Development of a structural fatigue life assessment framework for high-performance naval ships

Navies around the world have been using High Speed Light Craft (HSLC) in a wider range of military roles and ocean environments than ever before. This increasing range of operations, use of lightweight scantlings, and susceptibility to slamming necessitates the development of improved structural fatigue assessment methods. These improved methods are required to ensure that a HSLC will meet its intended life, and to evaluate the impact of through-life modifications to the structure or operational profile. This PhD thesis covers the development of an efficient structural fatigue life assessment framework for high-performance naval ships. The framework helps to inform risks through-life for decision-makers, by integrating real-world data and optimised tool selection. The thesis begins with a critical review of available approaches to fatigue assessment and their associated merits and limitations when applied to naval aluminium HSLC. The review is followed by fatigue analyses of a naval HSLC using the different approaches, to establish their accuracy, fidelity, and expense. This part of the research includes the utilisation of full-scale measurements to quantify and explore the characteristics of slam events. The significant contribution of slamming to the fatigue damage is also demonstrated. Though the S-N curve concept in fatigue analysis is commonly used in the maritime industry, there are diverse specifications for its use which leads to different fatigue life estimates. Therefore, a new implementation of the nominal stress approach to assess the fatigue life of different welded details on the naval HSLC is presented. In the later stage of the thesis, a robust, easily implemented hybrid method for naval HSLC fatigue assessment is presented. The method uses a tailored combination of in-service data, fleet maintenance reports, and Finite Element Analysis to predict ongoing fatigue life and hence support the management of the remaining service life of a ship. The merit of the framework is demonstrated through the determination of the optimum approach to providing advice regarding the structural Life of Type (LOT) of naval HSLC for decision-makers. This includes identification of the variables that reduce the confidence limits of the LOT answer. The impact of the research includes improved understanding of the uncertainties and interdependencies between the fatigue life and capability aspects of naval ships, and the establishment of an evidence-base for setting testable requirements for new ships to support both the fleet-in-being and naval shipbuilding.