Enhanced design and operation of small high-speed craft

Abstract

This thesis presents a functional approach to improve the structural design and operation of small high-speed craft through the direct calculation of seakeeping loads and structural response. A novel procedure, combining computational and experimental methods, is developed to superimpose static, slowly varying and transient loads on a structural model of the craft. The approach is validated on a 17-metre search and rescue vessel with a maximum operating speed of 25 knots. Current structural design methods for small craft follow a relatively simplistic process and do not explicitly calculate the loads sustained and the response of the structure. To better inform the level of structural adequacy in different possible operating conditions of the craft, both loads and structural response need to be explicitly analysed. In this thesis, hydrostatic and wave loads are determined with hydrodynamic simulations, validated against towing tank tests and seakeeping trials. Slamming loads are estimated by matching strain measurements, taken during the seakeeping trials, to local finite element models of the structure. The single load components are superimposed in the generation of a load case, which represents the extreme loads expected to be sustained within a given return period. The structural response is computed using a global finite element model of the vessel and results are presented as graphs showing the structural response as a function of vessel speed and sea state severity. These graphs contribute to defining the ‘operational envelope’ of the craft, which informs designers and crews on the limits to speed for the safe operation in waves. The study finds broad applicability in the design of small high-speed craft. The approach has the potential to provide designers with better control over the structural design and maintainers with a vessel-specific understanding of the lifetime load patterns and of the structural behaviour, hence aiding targeted inspections. It also increases the awareness of the implications of high speed to the structural integrity of the vessel, enabling coxswains to make informed decisions on the risks being taken and enhance the safety of the on-board crew.