Structural continuity provides for a continuous load path through the structure, e.g. the deck load is transmitted through the stiffened side shell to the bottom shell structure, which is eventually supported by buoyancy forces. Any discontinuity in this will result in stress concentration locally. In ship structure there are several unavoidable structural discontinuities for functional requirement. Hence adequate measures are to be taken to compensate for these structural discontinuities.
There are various unavoidable openings in a ship’s structure for different functional requirements. Structural discontinuity caused by these openings may lead to local stress concentrations as well as loss of structural strength. Hence appropriate structural measures should be taken to compensate for these discontinuities.
(i) Welding flat bar
In case of medium sized openings like doors, where the opening may cut through a stiffener, additionalflat bar stiffeners are used to properly terminate the cut stiffener as well as a flat bar is welded all around the opening as shown in the Fig.8.6.
Fig. 8.4 Limit of misalignment in butt joints
Fig. 8.5 Flange alignment of T-section stiffeners
(ii) Doubler plate
In case of small openings like various discharge openings on shell side, doubler plate is welded on the inner side of the shell plate as shown in Fig. 8.7.
(iii) Insert plate
In the areas of large openings as in case of cargo hatch opening, stress concentration is likely to occur at the corners of these openings because of sudden change in sectional area of the deck plate. Here plate of higher thickness compared to the adjacent plates is inserted at the corners. These plates are referred to as insert plates as shown in Fig.8.8. This is done to enable the corner plates to withstand higher levels of stress because of inevitable stress concentrations.
(iv) Beams and Girders of higher scantlings
Stiffening members of higher scantlings in the form of hatch side girders and hatch end beams are used to make up for the lost strength due to large hatch opening as shown in Fig. 8.8.
Fig. 8.6 Flat bar ring and additional stiffening around door opening
Fig. 8.7 Doubler plate in way of shell opening
Reference
1. 47 Shipbuilding and Repair Quality Standard, (Rev. 7, June 2013), Part A Shipbuilding and Remedial Quality Standard for New Construction, IACS “Bulk Carriers - Guidelines for Surveys, Assessment and Repair of Hull Structure”.
Fig. 8.8 Insert plate, hatch side girder and hatch end beam in way of cargo hold hatch opening
Chapter 9
Material of Construction
Abstract Wide variety of materials is used in ship construction. For ship hull construction several materials are available, however one needs to select the one which suits the intending purpose the best. The objective is to have a product that will be economical to manufacture and maintain, should also be durable and reli- able. Various characteristics of steels, marine grade aluminum alloys and glassfibre reinforced plastics as materials for ship construction have been dealt with in this chapter.
In ship construction one can see use of wide variety of materials for wide ranging requirements. Here material for hull and superstructure construction will only be dealt with. For ship hull construction several materials are available, however one needs to select the one which suits the intending purpose the best. The objective is to have a product that will be economical to manufacture and maintain, should also be durable and reliable. The following aspects need to be checked while selecting a material for a given construction.
Design Aspects
From the designer’s point of view, the material selection for hull fabrication depends primarily on the following three types of mechanical responses,
(i) Strength and stiffness of the parent material (σult,σyand E).
(ii) Time-dependent material behavior such as, • creep,
• stress corrosion cracking, • fatigue and
• corrosion wastage.
(iii) Fatigue and corrosion fatigue of parent material.
Traditionally, the design of marine structures is based on strength and stability criteria to resist yielding or buckling, with the emphasis on (i) and (ii) above. The primary material properties used in this approach are ultimate strength, yield strength, modulus of elasticity, and fatigue endurance behavior (S-N curves).
© Springer Nature Singapore Pte Ltd. 2017
N.R. Mandal, Ship Construction and Welding, Springer Series
on Naval Architecture, Marine Engineering, Shipbuilding and Shipping 2, DOI 10.1007/978-981-10-2955-4_9
Resistance to fracture was usually taken into account by assessing the impact properties of the materials as given by Charpy-V notch impact test. For fracture dominated failure, following macroscopic defects are most important,
• cracks, • inclusions, • weld flaws, • porosity.
The essential feature of the fracture process is connected with the interaction of the local stress-strainfield with the defect.
Manufacturing Aspects
From manufacturing point of view, the material of construction should possess the following attributes: • Ductility/Formability • Machinability • Weldability • Resistance to corrosion • Maintainability.
As ship’s hull has compound curvature plates, the material should have adequate ductility, such that necessary plate bending can be achieved without any defects occurring in the plate. The material should be amenable to machining, like drilling, punching or cutting using guillotine shear. At the same it can be cut using oxy fuel/plasma cutting methods. The material should be easily welded using conven- tional welding techniques. In case of non-metallic material, like fibre reinforced plastics, conventional moulding techniques can be used for fabrication. Since ships operate in harsh corrosive environment, it will be an additional advantage to have good resistance to corrosion.
Economic Aspects
The economic aspect is equally important in the selection of the material for construction. There can be options where the economic aspect may over right the justification of material selection. Using a particular material should be economi- cally viable. In the process some other aspect may have to be somewhat compromised.
Environmental Aspect
With ever growing concern on environmental issues related to all kinds of industrial products and production process, it is of utmost importance to select materials which have the least environmental impact. The environmental impact needs to be looked into during production, operation and decommissioning phase of the product.