T Roberts

The effect of slamming and whipping on the fatigue life of a high speed catamaran

Abstract In order to optimise the structural design of lightweight high-speed vessels, knowledge is required of the effect of sea loads on their structures, with respect to both ultimate strength and fatigue life. This paper reports on an investigation into the influence of slamming and whipping on the fatigue life of a large high-speed catamaran. Full-scale measurements of slam events were conducted on a 98m Incat catamaran to investigate its slamming behaviour in a variety of sea conditions. The full scale results were then used to determine the influence of the presence of slam events on fatigue life. In addition, the effects of significant wave height, slam occurrence rates, slam peak stresses and whipping behaviour on fatigue life were examined. The fatigue life was found to reduce significantly with the presence of slam events. Therefore, a reduction in slamming may prolong the fatigue life of a vessel markedly. The fatigue life was found to reduce significantly as wave height increases, as slam rate increases and slam peak stress increases. However, small slam events were found to have little or no influence on fatigue life. Whipping behaviour, in particular decay coefficient, may also strongly influence fatigue life.

Global and Slam Loads for a Large Wavepiercing Catamaran Design

Abstract Prediction of design loads for small high speed commercial craft has traditionally been governed by classification society, rule based formulae. Larger high speed vessels, particularly those greater than 100 m in length, require validation of rule based design loads. Validation methods include model tests or hydrodynamic studies using computer based motion and loads software. This paper presents the results of studies into the calculation of global loads for a 112 m, wavepiercing catamaran design. In particular, the preliminary design approach for assessment of loads is discussed. Longitudinal bending and pitch connection moment loads are derived from linear and nonlinear motion and loads software packages FASTSEA, SWAN and BESTSEA. The computed results are compared with rule based loads and empirically derived loads based on full-scale measurements from similar craft. Local wet deck slamming loads are discussed with reference to current rule based formulae and results of full scale tests on similar vessels. The effect of high local slam loads is discussed and how these loads may influence the design process. Conclusions are drawn on the presented results concerning the interaction of global and local loads in the design of a 112 m wavepiercing catamaran for Incat shipbuilders in Hobart, Australia.

Vibration of High-speed Ship Frames

Impact testing of a typical high-speed ship section has shown that the mode of vibration most likely to fall in the frequency range of excitation resulting from propeller or rotor blade passing is that where the ship frame rocks in a fore and aft direction about its base connection to the hull plate. This vibration has significant amplitude to either side of the keel and it is found that connection between the inboard and outboard sides is weak. As a consequence, vibration can occur in two modes in which the opposite sides of the hull either move together or in antiphase. This leads to the structural strain energy in the two modes being slightly different, the mode with rather less strain energy having slightly lower frequency. Because the frequencies of the two modes are close together, the transient response exhibits beating in which vibration energy is exchanged between the two sides of the hull at the low beat frequency. Vibration of the ship frames in this manner appears to have been the cause of minor weld cracking where the top of stiffeners pass through cutouts in the web of the ship frames within the fuel tank areas of the hull.