12-Ship Structure - Regulatory Controls (2012)

Ship Structure – Regulatory Controls SHIP STRUCTURE – REGULATORY CONTROLS 12 MODULE Introduction A vessel’s structural design and its integrity are amongst the most strictly regulated aspects of the design in general Furthermore, when a vessel is constructed under survey, regular inspection of fabrication and structural assemblies (e.g., alignment and welding standards) continues that same quality assurance which actually begins with the use of only approved materials This quality assurance regime is manifested in the national administration requirements and if applicable, classification society rules For commercial vessels in Australia for example, the national administration requirements for structure are embodied within the National Standards for Commercial Vessels (NSCV) issued and controlled by the National Marine Safety Committee (NMSC) and/or the Australian Marine Orders (AMO) The NMSC will, from 2013, be absorbed into the Australian Maritime Safety Authority (AMSA) Where a commercial vessel is to be under the survey of a classification society then compliance with both national administration and classification society regulations is necessary In the context of regulation of structure, if requirements of the national administration differ from those of the classification society, then deference to the classification society requirements is usually stipulated For Australian naval craft the national requirements are contained within the DEF(AUST) requirements which are prepared and controlled by the Directorate of Navy Platform Systems Engineering within the Department of Defence Approved Materials for Structure The materials employed in the construction of a vessel need to be of approved type and their areas of use are limited or specified Classification societies detail the required chemical and mechanical properties of the steels and aluminium alloys which are to be adopted for the structure within any vessel under their survey Furthermore, the standards of testing for these properties are outlined Most classification societies issue lists of approved manufacturers of pre-approved materials to facilitate ease of process in the procurement of a vessel’s construction materials The following extracts are provided to illustrate just some of the requirements of the materials adopted in steel, aluminium and GRP vessel construction in accordance with the NSCV and/or certain classification societies or standards Ship Structure – Regulatory Controls Extract from NSVC Part C Section 3illustrating the required standards of materials to be used in vessels which are not classed (under survey) with any classification society Ship Structure – Regulatory Controls Extract from American Bureau of Shipping (ABS) Rules for Materials & Welding (2007) illustrating the coverage of materials and applications in steel vessels Ship Structure – Regulatory Controls Extract from American Bureau of Shipping (ABS) Rules for Materials & Welding (2007) stipulating required chemical properties of hull steels (Grades A – E) Extract from American Bureau of Shipping (ABS) Rules for Materials & Welding (2007) stipulating required tensile properties of hull steels (Grades A – E) Ship Structure – Regulatory Controls Extract from American Bureau of Shipping (ABS) Rules for Materials & Welding (2007) stipulating required tensile & tensile properties of hull higher tensile steels Ship Structure – Regulatory Controls Extract from Det Norske Veritas (DNV) Rules for Ships/High Speed, Light Craft & Naval Surface Craft (2003) stipulating chemical composition limits for wrought aluminium alloys Extract from Det Norske Veritas (DNV) Rules for Ships/High Speed, Light Craft & Naval Surface Craft (2003) stipulating mechanical properties for 5000 series rolled aluminium alloys Ship Structure – Regulatory Controls Extract from Det Norske Veritas (DNV) Rules for Ships/High Speed, Light Craft & Naval Surface Craft (2003) stipulating mechanical properties for 5000 & 6000 series extruded aluminium alloys Extract from ISO 12215−5.2 Small Craft – Hull Construction & Scantlings, Part stipulating glass content by mass for GRP plies Ship Structure – Regulatory Controls Extract from ISO 12215−5.2 Small Craft – Hull Construction & Scantlings, Part stipulating GRP mechanical properties as a function of mass content (weight fraction) Extract from ISO 12215−5.2 Small Craft – Hull Construction & Scantlings, Part stipulating strength characteristics as a function of mass content (weight fraction) Ship Structure – Regulatory Controls Extract from ISO 12215−5.2 Small Craft – Hull Construction & Scantlings, Part stipulating mechanical properties for sandwich core materials used in GRP composite construction Hull Structure Requirements A vessel constructed and operating under the survey of a national administration and/or a classification society must satisfy the corresponding requirements for global and local strength Rules or standards applying define minimum requirements in terms of plating thicknesses, section modulus, moment of inertia, etc., in response to determined bending moments, shear loads, buckling stresses, slamming pressures and deck load ratings 3.1 Minimum Plate Thickness Minimum plating thicknesses stipulated may include allowances for gradual reduction due to corrosion and abrasion depending upon the vessel type and location within the vessel (e.g ice abrasion of the ice belt plating in ice-breakers, grab abrasion of the inner bottom in bulk carriers) Shock and ballistic loads on plating are considered within rules for naval platforms where appropriate Plating may constitute as much as 85% of the structural weight of a vessel Rule-minimum plate thickness is a function of the selected stiffener spacing and hence reduction of stiffener spacing will result in a reduced required plate thickness There will however be a lower limit set On occasion rule-minimum plate thickness determined may be so thin to the point where significant weld-induced distortion of the plate during construction would result and thus the designer or constructer may adopt thicknesses in excess of the minimum required 3.2 Minimum Stiffener, Frame & Girder Section Moduli Rule-minimum section modulus are applied to structural profiles forming plate stiffeners, frames, deck, side and bottom frames (transverses), deck and bottom girders and side stringers Due to the grillage nature of hull structure, the rule-minimum section modulus for these components is always to include the attached plating of the stiffened shell, bottom, deck or bulkhead structure The structural designer should ensure that the appropriate effective width of attached plating is used in the calculation; this effective width being stipulated by the rules in use Ship Structure – Regulatory Controls 3.3 Buckling Strength Compliance with rule-minimum section modulus is, in the first instance, compliance with the requirements to withstand stresses resulting from global longitudinal bending and/or local lateral loads (pressures) Rules governing these scantlings often include a caveat that compliance is only achieved if the resulting structure can also be shown to withstand buckling due to induced axial loads It is therefore beholden upon the designer to undertake the necessary analysis in order to prove that this is in fact the case 3.4 Hull Global Longitudinal Strength Vessels of length in excess of 60 metres have their global strength requirements dominated by longitudinal bending, i.e., the superposition of induced still-water and wave bending moments Following the determination of this total bending moment in compliance with the authority’s methods, satisfying the required global strength is via the achievement of a rule-minimum moment of inertia and/or section modulus at the keel (bottom) and at the strength deck In most cases the critical hull girder cross-section used in the determination of these values for the vessel’s structure as designed will be that section which contains the least amount of effective material, i.e., the section containing the largest hatches or other openings, but it also depends on the distance of these from the neutral axis If there is any doubt, the moment of inertia and section modulus should be calculated for all the potentially critical sections In general, the net sectional area of longitudinally continuous material (members) is to be used in the section calculation, and small isolated openings need not be deducted provided the openings and shadow area breadths of other openings in any one transverse section not reduce the section modulus by more than a few percent Classification society rules should be consulted in regard to included material and deductions for openings The following constraints on material to be included in the calculation are those employed by Lloyd’s Register of Shipping Rules & Regulations for the Classification of Ships (July 2005) They are presented here to function as a guide only, and the appropriate rules controlling any design should always be used Lloyd’s Register Constraints on Included Material All continuous longitudinal structural material is to be included in the calculation of the inertia of the hull midship section Openings having a length in the fore and aft direction exceeding 2.50 m or 0.1 BM , or a breadth exceeding 1.20 m or 0.04 BM , (whichever is the lesser), are always to be deducted from the sectional areas Smaller openings (including manholes, lightening holes, single scallops in way of weld seams, etc.,) need not be deducted provided they are isolated and the sum of their breadths or shadow area breadths) in one transverse section, does not reduce the section modulus at the deck or at the bottom by more than 3% When calculating deduction-free openings, the openings are assumed to have longitudinal extensions as shown by the shaded areas in Figure 12.1 The shadow area is obtained by drawing tangent lines to an opening angle of 30 10 Ship Structure – Regulatory Controls Isolated openings in longitudinal stiffeners or girders need not be deducted if their depth does not exceed 25% of the web depth with a maximum depth for scallops of 75 mm Openings are considered isolated if they are spaced not less than 1.0 m apart A actual small opening (manhole) b1 shadow area for manhole shadow area for hatch b2 30 actual small opening (hatch) b3 A Figure 12.1 Deduction-free openings and shadow areas Hulls of length less than 60 metres may not have rule-minimums for moment of inertia or section moduli as such vessels often possess such a low length/depth ratio that their moment of inertia and corresponding section moduli are significantly in excess of any calculated minimums necessary for the global bending moment sustained Local loadings on structure dominate the structural requirements of these vessels Sample Extracts from NSCV Part C Section − Construction The following extract from the Australian National Standard for Commercial Vessels (NSCV) is included to at least partially illustrate the NMSC/AMSA policy towards the regulation of strength of a vessel’s structure under AMSA survey (i.e., Australian registered (flagged) commercial vessels) Such vessels may, depending upon their size, type and category of operation, need to comply with Australian Marine Orders Parts 12 and 16 in regard to their structural design 11 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the general strength requirements for all vessels covered by the NSCV 12 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the approach adopted by the Australian administration to commercial vessels covered by the NSCV and in class 13 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the approach adopted by the Australian administration to commercial vessels covered by the NSCV and in class 14 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the approach adopted by the Australian administration to small commercial craft covered by the NSCV (Continued overpage) 15 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the approach adopted by the Australian administration to small commercial craft covered by the NSCV (Continued overpage) 16 Ship Structure – Regulatory Controls Extract from NSVC Part C Section illustrating the approach adopted by the Australian administration to the strength of small commercial craft covered by the NSCV Sample Extracts from Classification Society Rules The following extracts (from ABS Rules for Building & Classing Steel Vessels – 2007) are included to illustrate some of just one classification society’s approach to the determination of a vessel’s scantlings 17 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing the method to calculate wave bending moment 18 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing the method to calculate wave shear force 19 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing rule-minimum for hull girder section modulus (Continued overpage) 20 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing rule-minimum for hull girder section modulus and a means of calculating nominal shear stress in side shell plating 21 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing rule-minimum thickness for side shell plating 22 ... Ship Structure – Regulatory Controls 3.3 Buckling Strength Compliance with rule-minimum section modulus is, in the... 19 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing rule-minimum for hull... 20 Ship Structure – Regulatory Controls Extract from ABS Rules for Building & Classing Steel Vessels (2007) showing rule-minimum for hull