined. No casualty will then result either from loss of transverse stability or excessive sinkage and trim.

CONSTRUCTION OF WATERTIGHT BULKHEADS The plating of a flat transverse bulkhead is generally welded in horizontal strakes, and convenient two-dimensional units for prefabrication are formed. Smaller bulkheads may be erected as a single unit; larger bulkheads are in two or more units. 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 (see Figure 18.1). 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, and in this context are dealt with later.

It is not uncommon to find in present day ships swedged and corrugated bulkheads, 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 (see Figure 18.2). Both swedges and corrugations are arranged in the vertical direction like the stiffeners on transverse and short longitudinal pillar 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.

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. 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 to 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 bulkheads 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.

testing watertight bulkheads Both the collision bulkhead, as the fore peak bulkhead, and the aft peak bulkhead provided they do not form the boundaries of tanks are to be tested by filling the peaks with water to the level of the load waterline. All bulkheads, unless they form the boundaries of a tank which is regularly subject to a head of liquid, are hose

Konstruksi Kapal
Figure 18.1 Plain watertight bulkhead

Stiffeners t 1_l_

Corrugations

Ship's

Corrugations

Ship's

Stiffeners t 1_l_

Upper deck

Upper deck

Tank top

Watertight [

Upper deck l i

Tween floor

Tween stiffener deck Bracket

.-Hold stiffener

Tank top

LOOKING 'AFT

ELEVATION IN WAY OF STIFFENERS

Figure 18.2 Corrugated watertight bulkhead tested. Since it is not considered prudent to test ordinary watertight bulkheads by filling a cargo hold, the hose test is considered satisfactory.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Responses

  • jasmine
    Why transverse bulkhead are fitted in a ship?
    3 years ago
  • leila trentini
    Why stiffner fitted on the bulk head?
    3 years ago
  • MARK
    How pipes passing theough the water tight bulkhead on ship?
    2 years ago
  • George Harris
    Why strakes of plating on bulkhead horizontal?
    2 years ago
  • ARABELLA
    What is plain watertight bulkhead?
    2 years ago
  • simon
    How to determine no of transverse bulkhead on ship?
    2 years ago

Post a comment