11-Ship Structure - Framing Systems (2012)

Ship Structure – Framing Systems SHIP STRUCTURE – FRAMING SYSTEMS 11 MODULE Reference & extracts: Hughes, O.F., Ship Structural Design S.N.A.M.E., New Jersey 1988 Form of Marine Structure Land-based structures are designed primarily from an aspect of mass support where the primary loading is static and vertical; the mass of the structure and its contents at any level bearing at points under the influence of gravity Additional static loadings arise from accumulated rain, snow and ice Dynamic loadings for land-based structures include wind loadings and seismic phenomena The components of the land-based structure usually serve a singular role: sustaining and distributing imposed loads, often uniaxially, where those of greatest magnitude occur at terrestrial foundation points or surfaces Structural components are commonly limited to beams, columns and tubular sections configured either singly or as trusses Structural components of a marine vessel (surface or submersible) by contrast often serve roles additional to that of the provision of mere structural integrity The hull shell and strength deck (in a surface vessel) function together as the vessel’s principal strength member but must additionally be formed such that it provides the hydrostatic and hydrodynamic qualities prescribed by the vessel’s mission requirements and intended performance envelope Internal structural components may, in addition to distributing and resisting localised loads, contribute to the overall longitudinal and transverse strength, and furthermore, function as liquid-tight boundaries of internal compartments If structural efficiency was the singular consideration, then the same structural components as used in land-based applications (beams, columns, trusses, etc.,) would be adopted for marine vessels, however, the liquid-tight envelopes required (hull surface and internal boundaries) demand plating or composite sheeting with integral stiffeners in order to provide resistance to the multiple loadings sustained, both in the plane and normal to the structure Marine structural design diverges furthermore from land-based design as hydrodynamic requirements especially demand curvature (cylindrical and complex) in structural surfaces in certain regions of the hull Structural Loadings Generalised 2.1 Variability of Loadings The loads that a vessel’s structure must be designed to withstand have numerous sources Static components of loadings may include the mass and buoyancy forces of the vessel itself in calm water, cargo masses (both of solid, liquid or gaseous cargoes), and the masses of installations Ship Structure – Framing Systems (machinery, lifting appliances, etc Dynamic components generated by wave-induced motions of the vessel will cyclically or randomly alter the magnitudes of the apparent static loadings significantly Additional dynamic loads (often of higher frequencies than those associated with wave- or motion-induced loadings) are caused by slamming and springing, or by propulsion machinery Such loadings may result in the vibration of the vessel globally or locally Specific loadings unique to particular vessel types additionally apply Examples are ice loadings in icecapable vessels, thermally-induced loads stemming from heated or refrigerated cargoes, and ballistic loads on naval combatant vessels An important characteristic of all these loadings is their variability within the time domain Even the static mass and buoyancy loadings vary over the course of a voyage and from voyage to voyage In order to design the structure of a vessel for an intended life of 20 years or more, this time dependence of loadings needs to be considered Loads imposed through wave action and motion are random in nature and can therefore be expressed only in probabilistic terms Consequently it is generally impossible to determine with absolute certainty a single value for a peak loading that a vessel’s structure will be required to sustain Instead it is necessary to use a probabilistic representation in which a series of loads of ascending severity is described, each having a probability corresponding to the expected frequency of its occurrence during the vessel’s lifespan When conventional design methods are used, a design load magnitude may then be selected as the one having an acceptably low probability of occurrence In more rigorous reliability methods of design the load data in probability format can be used directly 2.2 Probabilistic Nature of Structural Behaviour Due to the complexity of the structure and the limitations of analysis capabilities and methods it is rarely possible to achieve absolute accuracy in the prediction of structural response even under circumstances where the loading may be precisely known It is essential for the designer to consider the probable extent and consequences of uncertainties in an analysis when making a judgement concerning the overall acceptability of any structural design Equally, it is essential for the naval architect to select an appropriate balance between an acceptable level of uncertainty within a structural analysis and the time and effort required to achieve a meaningful higher level of accuracy The uncertainties are therefore acknowledged and must be allowed for in the design via the application of safety factors Three primary sources of uncertainty exist in determining structural response under loading: Structural Idealisation The designer’s stress analysis is undertaken using an idealised form of the actual structure For example, beam theory may be employed to predict the stress distribution in part or the whole of the hull girder even though it is known that the hull geometry may not follow exactly the assumptions of beam theory .2 Material Properties Actual mechanical properties of the construction materials may not be exactly as those assumed by the designer Furthermore, steel or aluminium plates and sections as delivered from the manufacturer not agree precisely with nominal dimensions used in the design Similarly, chemical and physical properties of the materials may vary within tolerance limits Classification societies specify minimum standards for such properties via a range of acceptable values, however, the properties of the materials actually used in the construction depend heavily Ship Structure – Framing Systems on the quality control in the production process and are unknown by the designer in advance Additionally, degradation of the material’s physical properties resulting from corrosion may occur over the lifespan of the vessel .3 Standards of Workmanship Skill and workmanship is integral to construction integrity Perfect alignment of structural components and perfectly executed welds are assumed within a structural analysis Robotic welding and expensive jigging of components adopted during assembly may result in standards approaching the ideal, however, sub-assemblies and modules especially, and certain singular structural components cannot be jigged so easily (if at all) and much manual welding remains necessary Absolutely flawless welding and totally precise plate forming is rarely achievable in efficient and economical production Even optimal component alignment and welding standards still involves weld-induced distortion and associated stresses Failure Modes Avoidance of structural failure is the overriding objective in structural design and in order for the naval architect to achieve that it is necessary for the naval architect to be aware of the potential modes of failure and methods of predicting their occurrence These potential failures are generally those characteristic of any monolithic structure composed of stiffened plate panels assembled through welding and possessing significant structural redundancy, i.e., having numerous alternative distribution paths for lines of stress 3.1 Degrees of Failure Structural failure may occur with different degrees of severity At the low end of the scale there may be small cracks or deformations in minor structural members that not jeopardise the continued ability of the structure in the performance of its role(s) Such minor failure may have only aesthetic consequences At the high end of the scale is catastrophic failure (collapse) of the structure resulting in loss of part or the vessel, or the entire vessel and potentially involving injury or death of personnel Between these extremes are differing degrees of failure which may reduce the load-carrying ability of individual members or parts of the structure but, due to the redundant nature of the vessel’s structure, not lead to total collapse Such failures are normally detected and repaired prior to any continued failure extending to catastrophe 3.2 Failure Mechanisms Four principal mechanisms are recognised as causing most cases of marine structural failure, aside from collision or grounding These modes of failure are: • Excessive tensile or compressive yield; • Buckling due to compressive or shear instability; • Fatigue cracking; • Brittle fracture Ship Structure – Framing Systems The solution to brittle fracture has been the development of design details which avoid the occurrence of notches and other stress concentrations within structure, together with the development and selection of steels and alloys possessing a high degree of resistance to the initiation of cracks particularly at low temperatures Design Procedure The development of a completely rational structural design procedure has been pursued in several disciplines including civil, mechanical and aeronautical engineering, as well as in naval architecture Using such procedures it should be possible first to formulate a set of criteria to be met by the structure and then through the application of fundamental reasoning (rationales) and mathematical analysis arrive at a structural configuration and set of scantlings which simultaneously meet all of the criteria Although this ideal has not yet been obtained, the perpetual development of structural design has permitted significant improvements in structural efficiencies and optimisation The requirements imposed upon the vessel will include mission requirements of the client and, in addition, regulatory requirements such as those established by the applicable flag administration and classification society Overall dimensions and general arrangement of the vessel are selected to satisfy these requirements Thus in designing the principal members of the structure, it may be assumed that the principal dimensions and the subdivision of the vessel’s internal volume using bulkheads, decks and tank boundaries have already been determined The task of structural design then consists of the selection of material types, frame and stiffener spacing and sizing, and plate thicknesses, that when combined in this 3-D geometrical configuration, will enable the vessel to perform its function for its anticipated lifespan At this point, in selecting the criteria to be satisfied by the structural components, the designer must rely on either empirical criteria, including factors of safety and/or allowable stresses, or on the use of reliability principles An additional element is needed to complete the design synthesis; obtaining the optimal combination of the various elements By reason of the complexity of a vessel’s structure and the probabilistic nature of the data required for vital inputs, it is usually impossible to achieve an optimum solution in a single set of calculations Instead, some form of iterative procedure may be adopted The traditional method of marine structural design involving the extrapolation of previous experience can be considered as part of that iterative process Much of structural design, even when the most advanced methods are used, consists of a stepwise process in which the designer develops a structural configuration on the basis of experience, intuition and imagination, then performs an analysis of that structure in order to evaluate its performance If necessary, the scantlings are revised until the design criteria are satisfied The resulting configuration is then modified in some way that is expected to result in an improvement in performance or cost, and the analysis is then repeated to ensure compliance with the design criteria Thus a key element in structural design is the process of analysing the response of an assumed structure The process of finding a structural configuration having the desired performance is not nearly so straight forward, especially in the case of complex structures As a consequence it is only after the completion of several satisfactory design syntheses that the process of optimisation can take place In summary, key steps may be identified to characterise the structural design process, whether it be intuitive or mathematically rigorous: • Development of the initial configuration and scantlings; Ship Structure – Framing Systems • Analysis of the structural response of the assumed design; • Comparison with performance criteria; • Structural redesign through alteration of both configuration and scantlings to effect an improvement; • Repeat the above as necessary to approach an optimum Formally the final optimisation step consists of a search for the best attainable (usually minimum) value of some quantity such as structural weight, construction cost, or the so-called total expected cost of the structure The last of these quantities consists of the sum of the initial cost of the vessel, the anticipated total cost of complete structural failure multiplied by its probability, and a summation of lifetime costs of repair of minor structural damages Such an optimisation procedure forms the basis for a sound, economical design, whether it be undertaken using formal mathematical optimisation schemes using software or manually (for some sections of the process) Introductory Overview of Structural Loads Loads acting on a vessel’s structure may be categorised as in the following, where the categories are based partly upon the nature of the load and partly upon the nature of the vessel’s response: 5.1 Static Loads Static loads are loads that change only when the total mass (displacement) of the vessel changes, as a result of changes in deadweight, such as the loading or discharge of cargo or passengers, fuel consumption, or modification of the vessel itself Therefore static loads are those that have no accelerations applied or (it may be argued) have a duration of greater than several minutes Loads having a duration greater than several minutes may be cyclical or random phenomena but for the purposes of structural analysis may be regarded as static and are often referred to as quasi-static Examples of these include concentrated loads caused by dry-docking or grounding and thermal loads resulting from non-linear temperature gradients within the hull 5.2 Low-frequency Dynamic Loads These are loads which 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 components that there is no appreciable resonant amplification of the stresses induced in the structure The loads are regarded as dynamic due to their origins in wave action and are therefore perpetually changing with time They may be further subdivided into the following groups: • wave induced hull pressure variations; • hull pressure variations due to oscillatory vessel motions (pitching, heaving, rolling, etc); • inertial reactions resulting from accelerations of the vessel Ship Structure – Framing Systems 5.3 High-frequency Dynamic Loads This category of loads are time-varying loads of sufficiently high frequency that they may induce a resonant vibratory response in the structure Certain excitation forces may be quite small in magnitude but as a result of resonant amplification can produce large stresses and deflections Examples of such high-frequency dynamic loads include: • Hydrodynamic loads induced by propulsors on hull surfaces or appendages; • Loads imparted on structure by reciprocating or unbalanced rotating machinery; • Hydroelastic loads resulting from the interaction of appendages with flow over the hull; • Wave-induced loads due mostly to short waves whose frequency of encounter overlaps the lower natural frequencies of hull vibration and which may excite an appreciable resonant response termed springing 5.4 Impact Loads Impact loads are high magnitude ultra-short duration loads such as those resulting from slamming (wave impact on the forefoot, bow flare or stern counter), weapons firing, collision, impacts from cargo handling equipment such as grabs, and sloshing of liquids within part-filled tank spaces Landing of fixed- and rotary-wing aircraft on flight-decks and ice-breaking may also induce impact loadings on a vessel’s structure Transversely directed impact loads may induce transient hull vibration in the horizontal plane, termed whipping Functions of Hull Structural Elements The strength deck, bottom and side shell of a vessel act as a box girder in resisting bending and other loads in addition to forming a watertight envelope to provide essential buoyancy The remaining structure contributes directly or indirectly to these functions by maintaining the position and integrity of these main members and enabling their efficient function 6.1 Bottom Plating (incl inner bottom) The bottom plating is a principal longitudinal member constituting the lower flange of the hull girder and being part of the watertight envelope is subject to the local hydrostatic pressure In the forward region it must withstand the additional dynamic pressure associated with slamming When fitted, the inner bottom makes a significant contribution to the strength of the lower flange Inner and outer bottom plating, together with bottom girders and floors, function as a double-plate panel to distribute secondary bending effects (caused by external hydrostatic, internal fluid and cargo loads) to main supporting boundaries, i.e., bulkheads and side shell 6.2 Decks One or more strength decks form the principal members of the hull girder upper flange and usually the upper watertight boundary and may be subject to local water, cargo and equipment loadings Other decks, depending upon longitudinal extent, vertical distance from the neutral axis of the hull, and their effective attachment, contribute to a lesser extent in resisting the longitudinal bending Locally, internal decks are subject to the loads imposed by cargo, Ship Structure – Framing Systems machinery, stores, and liquid pressure if forming a tank boundary or barrier against progressive flooding 6.3 Shell Plating The side shell provides the webs for the main hull girder and is an important part of the watertight envelope, being subject to static water pressures and dynamic loadings due to wave action and vessel motion, particularly impact loadings (slamming, berthing and tug landings) In the stern region, extra plate thickness is beneficial in way of rudder and shaft strut mountings and stern tubes, for increased strength and panel stiffness and for the reduction of vibration In icecapable vessels the ice belt plating is required to withstand ice loadings and abrasion 6.4 Bulkheads Bulkheads are one of the major components of internal structure Their function in the hull girder depends on their orientation and extent Main transverse bulkheads act as internal stiffening diaphragms for the girder and resist in-plane torsion (racking) loads but not contribute directly to longitudinal strength Longitudinal bulkheads, if extending more than about 10% of the hull length, contribute to longitudinal strength and may be as effective as the side shell itself Bulkheads generally serve other structural functions such as tank boundaries, deck support, support of superstructures and major load-inducing installations (e.g., crane pedestals), and add rigidity to reduce vibration Transverse watertight bulkheads additionally provide subdivision to prevent progressive flooding, and both transverse and longitudinal bulkheads provide fire integrity forming divisions between fire zones 6.5 Double Bottom Construction Cargo vessels of gross tonnage 500 tons and greater, and passenger vessels (other than highspeed light craft) require a double bottom construction, in most cases between collision and aft peak bulkheads The inner bottom, other than contributing to the strength of the lower flange of the hull girder, provides improved watertight integrity and protection against flooding in the event of bottom damage The double bottom is given a cellular construction which enables the enclosed volume(s) to be utilised for ballast and fuel storage Vertical plating connects the bottom shell and inner bottom Those fitted transversely are called floors and those fitted longitudinally are centre girders or side girders, as appropriate These vertical orthogonally arranged plates, if watertight, may form the boundaries of tanks, and irrespective of watertight integrity additionally provide the main points of support for the vessel during dry-docking Floors and girders are stiffened vertically, usually employing flat bar stiffeners Systems of Framing There are systems available to the designer The framing system adopted is primarily driven by vessel length and type 7.1 Transverse System of Framing Essentially the transverse system of framing consists of a series of closely spaced ribs encircling the hull These ribs, comprising of vertical side frames, horizontal deck beams and floors in the bottom, provide the stiffening of the shell and deck plating upon which the longitudinal strength of the vessel primarily depends The encircling ribs and their integral components also provide Ship Structure – Framing Systems support of hydrostatic and local loadings and maintain the geometric integrity of the hull Side stringers and deck girders may be employed where deck spacing and beam span respectively necessitate support of side frames and deck beams Web frames or deep frames are fitted every or frame spaces to support stringers and girders with the side stringers and deck girders fitted intercostally Spacing of transverse frames is rarely permitted to be greater than 1000 mm in larger vessels and in smaller craft may be as little as 300 mm Floors in the bottom structure (whether single or double) should be aligned immediately below the side frames to provide support and structural continuity Transverse framing may be used in small pleasure craft, inshore and harbour service craft and in the smaller fishing vessels where the vessel’s length to depth ratio is small and coupled with low to moderate sea-states longitudinal bending stresses (and the associated buckling stresses) may considered insignificant In larger vessels the transverse system of framing provides insufficient resistance to buckling of deck and bottom plating induced by axial (in plane) compression arising from the sagged or hogged conditions respectively As the combined loadings of the still-water and wave bending moments and shear forces are frequently those of greatest magnitude to be sustained by the hull, the transverse system of framing has been superseded by the longitudinal and combined systems of framing deck girder bracket deck beam (transverse) deck plating stringer web frame side frame ‘tween deck plating deck girder deck beam (transverse) bracket web frame side frame transverse bulkhead stringer tank side bracket inner bottom plating 20 bracket floor 25 plate floor bottom plating reverse frame (inner bottom) 30 side girder (intercostal) bottom frame Figure 11.1 Longitudinal section of a transversely framed hull showing side structure Ship Structure – Framing Systems 7.2 Longitudinal System of Framing When frames which stiffen and support the shell and inner bottom, and members which stiffen and support the decks are run longitudinally instead of transversely, they contribute significantly to the section modulus of the hull girder and hence assist in resisting the longitudinal bending of the hull Where the primary plating is subject to high in-plane compressive stress, longitudinal stiffeners also increase the critical buckling strength of the plating to which they are attached It is in this regard that the longitudinal system is considerably more structurally efficient than the transverse system and hence is used exclusively in vessels such as tankers and bulk carriers and in vessels over 100 m in length unless the combination system of framing is preferable The longitudinal system of framing comprises longitudinal stiffeners attached to deck, side-shell, bottom and inner bottom plating The longitudinal deck stiffeners are supported by deck transverses and side-shell stiffeners are supported by side transverses These side transverses are normally significantly deeper than a web-frame used in the transverse framing system Bottom and inner bottom stiffeners are supported by floors Where spans of transverses are large, deck girders provide support, spanning intervals between transverse bulkheads The longitudinal system of framing is generally employed in naval combatants and high-speed commercial craft where strength and weight saving is of paramount importance deck girder deck longitudinal bracket side longitudinal deck transverse (beam) deck plating side transverse bracket deck longitudinal ‘tween deck plating deck girder bracket side transverse side longitudinal bracket transverse bulkhead inner bottom longitudinal floor stiffener inner bottom plating 20 transverse bilge bracket side girder 25 bottom longitudinal 30 plate floor bottom plating bracket Figure 11.2 Longitudinal section of a longitudinally-framed hull showing side structure Ship Structure – Framing Systems 7.3 Combination System of Framing Longitudinal framing is so efficient that it might be expected to have become standard practice For many types of commercial vessel, the deep side transverses required to support the longitudinal side framing can have serious disadvantages In certain cargo vessels, e.g., roll-on roll-off and refrigerated cargo vessels they may be regarded as interfering with the stowage and movement of cargo In large cruise vessels the deep side transverses may not readily facilitate the preferred arrangement of accommodation outfit and its integral joinery and deck girders may interfere with the transverse branches of piping and air-conditioning ducts In such cases a practical solution may then be to longitudinally frame the bottom shell, inner bottom and strength deck and to transversely frame the side shell and ‘tween decks In some vessel types all decks may be longitudinally framed and only the side shell is transversely framed In either case, this hybrid system of framing is referred to as the combination system of framing 10 Ship Structure – Framing Systems 11 Ship Structure – Framing Systems 12 Ship Structure – Framing Systems 13 Ship Structure – Framing Systems 14 Ship Structure – Framing Systems 15 Ship Structure – Framing Systems 16 ... sloshing of liquids within part-filled tank spaces Landing of fixed- and rotary-wing aircraft on flight-decks and ice-breaking may also induce impact loadings on a vessel’s structure Transversely directed... longitudinally-framed hull showing side structure Ship Structure – Framing Systems 7.3 Combination System of Framing. .. 11 Ship Structure – Framing Systems 12 Ship Structure – Framing Systems