Fatigue limit state design of fast boats

Abstract

Typically, the dominant load for the structural design of a planing hull is slamming, while sailing fast at head seas. The slamming is a violent fluid structure interaction, where dynamics, hydro-elasticity, and nonlinear structural effects may be important. Considering these effects in a rational design may reduce the scantlings as compare to design by rules, which assess the scantlings by applying a quasi-static design pressure and linear beam theory. More substantial reduction of the scantlings is expected in a limit state design, where extreme loading conditions and flexible structure increase the importance of hydro-elasticity and non-linearity. While applied rules specify allowable stresses that typically provide a service life of 20 years, without the need to check for fatigue; rational limit state design must incorporate the Fatigue Limit State (FLS). This paper presents a rational limit state design method for planing hulls, which apply hydro-elastic nonlinear dynamic structural analysis, to assess the fatigue service life, based on an operation plan specified by the client. The method integrates design rules with direct analysis of fluid-structure interaction and principles of representation of random sea, to a practical design procedure. Furthermore, a parametric analysis is applied to provide a database of 450 cases that may be used by designers for the assessment of fatigue service life in a practical range of geometrical parameters and service conditions. A design example demonstrates the potential of the method to reduce the scantlings.