Determination of ship roll damping coefficients by a differential evolution algorithm

The execution of the so-called extinction tests represents the classical experimental method used to estimate the damping of an oscillatory system. For the specific case of ship roll motion, the roll decay tests are carried out at model-scale in a hydrodynamic basin. During these tests, the vessel is posed in an imbalance condition by an external moment and, after the release, the motion decays to the equilibrium condition. When the damping is far below the critical one, the transient decay is oscillatory. Here a new methodology is presented to determine the damping coefficients by fitting the roll decay curves directly, using a least-square fitting through a differential evolution algorithm of global optimisation. The results obtained with this methodology are compared with the predictions from standard methods. This kind of approach seems to be very promising when the motion model of the system under investigation is established with any level of non-linearities included. The usage of the fitting procedure on the approximate analytic solution of the differential equation of motion demonstrates the flexibility of the method. As a benchmark example, two experimentally measured roll extinction curves have been considered and suitably fitted. The newly predicted results, compared with the ones obtained from standard roll decay analysis, show that the algorithm is capable to perform a good regression on the experimental data

Station Keeping and Seakeeping in Offshore Vessels Design

The major guidelines for examining and verifying the station keeping and seakeeping capabilities of offshore vessels are here presented together with a critical discussion of the main items to be considered in early design stages. To properly focus the sequential procedure of analysis, the experience gained by the author during design and testing of several offshore vessels will be discussed. Numerical examples will support the presentation

Dynamic Absorbers for an Externally Excited Pendulum

To an externally excited pendulum a dynamic absorber, which can move in transverse or longitudinal direction, is added. The quenching efficiency of two different systems is studied and compared. The absorber can be very efficient for slightly damped systems if it is correctly tuned. This is especially true when only small absorber mass can be adopted

Considerazioni sui sistemi di riferimento nello studio dei moti nave

In questo lavoro si prende in considerazione il problema dei moti nave con particolare riguardo al sistema di riferimento pi\uf9 adatto per la loro descrizione. Introdotto un riferimento fisso ed uno mobile legato alla nave, si descrive la posizione di quest'ultimo mediante gli angoli di Eulero. Vengono poi fornite le equazioni che permettono di ottenere, per ogni posizione della nave, le grandezze usualmente impiegate nei calcoli di geometria e statica della nave. Si ricavano, infine, le equazioni differenziali del moto e si esplicitano i termini che rappresentano l'interazione con il fluido, con particolare riguardo alla spinta idrostatica. Le equazioni vengono generalizzate per descrivere i moti di nave con falla con il metodo dell'imbarco di peso

Stabilit\ue0 di forma di bolle in risonanza ultrarmonica

In questa memoria viene studiata la stabilit\ue0 di forma sferica per una bolla di gas in acqua soggetta ad un campo di pressione acustica sinusoidale nelle zone di frequenza corrispondenti alla prima e seconda risonanza ultrarmonica dell\u2019oscillazione radiale. Ci\uf2 permette di ottenere la soglia di instabilit\ue0 di forma in funzione dell\u2019intensit\ue0 dell\u2019eccitazione

Metodi approssimati per la previsione dei moti nave, della resistenza aggiunta, e dei carichi idrodinamici

Metodi di Shiffrin, Moh