The US Coast Guard (USCG) operates its cutters in very challenging conditions. While on deployment, these ships always have to be ready for law enforcement or search and rescue activities in heavy weather. A combination of extensive monitoring and reliability analysis is executed to ensure the structural integrity of these vessels. For this reason, the USCG initiated a project in 2007 to assess hull structure fatigue design approaches for its new National Security Cutters (NSC). Following the initial Fatigue Life Assessment Project, two additional cutters were instrumented for fatigue life evaluations.This paper presents the setup of two types of hull structure monitoring (HSM) systems. One is an overall system focused on global load effects and strains. The other is a local system aimed at detecting cracks through acoustic emission (AE) monitoring.Long base strain gauges are an important part of the global HSM system that was installed on USCGCs STRATTON and BERTHOLF. The location of these sensors was optimized using hydro-structural calculation tools. A conversion matrix approach was used to transform global strains into internal loads and local strains; a validation of this procedure is performed against measurements. From the global HSM system it was found that the encountered wave heights are lower than assumed during design. Also the days at sea were lower than the design assumption. A forecast approach is presented based on the measured information. Global hull girder vibrations due to its flexibility increase the fatigue budget consumption by about 6%. With the acoustic emission HSM system installed on board USCGC MELLON the presence of a high AE activity source was demonstrated. This source is typical of a fatigue crack of a size which may be verified by non-destructive testing methods. The views expressed herein are those of the authors and are not to be construed as official or reflecting the views of the Commandant or of the U.S. Coast Guard.
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