ASV operability at sea: Size matters as much as hull form design
The influence of vessel size on the performance of a second generation Autonomous Surface Vessel (ASV) in irregular head waves is investigated. The hull of the SWATH type ASV is optimized using a combination of high fidelity hydrodynamic numerical models to achieve minimal propulsion powering requirement as well as lower motions in waves compared to state of the art catamaran type (CAT) surface vessels currently referred to as first generation. Autonomous Underwater Vehicles (AUV) assistance at sea requires the ASV to possess exceptional seakeeping qualities which hull optimization procedures aim to achieve. However, the performance of the ASV may be sensitive to the most probable wave period of the ambient environment due to the asymptotic peaks typical of SWATH vessel responses in long waves. It is the objective of this study to assess the seakeeping behavior of five different sizes of the ASV at sea to determine the limiting sea state for each vessel size. Parameters such as the significant heave and pitch motions are used to examine the operability of the vessels in five sea states. The results highlight the importance of selecting appropriate vessel size based on the predominant weather conditions of the location.
Influence of viscous effects on numerical prediction of motions of SWATH vessels in waves
The accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts, such as Autonomous Surface Vessels (ASV). Two different numerical methods to predict motions of a SWATH-ASV are considered: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and new a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous flow method are critically discussed against those obtained by potential flow strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, RAOs and motions response in irregular waves at zero speed. Some relevant conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH vessels like the one presented: amplitude of the pitch and heave motions predicted at the resonance frequency differ of 20% respectively and 50%; in this respect, the hybrid method with fully non-linear, viscous free surface calculation of the radiation forces turns out to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations.
Representation of systems disturbed by wide band noise
In this paper, a method of handling and working with wide band noise is developed. We represent wide band noise as a distributed delay of white noise and use it to reduce a nonlinear system disturbed by wide band noise to a nonlinear system disturbed by white noise. An application of this reduction to a nonlinear filtering problem under a wide band noise disturbance is discussed.