Sub division aims at finding the number of bulkheads required, and where they should be placed. Sub division of passenger ships into watertight compartments must be such that after assumed damage to the ship’s hull the vessel will remain afloat in a stable position.
The first SOLAS Convention in 1914 immediately after the Titanic disaster introduced new international requirements dealing with safety of navigation for all merchant ships; the provision of watertight and fire-resistant bulkheads; life-saving appliances and fire prevention and fire fighting appliances on passenger ships. Subsequent Convention revised the concepts ok practical experiences both on the backdrop of damage stability as well as fire protection.
The 1948 SOLAS brought about Important improvements in such matters as watertight subdivision in passenger ships; stability standards; the maintenance of essential services in emergencies; structural fire protection, including the introduction of three alternative methods of subdivision by means of fire resistant bulkheads, and the enclosure of main stairways.
The 1974 SOLAS introduced the concept of the degree of subdivision – measured by the maximum permissible distance between two adjacent bulkheads – varies with the ship’s length and the service in which it is engaged. The highest degree of subdivision applies to ships of the greatest length primarily engaged in the carriage of passengers.
The requirements for machinery and electrical installations are designed to ensure that services, which are essential for the safety of the ship, passengers and crew, are maintained under various emergency conditions.
Sub division on Passenger ships
Passenger Ship Construction rules 1977 are based on SOLAS vessels. 1974 and its amendments. These rules lay down regulations for the sub division of Passenger
1) Passenger ship is a ship, which carries more than 12 passengers.
2) Sub division load line is the water line used to determine the subdivision of the vessel.
3) Deepest subdivision load line is the waterline corresponding to the maximum draft permitted by the sub division requirements.
4) Sub division length is the length between perpendiculars taken at the ends of the deepest sub division load line.
5) Bulkhead deck is the uppermost continuous deck up to, which the transverse watertight bulkheads are carried. The degree of sub division specifies minimum number of compartments, such that and all circumstances the final water line will not be beyond a line tangent to the bulkhead deck. (i.e. bulkhead deck should never be submerged.)
6) Margin line – For the purpose of safety an imaginary line is drawn 76mm (3″) below the bulkhead deck. This is called the margin of safety line, and sub division should be such that in the worst condition of flooding, the vessel will not sink beyond the line tangential to the margin line.
7) Permeability is the percentage of any space, which can be occupied by water. For this purpose the vessel is divided into 3 sections — machinery spaces, area forward of machinery space and areas aft of machinery space.
8) Watertight – with regard to any structure, this means capable of preventing the passage of water through that structure in any direction under a head of water up to the ship’s margin line.
9) Weather tight – with relation to any structure means capable of preventing the passage of seawater through the structure under ordinary sea conditions.
10) Progressive flooding is what occurs when the vessel sinks to such an extent that Water enters the undamaged compartment through weather tight openings on the upper deck.
11) Floodable Length -At any point in the length of the vessel, that is determined by method of calculation, which takes into consideration the vessel’s block coefficient, draft and other characteristics, with regard to any part of the vessel a continuous bulkhead deck, the floodable length at that point means the maximum length with its centre at the same point which under certain definite assumptions of permeability, can be flooded without submerging any part of the ship’s margin line.
12) Permissible Length — The vessel shall be efficiently sub-divided as possible having regard to the nature of the service for which it is intended. The degree of sub-division shall therefore vary such that the highest sub-division corresponds to the vessel of greatest length primarily engaged in the carriage of passengers. The Permissible length of any compartment equals the floodable length at the centre of that compartment multiplied by a factor of sub-division.
13) Factor of sub-division is an arbitrary factor applied to the floodable length to obtain the permissible length of the compartment of the vessel. This depends upon:
- The vessel’s length
- Nature of service (Passenger, cargo & types of cargo)
The factor decreases in a regular and continuous manner as the vessel’s length increases and also from a factor ‘A’ for vessels only engaged in carrying cargo and a factor ‘B’ only for passenger vessels.
This factor insures that one or two or three compartments must be flooded before the vessel reaches the margin line. Vessels are therefore referred to as one, two or three compartment vessels, with regard to their damage stability characteristics.
14) Criterion of service numeral (Cₛ): – Vessels intended for service intermediate between the two extreme types (cargo and passenger) are given a factor of sub division between A and B called Cₛ. This is based on the volume of passenger spaces below the margin line, the number of passengers, the volume of machinery spaces below the margin line. Any increase in any of these factors will increase the criterion numeral and decrease the factor of sub-division, and thereby the permissible length. Cₛ is such that factor A (cargo vessels only) = 23 and factor B (passenger vessels) = 123.
Calculation of floodable length:
To find the number of bulkheads required, and where they should be placed, the permissible length of each compartment needs to be determined.
This is a function of the floodable length, which is obtained from a graph called the curve of floodable lengths. Every point on this curve has an ordinate representing the length of vessel, which may be flooded to the centre of the length at that point, without submerging the margin line. This curve is also used for assessing the vessels stability in damaged condition.
Without getting into the mathematics of it such a curve shall enable designers in the positioning of the bulkheads.
Damage due to flooding
Unsymmetrical flooding is that which occurs when the amount of water is more on one side of the ship, resulting in the ship heeling to one side.
The maximum angle of heel after unsymmetrical flooding of any compartment should not exceed 15°. Where it is necessary to correct this heel, any means adopted to reduce this should be self-acting, and the controls should be operable from above the bulkhead deck.
If cross flooding fittings are installed, the time for equalisation should not exceed 15 minutes.
After assumed critical damage to the ship, the final condition should be such that:
- If unsymmetrical flooding takes place in one compartment, the angle of heel should not exceed 7°, if in two or more adjacent compartments it should not exceed 12°.
- If symmetrical flooding takes, the final GM should not be less than 50mm as calculated by the constant displacement method.
Information available to the Master
The master should be supplied with suitable information concerning the use of cross flooding fittings.
The master should be supplied with the data necessary to maintain sufficient intact stability under service conditions to enable the ship to withstand critical damage.
This includes the maximum permissible KG, or the minimum permissible GM for a range of drafts or displacements which cover all service conditions, as well as the influence of the various trims taking into account the operational limits.
In case of ships that require cross flooding, the master shall be informed of the conditions of stability on which the calculations of heel are based e.g. moments due to crowding of passengers, moments due launching of fully loaded survival craft on one side of the vessel, moments due to wind pressure, the volume and surface permeability’s. The master should also be aware that the extent of damage is assumed within certain parameters.
The master should be warned that excessive heeling might result if the ship sustains damage when in a less favourable condition.
All passenger ships and all cargo ships above 24 meters in length must be inclined after its construction is completed so that its stability is determined.
The master must be provided with enough information so that he can rapidly and accurately obtain the ships stability under various conditions. If there are any alterations to a ship that affects her stability, he shall have amended information of the same. If required, the ship shall be re-inclined to determine her new condition.
At periodic intervals not exceeding 5 years, a ‘lightweight survey should be carried out on all passenger ships to verify if there are any changes in the light displacement and the longitudinal centre of gravity. If there is any change of over 2% in the light displacement or over 1% of the Length in the LCG, the ship will have to be re-inclined.
Sub Division Load Lines
In passenger ships sub-division is carried out with reference to a particular maximum draft called the subdivision draft and marked by a Subdivision Loadline. A vessel having spaces, which are specially adapted for accommodating passengers or cargo alternatively, may, if desired by the owner, have additional sub division load line. These load lines are marked by the letter —C.
- C1 – principally passenger carriers
- C2 – passengers in tween decks ( alternative passenger / Cargo)
- C3 – passengers in lower decks carrier. (alternative Passenger/ Cargo)
- Details of these load lines are recorded in the passenger ship safety certificate together with the spaces used alternatively for passenger or cargo. The deepest subdivision mark cannot be above the deepest saltwater load line as per international load line rule. If however a subdivision load line is above the appropriate International Loadline for a particular locality and season, the International loadline will apply. At no stage, in salt water, should the appropriate sub division loadline be submerged.
Unberthed passenger ships
- During the Simla Convention of 1931 due attention was given to special trade passenger vessel’s carrying unberthed deck passengers between Singapore, India — Gulf and Red Sea. To provide additional safety for such vessel another series of subdivision load lines D1, D2 and D3 are assigned which gave greater freeboard than the C lines. These ships are now called Special Trade Passenger ships.