1. Introduction
1.1 Ship structure A ship structure usually consists of a network of plates and supporting structure. The supporting structure consists of large members running both longitudinally and transversely and is often known as the Framing. The ship plating is attached to the framing. Framing 骨架，支撑外板、甲板底板等的一切桁材和型材的统称
Typical Transverse and Longitudinal Strength Members
Superstructure: poop, forecastle, deck house, etc 1.1.1 Structural components Shell plating：keel, bottom plating, bilge strake, bilge keel, side plating, sheer strake, etc Bottom: single bottom, double bottom, center(side) girder, center (side) keelson, bottom frame, floor, bottom longitudinal, etc Side framing: frame, web frame, side stringer, side transverse, side longitudinal, etc Deck: deck, deck framing, beam, deck girder, deck transverse, side longitudinal, web beam, etc Bulkhead: transverse bulkhead, longitudinal bulkhead, horizontal girder, vertical girder, etc Superstructure: poop, forecastle, deck house, etc
Keel A large center plane girder running longitudinally along the bottom of the ship. Plating Thin pieces closing in the top, bottom, and sides of the structure. Plating makes a significant contribution to longitudinal hull strength, and resists the hydrostatic pressure load.
Frame (肋骨) A transverse member running continuously from the keel to the deck. Resists transverse loads (ie - waves hitting the side of the ship) Floor （肋板） Deep frames running from the keel to the turn of the bilge. Frames may be attached to floors - the frame would be that part above the turn of the bilge.
Longitudinal Stringers Girders which run parallel to the keel along the bottom of the ship. Longitudinals intersect floors at right angles, and provide longitudinal strength. Stringers Girders running along the sides of the ship. Typically smaller than longitudinals, they also provide longitudinal strength.
Deck Beams Transverse members of the deck frame. Deck Girders Longitudinal members of the deck frame.
1.1.2 Framing systems There is a compromise between the requirements of strength and the conflicting but equally important requirements of buoyancy, space utilization and cost. Longitudinal framing Longitudinal strength members: keel, longitudinals, stringers, deck girders, plating
The longitudinal elements have a primary role in combating the longitudinal bending stress. Ships that are longer than about 300ft (91.44m) which is a typical wave length associated with an ocean wave tend to have a greater number of longitudinal elements to their structures than transverse elements.
A practical solution for modern cargo ships is to frame the bottom shell, inner bottom and strength deck longitudinally utilizing transverse floors every third frame. Transverse framing is then applied to the side shell and between decks. Longitudinal framing is generally used on warships
Transverse framing The transverse elements such as frames and hull plating have a primary role to combat the hydrostatic load. For ships shorter than 300 ft (91.44m) and those designed to operate at large depths, this is the primary load of concern. Hence short ships and submarines have structures consisting of many frames and fairly thick plating.
Combination Framing System Modern Naval vessels typically use a Combination Framing System which combines the other two methods in some creative manner. A typical combination framing network might consist of longitudinals and stringers with shallow web frames. Every third or fourth frame would be a deep web frame. The purpose of such a system is to optimize the structural arrangement for the expected loading, while minimizing weight and cost.
Double bottom construction
They are strong and can withstand the upward pressure of the sea in addition to the bending stresses. Double bottoms provide a space for storing fuel oil, fresh water (not potable), and salt water ballast.
The structure can withstand considerable bottom damage caused by grounding or underwater blasts without flooding the ship provided the inner bottom remains intact . Also, a double bottom provides a smooth inner bottom. This makes it easier to arrange cargo and equipment while providing better accessibility for cleaning..
Small vessel single-bottom construction
Watertight Bulkheads Large bulkheads split the hull of a ship into separate sections. In addition to their stiffening of the overall ship structure, they have a primary role in reducing the effects of damage on a ship .
1.2 Characteristics of ship structures A ship structure is a complicated three-dimensional shape. The shape is determined more by resistance, powering, and internal arrangement considerations than by the desire to optimize the structure's shape for load carrying capability. A ship's structure possesses certain distinctive features not found in other man-made structures .
Main dimensions: length, breadth, depth, draft, displacement, block coefficient Lines plan, General arrangement Stability, low resistance and high propulsive efficiency, and navigational limitations on draft
1.2.1 Size and complexity of ships Ships can be gargantuan in their proportions. 400m (1312 ft) in length, 63m (207 ft) in breadth, 35.9m (118 ft) in depth, a loaded draft of 28.5m (93 ft)
Knock Nevis Length: 458.45m breadth : 68.8m Draft: 24.6 m Knock Nevis is a floating storage and offloading unit (FSO) Knock Nevis was built in 1979 Length: 458.45m breadth : 68.8m Draft: 24.6 m DWT: 564,650 t From Wikipedia
USS Enterprise (CVN-65) From Wikipedia Length: 342 m Beam: 40.5m(waterline), 78.4m(extreme) Draught: 12 m Displacement: 73,858 tons standard 92,325 tons loaded
The external surface of the hull, or shell, must be a complex three-dimensional curved surface. Difficulties in analysis and construction.
1.2.2 Multipurpose function of ship structural component The shell plating The principal strength member A watertight envelope of the ship Bulkheads Contributing substantially to the strength of the hull. Serving as liquid-tight boundaries of internal compartments.
Decks Governed by the arrangement of internal spaces Resisting local distributed and concentrated loads (Local strength) Contributing to longitudinal and transverse strength
1.2.3 Variability of ship structural loads Static components (the weight and buoyancy of the ship in calm water) Dynamic components (simple wave-induced loads, slamming or springing, by the propellers, ice breaking, thermally-induced loads, underwater explosions, gunfire, blasts and projectiles, etc.)
The loads imposed by the sea, like the sea itself, are random in nature. Time-dependent or time-varying
1.2.4 Probabilistic nature of structural behavior In ship structural performance prediction there are at least three sources of uncertainty. Computational modeling Properties of the materials The quality of workmanship during construction and maintenance
1.3 Modes of ship structural failure The four basic modes of failure that we will consider are: Tensile or compressive yield Yield Buckling/Instability Fatigue Brittle Fracture .
1.3.1 Tensile or compressive yield “Slow plastic deformation of a structural component due to an applied stress greater than yield stress.” A factor of safety is applied during the design of a ship’s structure so that the largest expected stress is only 1/2 or 1/3 of the yield strength .
1.3.2 Buckling/Instability “An unstable condition caused by the compression of long slender columns resulting in substantial dimensional changes and a sudden loss of stiffness”
Instability failure of a structural member loaded in compression may occur at a stress level that is substantially lower than the material yield stress. The load at which instability or buckling occurs is a function of member geometry and material modulus of elasticity rather than material strength. Buckling is likely to occur on cross-stiffened deck panels on a ship due to large compressive stresses from longitudinal bending.
1.3.3 Fatigue “The failure of a material from repeated applications of stress, such as from vibration” The Endurance Limit is the stress below which the material will not fail from fatigue. Fatigue Characteristics
Fatigue failure in a real structure is greatly affected by such things as material composition (impurities, carbon content, internal defects), surface finish (smooth surfaces are best), environment (salt water is worse than air, moist air is worse than dry air), geometry (sharp corners and discontinuities are bad), and workmanship.
The most common consequence of fatigue in ships is the development and propagation of cracks. If such cracks are not repaired, they can result in catastrophic failure.
1.3.4 Brittle Fracture “The sudden catastrophic failure of a structure with little or no plastic deformation” The risk of brittle fracture occuring depends on the material, temperature, geometry, and rate of loading.
Material A material with low toughness is susceptible to brittle fracture. Low carbon steels are less brittle than high carbon steels. Temperature A material operating below its transition temperature is much more susceptible to brittle fracture because the toughness is very low.
Geometry Cracks having sharp edges are worse than those which are rounded. A smaller crack is better than a big one. Even the orientation of the crack with respect to the loading is a factor. Rate of Loading Impact loads are more likely to cause brittle fracture than loads applied gradually and smoothly.
1.4 Loads acting on ship structures It is convenient to divide the loads acting on the ship structure into four categories. 1.4.1 Static loads 1.4.2 Low-frequency dynamic loads 1.4.3 High-frequency dynamic loads 1.4.4 Impact loads
1.4.1 Static loads Loads that change only when the total weight of the ship changes. (loading or discharge of cargo, consumption of fuel, or modification to the ship itself)
Weight of the ship and its contents. Static buoyancy of the ship at rest or moving. Thermal loads resulting from nonlinear temperature gradients within the hull. Concentrated loads caused by dry docking and grounding.
1.4.2 Low-frequency dynamic loads Loads that vary in time with periods ranging from a few seconds to several minutes, and therefore occur at frequencies that are sufficiently low compared to the frequencies of vibratory response of the hull and its parts that there is no appreciable resonant amplification of the stresses induced in the structure.
Wave-induced hull pressure variations. Hull pressure variations caused by oscillatory ship motions. Inertial reactions resulting from the acceleration of the mass of the ship and its contents.
A ship’s axes and degrees of freedom surge （船舶）纵荡 Sway 横摆 Heave 升沉 Yaw 摇艏 A ship’s axes and degrees of freedom
1.4.3 High-frequency dynamic loads Time-varying loads of sufficiently high frequency that they may induce vibratory response of the ship structure. Probably loads with small magnitudes can give rise to large stresses and deflections due to resonant amplification.
Hydrodynamic loads induced by propulsive devices on hull or appendages. Loads imparted to the hull by reciprocating or unbalanced rotating machinery. Hydroelastic loads resulting from interaction of appendages with the flow past the ship.
1.4.4 Impact loads Loads resulting from slamming or wave impact on the forefoot, bow flare and other parts of the hull structure, including the effects of green water on deck. bow flare艏外飘
1.5 Design philosophy and procedure The issue of structural design involves the selection of material types, framing forms (longitudinal framing, transverse framing, mixed framing), frame spacing, frame and stiffener sizes, plating thickness.
Development of the initial configuration and scantlings Knowledge, experience, imagination, intuition, creativity Analysis of the performance of the assumed design Loads and responses
Comparison with performance criteria Safety, serviceability, durability, technology, economy Redesign the structure by changing both the configuration and scantlings in such a way as to effect an improvement Repeat the above as necessary to approach an optimum
Main contents of structural strength calculation The determination of loads Magnitudes, properties The responses of the structure (Load effects) Stresses, buckling loads, defections Strength criteria Allowable stresses or permissible stresses; Allowable deflections or permissible stresses