Structural Optimization: LBR-5 Software and its application

As well known already in earliest phases of a ship project many aspects and choices depend on the structural design which has been defined only at a preliminary level. This trend appears to be similar for merchant ships, passenger ships and motor yachts. Only in the final part of the project some shipyards begin to apply optimization processes, more or less sophisticated, in order to refine the structural design in view of reducing the weight and/or the construction cost. The weight in particular has a very important impact on pleasure vessels, both motor and sailing. Structural modifications suggested by such optimization procedures imply a number of second order changes in related items such as plant, outfitting and others. As a consequence the structural optimization could be particularly useful if it can be applied during the first stages of the project, this way avoiding very expensive time losses and changes caused by any structural modifications. The LBR-5 software has been successfully used to optimize hull structures of a 60 meters mega yacht. Differently from large ships, the mega yacht has not a “cylindrical body” in the central part of the hull. So, a new module of the software has been used in order to analyze several sections of the ship and to perform an overall optimization. For this application the optimization analysis has been carried out by different approaches: assuming the weight as the objective function a gain of about 8% has been achieved, while a least cost optimization allowed a reduction of 15%

Parametric Evaluation of the Effective Breadth for GRP Beams with FEM Calculation

The direct scantling of reinforced plates can be performed by simplified grillage theory or by FEM codes. The first approach gives satisfactory results and better matches Classification Society Rules. The only problem is represented by the determination of beam geometry which implies the determination of the shell portion cooperating with the reinforcement. The width of this strip, called \u201cEffective Breadth Be\u201d, depends on several geometric characteristics like the shell thickness t, the beam span L, the beam distance B and the dimensions of the beam transverse section. In the case of steel plates the relationships between the above parameters and Be is well known and several procedures are available in literature and provided by the Classification Societies rules for determining the effective breadth. More uncertainties exists for what GRP structures are concerned. In this case the only references are represented by semi empirical Classification Societies formulas which give Be just as a function of the shell thickness. For this reason, and in cooperation with the Italian Classification Society, an investigation on this matter has been started by the author considering both single skin and sandwich shells. The analysis has been performed by FEM calculations on a number of numerical models each of them obtained by a different combination of the values of the considered parameters that are: the shell thickness t, the L/B ratio and the transverse section dimensions of the beam. The results have been presented on diagrams with Be/B ratio versus L/B ratio with different combination of the evaluated parameters. A comparison of the curves with the values provided by Classification Societies has been performed as well. Finally all the results have been utilised to obtain a regression curve by which it is possible to evaluate the Effective Breadth by considering all the considered geometric characteristics

Optimization of Hull Structures for a 60 meters Mega Yacht

peer reviewedAs well known already in earliest phases of a ship project many aspects and choices depend on the structural design which has been defined only at a preliminary level. This trend appears to be similar for merchant ships, passenger ships and motor yachts. Only in the final part of the project some shipyards begin to apply optimization processes, more or less sophisticated, in order to refine the structural design in view of reducing the weight and/or the construction cost. The weight in particular has a very important impact on pleasure vessels, both motor and sailing. Structural modifications suggested by such optimization procedures imply a number of second order changes in related items such as plant, outfitting and others. As a consequence the structural optimization could be particularly useful if it can be applied during the first stages of the project, this way avoiding very expensive time losses and changes caused by any structural modifications. In this paper the structural optimization of a 60 meters megayacht is presented, performed by LBR-5 code developed by the University of Liege. This code is an optimization tool specifically designed for structures composed by stiffened plates and stiffened cylindrical shells. The optimal solution is reached through an optimization algorithm based on convex linearization and a dual approach. The LBR-5 software has been successfully utilized to optimize hull structures of a 60 meters megayacht. Differently from large ships, the mega yacht has not a “cylindrical body” in the central part of the hull. So, a new module of the software has been used in order to analyze several sections of the ship and to perform an overall optimization. For this application the optimization analysis has been carried out by different approaches: assuming the weight as the objective function a gain of about 8% has been achieved, while a least cost optimization allowed a reduction of 15%

Vibration analysis for the comfort assessment of superyachts

4siComfort levels on modern superyachts have recently been the object of specific attention of the most important Classification Societies, which issued new rules and regulations for evaluating noise and vibration maximum levels. These rules are named “Comfort Class Rules” and set the general criteria for noise and vibration measurements in different vessels’ areas, as well as the maximum noise and vibration limit values. As far as the vibration assessment is concerned, the Comfort Class Rules follow either the ISO 6954:1984 standard or the ISO 6954:2000. After an introduction to these relevant standards, the authors herein present a procedure developed to predict the vibration levels on ships. This procedure builds on finite element linear dynamic analysis and is applied to predict the vibration levels on a 60 m superyacht considered as a case study. The results of the numerical simulations are then benchmarked against experimental data acquired during the sea trial of the vessel. This analysis also allows the authors to evaluate the global damping ratio to be used by designers in the vibration analysis of superyachts.partially_openopenPais, T., Moro, L., Boote, D., Biot, M.Pais, T.; Moro, L.; Boote, D.; Biot, M

A simplified approach to the dynamic effective thickness of laminated glass for ships and passenger yachts

In the last years, after the economic crisis that modified the super and megayacht market and construction, the project focus of shipyards and owners has been moved from the research of higher performances and whimsical exteriors to more classic aesthetics and higher level of onboard comfort. This first aspect has led to the use of larger openings on superstructures\u2019 sides in order to enlarge windows and bring more natural light onboard vessels. The request of more comfortable units has granted a strong effort to find new solutions in order to reduce the noise and vibration level, dealing in particular to the structural elements with higher noise radiations, such as dampers, pillars and windows. The aforementioned trends is reflected on a more intensive use of glazed windows onboard, that have led to non-trivial problem of both structural response (since the structural component are reduced in order to enlarge opening) and NVH assessment, considering that glued glass panes act as an harmonic speakers, having natural frequencies closed to ones of the main superyacht excitations. In this paper, a simplified interactive design procedure for laminated glass used onboard super and megayacht is provided; in particular, authors developed a model for the calculation of a simplified dynamic effective thickness of laminated glasses, in order to increase the reliability of the modelling of glazed panes, with the same dynamic properties, in global numerical models of vessels

Stress Analysis of Bulk Carrier Hatch Corners

One of the most frequent damage cause for bulk carrier ships is represented by high stress concentrations occurring in deck plating close to hatch corners in way of coaming stay. In this area many fractures are found during surveys which are generated mainly by three types of problems: \u2022 stress concentration in correspondence of deck large openings; \u2022 bad design of stays; \u2022 different thickness between transversal and external deck plates. The typical approach followed by IACS classification societies consists in a wide range of qualitative suggestion (transversal stiffening for transversal deck strip, elliptical hatch corner etc.) and a very simple formula providing the extra thickness of the hatch corner compulsory insert. This solution seems to be inadequate to face this phenomenon and a better knowledge is mandatory to understand which parameters are more influent on stress distribution and concentration. The purpose of the analysis herein presented is to identify most important parameters and to study their influence, in order to develop a more sophisticate procedure to be used in preliminary design stage. The investigation has been developed starting from the analysis of five different bulk carriers, selected on the basis of their geometric and structural properties, and modeled with FEM software Leonardo hull 3D v. 2.4.1. The analysis has been divided into two sub-problems: \u2022 study of b/B ratio influence (hatch breadth versus ship breadth); \u2022 study of lw/b ratio influence (cross deck length versus hatch breadt).. Two different series of FEM models have been realized: Global models of the hull extended to a portion of three cargo holds; Simplified models with increasing refinement degree. All global models have a relatively coarse mesh and are loaded with a wide number of loading condition (homogeneous, heavy and light ballast, alternate ore) corresponding to relevant RINA load cases in order to maximize the effects. For what simplified models are concerned four different types of models have been analysed: a) The first model consists in a simple plate with a rectangular opening and two longitudinal box structures simulating the wing tanks. All the parts of the model have a constant thickness. b) A second series of models has then been created, similar to the first one, adding a coaming around the hatch opening. As in the first case the thickness has been maintained constant. c) The third series is similar to the first one but the thicknesses of the lateral deck strip and of the cross deck have different values, according to the real case. d) The fourth series is similar to the second one. Again deck plates have different thicknesses. For all the models the results have been processed to calculate stress concentration factors \u201ck\u201d (defined as longitudinal stress close to hatch corner versus theoretical hull girder stress) for all selected loading conditions. From the \u201ck\u201d coefficients obtained for every model, a \u201cstress concentration surface\u201d, as a function of b/B and w/b, has been drawn and expressed by a simplified formula

Fatigue Collapse of Ship Structural Components: a Case Study

Most damages to structural components of ships are due to fatigue phenomena. Particular care should then be devoted to fatigue evaluation since the first phases of any ship project. All main Classification Societies have in their Rules simplified procedures for fatigue evaluation, in order to make all designers able to perform structure scantling complying with fatigue principles. Apart from design necessities, there is another aspect for which it becomes important to assess and evaluate fatigue phenomena; this happens when it is necessary to ascertain the causes of occurred an accident. While for design purposes the aim is to calculate the fatigue life of a structural detail based on a probabilistic approach, in the case of an occurred accident the driving philosophy is to accurately analyse the life history of the structure and to calculate, by a deterministic approach, the cumulative damage of the component up to the event time. The aim of this paper is to set up a procedure to analyse and verify the causes of the failure of a ship structure component due to fatigue phenomena

Developments of Tuned Mass Damper for yacht structures

The comfort level on board superyachts depends on a variety of parameters all dealing with the human perception of various phenomena such vibrations, noise, ship motions and son on. In this work, the attention has been strictly devoted to vibrations and on possible solutions to reduce them. This kind of analysis is a challenging task for designers and a relevant subject for shipyards that are seriously engaged to find solutions aimed at improving the quality of their products. In this perspective, current numerical methods represent a reliable predictive tool in the design phase to identify natural frequencies of the hull and local structures. The adoption of \ue2\u80\u9cTuned Mass Damper\ue2\u80\u9d system could be a suitable solution both at the design stage, when for the sake of weight containment heavy structure reinforcements should not be advisable and after construction when any other intervention is not possible. In this paper, the application of such device on decks of yacht is presented. TMD numerical models are created and tested to identify the main parameters characterizing a dynamic absorber, then two passive control devices, able to easily conform to the yacht deck geometry, are tuned to decrease the vibration level of the sun deck zone