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Journal Article•DOI•

Performance characteristics of a conceptual ring-shaped spar-type VLFS with double-layered perforated-wall breakwater

Yufeng Kou1, Longfei Xiao1, Longbin Tao2, Tao Peng1•
Shanghai Jiao Tong University1, University of Strathclyde2
01 May 2019-Applied Ocean Research (Elsevier BV)-Vol. 86, pp 28-39
TL;DR: In this article, a ring-shaped spar-type very large floating structure (VLFS) is proposed as an intermediate base for supporting deepwater resource exploitation far away from the coast line, where a double-layered perforated-wall breakwater is vertically attached to the VLFS and pierces through the water surface to attenuate the wave energy in the inside harbor.
Abstract: A ring-shaped spar-type Very Large Floating Structure (VLFS) is proposed as an intermediate base for supporting deepwater resource exploitation far away from the coast line. The proposed VLFS is composed of eight rigidly connected deep-draft spar-type modules and an inside harbor. A double-layered perforated-wall breakwater is vertically attached to the VLFS and pierces through the water surface to attenuate the wave energy in the inside harbor. The hydrodynamic performance characteristics of the ring-shaped VLFS was experimentally evaluated in the present study, focusing on the motion responses, wave elevations, and wave run-ups. The natural periods of the motions in vertical plane were determined to be larger than 40 s, which is much larger than common wave periods. This enhanced the motion performance in vertical plane and afforded favorable habitation and operation condition on the VLFS. A large surge damping was induced by the vertical breakwater, which tended to significantly affect the surge and pitch motions, but had a negligible effect on the heave motion. The component frequencies of the wave elevations in the inside harbor and the wave run-ups were identical with those of the incident waves. The wave attenuation was frequency-dependent and effective for the common wave frequencies, with a smaller loss coefficient observed in higher sea state. The wave attenuation and wave run-ups tended to improve in the absence of the leeward walls.

Summary (3 min read)

Jump to: [1. Introduction] – [2.1 Conceptual design and models] – [2.2 Experimental campaign or set-up] – [3.1 Natural period and damping] – [3.2 Motion transfer function] – [3.3 Motion response] – [3.4 Wave elevation and attenuation] – [3.5 Wave run-ups] and [4. Conclusions]

1. Introduction

  • Various concepts of Very Large Floating Structure (VLFS) composed of different types of modules have been proposed for a variety of applications such as floating airports, mobile offshore bases, floating bridges, floating fuel storage facilities, floating piers, floating offshore aquacultures, offshore renewable energy bases, and even floating cities [1].
  • In addition to studies on the traditional rectangular type [5,6], Hamamoto and Fujita [7] also analyzed L-shaped, T-shaped, C-shaped, and X-shaped VLFSs.
  • The semisubmersible-type VLFS not only utilizes the low waterline area columns to support the long-length deck but also uses the submerged pontoons to maintain the constant buoyancy, bringing about the reduction of the effects of waves and the wave-induced motion response.
  • The VLFS has self-mobile and stationkeeping capabilities, which are enabled by a sail control system and the semi-submersible hull structure.
  • Owing to the complicated wave-wave and wavestructure interactions, the breakwater tends to significantly affect the hydrodynamic performance of the VLFS, including wave reflection, diffraction and transmission, energy dissipation, wave run-up, motion response, and mooring loads.

2.1 Conceptual design and models

  • It consists of eight deep-draft spar-type modules with rigid connectors.
  • The VLFS can be opened as two semiring parts to let the work boat come into the inside harbor.
  • Every module has a breakwater consisting of one set of double-layered porous plates with rectangular openings, and the porous plates are vertically attached to the columns of the module and pierce through the water surface.
  • The VLFS and module models are built for wave test according to the linear scale ratio of 1:80, as shown in Fig.1 (a) and (b).
  • The sizes and distributions of the rectangular openings of the outer and inner porous plates are also different, resulting in different porosities.

2.2 Experimental campaign or set-up

  • To investigate the hydrodynamic performance characteristics of the ring-shaped VLFS, an experimental study was conducted in the Deepwater Offshore Basin of Shanghai Jiao Tong University in China.
  • The length, axial stiffness and pretension of each slack line are 34 m, 12.82 kg/m and 2 kg in model scale, respectively.
  • The motion response, wave elevation, and wave run-up were measured in all wave tests.
  • The wave probes WP1 and WP7 were fixed 5 m away from the VLFS model in the basin to measure the wave elevations at the upstream and the lateral side of the structure, while the wave probe WP4 was located at the center of the VLFS model to measure the wave elevation in the inside harbor.
  • The frequency-)(R S dependent transfer function of the motion can be calculated by the similar method.

3.1 Natural period and damping

  • The natural periods and non-dimensional damping coefficients were determined by the decay tests.
  • The results for both the VLFS and the module with the breakwater are compared in Table 3.
  • As can be observed, the natural periods of the motions in vertical plane are substantially larger than dominant wave periods owing to the very large masses of the structure.
  • Based on the test results of the module, the breakwater increased both the natural period and the damping of the roll and pitch motions, while the effect on the heave motion was negligible.
  • Fig.5 obviously shows that the pitch of the module decays rapider with a larger period than that without the breakwater.

3.2 Motion transfer function

  • The result of the white noise wave test Case 1 was used to obtain the linear transfer functions of the motions for a wide range of wave frequencies, as shown in Fig.7 for the VLFS.
  • With regard to the heave and pitch motions, it can be seen that the values of the transfer functions for the frequencies higher than 0.2 rad/s are quite small.
  • For wave frequencies higher than 0.25 rad/s, the effect of the breakwater on the motion response is negligible.
  • For the lowfrequency motions, the values of the transfer functions for the module with the breakwater are obviously smaller than those for the module without the breakwater.
  • With regard to the wave frequency motions, there is no obvious difference in the transfer functions, which means the damping effect of the breakwater on reducing the motion response tends to be counterbalanced by the increment of additional wave forces due to the breakwater.

3.3 Motion response

  • The results of the irregular wave tests were used to determine the total motion responses corresponding to full frequency range, and were further filtered to obtain the Low-Frequency (LF) and High-Frequency (HF) responses.
  • The table reveals that, not only the surge motion, but also the heave and pitch motions are mainly low-frequency responses.
  • The good motion performance in vertical plane tends to favor habitation and operations on the VLFS.
  • Both the mean value and variance of the surge motion clearly decrease with decreasing the breakwater height and without the leeward walls, which indicates that the breakwater with the vertical porous walls significantly affects the horizontal motion response owing to the change in the wave interaction forces.
  • The peak spectrum frequency of the surge motion varies with the conditions in Fig.10 (a), while those of the heave and pitch motions are constant in Fig.10 (b) and (c).

3.4 Wave elevation and attenuation

  • As an intermediate base for supporting offshore resource exploitation, the ring-shaped VLFS is used for the mooring and sheltering of work boats.
  • At the frequencies higher than 1.4 rad/s, the transfer function values become close to 1.0 again, indicating the disappearance of wave attenuation.
  • Since the wave frequencies commonly range between 0.4 and 1.4 rad/s, the wave attenuation tends to be evident and the breakwater works well as a result of the wave dissipation by the two-layered perforated walls.
  • Under the 1-year condition as shown in Fig.12 (a), the outside wave is close to the incident wave at the upstream side, while it is smaller in the lateral direction.
  • The energy is particularly low in Case 4 for the VLFS without the leeward walls, in which case the inside waves tend to be transmitted downstream and not reflected in the inside harbor.

3.5 Wave run-ups

  • Wave run-up is one of the critical issues of both the design and utilization of deepwater structures exposed to harsh environments.
  • In addition, a uniform peak frequency of 0.16 rad/s for all four measurements exists and is equivalent to the natural frequency of heave motion.
  • The outer wave run-ups are more significant than the inner ones owing to the wave attenuation by the breakwater, as shown in both Fig.15 (a) and (b).
  • The determined coefficients are obtained by Eq. (8) and presented in Table 7.

4. Conclusions

  • The motion performance, wave elevations, and wave run-ups of a ring-shaped VLFS with a perforated-wall breakwater were experimentally investigated.
  • The vertical breakwater has a significant effect on the low-frequency motions.
  • The wave run-up energy is observed to be concentrated within the same frequency range as that of the incident wave, and there are small additional parts at the natural frequencies of the heave and pitch motions.
  • The wave run-up coefficients are slightly larger in higher sea state owing to the wave reflection in the inside harbor.
  • The ring-shaped spar-type VLFS experiences excellent motion performance in vertical plane which can afford favorable habitation and operation conditions.

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Figures (6)
Citations
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Potential flow theory-based analytical and numerical modelling of porous and perforated breakwaters: A review

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TL;DR: A comprehensive review on analytical and numerical applications of potential flow models on porous/perforated breakwaters, including the theories of porosity models, novel developments and applications of various analytical approaches can be found in this article .
Abstract: Analytical approaches and numerical tools play a significant role in the research and design of porous and perforated breakwaters. Before a new porous or perforated breakwater concept enters into the stage of physical model testing, careful parametric studies and mechanism studies need to be carried out to examine its feasibility and to ensure the success of the model test. Among the non-physical modelling approaches, analytical and numerical methods based on potential flow theory are often used as they are more efficient than those allowing for flow viscosity. Although the conditions of non-viscous, incompressible fluid and irrotational flow are assumed, the potential flow models can assist in understanding major mechanisms that determine the performance of porous/perforated breakwaters and provide satisfactory estimates on important hydrodynamic parameters. Consequently, they are a popular choice in the study of porous/perforated breakwaters until today. Despite the long history of application, the potential flow models continue to witness recent developments and improvements that aim to increase the efficiency and accuracy of the modelling. This paper gives a comprehensive review on the analytical and numerical applications of potential flow models on porous/perforated breakwaters, including the theories of porosity models, novel developments and applications of various analytical and numerical approaches.

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Jiabin Liu1, Qinghe Fang1, Anxin Guo1, Hui Li1•
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15 Jul 2020-Ocean Engineering
TL;DR: In this paper, empirical formulae of wave loads on quasi-ellipsoid-type foundations coaxially surrounded by perforated breakwaters were developed by using mathematical models in the framework of potential theory.
Abstract: This study provides empirical formulae of wave loads on quasi-ellipsoid-type foundations coaxially surrounded by perforated breakwaters. The variation of the wave load on each structure versus wave number is characterized as a polygonal line by four key points. The coordinates of these points can be directly obtained from the empirical formulae, which are developed by using mathematical models in the framework of potential theory. The empirical formulae can consider not only the geometric parameters and porous parameters but also the wave exposures and wave numbers. Comparison with the experimental results indicates that the empirical formulae can effectively determine the wave loads on the models under different wave scenarios. Further investigation is conducted on circular foundations, which are the most distinct case of quasi-ellipsoid-type foundations. The wave load magnitudes and phase differences are investigated by experimental and theoretical methods with the consideration of porous parameters and geometric parameters. The experimental results also validate the occurrence of the “phase jump” when the wave loads on the breakwater are minimum. Empirical formulae for the distribution of the “phase jump” points are also presented.

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Modelling wide perforated breakwater with horizontal slits using Hybrid-BEM method

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Mengmeng Han1, Chien Ming Wang1•
University of Queensland1
15 Feb 2021-Ocean Engineering
TL;DR: In this article, a hybrid boundary element method has been developed to efficiently solve the hydrodynamic response of a wide perforated breakwater with arbitrary hull geometry and multiple horizontal slits, where fluid subdomains inside the slits were considered as rectangular and analytical eigenfunctions of the slit area were applied to derive the boundary condition at the interface.
Abstract: A hybrid boundary element method has been developed to efficiently solve the hydrodynamic response of a wide perforated breakwater with arbitrary hull geometry and multiple horizontal slits. The fluid subdomains inside the slits were considered as rectangular and analytical eigenfunctions of the slit area were applied to derive the boundary condition at the interface. The involvement of analytical solutions in the numerical boundary element method (BEM) is the novelty of the present approach. The 2D numerical solution was verified with analytical solution of perforated rectangular breakwater to show the validity of the proposed hybrid BEM. Parametric studies have been performed to study the effect of porosity, position and layout of the slits. It has been found that the slits generally reduce the diffracted and radiation potentials, and would be most effective when placed near the free surface. The approach was also extended to 3D channel flow and the results were calibrated against an experimental study performed in the wave flume.

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Cites background from "Performance characteristics of a co..."

  • ...From equations (4) and (19), the boundary condition on the plate covered region is achieved by eliminating the plate deflection h3 is as follows "...

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  • ...Kou et al.19 proposed a ring-shaped VLFS to support deepwater resource exploitation....

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Journal Article•DOI•
Characteristics of Floating Arrays from Aligned to Staggered Patterns

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Rui Ding, Haicheng Zhang, Daolin Xu, Jiarui Liu, Leilei Liu, Yousheng Wu 
01 May 2023-Journal of Environmental Engineering
TL;DR: In this article , a floating array with 12 identical semisubmersible modules in a 3×4 configuration was used to study the effects of staggered structure on the dynamic characteristics of floating systems.
Abstract: In nature, staggered structures, such as knitted baskets and brick patterns in the structure of buildings, always exhibit the feature of structural reinforcement. Can floating arrays become more dynamically stationary if floating modules of an array are interconnected in a staggered pattern rather than in an aligned pattern? This work examined the dynamic characteristics of staggered floating arrays. A floating array with 12 identical semisubmersible modules in a 3×4 configuration was used to study the effects of staggered structure on the dynamic characteristics of floating systems. A network modeling method considering the geometric effect of connectors was used to establish the analysis model. The numerical analysis was conducted in three main aspects. First, the impact of the staggered deployment of modules on the stiffness feature of the floating structure was investigated. Subsequently, the variation of the stability of floating array with increasing staggered displacement was studied, and the corresponding dynamic mechanism is discussed in detail. Furthermore, the characteristic transition of the floating array evolving from aligned pattern to brick-pattern with different staggered displacements is studied. The numerical results demonstrate that the staggered deployment of modules could increase the floating system’s stiffness along the longitudinal direction and improve the stability of heave and pitch motions near the head wave. This work could provide guidelines for the configuration design of floating islands for optimal ocean space utilization.
References
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Journal Article•DOI•
Hydroelastic analysis of pontoon-type VLFS: a literature survey

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Eiichi Watanabe1, Tomoaki Utsunomiya1, Chien Ming Wang2•
Kyoto University1, National University of Singapore2
01 Jan 2004-Engineering Structures
TL;DR: A literature survey of the research on hydroelastic analysis of pontoon-type very large floating structures (VLFSs) is presented in this paper, where the reader is provided with the basic assumptions, equations and boundary conditions.
Abstract: 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.

297 citations

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Journal Article•DOI•
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Miguel Lamas-Pardo, Gregorio Iglesias1, Luis Carral•
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TL;DR: In this article, a review of the concept of VLFS, showing how they are deployed for both coastal and offshore areas, is presented, where the MOB project (Mobile Offshore Base) is the design that has been most fully developed.
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159 citations

"Performance characteristics of a co..." refers background in this paper

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Very Large Floating Structures: Applications, Research and Development

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Chien Ming Wang1, Zhi Yung Tay1•
National University of Singapore1
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Abstract: Very large floating structures (VLFS) have attracted the attention of architects, city planners, and engineers because they provide an exciting and environmentally friendly solution for land creation from the sea as opposed to the traditional land reclamation method. The applications of VLFS as floating piers, floating hotels, floating fuel storage facilities, floating stadia, floating bridges, floating airports, and even floating cities have triggered extensive research studies in the past two decades. The VLFS technology has developed considerably and there are many innovative methods proposed to minimize the hydroelastic motion, improve the mooring system and structural integrity of the VLFS. This keynote paper summarizes the applications, research and development of VLFS over the past two decades.

158 citations

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A B-spline Galerkin scheme for calculating the hydroelastic response of a very large floating structure in waves

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Masashi Kashiwagi1•
Kyushu University1
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TL;DR: In this article, an effective scheme for computing the wave-induced hydroelastic response of a very large floating structure is presented, and a validation of its usefulness is provided. But the scheme uses bi-cubic B-spline functions to represent unknown pressures, and the Galerkin method to satisfy the body boundary conditions.
Abstract: This paper presents an effective scheme for computing the wave-induced hydroelastic response of a very large floating structure, and a validation of its usefulness. The calculation scheme developed is based on the pressure-distribution method of expressing the disturbance caused by a structure, and on the mode-expansion method for hydroelastic deflection with the superposition of orthogonal mode functions. The scheme uses bi-cubic B-spline functions to represent unknown pressures, and the Galerkin method to satisfy the body boundary conditions. Various numerical checks confirm that the computed results are extremely accurate, require relatively little computational time, and contain few unknowns, even in the region of very short wavelengths. Measurements of the vertical deflections in both head and oblique waves of relatively long wavelength are in good agreement with the computed results. Numerical examples using shorter wavelengths reveal that the hydroelastic deflection does not necessarily become negligible as the wavelength of incident waves decreases. The effects of finite water depth and incident wave angle are also discussed.

124 citations

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Longbin Tao1, Daniel Dray1•
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01 Jul 2008-Ocean Engineering
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Abstract: Heave plates have been widely utilized in floating offshore structures as they can provide additional damping and added mass to improve the hydrodynamic response of the system. This study investigates the hydrodynamic characteristics (added mass and damping) of oscillatory solid or porous disks using model scale experiments. All experiments were conducted via forced oscillation model tests using a planar motion mechanism (PMM). The hydrodynamic coefficients of the solid or porous disk obtained from the force measurements are analysed and presented. The sensitivities of the damping and added mass coefficients to both motion amplitude and the disk porosity are examined.

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Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Performance characteristics of a conceptual ring-shaped spar-type vlfs with double-layered perforated-wall breakwater" ?

The hydrodynamic performance characteristics of the ring-shaped VLFS was experimentally evaluated in the present study, focusing on the motion responses, wave elevations, and wave run-ups.