Original Date: 04/24/2007
Revision Date: / /

The University of New Orleans, College of Engineering teamed with the University of Maryland, College Park to develop new design rules utilizing probabilistic methods to reduce weight in future commercial and naval vessel structures.

The University of New Orleans, College of Engineering (UNO COE) saw a need for weight reduction in today’s shipbuilding market. The U.S. Navy is now requiring the use of light-gage plate in nonstructural and structural bulkheads on their LPD-17 and DDG-21 next-generation warships. Ongoing committee studies were considered by the team in the development of these new procedures for the design of ship structures. The Load and Resistance Factor Design (LRFD) criteria were initially envisioned to be used in parallel with currently used procedures. The applications and limitations of this procedure are currently limited to the design of fine-bow, bow-with-flare, or flat- bottomed vessels and conventional displacement-type monohull surface vessels made of metallic materials, and those with a length of 300' to1000' between perpendiculars.

The old process to design vessels followed the rules of the Navy or the American Bureau of Shipping. New designs do not have historical information to use as a basis. The team’s objective in the first year was to develop LRFD criteria for hull girder bending of surface ship structures for both commercial and naval vessels. In the second year, the team developed LRFD criteria for unstiffened panels of surface ship structures; in the third year of the project, the team developed LRFD criteria for fatigue of surface ship structures.

The development of this new method for the structural design of conventional displacement-type surface monohull ships is based on a structural reliability theory in an LRFD format. The LRFD rules can be used for early design of ship structures to check adequacy in the detailed design stage. For marine applications, LRFD development is based on special and extreme analysis of wave loads with a combination of partially correlated loads. Nominal strength and load values, along with achieving target reliability levels, are also considered. The LRFD method builds on conventional marine and structural codes. The design philosophy must provide for ship structural designs that have adequate safety and allow for the proper functioning of each component. The Allowable Stress Design (ASD) and the LRFD design philosophies for designing structures are currently in use. The LRFD philosophy utilizes probabilistic methods versus the pure-static and dynamic formulas used in the ASD philosophy.

Advantages of the LRFD philosophy include a more rational approach for new designs and configurations, consistency in reliability with the potential for a more economical use of material, calibration of building codes that allows for future changes from information acquired from predicted models, and material and load characterization.

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