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Structural analysis for the design of VLFS
Abstract
A review is presented here with respect to linear and nonlinear structural analyses for the design of very large floating structures (VLFS), with a particular focus on the pontoon-type VLFS, developed during the Mega-Float project in Japan. The role of structural analyses for the design of VLFS is first described through a structural design flow typical of VLFS and design limit states. Structural modeling techniques for hydroelastic global response analysis and a two-step approach for stress analysis of detailed structures, based on the results of global response analysis, are outlined. Regarding the assessment of structural safety for accidental loads, a simulation of airplane collision on VLFS is presented. Finally, a method of progressive collapse analysis of VLFS under abnormal wave loads, based on the idealized structural unit method (ISUM), is introduced.
... This model has been adopted by the Japanese Mega-Float project also [1,2]. Recently, the authors pointed out that the torsional sti!ness which is traditionally being used for the plane grillage model is incorrect, and proposed a modi"ed formula of torsional sti!ness [3]. However, the formulation did not have a sti! theoretical background. ...
... In the present study, the proposed formula of torsional sti!ness in Ref. [3] is derived based on the concept of equivalent strain energy. Then, a series of hydroelastic response analyses of pontoon-type VLFS test models [4,5] is carried out using grillage beam models and a homogeneous plate model. ...
... The proposed torsional sti!ness of Eq. (8) is two times as large as that given by the existing formula [3]. This is because only a half of the shear strain of Eq. (3) has been considered in the existing formula without considering the biaxial torsional deformation expressed by Eq. (2). ...
For the structural design of very large floating structures (VLFS) of several thousand meters long, a hierarchical system of structural analysis must be developed, in which an idealized structural modeling is employed for the global response analysis while the local structural response is analyzed using a zooming technique. In this study, two types of grillage beam models, a plane grillage model and a sandwich grillage model, are considered and their applicability to the global response analysis of a pontoon-type VLFS in waves is examined. The influence of structural modeling on the buckling strength evaluation of VLFS is also studied.
... In practice, the design of Mega-floats can be divided into three basic stages, see Fujikubo (2005): ...
... Because of their evolutionary character, based upon a combination of engineering fundamentals, empirical information and historical precedent, it is stated by several authors as e.g. Palo (2005), Watanabe et al (2004), Suzuki (2005) and Fujikubo (2005), that Classification guides only rarely apply to the design of new, innovative structures, as it is not clear in advance which phenomena and failure modes will be dominant. This means that the structural design of VLFS's needs a first-principle approach that requires a performance rather than a descriptive format, and is to be based upon rational analysis methods to predict structural responses, as well as explicit design criteria incorporating operating conditions, strength and serviceability requirements, safety requirements, durability and cost effectiveness. ...
Concern for the quasi-static response of ship and offshore structures, as required for safety and serviceability assessments. Attention shall be given to uncertainty of calculation models for use in reliability methods, and to consider both exact and approximate methods for the determination of stresses appropriate for different acceptance criteria.
... Unless M u is large enough to satisfy the required safety criteria, the design procedure has to be iterated from step 2 (see Fig. 10). The proposed design procedure is based on the typical design flows in Fujikubo (2005) and Wang et al. (2008), which require high computational cost for hydroelastic analyses. Mooring effect is not considered in the preliminary design stage. ...
... Mooring effect is not considered in the preliminary design stage. The detailed structural response of the VLFSs to evaluate safety against fatigue and corrosion is considered in the detailed design stage, see Fujikubo (2005) and Wang et al. (2008). ...
... In order to calculate the VLFS structural response for strength analysis, the researchers pro• posed some structural models with the characteristics of VLFS, such as isotropic plate model, ortho• tropic plate model, plane-grillage model [59] , sandwich-grillage model [59] and three dimensional shell finite element model [60] . The related review papers and comparison papers are cited here [44,[61][62] . The simple structural models have a small amount of calculation and can be used for preliminary analy• sis of the structure at the initial stage of design to obtain approximate design parameters. ...
The application of Very Large Floating Structures (VLFS) has been a research hotspot in the field of floating structures in recent years. This paper mainly reviews the important conceptual de• signs and key technologies of VLFS. First of all, VLFS are divided into four categories according to the purpose: vehicle platform, resource platform, living platform, and special platform. According to the different categories, some examples of significant conceptual designs for VLFS are introduced, includ• ing Mobile Offshore Base (MOB), Mega-Float, Ultra Large Floating System, Floating Fuel Storage Base, Freedom Ship, Lilypad Floating Ecopolis, Ocean Heart, Rocket Launch Platform, and Scientific Experimental Platform. Then, based on the current development of VLFS, several key technologies in• volved are reviewed and analyzed, including hydroelasticity, multi-module and connector, mooring system, structural strength and reliability, and practical technologies. Finally, some comments and rec• ommended conferences on VLFS are given. Earth, because 71% of the surface is covered by water, is also known as"blue planet". The blue area is mostly ocean, and the Very Large Floating Structures (VLFS) are the ideal mediums for human beings to explore the ocean. With the increasing importance on marine resources, research on VLFS has become a research emphasis in the field of ocean engineering. So far, the research on VLFS has been popular in some countries such as the United States [1] , Japan [2] , China [3-4] , Norway [5-6] , Singapore [7-8] , Malaysia [9] , South Korea [10] and the Netherlands. They have done a lot of pioneering work in this filed and obtained a lot of remarkable results, making sig• nificant contributions to the development of VLFS. This paper mainly reviews the important conceptual designs and key technologies of VLFS.
... Many investigations have been carried out to investigate the effect of WECs on the response of very large floating structures (VLFS) (Fujikubo, 2005;Ding et al., 2021). Jiang et al. (2021) provided a comprehensive review of connector systems for modularized VLFS, highlighting the research and development in this area. ...
... For the UHPC-FRP composites, the configuration of the materials UHPC and FRP is the most critical issue to utilize the high compressive strength of UHPC and high tensile strength of FRP. Hydroelastic analysis is required to evaluate the responses of the floating structures in waves, because the MFS has relatively small height compared to the overall length of the structures (Fujikubo, 2005;Suzuki et al., 2007), resulting in significant bending moment. ...
This paper proposes a hydroelastic and structural analysis method for modular floating structures (MFS) constructed from fiber-reinforced polymer (FRP) reinforced ultra-high performance concrete (UHPC) for floating photovoltaic (FPV) systems. Initial structural design, hydroelastic analysis of an equivalent plate model as well as structural analysis are performed in the approach. We investigate the hydroelastic responses of a continuous very large floating structure (VLFS) and three hinge-connected modular floating structures. The floating module is designed to be carbon fiber-reinforced polymer (CFRP) reinforced UHPC box-pontoon. Hinge connections are applied to neighboring modules to form the floating structure. In the hydroelastic analysis, an equivalent plate model was conducted, and the effects of the number of hinge connections and incident wave conditions on the bending moment and vertical deflection were studied. We found that the hinge connection had a great effect on reducing the bending moment. The aspect ratio of the module should also be considered. Cross-sectional analysis was performed to estimate the bending capacity. Nine cross-sectional trials were compared to verify their safety probability in the specific static analysis. Prestressing was found very effective in enhancing the bending capacity. Non-prestressed UHPC is not recommended owing to its limited bending capacity.
... Many investigations have been carried out to investigate the effect of WECs on the response of very large floating structures (VLFS) (Fujikubo, 2005;Ding et al., 2021). Jiang et al. (2021) provided a comprehensive review of connector systems for modularized VLFS, highlighting the research and development in this area. ...
... The procedure divides the structure into specific idealized units (Paik and Thayamballi 2003): plate unit, stiffened plate unit, beam-column unit etc. Each unit is modeled to represent the part of the ship structure with respective loads, constraints (Paik et al. 2001;Fujikubo and Kaeding 2002;Fujikubo 2005) and used for nonlinear limit state analysis of large structures. Similar approach to one presented in this paper was done in following papers Nagiar et al. (2013) and Sertic et al. (2013). ...
The paper presents a procedure for modeling an orthotropic thin plate finite element so to have an equivalent structural response as the steel stiffened plate of same overall dimensions. New, equivalent element, homogenizes the structural properties of plates and stiffeners in element’s stiffness matrix. By use of such elements, a significant reduction in overall size of a global finite element model of large stiffened panel structures could be achieved. The algorithm of proposed procedure is fully explained and, as an example, applied to a standard global model of steel ship hull structure. Structural response comparison is made between reduced model made of newly defined thin finite elements and conventional ones. Advantages are emphasized and remedies of some recognized shortcomings are suggested.
... Hydroelasticity, i.e., the influence of structural deformations on the hydrodynamic loads, can be relevant for some marine structures, such as fishing farms [10] and very large floating structures [11]. Some researchers have evaluated the relevance of hydroelasticity for typical FWT hulls. ...
Dynamic analysis of floating wind turbines often considers the hull as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible beams. The analysis is based on numerical simulations of free decays, structural response to wave excitation and short-term fatigue damage accumulation at tower base and tendons. In addition, the importance of hydroelastic effects due to the pontoons' vertical deformations is evaluated. Pontoon flexibility changed the platform natural periods and motion amplitude significantly, and the adoption of flexible pontoons reduced the predicted fatigue damage in the tower base and tendons. On the other hand, hydroelasticity had negligible consequences for motion and load responses considered here.
... Failure of the important local components may lead to the lack of global strength, resulting in the loss of life and property in safety accidents (Ma et al., 2008). Therefore, a large number of researches have been conducted for strength analysis and life prediction of typical structural nodes, especially for brackets, on ships and offshore plat-forms (Tamehiro et al., 1982;Fricke et al., 2004;Fujikubo, 2005;Fricke and Kahl, 2007;Xie and Xie, 2009;Stoschka et al., 2010). ...
For a semi-submersible platform in repair, the eight old main brackets which connect columns with pontoons need to be replaced by new ones. In order to ensure the safety of the cutting operation of the old main bracket and calculate the initial stress condition of new main bracket, the structural stress monitoring of eight key spots is carried out, and then the calibrated finite element model is established according to the field monitoring results. Before cutting the main bracket and all associated structures, eight rectangular rosettes were installed, and a tailored cutting scheme was proposed to release the initial stress, in which the main bracket and associated column and pontoon plates were partly cut. During the cutting procedure, the strains of the monitoring spots were measured, and then the structural stress of the monitored spots were obtained. The stress variation characteristics at different spots during the initial cutting operation were shown and the initial stress condition of the monitored spots was figured out. The loading and support conditions of the semi-submersible platform were calibrated based on the measured initial stress condition, which made the finite element model more credible. The stress condition with the main bracket and associated structures entirely cut out is analyzed by the Finite Element Method (FEM), which demonstrates the cutting operation to be safe and feasible. In addition, the calibrated finite element model can be used to calculate the initial stress condition of the new main bracket, which will be very helpful for the long-term stress monitoring on the main bracket.
... Prinsip dasar yang perlu diketahui untuk melakukan analisa terhadap dermaga apung adalah struktur terapung (Floating Structure) merupakan suatu struktur yang fleksibel dan elastis sehingga untuk perhitungan dasar dapat dianalogikan sebagai balok memanjang dengan kekakuan EI ditempatkan diatas pondasi elastis atau ditumpu oleh pegas secara merata [5]. Dalam system koordinat X-Y dapat diilustrasikan seperti gambar 2 dan 3. ...
Indonesia sebagai negara kepulauan terbesar di dunia memiliki sekitar 17 ribu pulau dengan panjang garis pantai mencapai 95.181 kilometer. Secara geografis posisi Indonesia sangat strategis terhadap lalulintas perdagangan karena terletak antara dua benua dan dua samudera. Kondisi tersebut perlu didukung oleh sarana dan prasarana transporatsi antar pulau termasuk pelabuhan yang memadai untuk menunjang kegiatan perdagangan serta menjaga kedaulatan kemaritiman Indonesia. Perencanaan pelabuhan perlu disesuaikan dengan kondisi alam Indonesia yang berada pada daerah rangkaian cincin api yang merupakan lempeng tektonik paling aktif dan memberikan kontribusi besar terhadap terjadinya gempa di bumi. Salah satu alternatif desain pelabuhan yang bisa dikembangkan adalah dermaga apung yang didesain dan direncanakan untuk mampu menahan beban baik beban internal akibat muatan maupun beban eksternal dari lingkungan berupa tumpuan air, hempasan gelombang dan gaya tumbukan kapal saat sandar. Struktur dermaga apung memilki sifat yang dinamis dan tidak berhubungan langsung dengan dasar perairan. Kondisi ini membuat dermaga apung memiliki kapasitas yang terbatas jika dibandingkan dengan dermaga konvensional beton. Ukuran struktur dermaga akan menjadi bagian dari beban daya apung dermaga, sehingga semakin besar berat struktur maka akan semakin kecil kapasitas dermaga. Untuk itu analisa terhadap kekuatan struktur menjadi sangat penting dan harus dilakukan dengan metode yang memiliki tingkat akurasi yang tinggi. Pada tulisan ini analisa awal terhadap struktur dermaga apung berbasis pada Finite Element Methode.Kata Kunci : struktur, dermaga terapung, pembebanan.
... In the renewable energy field, the structural and fatigue analysis is mainly dedicated to wind systems [6,7,8]. Still, its application to the wave energy field is not so much exploited, being primarily devoted to mooring systems [9] and inspired by the knowledge gathered from offshore floating structures, like oil platforms and shipbuilding [10,11,12]. ...
The demand for electricity production has been consistently raising since the last century. In the future, the tendency is to grow even further. Concerning this fact, renewable energy and specifically, wave energy should be considered as an alternative for energy production. However, devices suitable to harness this renewable energy source and convert it into electricity are not yet commercially competitive. This paper is focused on the structural analysis of a wave energy converter (WEC) through the numerical study of several design parameters. Tridimensional computer aided design (3D CAD) numerical models were built and several Finite Element Analyses (FEA) were performed using a commercial finite element code. The main components of the WEC were simulated assuming different materials. The Von Mises stress gradients and displacement fields determined by FEA demonstrated that, regardless of the WEC component, materials with low Young’s modulus seems to be unsuitable for this application. The same is valid for the material yield strength since materials with higher yield strength lead to a better structural behavior of the WEC components. The developed 3D CAD numerical model showed to be suitable to analyze different combinations of structural conditions.
... The section properties of the elements were specially defined in terms of the spaces between adjacent girders or floors, the height of double bottoms, thickness of plates, thickness of girders or floors, etc. Since this simplified grillage analysis does not determine the stress inside the elements, but does determine the reaction forces at the boundary constraints, the strength of the bottom structures and any buckling problems were separately investigated against reaction forces at the docking supports (Chun et al., 2006;Chun & Seo, 2007;Fujikubo, 2005). The work by Cheng et al. (1995Cheng et al. ( , 2004 and Su (2007) focuses on designing optimal positioning and stiffness allocation for docking blocks for a single-ship docking. ...
When blocks are supported on a dock, huge reaction forces concentrated at the supports cause structural damage owing to local stress concentrations. Thus, the supports should be arranged to avoid local failure from the reaction forces by redistributing those forces. Docking analyses to determine the proper blocks and their support arrangements are introduced so that the local stresses are minimized to warrant the safety of the docking supports. Local stresses enforced by the support arrangement should be evaluated by finite element analysis (FEA). However, it is difficult to consider an accurate 3D geometry of the blocks in the finite element model because the structural design information is too complicated to determine within several days using the FEA model. This paper presents a simplified FE model to evaluate the safety of the arrangement of supports using a simplified grillage element. The grillage element can be efficiently used to obrain the reaction forces in docking analysis becasuse the reaction forces at the supports are enough to assess the safety of block. Since a simplified grillage model of the entire ship cannot accurately calculate the local stresses, an optimized modeling method based on the grillage element was introduced. The local reaction forces obtained by the proposed approach and three-dimensional FEA were discussed for typical types of ships. It is shown that the reaction forces obtained by the present grillage model are in reasonably good agreement with the FEA model.
... Ali, 2005;Alarcon, 1997;Andrianov, 2005b;De Boo, 2005;E. Watanabe, et al., 2004;Fit, 2006;Fujikubo, 2005;GRAAF, et al., 2006;Gunnar Rognaas, 2001;Holdsworth, 2007a;Schuwer, 2007;VREUGDENHIL, et al., 2006) • Suitable mooring and movement system (A.Ali, 2005;De Boo, 2005; H. S. Koh, 2008;Kuijper, 2006;Rijcken, 2006). ...
... Because of their evolutionary character, based upon a combination of engineering fundamentals, empirical information and historical precedent, it is stated by several authors as e.g. Palo [82], Watanabe et al [80], Suzuki [78] and Fujikubo [92], that Classification guides only rarely apply to the design of new, innovative structures, as it is not clear in advance which phenomena and failure modes will be dominant. Recently, structural behaviour of Mega-floats have been investigated by many researchers, see Takagi et al [93]; Murai et al [94]; Takaki et al [95]; Okada [96]; Yasuzawa [31] ; Ohta et al [97] . ...
This paper presents a summary of the recent advances on Quasi-static response of ship and offshore structures as discussed by the Technical Committee II.1 - Quasi-Static Response of International Ship and Offshore Structures Congress (ISSC), 2006. The technical committee’s mandate was concern for the quasi-static response of ship and offshore structures, as required for safety and serviceability assessments. Attention was given to uncertainty of calculation models for use in reliability methods, and both exact and approximate methods for the determination of stresses appropriate for different acceptance criteria were considered.
... Also the LR proposed how to put a load distribution to bottom grillage model. Since this simplified grillage analysis really gives not the stress inside the elements but reaction forces at the boundary constraints, the strength of bottom structures or the buckling problem was separately investigated against reaction forces at the docking supports(Chun et al., 2006; Fujikubo, 2005). For a couple of decades, the outstanding development of computers and numerical methods has made a three dimensional finite element analysis of a big and complex marine structure possible. ...
The docking analysis of a global ship structure is requested to evaluate its structural safety against the reaction forces at supports during docking works inside a dry dock. That problem becomes more important recently as the size of ships is getting larger and larger. The docking supports are appropriately arranged in a dock to avoid their excessive reaction forces which primarily cause the structural damages in docking a ship and, up to now, the structural safety has been assessed against the support arrangement by the finite element analysis (FEA) of a global ship structure. However, it is complicated to establish the finite element model of the ship in the current structural design environment of a shipyard and it takes over a month to finish the work. This paper investigates a simple and fast approach to carry out a ship docking analysis by a simplified grillage model and to assign the docking supports position on the model. The grillage analysis was considered from the motivation that only the reaction forces at supports are sufficient to assess their arrangement. Since the simplified grillage model of the ship cannot guarantee its accuracy quantitatively, modeling strategies are proposed to improve the accuracy. In this paper, comparisons between the proposed approach and three-dimensional FEA for typical types of ships show that the results from the present grillage model have reasonably good agreement with the FEA model. Finally, an integrated program developed for docking supports planning and its evaluation by the proposed approach is briefly described.
... The ultimate capacity predictions based on the simplified grillage model were found to be within 3-6% of the more detailed finite element models but with significantly reduced solution time. Furthermore, Fujikubo [17] used a sandwich-grillage model to analyse the linear and nonlinear hydroelastic response of very large floating structures. In his model, the top and bottom deck plates of the floating structures are modelled by rectangular membrane elements, while the bulkheads are modelled by beam elements. ...
A simplified grillage beam analogy was performed to investigate the behaviour of railway turnout sleeper system with a low value of elastic modulus on different support moduli. This study aimed at determining an optimum modulus of elasticity for an emerging technology for railway turnout application - fibre composites sleeper. The finite element simulation suggests that the changes in modulus of elasticity of sleeper, Esleeper and the sleeper support modulus, Us have a significant influence on the behaviour of turnout sleepers. The increase in Us from 10 to 40 MPa resulted in a 15% reduction in the bending moment while the increase in Esleeper from 1 GPa to 10 GPa has resulted in almost 75% increase in the bending moment. The shear forces in turnout sleepers is not sensitive to both the changes of the Esleeper and Us while the sleeper with low Esleeper tend to undergo greater settlement into the ballast. An Esleeper of 4 GPa was found optimal for an alternative fibre composite turnout sleeper provided that the Us is at least 20 MPa from the consideration of sleeper ballast pressure and maximum vertical deflection. It was established that the turnout sleeper has a maximum bending moment of 19 kN-m and a shear force of 158 kN under service conditions.Highlights► The simplified grillage beam analogy is reasonable to model a complex railway turnout system. ► The behaviour of sleepers in a railway turnout is most critical between the switch and the frog. ► Modulus of elasticity and support stiffness has a significant influence on the behaviour of sleepers. ► The FEM analysis provided a basis for an optimum design of composite turnout sleeper alternative.
A novel material-structure-hydroelasticity coupling analytical model is proposed for marine structures, which is utilized for the calculation and optimization of a very large floating sandwich structure (VLFSS) with a hierarchical ultrahigh-performance concrete (UHPC) core in this study. For the coupling material and structure analysis, three-dimensional representative volume element and self-consistent methods are developed to reveal the physical relations between the UHPC core's macroscale mechanical properties (e.g., modulus and density) and mesoscale hierarchical characteristics (e.g., aggregate and porosity) and to obtain the corresponding parameterized formulas. For the coupling material-structure-hydroelasticity analysis, a sixth-order dynamical equation for the potential flow model of the VLFSS, in which the hierarchical core's parameters are introduced through the material-structure coupling formulas, is developed. The hydroelasticity equations containing multiscale parameters are solved, and the mechanical responses are calculated. Using this coupled multiscale method, the shear force in the representative VLFSS is optimized for a smaller amplitude, which relies on the interactivity of the hierarchical structural parameters and wave conditions. These results demonstrate the potential of the multiscale coupling methodology to achieve the physically significant optimization of a floating composite structure in ocean engineering.
p>To overcome the shortcomings of traditional very large floating structures constructed either of steel or concrete, a new type of very large floating structure constructed of steel-concrete composite components is developed. First, the concept of very large steel-concrete composite pontoon-type floating structure is proposed and the structural characteristics of the structure is described in detail, which has significant advantages and potential compared to steel or concrete floating structures, such as anti-explosion, durability, reliability, functional flexibility and economy. Then, on the basis of three-dimensional potential theory, the computer program THhydro is developed to calculate the three-dimensional hydroelastic response of floating structures. The program is substantially verified by the experimental and numerical results of previous literature. Finally, the parameter analysis on the hydroelastic response of very large steel-concrete composite pontoon-type floating structure is conducted, which shows that the usage of steel-concrete composite components can improve the performance of very large floating structures to an extent.</p
The concept of Modular Floating Structures (MFS) offers a unique avenue to explore new and sustainable ways for addressing issues of coastal urbanization and sea level rise in the proximity of coastal cities. This concept is easily implemented in calm waters without the interference of waves. Yet, its implementation in open water, poses greater challenges, particularly in terms of habitability and comfort. The current study examines the feasibility of the concept in two locations: in mild sea zone near Singapore and in open water conditions at the Eastern Mediterranean Sea. Both conditions are examined in operational and extreme storms. It is shown that the MFS configuration can attenuate incident waves of short periods. This reduces the motion amplitudes of the inner modules with respect to the exterior modules facing the waves. It is also presented that during extreme storms, the chosen configuration is less effective, and the motion amplitudes of all modules within the MFS fabric are almost identical. To further increase the acceptable sea states, the study proposes a unique floating seawall design, which provides a substantial wave reduction in long wave periods. The study presents the efficiency of the new configuration in operational weather and a 100-years storm.
This paper is the second of two companion papers concerning the ultimate hull girder strength of container ships subjected to combined hogging moment and bottom local loads. The nonlinear finite element analysis in Part 1 has shown that local bending deformation of a double bottom due to bottom lateral loads significantly decreases the ultimate hogging strength of container ships. In this Part 2, extending Smith's method for pure bending collapse analysis of a ship's hull girder, a simplified method of progressive collapse analysis of ultimate hogging strength of container ships considering bottom local loads is developed. The double bottom is idealized as a plane grillage and the rest part of the cross section as a prismatic beam. An average stress-average strain relationship of plate/stiffened plate elements employed in Smith's method is transformed into an average stress-average plastic strain relationship, and implemented in the conventional beam finite element as a pseudo strain hardening/softening behaviors. The extended Smith's method is validated through a comparison with nonlinear finite element analysis.
The concept of Modular Floating Structures (MFS) presents a unique alternative to increase the available land resources in the adjacent marine environment of coastal cities. It offers a sustainable technological adaptation that can help mitigate overdevelopment and urban growth limitations. This study examines the comfort range of a suburban offshore MFS module, by using a novel methodology that reconciles residential comfort criteria with seakeeping. The investigation is performed by characterising the hydrodynamic structural response and analysing its compatibility for offshore dwellings. It is demonstrated that the MFS module complies to marine regulations, including seakeeping and comfort, but when evaluating its hydrodynamic response to the accepted accelerations in residential buildings, it reaches performance limits at a certain sea state. This may influence the choice of environment, based on the allowable significant wave height, or call for better hydrodynamic performance using multi-body configuration.
One of the most important research topic in the field of ship strength analysis is the problem of structural behavior of stiffened plates (panels), as the basic units of ship structures.
In this paper, stiffened plates are modeled as unstiffened ones, with the purpose to achieve the compliance in their structural response, as much as possible. Therefore, the procedure (equivalence method) is developed in order to obtain elasticity parameters of unstiffened plates so that maximum displacements and stresses would be the same in both plates. Furthermore, an equivalent finite element is defined. This element has stiffness matrix that homogenize the mechanical properties of both plate and stiffener. Developed mathematical algorithm takes into account application of orthotropic plate theory and finite element method.
Considering that ship hull consists of large number of stiffened plates, the goal of equivalent plate strength modeling is to form a numerical model of ship structure with a reduced number of elements and degrees of freedom. This model, apart from conventional one, does not have stiffeners, just equivalent unstiffened plates. Comparing the specific numerical model of ship structure that consists of conventional finite elements and one that consists of new, equivalent elements, one can get more clear view on application, advantages and disadvantages of developed method. The goal of this thesis is to simplify structural modelling of large numerical models of ship structures, without any significant loss in quality of the analysis, namely resulting displacements and stresses.
Furthermore, a series of measurements is performed on specific models of stiffened plates in order to achieve general comparison of the structural response of real structure as opposed to numerically modeled one, especially having in mind that finite element method is incortporeted in equivalence method.
Land scarcity in and around coastal cities is a growing problem in both industrialized and developing nations. The lack of development areas increases the tension between infrastructure needs, urban needs and nature – impacting both growth and quality of life. This study advocates that floating structures can offer a unique avenue to explore new and sustainable ways of addressing these issues. Recognizing that no comprehensive analysis or study on the legal requirements needed for the realization of such projects has yet been conducted, the study's first aim is to define the required design guidelines by synthesizing statutory requirements, building codes and international regulations. From a statutory perspective, these encompass two disciplines: civil engineering and naval architecture. To this end, a preliminary design of a Modular Floating Structures (MFS) module is presented, reconciling the design requirements of the two disciplines, in order to proof the feasibility of the MFS technology for urban use offshore. The study mainly focuses on structural and safety aspects, and sheds light on other crucial factors for offshore dwelling feasibility, such as occupant comfort.
This article outlines methods for hydroelastic response evaluation required for the design of a VLFS (very large floating structure). Since the hydroelasticity is a problem of interference between fluid actions and structural deformation, the methods need modeling both in fluid and structural domains. Further, they need to combine techniques to enhance the computational efficiency as the analysis domain of the VLFS, or the number of unknowns to be solved, is large. On the other hand, a modeling with an appropriate complexity is necessary depending on which design stage you are at and what type of output is anticipated. Various typical methods proposed so far are reviewed after the theoretical backgrounds are given. The characteristics of the hydroelastic response are also explained by using an analytical solution to a simple VLFS model. The hydroelastic response is characterized by the response to a wave whose wavelength is equal to the characteristic length defined by a few parameters for the VLFS model and the resonant response with respect to the flexible modes.
The Mega-Float project is a pontoon-type Very Large Floating Structure that was designed for deployment in protected waters such as in a large bay. It consists of a floating structure, a mooring system, and access infrastructure. While construction of a breakwater was contemplated, the existing breakwater around the site was found to be sufficient. Unlike conventional ships where the response is dominated by rigid-body motions, the response of the Mega-Float is dominated by hydroelastic responses because of its thin mat-like configuration. A 6-year research project was conducted in Japan with the aim of developing the fundamental design, construction, and operational technologies in Phase I (1995–1997) and a corroborative study on the use of the Mega-Float as an airport in Phase II (1998–2001). This chapter gives an overview of the Mega-float research project including its purpose and achievements.
The Marina Bay was identified as the venue for the National Day Parade for 2007 and subsequent years. The concept was to create a large usable space on water for mass spectator events and to design it as a multi-purpose facility to be used for sporting activities, mega festivities, cultural performances and water activities. The pontoon-type floating platform technology was adopted to create the large-scale floating performance stage. The Marina Bay floating platform measured 120 m long, 83 m wide and had a depth of 1.2 m and was built by assembling fifteen modular steel pontoons. This floating stage, completed in April 2007, is believed to be the world’s largest floating performance stage.
Floating airport consisting of multiple flexibly connected modules is a typical dynamic network with flexible-rigid-fluid coupling. A new method is proposed for modeling and a chain-topological network model for the floating airport is developed. In numerical simulations, the nonlinear responses of surge, heave, pitch motions and loads of connectors are analyzed implying that the classical linearization approach may severely underestimates the actual results. Further, this paper studies synergetic dynamics of the network and amplitude death phenomena. The onset of amplitude death associated with coupling stiffness and wave period are illustrated, which is important for the stability safety design of the floating airport. This work provides a new methodology and an application example in the study for network structural dynamics, including very large scale floating structures. ©, 2015, Chinese Journal of Theoretical and Applied Mechanics Press. All right reserved.
Loads generated after an air crash, ship collision, and other accidents may destroy very large floating structures (VLFSs) and create additional connector loads. In this study, the combined effects of ship collision and wave loads are considered to establish motion differential equations for a multi-body VLFS. A time domain calculation method is proposed to calculate the connector load of the VLFS in waves. The Longuet-Higgins model is employed to simulate the stochastic wave load. Fluid force and hydrodynamic coefficient are obtained with DNV Sesam software. The motion differential equation is calculated by applying the time domain method when the frequency domain hydrodynamic coefficient is converted into the memory function of the motion differential equation of the time domain. As a result of the combined action of wave and impact loads, high-frequency oscillation is observed in the time history curve of the connector load. At wave directions of 0° and 75°, the regularities of the time history curves of the connector loads in different directions are similar and the connector loads of C1 and C2 in the X direction are the largest. The oscillation load is observed in the connector in the Y direction at a wave direction of 75° and not at 0°. This paper presents a time domain calculation method of connector load to provide a certain reference function for the future development of Chinese VLFS
This paper presents the systems engineering solutions implemented in developing a large floating performance stage that was constructed at the Marina Bay of Singapore. The Marina Floating Platform is designed to be a multi-purpose facility on the bay for mass spectator events, sporting activities and cultural performances, as well as be a re-configurable “piers” for water sports and boat shows. This floating platform was completed in 2007 and is believed to be the world’s largest floating performance stage on water. It hosted the nation’s National Day Parade 2007, the first parade to be held on water. Since then, the floating platform and its seating gallery have been used as the venue for lifestyle events, extreme sports, the Singapore Fireworks Festival and Water Carnival. The floating platform opens up new possibilities in space creation in land-scarce countries such as Singapore.
A pFFT–FE coupling method, which can calculate the hydroelastic behavior of floating flexible structures, has been developed. The method can handle a very large number of constant hydrodynamic panels in a reasonable CPU time. The scheme uses a consistent way of the data passing in which the energy is conserved between the generalized modal damping and the radiation waves if the hydrodynamic analysis is accurate enough. In addition, the scheme satisfies the generalized Haskind–Newman relation between the modal diffraction force and the Kochin function. These properties are important to ensure the numerical accuracy. The numerical convergence and the accuracy of the method are demonstrated in various ways including the comparison with experimental data. Finally an application to the sailing type offshore wind-power plant is shown to demonstrate the applicability of this method to the challenging problem.
To assure the safety of large structures such as ships and offshore structures, it is important to clarify the entire process leading to the overall collapsing state of the structures. For this purpose, several computational methods are developed in the 2 main fields of mechanics with the aid of the finite element method (FEM): Computational Welding Mechanics for Construction Stage, and Computational Structural Mechanics for Ultimate Strength Analysis - ISUM (Idealized Structural Unit Method). These new computational methods form an essential basis for the idea of design by analysis and production planning based on computational simulation. Strength Analysis - ISUM (Idealized Structural Unit Method). These new computational methods form an essential basis for the idea of design by analysis and production planning based on computational simulation. Copyright © by The International Society of Offshore and Polar Engineers.
Very Large Floating Structure (VLFS) is a unique concept of ocean structures primary because of their unprecedented length, displacement cost and associated hydroelastic response. International Ship and Offshore Structures Congress (ISSC) had paid attention to the emerging novel technology and launched Special Task Committee to investigate the state of the art in the technology. This paper summarizes the activities of the committee. A brief overview of VLFS is given first for readers new to the subject. History, application and uniqueness with regard to engineering implication are presented. The Mobile Offshore Base (MOB) and Mega-Float, which are typical VLFS projects that have been investigated in detail and are aimed to be realized in the near future, are introduced. Uniqueness of VLFS, such as differences in behavior of VLFS from conventional ships and offshore structures, are described. The engineering challenges associated with behavior, design procedure, environment, and the structural analysis of VLFS are introduced. A comparative study of hydroelastic analysis tools that were independently developed for MOB and Mega-Float is made in terms of accuracy of global behavior. The effect of structural modeling on the accuracy of stress analysis is also discussed. VLFS entails innovative design methods and procedure. Development of design criteria and design procedures are described and application of reliability-based approaches are documented and discussed.
This paper discusses the engineering design and construction of a large floating steel platform at the Marina Bay of Singapore. The floating platform was designed to be a multipurpose facility on the bay for mass spectator events, sporting activities and cultural performances. This floating platform, completed in April 2007, is believed to be the world's largest floating performance stage and hosted the National Day Parade (NDP) in the last 2 years. It has been an ideal venue for mega festivities, water sports and boat shows since; and it is a floating icon on the Singapore waters. It has also been earmarked as the venue to stage the opening and closing ceremonies for the inaugural 2010 Youth Olympics to be held in Singapore. Building the floating platform involved numerous challenges and innovatively adopted new engineering techniques, which will be described herein.Singapore constructed its first large floating platform in response to the requirement for a temporary venue to hold the NDP from 2007 to 2011, while the National Stadium was demolished. The floating platform generates a usable space of 120 m × 83 m on the water, at the Marina Bay, and was designed to carry a heavy load comprising at least 9000 people, 200 t of stage props and three 30-t vehicles. A 27 000 seating capacity gallery along the shoreline faces the floating platform and allows the spectators to view the various events on the platform as well as on the water against the backdrop of the Singapore City skyline. This floating platform is a floating icon and a new landmark on the Singapore waters. The floating platform hosted the nation's spectacular NDP in August 2007 (see Fig. 1). It was the first time the NDP was held on water and Singaporeans were thrilled by the unique structure of the floating platform. The floating platform was made configurable for boat exhibition and relocatable so as to make way for boat racing event in the bay. Besides meeting all these requirements, the design and construction had to overcome many environmental constraints and technical challenges. At about the same time, the Marina Bay was being developed into a fresh water reservoir and planned as the new downtown and financial centre. This led to more restrictions as the floating platform must be environmental friendly and the architectural design needed to blend with the surrounding infrastructure developments. The building needed to take into consideration the construction of the barrage, the Marina Barrage, across the mouth of the bay, which limited the access and transportation by sea of the floating platform into the Marina Bay. The novel solution of using pontoon-type very large floating structure (VLFS) technology was adopted to create large usable space on water. This pontoon-type VLFS technology simply harnesses the inherent buoyancy force of the water to support itself. The floating structure was neither a ship nor a building infrastructure and its development and construction required multidisciplinary engineering. Engineers in civil, mechanical, marine engineering as well as naval architecture, had to work together to synthesise and balance the demand involving marine construction standards as well as building infrastructure rules and regulations.
The estimation of wave-induced response of a very large floating structure has been calculated by a coupled structure fluid interaction program. The evaluation of the calculated responses is mainly carried out on amplitude of motions, such as vertical displacements, accelerations. Although the nominal stress on the structure can be calculated by the program, it is very hard to obtain a detailed structural stress because the scale ratio of the detailed structure to whole floating structure is very large.This paper is concerned in the wave-induced stress response calculation procedure for the detailed vstructure on a very large floating structure. The calculation method consists of two steps. First step; Internal loads such as wave-induced bending moment and shear force and external load such as wave -induced water pressure, Aikc, s(ω, χ;x, y), are calculated by 3D coupled structure-fluid interaction program. Second step ; When these loads act in the same time on the structure, wave-induced stress for the detailed structure, σikc, s (ω, χ;x, y), can be calculated by using stress factor, kj.σi, s (ω, χ;x, y)=∑Nj=1kjAijc, s (ω, χ:x, y)To verify the procedure, we conduct a full scale measurement of 200×100m scale floating structure for 2 months. The estimated stress of detailed structure, for example stress around scallop at transverse bulkhead, shows very good agreement with measured stress.
Very Large Floating Structures (VLFS) of several thousand meters long are being considered for various applications such as floating airports, offshore cities and so on. The VLFS recently designed have a thin mat-like configuration and very large horizontal size. A typical design, for example, is 5 Km long, 1 Km wide and 5 m high. Therefore this type of structure will be very flexible and the elastic deformation due to wave action will be more crucial than the rigid body motions.For the analysis of the hydroelastic behaviors of VLFS in waves, the conventional numerical techniques can be applied in principle. However, the direct application of such techniques is practically impossible, because the wave length is very small relative to the horizontal size of the structure and it requires enormous computational burden. There are many calculation methods proposed to overcome this difficulty. However, it is still very difficult for the case of the length of structure is more than one hundred times of incident wave length.Then the author propose a new numerical calculation method based on the 3-dimensional eigenfunction expansion method for the diffraction and radiation problems and the modal expansion method for the elastic response representation.This paper describes, at first, a new calculation scheme proposed then the numerical results compared with the model experiments carried out at various institutions which indicate good agreement each others. Finally, based on the calculations, it shows the various features of hydroelastic behaviors of typical VLFS including the effects of water depth, elastic stiffness and so on.
Research and development on the Mega-Float, a very large floating structure (VLFS), was carried out for 6 years from 1995 to 2000 as a Japanese national project. In the first stage of the project (Phase-Ι:1995-1997), the general and basic characteristics of a VLFS such as hydroelastic responses in wave were studied. This required the development of methods for calculating the hydroelastic response of a VLFS with complicated shapes and structures. In the second stage of the project(Phase-ΙΙ:1998-2000), problems specific to a floating airport were studied. This also involved planning a model of a prototype floating airport with two 4000m long runways assumed to be located in Tokyo Bay. This paper presents the procedures used to analyze the structural safety and functionality of a VLFS to be used as an airport and reports on the analyses applied to the model of a prototype floating airport in Tokyo Bay. Design wave load conditions were determined by analyzing Tokyo Bay wave data and hydroelastic response analysis of a large floating airport model with a length of 4770m was carried out under such design wave conditions. Structural stresses under combined load conditions consisting of permanent loads, live loads and wave loads were evaluated and a structural safety assessment was made. In addition to stresses, slopes and radii of the curvature of the runways, and fluctuation of the PAPI(Precision Approach Path Indicator) were also evaluated in order to assess functionality for airport facilities. This assessment employed the same criteria as used for airport facilities on land. Consequently, it was found that a floating airport is likely to be as adequate as an airport on land.
This paper deals with a simplified method for the preliminary design of pontoon-type very large floating structures (VLFS), which are supposed floating airport, based on collapse behavior and reliability analysis in irregular waves. Firstly, a simplified estimation method is presented for the probabilistic load effect model of VLFS under irregular sea-state conditions. Next, limit state conditions are shortly presented for the buckling and ultimate collapse strength of stiffened plates under combined compression, shear and lateral pressure in the deck, bulkhead and bottom parts of VLFS, especially, by using a simplified estimation formula. Then, the validity is shown by non-linear finite element method. Finally, dominant limit state modes of 5,000m-class VLFS under combined loads with bending moment, shear force and lateral pressure are obtained by applying the above methods. Then, the features of the collapse behavior and reliability level are investigated by using above calculation results. Effects of design parameters such as yield stress, plate thickness, stiffener and bulkhead space are also investigated using sensitivity analysis.
This paper deals with the limit state and reliability analysis of huge barge structures (HBS) which are supposed floating refuse storage and incineration plant based on collapsing behavior analysis in irregular waves as a part of studies on structural reliability-based design methods. First, a limit state and reliability analysis method is shortly presented for the buckling and ultimate collapse strength of deck, bulkhead and bottom panels of HBS. Next, a simplified method is briefly introduced for collapsing behavior and reliability analysis of HBS under extreme sea loads by using a developed system combined with a finite element method and plastic node method using hexahedral element models. Moreover, a simplified estimation method is shortly introduced for the probabilistic load model considering the hydro-elastic response of the structure in irregular waves. Finally, dominant limit state modes of 1,000m-class HBS under combined loads with bending moment, shearing force and lateral pressure are obtained by applying the above methods. Then, the features of the collapsing behavior and reliability level are investigated by using above calculation results. Effects of statistical values such as reduction of thickness due to corrosion, yield stress and design parameters are also investigated using sensitivity analysis.
A new simplified model for collapse analysis of stiffened plates is developed in the framework of the idealized structural unit method (ISUM). By idealizing material and geometrical nonlinearities, larger structural units are defined as an element in ISUM than in conventional finite element analysis (FEA). The proposed stiffened plate model consists of ISUM plate elements and beam-column elements. The formulation of the plate element is performed by introducing accurate shape functions to simulate the buckling/plastic collapse behaviour of plate panels. Combining plate and beam-column elements allows for both local buckling of the plate panel and overall buckling of the stiffener.Fundamental collapse modes of plate panels and stiffened plates are investigated by conventional FEA. According to the observed characteristics, the new simplified model is formulated. Comparisons with FEA demonstrate the accuracy of the simplified model and its high applicability to typical stiffened plates in marine structures.
An efficient method of analysis of non-linear behavior until collapse of large size redundant structures is presented. The method is named as The “Idealized Structural Unit Method”. In this method the structure is divided into the biggest possible “Structural Units” whose geometric and material nonlinear behavior can be idealized and described in a concise analytical-numerical form. The structure is reassembled and load is applied incrementally until ultimate strength is attained.In this method, modeling is an obvious operation of dividing the structure into its actual structural units. This eliminates the effort required for the choice of type of element or size of mesh as in the finite element method. The number of structural units and overall degrees of freedom required for the analysis of a structure by this method are less in order than the number of elements and overall degrees of freedom required to analyze the same structure by the FEM. Costs of computer and data preparation may be drastically reduced.Application of the method to metal structures built up of deep I girders, such as bridge girders, ships deep girders and grillage structures is considered. The “Girder Structural Unit” is defined and its behavior under increasing loads is idealized based on existing as well as newly developed theoretical and experimental studies. Conditions for web bucking, ultimate strength and full plastic strength are established and expressions for stiffness during various stages are derived. A deep girder structure may then be treated as an assembly of such “Girder Structural Units”. Results of analysis of example structures are presented. The consumed computer time is found to be very short, as expected. Comparisons with results of experimental studies show good agreement.
Detailed measurements are presented from a parametric wind tunnel study into the influence of free board, deck layout and heel angle for a range of azimuth or wind angles on floating offshore structures. Results show that lift forces and platform free board have a major influence on the overturning moment. A further investigation was then carried out to determine the influence of the sea surface. (from authors' abstract)
When a super large floating structure, constructed with the steel and concrete slabs, is used as airport, there is a slight possibility for airplane and engine to drop on it. For designing the super large floating structure, it is important to verify the safety of it against the airplane collision. In order to clarify the behavior of deck structure and concrete pavement during collision, numerical calculations are performed utilizing non-linear dynamic FEM program `LS-DYNA-3D'. The minimum thickness of deck plate and concrete pavement, which are necessary to prevent the penetration of the airplane or engine into the deck plate, are shown based on the results of calculation. And also, it is shown that the concrete pavement is effective to absorb the dynamic energy produced by an airplane collision. As the above numerical calculation requires an enormous computing time, it is not economical to apply this at the preliminary structural design stage. So, a simplified model for simulation of the phenomena at airplane collision is developed to save the computing time.
A new simplified model for stiffened plates in collapse analyses is developed. The model consists of large plate elements and beam-column elements. It is formulated according to the typical collapse modes of stiffened plates under longitudinal thrust, which are investigated by conventional Finite Element Analysis (FEA). Comparisons with FEA show the excellent applicability of the model to typical stiffened plates in marine structures and its accuracy. To demonstrate the application of the model to offshore structures, the collapse behaviour of a transverse strip of a Very Large Floating Structure (VLFS) in longitudinal waves is examined.
For the structural design of very large floating structures (VLFS) of several thousand meters long, a hierarchical system of structural analysis must be developed, in which an idealized structural modeling is employed for the global response analysis while the local structural response is analyzed using a zooming technique. In this study, two types of grillage beam models, a plane grillage model and a sandwich grillage model, are considered and their applicability to the global response analysis of a pontoon-type VLFS in waves is examined. The influence of structural modeling on the buckling strength evaluation of VLFS is also studied.
A structural safety assessment of a pontoon-type very large floating structure (VLFS) surrounded by a gravity-type breakwater
was carried out for extreme wave conditions by considering the damage to the breakwater. Bending and shear collapses are considered
to be a failure mode of the floating structure, while overturning damages the breakwater. The probability of the breakwater
overturning, and the transmitted wave height before and after damage to the breakwater, are evaluated using design formulae
for port and harbor facilities in Japan. The ultimate bending and shear strengths of the floating structure are calculated
by the idealized structural unit method (ISUM) and FEM, respectively. The calculated failure probability for the floating
structure is compared with the specified target safety level. It was found that the floating structure under consideration
is most likely to fail by bending in transverse waves, and that the corresponding failure probability satisfies the target
level.
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