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Abstract
Maintenance, inspection, and repair are key aspects of managing the structural integrity of ship systems in a life cycle framework. For example, an inspection program can be developed with the objective of maintaining the structural integrity of a ship. It can start with system definition, followed by qualitative reliability assessment, and then quantitative reliability assessment with the objective of performing reliability-based design for maintaining system integrity. Doubler plates currently offer a temporary solution for plate damage in ship structures. The temporary nature of this fix stems from the lack of data on their performance and an engineering design guidance. In this study, a reliability-based design model for an unstiffened panel with doubler plate(s) was developed using both finite difference (FD) and finite element (FE) procedures. Partial safety factors were also determined to account for the uncertainties in strength and load effect. The First-Order Reliability Method (FORM) was used to develop the partial safety factors.
Introduction Doubler plates currently offer a temporary solution for plate damage in ship structures. The temporary nature of this fix stems from the lack of data on their performance and an engineering design guidance [3]. There is a need to survey the use of doubler plates, document experiences with their use, and develop guidance on their design and uses with any associated limitations. This study will survey such experiences, and develop the needed guidance. Repair actions can be classified into two categories: (1) temporary, and (2) permanent. The use of doubler plates, or lapped plating, has become extensive, and is an inexpensive method of repairing corroded plating, cracked plates, or defective welds. A doubler plate is nothing but a plate that is added to top of the defective area and welded around the plate’s perimeter (see Figure 1). While this method of repair has economic advantages, it falls in the temporary repair category and its use has never been accepted as a permanent repair. This temporary repair method would maintain structural integrity until the ship is either in dry-dock or in restricted availability, and is followed by permanent repairs made to the original corroded structure. In the maintenance of commercial ships the use of doublers for anything other than temporary repair is currently not recommended. The objections to their use are both on technical and operational grounds. Hull girder structural components of a ship are basically rolled shapes or built-up sections that are composed of plate elements (flat plates). The strength of these structural components is usually governed by local buckling of these plate elements or flat plates that make up the cross-section. Such local buckling means that the buckled element will no longer take its proportionate share of any additional load the column is to carry. This also means that efficiency of the cross section is reduced. This situation resembles the case where there is a crack or a hole in one of the plate elements of stiffened panels of a ship, which needs an immediate remedy such as adding a welded doubler plate on top of the crack or the hole (see Figure 1b). There are numerous factors that can affect the capacity (or strength) of flat plate with doublers plate. Among these factors are [3]:
Figure 1a. Base plate
Figure 1b. Unstiffened panel with doubler plate
The type of welding along the perimeter of the doubler plate.
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