The container ships of early 60s were very similar to the general cargo ship of that time. Benefit from the fast turnaround period multiplied by the capacity to carry revolutionized the concept of carrying dry cargo. This lead to the evolution of ships of more than double the previous dimensions, with the design made to carry several tiers on deck, high forecastle and a flared bow.
Cellular container ships are ships of one of the fastest turnaround period. They are made for the carriage of containers at fixed positions in holds / deck with cell guides. The ships may have double or single sides, together with strengthening at the upper deck. Usually the larger ones have double wall.
Strong longitudinal box girders are formed on port and starboard by the upper deck, the second deck, sheer strake and the longitudinal bulkhead. The box girder provides stiffness against racking stresses. A large container ship would have large discontinuity due hatch openings to provide efficiency in loading and discharging. To have versatility for stowage in cellular guide type ships, ship would get subjected to large tortional stresses. The formation of tortion box as symbolically shown in the diagram would help. This is similar to the lip formation (rounding of tip) at the upper part of a bucket. The total cross-sectional area of the topside box girders or equivalent structure for both sides of the ship, including all effective material within 0.19D of the upper deck, but excluding hatch side coamings, over the range of cargo hold is to be not less than 0,15 Δ cm2, where Δ is the full load displacement of the ship at summer moulded draught, in tonnes.
The maximum number of twenty foot equivalent units (TEU) that may be carried on board, is determined by the naval architect and is entered into the ship’s Certificate of Class. The container guides consist of angle bars about 150 mm x 150 mm x 14 mm thick connected to vertical webs and adjoining structure spaced 2.6 m apart, which is the standard width of a container. The bottom of the guides is bolted to brackets welded to the tank top and beams, through doublers.
Structural arrangement of ships, which may be adapted for carriage of containers or the ships, which are pure container carriers have the arrangements to support and secure the containers to hull structure. Technical documentation in respect of stowage arrangement of containers, cells in container holds and fixed equipment for securing the containers shall be approved by class.
Maximum continuity of structure is provided, using longitudinal girders and avoiding abrupt changes of shape and dimensions of structural members. In ships with the length L0 ≥ 100 m, it is recommended that longitudinal framing should be applied for bottom, strength deck and the upper parts of sides. Thus, longitudinal framing may be used throughout the main body length of the ship and transverse framing may be used on the fore part and the after part. There can be mixed framing employed. Thus, longitudinal for longitudinal strength and transverse to resist tortion. Double bottom longitudinal girders are arranged in line with container sockets on the inner bottom. Double bottom plate floors and web frames are also made coplanar. The transverse structures such as box beam improves the hull’s torsional rigidity. The connectivity of double bottom to the transverse bulkheads should also consider this aspect.
Hatchless container ship
These ships would have extra deadweight capacity owing to the hatch covers being absent. The removal of covers also improves stability. The container guides extend above the deck to the relevant permissible height of the deck cargo. This eliminates the need for manually installed lashing cables or rods as are required when the on-deck containers are stowed on top of the hatch covers. An International Load Line Exemption Certificate shall be issued to any ship to which an exemption has been granted. Heavy lift vessels and semi-submergible vessels also fall in this category, where they were allowed to sail without hatch covers. MSC in its sixty-second session in 1993, approved the interim guidelines for open-top containerships, which provided a set of requirements for the design of this type of ship. The points which were stressed upon include: model tests; damage and intact stability; pumping system of hold bilges; and fire protection and dangerous goods.
The model tests must consider: own propulsion; the wave heights to be 8.5 m high with most unfavorable realistic wave period; simulation of sea spray from different directions; most unfavorable hold location; and unfavourable heading.
The open-top ships are equipped with 200 to 300 mm high pedestals on the tank top, on which the bottom containers are stowed. The ships are equipped with at least three independent bilge pumps each capable of pumping the maximum hourly rate of sea water in seagoing conditions as established in the model tests. The bilge system for cargo holds is independent of the machinery space bilge system and located outside of the machinery space. IMO regulations require that the maximum hourly rate of ingress of sea water in any one open hold determined from model testing should not exceed the hatch opening area multiplied by 40 cm/hour. All open cargo holds must further be equipped with bilge high level alarms. All these features should prevent cargo at the bottom from being damaged in the case of water ingress. The forward two hold may still have the conventional hatch covers.
Sometimes, “rain shelters” concept can provide certain degree of protection to the cargo. Hatchless container ship thus, has tall sidewalls which restrain the containers and provide shelter. The effective freeboard therefore is obviously very large.
Special consideration is given to the ships having hatch openings with a width greater than 0,85B or where the average rate of the torsional deformation exceeds 0,006o per metre or where the elongation of the hatch opening diagonal exceeds 35 mm under hydrodynamic torque loading.
Longitudinal underdeck girders
Longitudinal underdeck girders are, in general, to be fitted at deck level to support the hatch coamings. The arrangement may consist of one girder on the cetreline, one girder each, on port and starboard in the breadth of the hatchway and the girder. Where the girders are omitted, the construction must suitably compensate.
The top and bottom flanges of girders should have a minimum width suitable to accommodate the hatch coamings and the hatch cover securing arrangement and the scantling will be specially considered. They are, , to be continuous throughout the container hold area, including the engine room where this is suited between container holds.
The corners of main hatchway opening are to be rounded with a radius which, within the region of the container holds with wide hatches, is specified. The radius of the hatch corners of the main hatchway opening adjacent to the engine room is to be made as large as practicable with a radius of approximately 40B mm. The corners of main hatchway openings are generally to be rounded.
Transverse and horizontal perforated flats in double skin construction should have a thickness t, not less than lesser of following:
t = 7.5 + 0.015L or 10mm
The flats and transverses are to be efficiently stiffened and thickness increased locally where necessary.
DBs and Side Girders
For ships of over 100m in length, the DBs should be longitudinally framed. Plate floors should be fitted under bulkheads, mid-hold supports and stress points.The maximum spacing is not to be more than 3.8 metres.
The side girders are in general, to be arranged under container corner sittings. For DBs having a depth greater than 1.6 m, additional longitudinal stiffening is done.
The inner skin longitudinal bulkheads may be transversely or longitudinally framed. The continuity of longitudinal material must be maintained in the way of container holds and machinery space.
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