Explain the different types of framing systems used in ship construction. Discuss their design, structural arrangement, and advantages in ensuring the vessel's strength and stability. (16)
Structured for DG Shipping MEO Class II examination scoring criteria.
Framing systems used in ship construction.
The framing system is the arrangement of the transverse and longitudinal stiffeners that support the shell and deck plating. The three main systems are:
In this system the principal stiffening members run transversely (athwartships), i.e. the frames are transverse rings spaced closely (about 0.6-0.9 m apart) running from the keel up the side to the deck, with the longitudinal members (stringers, keelsons) spaced widely. The shell plating is supported by the transverse frames. It is used for the ends of ships (fore and aft peaks) and for ships where the transverse strength is important and the length is moderate (e.g. general cargo ships, tugs, and the ends of most ships). Advantages: simple, cheap, good transverse strength, easy to build; the transverse frames resist the transverse pressure of the water and the racking loads. Disadvantage: the longitudinal strength is provided mainly by the shell plating and stringers, so it is less efficient for long, deep ships.
In this system the principal stiffening members run fore-and-aft (longitudinally), closely spaced (about 0.6-0.9 m apart) along the deck, bottom and sides, with widely spaced transverse webs (transverse frames/ring frames) every few metres. The shell and deck plating is supported by the longitudinal stiffeners. It is used for the middle body of large ships (tankers, bulk carriers, container ships) where the longitudinal bending strength is critical. Advantages: high longitudinal strength and section modulus for a given weight, good resistance to buckling, and efficient use of material; the closely spaced longitudinals give a strong, stiff hull girder. Disadvantage: more complex and expensive to build, and the transverse strength must be provided by the widely spaced webs.
This uses longitudinal framing in the middle body (where longitudinal bending is greatest) and transverse framing in the ends (where the hull is finer and transverse strength/racking is important). It combines the advantages of both: high longitudinal strength amidships and good transverse strength and economy at the ends. It is the most common arrangement for large merchant ships.
Design and structural arrangement: the framing must support the plating against the hydrostatic and wave pressure, transmit the loads to the hull girder, and provide the required section modulus for longitudinal bending. The spacing, scantlings and the connection of the frames to the keel, stringers, deck and bulkheads are arranged to give continuity of strength and to avoid stress concentrations. The framing also provides the transverse rings that resist racking and the local loads of cargo and machinery.
Advantages in ensuring strength and stability: the framing system determines the hull girder's section modulus (longitudinal strength), the local strength of the plating, and the distribution of material. A well-designed framing system gives adequate strength with minimum weight, resists buckling and fatigue, and maintains the watertight integrity and the stability of the hull.