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.
Heave, Roll And Pitch Damping of a Deepwater CALM Buoy With a Skirt
The heave, roll and pitch damping of a CALM buoy contains linear contributions (wave radiation) and quadratic contributions (drag loads). The linear damping can be determined by diffraction-radiation calculations, but for the quadratic contributions model test data are required. In this paper a semi-empirical model is formulated for the calculation of the quadratic heave, pitch and roll damping. Forced oscillation tests were carried out using a CALM buoy model with several different skirts. Different oscillation amplitudes and frequencies were considered. From the test results added mass and damping coefficients were determined. These results were used to validate the semi-empirical model.
Viscous Free Surface Numerical Simulations of Oscillating SWATH Ship Sections
This study presents a solution method based on a fully viscous non-linear flow solver used to evaluate the two dimensional hydrodynamic coefficients of damping and added mass appearing in the generalized linear equations of motion of a ship in a seaway. Experiments made on a circular and a SWATH section were used to validate the direct numerical solver of Navier Stokes equations, based on finite volume technique which is implemented using the open-source libraries of OpenFOAM. Numerical results agree very well with experimental measurements for these two quite different section shapes, validating the satisfactory accuracy of the proposed method.