Gabriele Bulian

Smoothed particle hydrodynamics (SPH) simulation of a tuned liquid damper

The roll motion response of a single degree of freedom (SDOF) structural system to which a rigid rectangular partially filled liquid tank has been attached is considered. The SDOF structural system with the empty tank is first described with a mathematical model and this model is validated by performing decay experiments as well as experiments in which periodic excitations are applied to the system. The responses are accurately predicted by the model. The accuracy of these predictions allows us to study both experimentally and numerically, with weakly compressible SPH, the performance of the partially filled tank as a tuned liquid damper (TLD). The sloshing flows inside the tank comprise the onset of breaking waves which make the TLDs devices extremely difficult to model, especially for the potential flow multimodal approaches commonly used to simulate these sorts of coupled systems. In order to characterise the wave breaking effects on the response curves, tests have been performed with liquids of different viscosity, the increasing viscosity preventing the onset of breaking waves. The capabilities of SPH to treat this coupling problem are assessed and the results show that SPH is able to capture a substantial part of the physics involved in the addressed phenomena but further work remains still to be done relating to a more accurate treatment of the laminar viscosity and turbulence effects.

Review of Available Methods for Application to Second Level Vulnerability Criteria

The International Maritime Organization (IMO) has begun work on the development of next generation intact stability criteria. These criteria are likely to consist of several levels: from simple to complex. The first levels are expected to contain vulnerability criteria and are generally intended to identify if a vessel is vulnerable to a particular mode of stability failure. These vulnerability criteria may consist of relatively simple formulations, which are expected to be quite conservative to compensate for their simplicity. This paper reviews methods which may be applicable to the second level of vulnerability assessment, when simple but physics-based approaches are used to assess the modes of stability failure, including pure-loss of stability, parametric roll, surf-riding, and dead-ship condition.

A Numerical Feasibility Study of a Parametric Roll Advance Warning System

This work studies the practicality of using finite-time Lyapunov exponents (FTLEs) to detect the inception of parametric resonance for vessels operating in irregular longitudinal seas. Parametrically excited roll motion is modeled as a single-degree-of-freedom system, with nonlinear damping and restoring terms. FTLEs are numerically calculated at every integration time step. Using this numerical model of parametric roll and through tracking trends in the FTLE time series behavior, warnings of parametric roll are identified. This work serves as a proof of concept of the FTLE technique’s viability in detecting parametric resonance. The ultimate aim of the research contained in this paper, along with future work, is the development of a real-time, onboard aid to warn of impending danger allowing for avoidance of severe, even catastrophic, vessel instabilities.

Second Generation Intact Stability Criteria: on the validation of codes for direct stability assessment in the framework of an example application

ABSTRACT The Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety (SLF) of the International Maritime Organization (IMO) has undertaken the development of so-called “Second Generation Intact Stability Criteria” (SGISC) with the intention of providing a new set of rules covering those phenomena which are not properly covered by present, mostly semi-empirical, requirements. The first two levels of the envisioned 3+1 tiers structure of SGISC are so-called “vulnerability assessment” levels: most of the discussion has so far been dedicated to these levels. At the highest level there is the so-called “Direct Stability Assessment”, which is also strictly linked with the development of ship-specific “Operational Guidance”. Recent discussion on the topic of “Direct Stability Assessment” (DSA) has touched the issue of “validation” of numerical codes to be employed at this level. Stimulated by, and in view of, the ongoing IMO discussion, this paper presents the results of a recent series of experiments in beam waves (mono-/bichromatic, irregular) and associated simulations based on a 6-DOF blended code. Nonlinear harmonic and sub-harmonic resonances are observed and simulated.

Stability and roll motion of fast multihull vessels in beam waves

In this article, the results of a numerical and experimental study of the stability and roll motion of different advanced multihull vessels are presented. The study concerns the effect of the lines, transverse and longitudinal spacing for two catamarans – one with rounded bilge and one with a hard chine, an innovative bulbcat hull in comparison to the basic catamaran and two trimarans. The experimental results in beam waves are analysed by the means of different mathematical models.

Large amplitude rolling and strongly nonlinear behaviour of multihull ships in moderate beam waves

A trimaran and a pentamaran ship have been investigated to study the effect of transversal hull separation on roll motion in waves using typical outriggers with small beam and draft. The experiments in regular waves revealed the extreme sensitivity of these hull typologies to roll motion in beam waves, even in mild sea conditions. Two configurations of the trimaran and one of the pentamaran exhibited a multivalued roll response curve in the low frequency range. This is, to the best authors’ knowledge, the first published experimental evidence of bifurcations in roll motion of multihull ships, which are usually designed and selected for their superior stability characteristics and low rolling amplitude. Simulations have been performed by means of a linear seakeeping approach extended to take into account nonlinearirites for damping and restoring, and by using a nonlinear 1-DOF model for roll.

A Combined Experimental and SPH Approach to Sloshing and Ship Roll Motions

Passive anti-roll tanks have been used for a long time in ships to damp their roll motion. The coupled roll motion response of a single degree of freedom (SDOF) system to which a passive anti-roll tank has been attached is considered in the present paper. The performance of the anti-roll tank has been studied both experimentally and numerically, with weakly compressible SPH. The sloshing flows inside the tank comprise the onset of breaking waves. In order to characterise the wave breaking effects on the response curves, tests have been performed with liquids of different viscosity, the increasing viscosity preventing the onset of breaking waves. The capabilities of SPH to treat this coupling problem are assessed and the results show that SPH is able to capture a part of the physics involved in the addressed phenomena but further work remains still to be done.

Study of trimaran stability in longitudinal waves

The design constraints connected with speed, safety and environmental compatibility for waterborne transportation craft are often contradictory and, as a rule, lead to design compromises. In this paper, attention is focused on fast transportation operated by means of trimaran vessels. The need for energy saving and environmental sustainability leads to the reduction of the beam and the wetted surface of the outriggers as a general trend. In this paper, the behaviour of a trimaran in longitudinal regular waves is studied on an experimental basis as a function of speed for different wave conditions. The experimental results indicate an extreme sensitivity of trimarans of this type to the danger connected with the onset of parametric rolling.

Water bouncing robots: a first step toward large-scale water running robots

Robots running on water have attracted the attention of researchers in the last decades as an alternative to conventional aquatic propulsion mechanisms. Up to now, a large scale robot capable of running on water has not been realized. Bouncing on water is a prerequisite for running on water. For this reason, the development of a water bouncing robot represents a necessary first step. The paper presents the model of a 2-degree-of-freedom water bouncing robot inspired by the pogo-stick, a device for jumping off the ground in a standing position. An analytical model of the impact force between “robots foot” and water is provided for both water-entry and water-exit phases. Such a model has been integrated in a dynamic simulation of whole robot. The model represents a useful and general framework to gain an insight into the parameters that characterize the efficiency of robot.

Trimaran Vessels and Parametric Roll

Parametric roll can occur in case of trimaran vessels, in particular when the outriggers have limited draught and/or limited transversal separation. Due to the significant nonlinearities of the restoring moment and due to the often complex shape of the metacentric height in waves for this type of ships, parametric roll clearly shows quite peculiar characteristics. Starting by the description of a simplified 1-DOF nonlinear mathematical model for parametric roll in longitudinal regular waves the paper then describes how the Floquet theory can be directly applied to the linearized model in order to determine the instability regions for the upright position. An example of calculation of stability map for the upright position is provided. Also another example for prediction of rolling amplitude in the nonlinear range to highlight the peculiar aspects relevant to the considered trimaran configuration is discussed. In both cases experimental data are also reported for comparison. Finally the paper describes how the position of the outriggers can be optimally chosen in order to minimize the variations of metacentric height in waves.