The Surface Forces Logistics Center’s Naval Architecture Section was tasked to review the operational history and conditions of the US Coast Guard’s 47 MLBs and determine if they will have sufficient remaining hull structural fatigue life to extend their service life to 50 years as part of a business case analysis. In this process, a fatigue analysis approach was developed to provide guidance to support a feasibility level decision.
Fatigue analysis of small craft is not common and the Naval Architecture Section developed a relatively quick and easy method for performing the analysis. The analysis utilized regional wave buoy information, operation profiles, operating hours from Coast Guard databases, ABS and Savitsky methods for predicting pressure and slamming loads, and simple finite element models to predict the structural response. An approach is presented to infer long term loading history using the statistical relationships of the Rayleigh statistics and operational wave conditions.
In this analysis, the boats were categorized into three different operating regions, East Coast, West Coast, and Great Lakes based on differences in wave environment and further subdivided based on a number of annual operating hours. Results of the analysis indicate that the 47 MLBs will have a fatigue life of over 25 years from delivery; however, approximately one third of the 47 MLBs will develop fatigue cracks before 50 years of service based on current utilization rates.
Recommendations are provided to evenly distributing the hours of operation within districts and by home port rotations between different regions on a 10 to 15 year rotation cycle. If fully implemented by the operators and maintainers, this approach would extend the hull structure service life of all of the 47 MLBs to 50 years without requiring structural modifications.
