Cesare Mario Rizzo

EFD and CFD Design and Analysis of a Propeller in Decelerating Duct

Ducted propellers, in decelerating duct configuration, may represent a possible solution for the designer to reduce cavitation and its side effects, that is, induced pressures and radiated noise; however, their design still presents challenges, due to the complex evaluation of the decelerating duct effects and to the limited amount of available experimental information. In the present paper, a hybrid design approach, adopting a coupled lifting line/panel method solver and a successive refinement with panel solver and optimization techniques, is presented. In order to validate this procedure and provide information about these propulsors, experimental results at towing tank and cavitation tunnel are compared with numerical predictions. Moreover, additional results obtained by means of a commercial RANS solver, not directly adopted in the design loop, are also presented, allowing to stress the relative merits and shortcomings of the different numerical approaches.

An analytical/numerical study on buckling behaviour of typical composite top hat stiffened panels

This paper deals with the buckling strength of typical ship stiffened panels built in fibreglass composite laminates. The results obtained applying analytical formulations based on equilibrium and potential energy equations are compared with the ones obtained by linearised eigenvalue analyses of rather detailed finite-element models. Analytical formulations consider an elementary panel only, accounting for the boundary effects of stiffeners surrounding its edges, while FE models fully describe the interactions among plate and stiffeners as a whole. A sensitivity analysis to highlight the most influencing parameters in the problem is also presented. So far no systematic scantling assessment of buckling behaviour of hull shell and deck panels seems to be recommended by classification societies’ rules for pleasure crafts made in composite materials. Thus, this paper aims at illustrating the application limits of analytical methods, natural candidates to become the basis of rule requirements. Difficulties in the evaluation of torsion behaviour of stiffeners and of corresponding buckling modes involving both the plate and the stiffeners are highlighted. Some concluding suggestions for rule scantling assessment formulations are eventually provided.

Fatigue Strength Assessment of Propellers by Means of Weakly Coupled CFD and FEM Analyses

Propeller design considerably evolved during years, continuously imposing new challenges to the designer, from usual high efficiency and avoidance of erosive cavitation up to reduction of radiated noise and pressure pulses for “high added value” ships. New more stringent limits, together with a general requirement for performances optimization, leads to the need of a continuous review of design procedures which, in some cases, still rely on rather “old-fashioned” approaches.The aim of this paper is to analyse in detail the importance of considering fatigue phenomena in the propeller design, proposing a procedure for the evaluation in time of the stress field acting on a blade during one revolution, which is then considered for fatigue assessment.Copyright

Fatigue assessment of bulb stiffener joints according to local approaches

This paper deals with the fatigue strength assessment of attachments of bulb plate stiffeners (Holland profile), typical of ship structures, applying local approaches proposed by the International Institute of Welding (IIW) and in open literature. The application of these methods in the captioned cases is not always straightforward nor sometimes possible, neither effective. Difficulties in following the suggestions of IIW for the finite element (FE) modelling of relatively complex geometries are shown. Basically, the bulb itself is a three-dimensional component interacting with two-dimensional shells surrounding it: the three-dimensional versus two-dimensional mismatch is the main source of complexity. Since a reference thickness of the bulb cannot be identified, even the identification of stress points in post-processing of FE analyses is difficult, often being a cause of scatter and/or disagreement among different approaches. Comparisons among approaches and with available experimental data of tests carried out in the Ship Structures Laboratory of the University of Genova and in the Institute of Ship Structural Design and Analysis of the Hamburg University of Technology provide a general view of this topic and offer a critical review of the application of local approaches in rather challenging test cases, evaluating, as far as possible, effects of weld geometry on fatigue strength.

Dynamic buckling of masts of large sail ships

The design of sail systems is a fascinating topic that naval architects have been facing since many centuries and nowadays is revived because of the increasing size of modern pleasure and racing yachts. It is hence surprising that empiricism largely drives mast and rigging design even at the present time. Actually, while structural dynamics is explicitly considered in current scantling design of hull structures, this is not usually the case for mast and rigging. However, numerical simulations can now be successfully applied to assess effects of load variations in time. Considering the governing limit states and the typical behaviour of a pre-tensioned slender structure, the dynamic buckling of the bottom panel of a typical large mast is evaluated in this paper, showing significant differences from the widely applied quasi-static approach. The obtained results provide a new perspective for the scantling assessment of sail systems, overcoming the current empirical and prescriptive approach proposed by rules of classification societies and international standards.

A Review of Ship Surveys Practices and of Marine Casualties Partly Due to Aging Effects

The first part of this paper overviews the present international regulatory regime and the future expected developments of ship surveys, from the viewpoint of methods and criteria, identifying different degradation phenomena mainly related to aging of structures and hull equipment. The current onboard practice is recalled, both of mandatory surveys (including class surveys and particularly the new requirements introduced by the IACS Common Structural Rules) and of non mandatory industry-driven inspections. Then, reviews of few databases of marine casualties, available to authors, are presented aiming at assessing the role of aging effects in the marine accidents. Even if it should be admitted that available statistical samples are not always complete and accurate, the intent is to identify trends in the pattern of structural failures and to develop recommendations for improving the inspection regimes and procedures for condition assessment analyses by which to reduce the number and severity of such occurrences.Copyright

On mooring line tension and fatigue prediction for offshore vertical axis wind turbines: a comparison of lumped-mass and quasi-static approaches

Despite several potential advantages, relatively few studies and design support tools have been developed for floating vertical axis wind turbines. Due to the substantial aerodynamics differences, the analyses of vertical axis wind turbine on floating structures cannot be easily extended from what have been already done for horizontal axis wind turbines. Therefore, the main aim of the present work is to compare the dynamic response of the floating offshore wind turbine system adopting two different mooring dynamics approaches. Two versions of the in-house aero-hydro-mooring coupled model of dynamics for floating vertical axis wind turbine (FloVAWT) have been used, employing a mooring quasi-static model, which solves the equations using an energetic approach, and a modified version of floating vertical axis wind turbine, which instead couples with the lumped mass mooring line model MoorDyn. The results, in terms of mooring line tension, fatigue and response in frequency have been obtained and analysed, based on a 5 MW Darrieus type rotor supported by the OC4-DeepCwind semisubmersible.

Special issue on passenger ships and yachts

When many months ago the Editor in Chief of the journal asked me to be the guest editor of a Special Issue about yachts, it seemed to me too challenging although my teaching activities were at that time mostly devoted to the Pleasure Crafts and Yachts Design courses of the University of Genova, Italy. Later on, when passenger ships were additionally included in the scope of the Special Issue, I was really scared to broaden the subject, even if the continuously increasing size of pleasure crafts make them rather similar to medium sized cruise ships. However, some particular issues are still peculiar of the yacht industry and, especially as far as the structural design is concerned, dedicated procedures and regulations are needed, deviating from the ones currently applied to large cruise ships. This Special Issue opens with general views on megayachts structural related topics and classification requirements for the structural design of cruise passenger ships. Afterwards, a few specific structural problems are dealt with. In the third paper, a smart idea to speed up structural analysis of passenger ships takes advantage of capabilities of modern computation facilities. The main concepts of the classical laminate theory used for the structural analysis of composites hulls is transferred to the structural idealization of cruise ship structures. Implementing similar procedures into the design process may well improve design and production efficiency. Windows are weak components of both passenger ships and mega-yachts. The subject is addressed in the fourth paper, showing that numerical analyses validated by experimental testing can lead to useful simplified formulations, which are now available to understand in detail the structural behavior of complex hull fittings. The concept of effective breadth is a basic one in ship structural design, conveniently applied by naval architects since decades either in preliminary and advanced design of ship structures. Though, its validity is still under discussion for some special cases, e.g. marine composite structures. A novel definition of effective breadth is proposed in the fifth paper, based on strains instead of stresses, for the case of composite stiffened panels. The above mentioned papers highlight that modern methodologies applied to ship structural design and supported by nowadays available computation tools can be used to improve the traditional scantling approaches. Moreover, academia should strengthen the cooperation with industry in developing simplified and cost-effective scantling approaches, though based on sound engineering principles and validated by testing and analyses. Dynamic buckling is the subject of the sixth paper presented in this Special Issue: it represents a rather new topic not only regarding structural analysis of ships but also of other vehicles and structures. Indeed, the captioned case is typical of large sail ships and results are quite surprising, as often happens when buckling is involved. It must be admitted that sail ships are a niche, though a wonderful one. Hence, scantling of sails systems was mainly developed according to rules of thumb and empiricism. In this case, it was shown that safety factors currently applied can be reviewed provided the involved phenomena are fully understood. Comfort and noise issues are becoming more and more important in the structural design of ships. The papers you are going to read show the efforts made in mitigating underwater radiated noise, which represents a subject that is now going to be fully covered by rules and standards. One paper consists in a general review of methods and approaches outlined by a worldwide recognized expert, whereas the other one presents an advanced design application concerning a cruise ship proposed by a young researcher. It is interesting to note that noise issues have been faced by naval ship designers in the first place. As a matter of fact, shipyards involved in building very large motor yachts in last years are those that traditionally build ships and crafts for the national Navies. This trend can be noted in both Northern and Southern Europe, as well as in the U.S.A. The service conditions of a cruise ship and a pleasure craft are finally discussed in the last two papers of this Special Issue, dealing with passive fire protection, which largely governs also the passenger ship structural design, and with the control of motions of high speed planing boats. Thus, involving performance and safety issues. These papers come from countries where interest in passenger ship and yacht engineering is still growing up. Eventually, it is worth noting that this Special Issue has not been derived from conference papers. Indeed, most pa-

On the shear lag effective breadth concept for composite hull structures

The aim of this paper is to study the concept of shear lag effective breadth of plating, widely applied in the scantling assessments of metallic ship structures, for the special case of composite hull stiffened plating. Indeed, geometries and material behaviour are rather different and the definition of the effective breadth is worthy of investigation. A broad literature survey highlighted that the case of composites is almost not covered by previous studies and that only simplified empirical formulations are proposed for effective breadth evaluations in the rules of classification societies. Thus, suitable finite element models were created and validated to investigate the behaviour of the effective breadth of stiffened laminates when varying geometrical and other typical parameters of composite made ship structures. It was found useful to switch from stresses to strains in the effective breadth definition, considering the anisotropy of the material. An extensive sensitivity analysis allowed assessing the effect of parameters governing the phenomenon. Eventually, regression formulae are proposed summarising the outcomes and possibly applicable in design scantling practice.