On Ship Structure Risk and Total Ownership Cost Management Assisted by Prognostic Hull Structure Monitoring

Abstract

Ships must perform their missions with a high degree of reliability to maximize availability through their service life. The ultimate safety of the hull structure is time-dependent with degradation caused by the operational environment. Achieving the fore mentioned reliability and mission availability requirements are complicated because ships operate in random seaways producing random loading on the hull structure. The subsequent strength degradation also involves random processes including the material properties themselves. Furthermore, the models used to estimate the loading and responses are not perfect and result in additional randomness and related uncertainty. The potential Risks involved are very high, given the combination of uncertainties and high value of the assets, crews, and related resources. The primary research questions posed by this dissertation include; 1) what approaches are needed to make Risk informed decisions in Ship Structure Life Cycle Management (SSLCM) and, 2) how can Hull Structural Monitoring (HSM) be used effectively to support these decisions? This dissertation addresses these research questions by building on the fundamentals of hull structural loading and failure mechanisms on both component and systems-levels that are unique to ship structure. This fundamental research includes a correlation analysis of the system loading to support new definitions of ship structural system response. This new definition of structural system response provides insights into definitions of serviceability failure, reserve strength, and redundancy. Following the structural systems definition development, this dissertation proposes a Risk and Total Ownership Cost (TOC) trade-space perspective for making informed decisions and managing both Risk and costs associated with SSLCM and fundamental characterization of Risk and uncertainty. The development of Risk-TOC approach provides tangible and relatable benefits for understanding uncertainty in Risk terms required to make informed decisions. The Risk-TOC approach provides a more informed perspective than prior proposals for Decision Theory-based Optimal Inspection approaches with assumptions and parameters that do not fully quantify the uncertainties involved in the SSLCM processes. The Risk-TOC approach also provides a quantitative means for assessing the consequences of different failure modes (i.e., fatigue cracking and corrosion). The Risk-TOC approach provides a quantified basis for comparing Risk and costs given the magnitude of resources at Risk by monetizing uncertainty. In this manner, the Risk - TOC approach provides a framework for fundamental definitions, including monetized uncertainty, analysis of alternatives (AoAs), Return on Investment (RoI), and Value of Information (VoI). The benefits of prognostic HSM are presented in the context of reduction of uncertainty in the SSLCM processes; thereby, reducing Risk and TOC with favorable RoI and VoI. The Risk-TOC approach is verified as demonstrated in example applications involving a US Coast Guard Cutter. A discussion is provided on the implications of the Risk-TOC approach on SSLCM and sustainability. Conclusions and recommendations are presented for further development of the Risk-TOC approach for SSLCM.