Hull Structures as a System: Supporting Lifecycle Analysis

As modular weapon systems allow cost-effective upgrades of a vessel’s war-fighting capability, the degradation of the difficult-to-upgrade structure of the vessel may soon become one of the key drivers of vessel retirement and lifecycle maintenance costing. Existing structural design approaches are reviewed, along with recent developments in this field. It is argued that recent research has produced a number of ad hoc metrics for structural design, such as producability; however, to truly address the needs of future ship design teams it is necessary to integrate several such metrics in a systems-engineering view to evaluate how the structural system contributes to the overall capabilities and costs of a proposed vessel. Potential architectures for this approach are discussed, along with key shortcomings. A comparative example is given for structural fatigue of a strength deck under global bending loading, comparing the traditional design approach with a systems-oriented view. Introduction The US Navy is currently faced with the challenge of approving both innovative structural designs, such as the all-aluminum trimaran LCS, and maintaining the structure of existing conventional vessels, such as the CG-49 and DDG-51 classes. Despite extensive research into structural analysis and structural healthmonitoring technologies, naval architects currently lack the tools to forecast lifecycle structural performance and maintenance costs during either design or through-life extension studies. Such lifecycle considerations are expected to grow in importance as modular weapon and combat information systems allow cost-effective through-life upgrades, removing weapon system obsolescence as a reason to retire vessels from service. Furthermore, for naval structures, the structural system is typically supporting an investment of weapons, sensors, machinery, and other vessel systems worth many times the value of the structure itself but effectively permanently tied to the structure. In the future, it is highly likely that degradation of difficult-to-upgrade hull structure will be one of the primary driving causes for retiring a vessel from service, and thus directly impacting the overall platform lifecycle cost (LCC). Thus, the ability to predict and control structural lifecycle and maintenance costs will become increasingly important as the Navy considers service life extensions or evaluates suggestions for radical departures from conventional designs, such as the proposed 100-year service life ship. To effectively support future ship design, maintenance, and service-life extension decisions, it is proposed that it is necessary to extend the existing semiempirical component-based structural design rules based primarily on safety concerns to a system performance model for ship structures. This system-based approach extends the existing rule-based approach by formally stating performance requirements for the structure T E C H N I C A L P A P E R & 2011, American Society of Naval Engineers DOI: 10.1111/j.1559-3584.2011.00329.x 2011 #3&45 based on the vessel systems that the structure supports, and considering maintenance and repair requirements during structural design and analysis. By explicitly setting such performance targets, it will be possible to assign key performance parameters (KPP) and key system attributes (KSA) to the structural system, and allow the overall vessel design synthesis work to evaluate different structural concepts and weight budgets against achieved vessel performance attributes. This would allow the impact of designing for increased structural performance (such as extended lifetime or a reduction in the estimated repairs) to be traded off against impacts on other aspects of design such as displacement or powering requirements. Beyond design, these formal system targets would allow real-world experience with the vessel to be interpreted as an ongoing validation trial of the structural system, and observed failures, maintenance actions, and potential service-life extension options can be compared in terms of the system performance they provided for the time and monetary resources that they consume. However, moving to such a model is complex, as many performance requirements are built into the existing structural semiempirical rule sets without being formally identified, making it difficult to develop a complete set of requirements for new structural designs. This is especially true if service life is to be treated as a design variable, many rule sets implicitly assume a 20–30-year service life and cannot evaluate the impact of stretching the service life to 40, 50, or 100 years. Likewise, existing structural maintenance activities and costs are rarely fed back to naval ship designers, and thus it is rare to formally consider the impact on maintenance downtime or cost when reviewing different structural design options. In this paper, existing structural design, production, and maintenance philosophy for military and commercial vessel structures are reviewed. Achieved service lives under the existing approaches are also documented. Based on the existing state-of-the-art, developments required for a system-based model of structural performance are discussed. By adapting elements of system engineering to the structural design problem, an initial architecture for a structural system performance model is presented. Conclusions on the applicability of the systems view to structural design and analysis are presented, along with recommendations for future technology development to support the systems modeling approach.