Reducing Total Ownership Cost: Designing Robust Ship Structures
By Robert G. Keane, Jr., Tobin McNatt, and Dr. Jeffrey E. Beach, Consultant
The ship design philosophy in early stage naval ship design in the 1970’s and early 1980’s was that “smaller is better” and “lighter weight is cheaper”. This philosophy was incorporated in initial ship synthesis computer models developed during that time and is still embedded in many ship design tools in current use. This “smaller is better” design philosophy also influenced a generation of surface ship designs that are in the fleet today as well as legacy designs that continue to be used as parent designs for future ships. This “lighter is cheaper” design philosophy is manifested in many ships in the fleet today in terms of overly dense outfitted ships, highly dense machinery spaces, ship arrangements that do not contribute to efficient operation and repair, persistent structural problems; and in general, unnecessary complexity in the design and construction of new ships and the operation, repair and modernization of in-service ships. Ship synthesis computer models are used during Concept Design to evaluate and compare numerous ship designs to one another and against high level requirements. Designs must evolve quickly if numerous concepts are to be considered. Structural integrity issues during this stage relate to establishing some acceptable design stress(es) corresponding to a given condition or set of conditions to enable a ship structural weight to be determined. For the most part, during Concept Design this is considered a ROM estimate. Unfortunately, because structural weight represents the heaviest light ship sub-system, estimates made during Concept Design are critical to design feasibility and those weights are often established as limits and not just initial conditions. Since the predominant weight of a ship is the ship structure, limiting weight limits the size of the ship and drives the design to lightweight structures. The complications and problems of lightweight tructures have been a major contributor to excessive total ownership costs of naval ships. In addition, the geometric complexity of traditional ship structure has significant impact on labor costs of installing and maintaining internal fixed and distributed systems. However, since the late 1980’s the Naval Sea Systems Command (NAVSEA) has invested in a number of Design for Producibility initiatives the results of which have been applied on a design-by-design basis but still need to be incorporated in early stage ship design tools. With dramatic advances in high performance computing, higher fidelity, physics-based models can now be integrated into early stage integrated design environments. These will provide the early stage design engineer the capability to assess alternative ship concept designs and “optimize” for adequate structural performance throughout their in-service life, minimal construction work content and minimal weight, thus producing robust ship structures and significantly reducing the total ownership costs of naval ships.