Pontoon-type Very Large Floating Structures Research Articles

Wave energy extraction and hydroelastic response reduction of modular floating breakwaters as array wave energy converters integrated into a very large floating structure

Combing floating breakwaters with wave energy converters (WECs) and integrating them into very large floating structure (VLFS) can provide a viable option to explore economically offshore wave energy resources and simultaneously to protect marine structures. In this paper, the time-domain numerical model is developed based on the modal expansion theory with nonlinear consideration to optimize the design and layout of an integrated system of modular WEC-type floating breakwaters and a pontoon-type VLFS, with emphasis on the effects of the WEC geometric size and shape, the WEC-VLFS gap distance and the wave nonlinearity. A hybrid finite element (FE)-boundary element (BE) method is presented to simulate the structures as Mindlin plate elements and the water waves as fully nonlinear potential flow boundaries, respectively. Breakwaters as WECs with deeper draft and larger length are found to more fully interact in phase with long-period waves, and receive more wave energy extraction and larger hydroelastic response reduction. The addition of breakwaters has a favorable effect on the wave energy extraction, but a destructive effect on the hydroelastic reduction. Importantly, wave resonance induced by the multi-modal scattering waves in the WEC-VLFS gap leads to multiple peaks of the power capture efficiency. Compared to the symmetric-shape WECs, the asymmetric-shape WECs strengthen the gap resonant effect, which improves both the wave energy extraction and hydroelastic reduction for a broader frequency bandwidth. The findings of this study indicate the synergistic benefits of wave energy exploitation and transmitted wave attenuation at the fore-end of VLFSs.

Hydroelastic analysis of a truss pontoon Mobile Offshore Base

Very Large Floating Structures (VLFS) are one among the solution to pursue an environmentally friendly and sustainable technology in birthing land from the sea. VLFS are extra-large in size and mostly extra-long in span. VLFS may be classified into two broad categories, namely the pontoon type and semi-submersible type. The pontoon-type VLFS is a flat box structure floating on the sea surface and suitable in regions with lower sea state. The semi-submersible VLFS has a deck raised above the sea level and supported by columns which are connected to submerged pontoons and are subjected to less wave forces. These structures are very flexible compared to other kinds of offshore structures, and its elastic deformations are more important than their rigid body motions. This paper presents hydroelastic analysis carried out on an innovative VLFS called truss pontoon Mobile Offshore Base (MOB) platform concept proposed by Srinivasan and Sundaravadivelu (2013). The truss pontoon MOB is modelled and hydroelastic analysis is carried out using HYDRAN-XR for regular 0 waves heading angle. Results are presented for variation of added mass and damping coefficients, diffraction and wave excitation forces, RAOs for translational, rotation and deformational modes and vertical displacement at salient sections with respect to wave periods.

Reducing hydroelastic responses of pontoon-type VLFS using vertical elastic mooring lines

This paper is concerned with the reduction of hydroelastic responses of pontoon-type very large floating structures (VLFS) via the use of vertical elastic mooring lines connected at the fore and aft edges of VLFS to the seabed. These vertical mooring lines are used in addition to the mooring dolphin-rubber fender system that is used to restrict the horizontal movement of VLFS. The considered rectangular shaped VLFS is modeled as a Mindlin plate floating on an ideal fluid in which the linear potential theory is applicable. The hydroelastic analysis of VLFS with vertical mooring lines modeled as vertical elastic springs is performed in the frequency domain by using the hybrid finite element–boundary element (FE-BE) method. This study examines the effectiveness of vertical elastic mooring lines in reducing hydroelastic responses for various wavelengths, incident wave angles and aspect ratios. The stiffnesses of the mooring lines are optimized for maximum reduction in the hydroelastic response by using the Differential Evolution algorithm. It was found that hydroelastic responses of VLFS could be significantly reduced by using vertical elastic mooring lines with optimal stiffnesses.

Hydroelastic analysis of oblique irregular waves with a pontoon-type VLFS edged with dual inclined perforated plates

The fluid-structure interaction of oblique irregular waves with a pontoon-type very large floating structure (VLFS) edged with dual horizontal/inclined perforated plates has been investigated in the context of the direct time domain modal expansion theory. For the hydroelastic analysis, the boundary element method (BEM) based on time domain Kelvin sources is implemented to establish water wave model including the viscous effect of the perforated plates through the Darcy’s law, and the finite element method (FEM) is adopted for solving the deflections of the VLFS modeled as an equivalent Mindlin thick plate. In order to enhance the computing efficiency, the interpolation-tabulation scheme is applied to assess rapidly and accurately the free-surface Green function and its partial derivatives in finite water depth, and the boundary integral equation of a half or quarter VLFS model is further established taking advantage of symmetry of flow field and structure. Also, the numerical solutions are validated against a series of experimental tests. In the comparison, the empirical relationship between the actual porosity and porous parameter is successfully applied. Numerical solutions and model tests are executed to determine the hydroelastic response characteristics of VLFS with an attached anti-motion device. This study examines the effects of porosity, submerged depth, inclined angle and gap distance of such dual perforated anti-motion plates on the hydroelastic response to provide information regarding the optimal design. The effects of oblique wave angle on the performance of anti-motion and hydroelastic behavior of VLFS are also emphatically examined.

Optimal Layout of Gill Cells for Very Large Floating Structures

A pontoon-type, very large floating structure (VLFS) undergoes uneven deformation when loaded unevenly. The resulting differential deflection may lead to a cessation of equipment operation, and even compromising the structural integrity of the VLFS. One cost effective solution for reducing the differential deflection is by introducing the innovative gill cells at appropriate locations in the VLFS. Gill cells are compartments in VLFS with holes or slits at their bottom surfaces to allow water to enter or leave freely. At these gill cell locations, the buoyancy forces are eliminated and this allows uneven buoyancy forces acting at the bottom hull of the VLFS to somewhat counterbalance the applied nonuniform loading. In this paper, we investigate the effectiveness of gill cells in pontoon-type VLFS in reducing the differential deflection and von Mises stresses as well as the optimal layout (i.e., the number and locations) of gill cells that minimizes the differential deflection subject to a draft constraint. As the decision variables, objective functions and the constraints are not continuous and differentiable, genetic algorithms are adopted as an optimization tool. The optimal layouts for gill cells are determined for various VLFS shapes such as square, rectangular and I-shape and loading configurations.

Hydroelastic analysis of pontoon-type circular VLFS with an attached submerged plate

This paper is concerned with the hydroelastic analysis of a pontoon-type, circular, very large floating structure (VLFS) with a horizontal submerged annular plate attached around its perimeter. The coupled fluid–structure interaction problem may be solved by using the modal expansion method in the frequency domain. It involves, firstly, the decomposition of the deflection of a circular Mindlin plate with free edges into vibration modes that are obtained analytically. Then the hydrodynamic diffraction and radiation forces are evaluated by using the eigenfunction expansion matching method which can also be done in an exact manner. The hydroelastic equation of motion is solved by the Rayleigh–Ritz method for the modal amplitudes, and then the modal responses are summed up to obtain the total response. The effectiveness of the attached submerged annular plate in reducing the motion of VLFS has been confirmed by the analysis.

A boundary-integral method for the interaction of large-amplitude ocean waves with a compliant floating raft such as a sea-ice floe

The interaction of large-amplitude water waves with a compliant floating raft such as a sea-ice floe or a pontoon-type VLFS (very large floating structure) is considered. The solution is expressed as a series using a perturbation expansion, the first two components of which are solved inductively using a boundary-integral method. The primary interest of this paper is to the ways in which the second-order potential can be modified in order to apply the boundary-integral method and to the comparison of results with those derived using eigenfunction matching methods.

Analysis of the slowly varying drift force on a very large floating structure in multidirectional random seas

In designing the mooring system of a very large floating structure (VLFS), it is essential to estimate the slowly varying drift force in random seas. For a small vessel, Hsu's method or Newman's approximation may be used to simulate this slowly varying drift force. However, based on experiments and/or field observations, it was found that the slowly varying drift force acting on a VLFS could be reduced to a great extent from the simulated values based on those methods. Thus, the conventional methods are not applicable for a VLFS. This discovery led to the development of several methods for estimating the slowly varying drift force on a VLFS, e.g., Namba et al. (J Soc Nav Archit Jpn 186:235–242, 1999), and Shimada and Maruyama (J Soc Nav Archit Jpn 190:347–351, 2001). However, Namba's method is only applicable to a pontoon-type VLFS with a shallow draft, and Shimada's method is too simplified to account for the general shape of a VLFS and elastic deformation. These methods have been expanded in this article, and by our proposed method, any shape of VLFS and the effect of elastic deformation of the VLFS can be included. Formulations and several numerical examples are given.

폰툰식 VLFS의 초기구조설계에 관한 연구

In general the loads due to ocean wave are considered as main design parameters governing the global structural safety of VLFS (Very Large Floating Structure). In order to predict design wave loads accurately, hydro-elastic analysis must be conducted considering the initial global flexural rigidity of VLFS. However, in order to determine the structural scantling of major members (deck, bottom, side panels and longitudinal / transverse BHD etc.), static load and design wave loads must be given as explicit form generally. Therefore in order to determine a proper structural arrangement and scantlings of VLFS at initial design stage, both calculations of structural scantling and hydro-elastic analysis for wave conditions must be conducted iteratively and the convergence of their results must be checked. On this paper, based on the case design of a 500×300 m size's floating marina resort, the details of structural design technique using hydro-elastic analysis are explained and discussed. At first, the environmental conditions and the system requirements of the design of marina resort are described. The scantling formulas for the major members of pontoon type VLFS are proposed from the local and global design points of view. Considering the design wave loads as well as static design loads, the structural safety is checked iteratively.

Structural analysis for the design of VLFS

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.

불균일 강성을 갖는 폰툰형 구조물의 유탄성 응답 특성에 관한 실험 연구

Very Large Floating Structure(VLFS) is regarded as one of promising candidates for the future utilization of ocean space. VLFS has the merits of small environmental effect. short construction term, easiness for extension and removal. It is well known that hydroelastic response is one of major design concerns of such a huge structure. Most of studies on the hydroelastic analysis of VLFS assumed uniform mass and bending stiffness. In case of a floating hotel where noticeable change of mass and stiffness at the hotel part is expected. it is necessary to investigate the effect of nonuniform mass and bending stiffness on the hydroelastic response. A model test of a pontoon type VLFS with nonuniform bending stiffness carried out for performance evaluation of a floating marina-hotel-convention center is described in this paper. Through investigation of model test results and comparison with numerical analysis using eigenfunction method, effect of the variation of bending stiffness is discussed.

弾性変形を考慮したポンツーン型浮体に作用する変動波漂流力の解析

A pontoon type Very Large Floating Structure (VLFS) is expected to be implemented in the near future. Design of mooring system of VLFS requires an estimate of slowly-varying wave drift force. However, slowly-varying wave drift force on elastic VLFS has not been precisely computed yet. Slowly-varying wave drift force is second order wave force with respect to the wave height. Computation of second order wave force is very complicated and difficult because it requires computation of second order velocity potential. The authors have resolved this problem by using assistant radiation potential method, restricting the model for analysis to a pontoon type VLFS. In this paper, computation of slowly-varying wave drift force on elastic VLFS has been made, and comparison between direct computation and well-known approximation (Newman's approximation) is presented.

Hydroelastic analysis of pontoon-type VLFS: a literature survey

Presented herein is a literature survey of the research on hydroelastic analysis of pontoon-type very large floating structures (VLFS). After a brief introduction of VLFS, the reader is provided with the basic assumptions, equations and boundary conditions for a hydroelastic analysis of VLFS and the commonly used approaches for solving the problem. Based on a comprehensive search, research papers that contain significant contributions to the aforementioned topic are grouped under the following topics: wave forces, drift forces and other forces, VLFS models, VLFS shapes, mooring system, breakwaters, profiles of seabed, and anti-motion devices. More importantly, some future directions for VLFS research are articulated. In addition to providing a long list of papers, we also include a list of relevant conference proceedings, and websites containing valuable information on VLFSs.

해수순환 방파제를 고려한 폰툰형 구조물의 유탄성응답 해석

Ocean space utilization using VLFS(Very Large Floating Structures) can provide environmental impact free space by allowing sea water flow freely through the floating structure. Use of Pontoon type VLFS for that purpose needs employment of breakwaters for reduction of wave effects. Therefore, in order to maximize advantage of environmental impact free structure, the breakwater should be the one that can allow water flow freely through it, too. In this paper hydroelastic response of a pontoon type structure is analyzed considering breakwaters which allow water flow through its opening at bottom of the breakwaters. Mode superposition technique is used for solving equation of flexible body while interactions between the pontoon and breakwaters is considered based on generalized mode concept. Bi-quadratic nine node higher-order boundary element method is adopted for more accurate numerical treatment near sharp edged body shape. Performance of various combinations of breakwaters is investigated.

Structural safety assessment of a pontoon-type VLFS considering damage to the breakwater

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.

흘수가 폰툰형 초대형 구조물의 유탄성 응답에 미치는 영향 해석

본 논문에서는 고차경계요소법을 사용하여 폰툰형 초대형 구조물의 유탄성 응답을 해석하였다. 폰툰형 구조물의 설치가 예상되는 천수심 해역에서 유탄성 응답 해석에서 흘수의 영향을 고찰하였다. 초대형 구조물인 경우 실제 해상파의 파장 범위인 단파장 구간일 때 흘수의 영향을 무시한 해석은 구조물의 유탄성 응답과 상대파고를 과소평가 하는 것으로 나타나 폰툰형 구조물에서도 흘수 영향을 고려한 해석이 중요함을 보였다. Present study aims to investigate draft effects on hydro-elastic response of pontoon type VLFS(Very Large Floating Structure). A three dimensional higher-order boundary element method(HOBEM: Hong et al;1999, Choi, Hong and Choi; 2001) is extended to analyze elastic response of structures. Intensive numerical calculations were carried out for box type structure to investigate the draft effect on hydrodynamic forces on pontoon type VLFS. Main attention was paid to wave run-up along the waterline for various cases of draft scantling. It is found that the draft effects on the hydro-elastic response of pontoon type VLFS are important especially in short wave range and shallow water region.

On the prediction method of a hydroelastic responses of a VLFS considering a seabed topography

In recent years, there are a lot of researches on hydroelastic responses of a Very Large Floating Structure (VLFS) which has several kilometers both in length and width. In the most of these researches, a geometric shape of the model is pontoon-type rectangle VLFS and it assumes that the sea depth is constant. But if we have a plan to construct such large structure in actual coastal area, it is quite difficult to find a vast flat bottom of the sea in Japan. So, in this paper we propose a prediction method on a hydroelastic behavior of VLFS extending the eigen functions expansion method to that including the sea bottom topographical effects. In the analyses the hydroelstic effect of the VLFS is also analyticaly taken into account. One of the improve points of the present method is that the problem of any supportive type of VLFS (i.e. pontoon supportive, column-supportive etc.) can be analyzed by this method. In other words, we can treat the pontoon type problem as a box-like shape column supportive type.Finally, some numerical results are carried out and discussions on the validity of the present method and the effects of seabed topography are made in this paper.

The hydroelastic response of very large floating structure suppoted by footing-columns (2nd report)

A Very Large Floating Structure (VLFS) is expected as a new utilization of the ocean space. Rigidity of VLFS is, however, relatively small, so that an hydroelastic response of VLFS in waves should be considered. Many researchers studied on the elastic response of a pontoon type VLFS, in addition, on that of column support type VLFS. The VLFS supported by columns shows the smaller elastic response than that of the pontoon type VLFS. In the 1st report of this paper, we have discussed about hydrodynamic responses of a column with footing which has a wave-free frequency on wave exciting force and also examined the hydroelastic responses of a VLFS supported by a large number of the columns in regular waves.In this paper, we investigate on the charcteristics of hydroelastic behaviors of a VLFS supported by a large number of footing-columns which have two wave-free frequencies. And then, we disccuse about the hydroelastic behaviors of a VLFS in irregular seas based on spectral analyses. Furthermore we compare the hydroelastic responses of VLFSs among supported by columns without wave-free frequency, supported by columns with one wave-free frequency and supported by columns with two wave-free frequencies. In the analyses, the hydrodynamic forces of the single column are calculated by using 3-dimensional source sink method, and the hydrodynamic characteristics of VLFS are obtained by using hydro interaction theory.

Structural modeling for global response analysis of VLFS

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.

Study on Critical Condition of VLFS with Large Aspect Ratio

In recent years, many researchers investigated the structural response of pontoon type VLFS (Very Large Floating Structure). They made clear, that the waves often excite the structure outstandingly in the specific wave directions. For the design of VFLS, we have to understand sufficiently the relation between the structural response and incident wave angles.In this paper, the structural response of the VFLS, which has a rectangular plane configuration with large aspect ratio, is examined. In the results, the critical condition, that induces peak strain energy of the floating body, is presented.