When a new ship is designed, one of the primary steps involved in the development of the concept, is designing its General Arrangement and allocating proper spaces according to the requirements of the owner and functionality of the ship. It is up to the naval architect to decide on the ship’s general arrangement, depending on many design constraints that are mentioned in the technical specifications of the contract.
So how exactly do you decide on a ship’s general arrangement, and design the most optimum one for that particular project?
Since almost all ships have something unique in their general arrangement, there is no specific procedure with rules stated for the same. But in spite of that, a naval architect needs to know the basic procedure to follow in order to come down to an optimum design. In this article, we shall discuss a general procedure that is followed in the ship designing industry.
Initially, in order to have a visual approximation of the ship dimensions, draw an outline of the profile view, main deck or uppermost deck that contributes to longitudinal strength, and the forecastle deck. In some ships, the upper deck is stepped, i.e. it has a poop deck at the aft. Make sure you show that in the profile view and the deck outline view.
The reasons behind deciding the particulars (Especially height) of the forecastle deck at this stage are as follows:
- Minimum bow height has to be attained (according to ILLC Regulations) in order to reduce the deck wetness
- To provide forecastle deck area for anchoring and mooring equipment
- Adequate volume underneath for storage and chain locker, etc.
- To provide additional cargo space (in lower decks) in case of certain ships
After having drawn the profile plan, the first thing a designer should do is decide on the framing and frame spacing of the ship. The framing, whether longitudinal or transverse is decided on the basis of the length of the vessel. Generally, all ships longer than 120 m are longitudinally strengthened.
The frame spacing is then calculated by the formula specified in the rule book of the authorised classification society. The value obtained from the formula is generally rounded off to the nearest hundreds or fifties, so as to attain ease of production and design.
Next is to mark the decided frame spacing of the drawing. This frame spacing will now act as scale on the drawing, helping you to locate every point on the ship.
You must now divide the ship into certain number of watertight compartments, which is decided by the subdivision rules prescribed by the classification society. The rules specify the total number of watertight transverse bulkheads that are necessary to maintain watertight integrity of the ship. A ship generally has four types of transverse bulkheads:
- A fore peak collision bulkhead
- An aft-peak bulkhead
- A bulkhead at each end of machinery space
- Transverse bulkheads in cargo hold regions
Once the number of bulkheads have been decided, the length and number of holds should be planned accordingly. The ordinary transverse watertight bulkheads in the holds should be spaced at reasonably uniform intervals. Where non-uniform spacing is unavoidable and the length of a hold is unusually large, the transverse strength of the ship is to be maintained by providing additional web frames, increased framing etc.
In some cases, the decided number of bulkheads may interfere with the functionality of the ship or the specific requirements of that particular type of trade. Proposals to dispense with one or more transverse bulkheads in such cases can be considered by the classification society, provided it does affect the watertight integrity of the ship.
How to decide the position of the fore peak collision bulkhead?
- The distance of the forepeak collision bulkhead from the forward perpendicular is decided based on formulae prescribed by the authorised classification society. Generally, the class society would provide you with two formulae. One, to specify the minimum distance of the forepeak bulkhead aft of the forward perpendicular. Other, to specify the maximum distance of the forepeak bulkhead aft of the forward perpendicular
- It is up to you, as a designer, to provide the forepeak collision bulkheads within the above limits, depending on the dimensions of the forepeak ballast tank, anchor equipment, and chain locker dimensions
How to decide the position of the fore peak collision bulkhead?
The following considerations are taken during deciding the position of the aft peak bulkhead or the engine room aft bulkhead. First, the position of the engine room forward bulkhead is fixed according the position and length of the holds. Once that is done, about four frame spaces need to be left out before placing the main engine aft of the engine room forward bulkhead. That is to leave space for maintenance and crew operations.
Aft of the empty space, the length of the engine room is to be decided depending upon the length of the main engine, and the length of the intermediate shaft. Now, the intermediate shaft is coupled with the propeller shaft by a flanged connection. The coupling flange between the intermediate shaft and the propeller shaft is to be housed within the engine room itself. It is just aft of the coupling flange that the engine room aft bulkhead is positioned.
The propeller shaft runs from aft of the engine room bulkhead connecting to the propeller through the stern tube.
In many cases, the position of the engine rom aft bulkhead is also governed by the decided capacity of the aft peak ballast tank, which is always aft of the aft peak bulkhead. The capacity of the tank is estimated by trim and stability calculations, which is a very preliminary stage of design. But the engine and shaft lengths are decided at a comparatively later stage. This should give you an idea of how iterative the ship design process is.
How to arrange the cargo spaces?
The entire cargo space needs to be divided into cargo holds by placing the specified number of transverse watertight bulkheads. The longitudinal position of the bulkheads may be decided according to a few principles of cargo requirement:
- Holds should be kept of equal lengths wherever possible
- In some cases where necessary, alternate large and small holds are designed to meet the cargo requirements for different voyage and cargo conditions. This is normally done for bulk carriers, product tankers, and container ships
- Sometimes, a single large cargo hold (for large multipurpose carriers)
In cases of oil tankers and container ships, decisions on longitudinal bulkheads are to be taken, with respect to prevention of free surface effect, ensure proper cargo distribution and handling characteristics.
In case of bulk carriers, the slope of the tank top sloping bulkhead is to be taken care of. The tank slope must be more than the angle of repose of the cargo, which is generally around 30 degrees. The slope of the bottom tank is generally maintained at 45 degrees.
In a general arrangement, the double bottom height needs to be shown clearly, so as to ensure proper estimation and representation of the tank plan. Therefore the designer is required to estimate the height of the double bottom using the corresponding formula specified in the rules of the authorised class society.
Decide on the height of the tween decks. Ships that carry packed cargo and cars, require more deck space to attain maximum stowage capacity. In order to increase the overall deck area, these ships are provided with a number of tween decks. The height of each tween deck should be sufficient to accommodate the cargo that is to be stowed on it.
This consideration of tween deck is however not required for volume based cargo carrier, like oil tankers, chemical carriers and bulk carriers. And in case of container ships, the top of each container serves as the floor for the next container to be stowed above it, hence container ships do not require tween decks for cargo stowage.
After having decided on the capacities and sizes of the cargo holds, the size of hatch openings and hatch covers.
Ballast Tank Capacities and Tank Plan Design
Ballast water is required for empty voyage to have proper sinkage, trim and stability.
Excessive ballast capacity is bad since it is expensive and takes up useful space. Ballast capacity should be such that full propeller immersion is obtained at the aft end and forward draught is not too low to avoid the harmful effects of slamming.
Approximately in a ballast voyage, displacement is 0.5 of fully loaded displacement which is about 0.55 of full draught. Ballast distribution should be such that excessive hogging moment is avoided in this condition. So a designer should always ensure to segregate the ballast water tank from any other liquid tank.
There are a few other points that a designer must consider while making tank arrangements for ships:
- No access is required except for cleaning and maintenance. Minimum two manholes are to be provided on top, and preferably at the diagonal corners of the tanks so that they are maximum distance from each other
- Tanks and pipes carrying a particular type of liquid must be segregated from those carrying another type of liquid. They should also be colour coded differently
- Fresh water tank should not have any tank adjacent to itself. So a fresh water tank and any other tank must be separated by a cofferdam. For the same reason, FW tanks cannot be placed below the load waterline
- Since total liquid carried is relatively low, the tanks may conveniently be situated in the lower portions to increase transverse stability of the ship
- To simplify piping arrangements, and the total length of piping, fresh water tanks should be near the engine room, as well as accommodation. Dirty oil and heavy fuel oil tanks should be near the engine room. Dirty oil and sludge tanks can be conveniently located in the double bottom of the engine room
- W. tanks should be well distributed all over the length and breadth of ship to help the ship attain its stability and trim requirements. Pipes should not run inside tanks carrying another liquid, i.e. fuel oil pipe should not run through any ballast water tank
- Consumable tanks (Heavy Fuel Oil, Dirty Oil, and Fresh Water) should be so located that their consumption does not cause unnecessary adverse trim. They should not cause unduly adverse free surface effects. So these tanks should be divided into smaller tanks with reduced breath. Too many small tanks, however, will make complicated piping system
- W. tanks are either fully pressed or empty ballast water tanks should be distributed all over the length of ship with sufficient capacity in the peak tanks to adjust for the required trim and stability
- Tanks should be distributed symmetrically about centreline of the ship, so that adverse heel effects are not felt. If there is any such effect (damage stability) cross-connection between port and starboard tanks may be provided
- The boundaries of double bottom tanks, deep tanks etc. should be designed to withstand the applied hydrostatic pressure
- The tank distribution should not adversely affect the longitudinal strength of hull girder
Lastly, it is important to understand and know, that a general arrangement of any ship will consist of the drawings of the following views:
- Profile View (generally looking from starboard side)
- Midship sections (looking from aft, and looking from forward)
- Main deck plan (also shows the accommodation layout)
- Navigation deck plan.
- Forecastle deck plan
- Tank top plan
- Tank plan
It should also be noted that a the process of developing the general arrangement drawing is slightly different for various design firms, depending on their procedures and practices, however the underlying principle always remains the same. It is an iterative process, and the final GA is a arrived at, after repeated approvals by the classification society and the owners party.