Feature 1 | CONSTRUCTION MATERIALS Optimising designs
Frazer-Nash marine engineering expert, Richard Lawrence, explains how high-speed lifeboat designs can be structurally optimised to increase damage tolerance, and ensure that hulls are able to withstand severe loadings in service.
T
he manufacture and design of Royal National Lifeboat Institution (RNLI) all-weather
lifeboats (ALB) has fundamentally changed over the last 15 years, leading to vastly increased speed and power capabilities, which enable the charity’s volunteer crews to reach casualties at sea more quickly and efficiently. With faster boats available, emphasis is
now being placed on elements of structural and ergonomic design that allow the crews to achieve the full potential of these modern craft. The new design philosophy focuses on lighter weight and stiffer composite hulls, improved crew space and seat designs and advanced integrated bridge systems. The RNLI’s high-speed lifesaving
operations put increasing pressure on the ultimate strength and structural durability of its lifeboats. In particular, the advanced design of modern high-speed rescue craft may put hull structures under loadings that
are closer than predicted to their design limits. The extreme environmental loads, and difficulty of predicting the situations in which lifeboats operate, has led to a preference for a design methodology that goes beyond classification society rules. For the last ten years Frazer-Nash
Consultancy has been working with the RNLI to assess the effects of these new design and manufacturing processes on the structural durability of new classes of light-weight, high-speed rescue craft. The RNLI wants to ensure that its fleet of boats is assessed rigorously to ensure that the optimum operating conditions are created for the crews on board. The objective is to increase the capability of crew to gather information, and enhance their capacity to carry out search and rescue in the safest manner possible. In collaboration with the RNLI,
Frazer-Nash ran a series of assessments on a new class of lifeboat in order to understand the implications of modern
Crew onboard a RNLI lifeboat (credit: Nicholas Leach).
manufacturing techniques – and to establish the loads that modern lifeboats can ultimately withstand.
In-service environment Lighter weight and stiffer composite hulls, improved crew space, better seat designs, and advanced hull forms are allowing high-speed rescue craft to be driven harder. Conventional design approaches have proven to be inadequate for predicting the extreme operating environments the RNLI crews encounter during their normal operations. For some time now, Frazer-Nash has been working with the RNLI on the development of new modelling methods that correctly predict the structural loads its boats encounter in service. The simulation process involves the
determination of the boat’s hydrodynamic properties, using either tank test data or Computational Fluid Dynamics (CFD) models. These performance characteristics are used to define a dynamic Finite Element Analysis (FEA) model of the boat response to rough seas. Sea state definitions are generated from the normal data provided by measurements taken around the coasts of the British Isles, and reproduced in the model in the form of wave energy spectra. This type of analysis produces detailed predictions of the peak stresses in the hull, and allows the designer to undertake more detailed optimisation of the structural design of the vessel. The technique has proven particularly useful when new materials are considered, as the effect of features such as the lay-up of composite materials can be taken into account. With this detailed information,
gathered from simulations of a huge range of different sea states and events, a comprehensive picture of the loads to
20 Ship & Boat International November/December 2008
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