Abstracts:A hydroelastic model is considered to examine the proliferation of water waves over little deformation on a versatile seabed. The versatile base surface is modelled as a thin large plate and depends upon Euler-Bernoulli beam equation. In such circumstances, two different modes of time-harmonic proliferating waves exist rather than one mode of proliferating waves for any particular frequency. The waves with smaller wavenumber proliferate along the free-surface and the other with higher wavenumber spreads along the versatile base surface. The expression for first and second-order potentials and, henceforth, the reflection and transmission coefficients upto second-order for both modes are acquired by the strategy in view of Green's function method. A fix of sinusoidal swells is considered for instance to approve the scientific outcomes. It is seen that when the train of occurrence waves engenders because of the free-surface unsettling influence or the flexural wave movement in the fluid, we generally acquire the reflected and transmitted vitality exchange from the free-surface wave mode to the flexural wave mode. Further, we understand that the practical changes in the flexural unbending nature on the versatile base surface have a remarkable effect on the issue of water wave proliferation over small bottom distortions.

Abstracts:The aim of this paper is to develop a coupled nonlinear time-domain simulation scheme for nonlinear interactions among sloshing flows and floating body motion for both regular and irregular wave excitation. The contributions of a variety of nonlinear factors, outside waves and inside sloshing induced forces, as well as their influences on body coupled sway and roll motion were investigated. The induced forces are due to the changes in the transient wet surface of the floating body and full nonlinear sloshing. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion, as well as the relationship between sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions were also investigated and the results are presented. Finally, the numerical solutions are compared with existing experimental result. The fully nonlinear sloshing and floating body coupled motion are simulated based on the potential flow theory, with the transient position hydrodynamic assumption. The boundary value problem is solved by the B-spline higher-order panel method. The ISITIMFB (iterative semi-implicit time integration method for floating bodies) is applied to solve for the body's velocity and displacements. The sloshing energy dissipation is modeled by changing the boundary condition on the tank's solid boundaries. An extended principle to determine the energy dissipation coefficient for both regular and irregular cases is extracted. Then, the sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions are investigated, and the numerical solutions are compared with existing experimental results. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion is also investigated.

Abstracts:A time domain method is developed for estimating the transient response of very large floating structures under unsteady external loads. The hybrid frequency-time domain approach based on Cummins equations is adopted. First, the discrete-module-beam-bending-based hydroelasticity method in which a flexible structure is discretised into several rigid submodules connected by beam elements is used to establish the equations of motion for a flexible structure in frequency domain. The equations of motion in frequency domain are transformed into time domain following the idea of Cummins equations. The unsteady external loads at any point of the flexible structure are transferred to the centre of gravity of each submodule using the hydrostatic analysis of a beam in calm water. Good agreements are obtained between the numerical and experimental data for a weight drop test and a moving point load test of a continuous flexible structure in calm water, which validates the present time domain method. Finally, the time domain method is used for simulating the transient response of an interconnected flexible structure under the combination of wave and unsteady external loads, which highlights the significant effects of moving point loads on both the vertical displacement and bending moment of the structure.

Abstracts:This paper experimentally investigates the performances of two bottom-hinged-flap wave energy converters (BHF-WECs) operating in tandem. Firstly, the inertia effects of a single BHF-WEC operating in isolation are theoretically and experimentally examined to show their negligibility in coastal-water applications with the non-dimensional wave number ∼ O(1). Experiments of two in-tandem BHF-WECs with separation distance (d) varing from 0.5λ to λ (incident wavelength) with increment of λ/16 are then conducted in a small wave basin. Hydraulic pumps are used as the power take-off and their forcing characteristics as an energy damper are determined using a rating platform. Time-series images of flapping motions of the BHF-WECs, recorded with high-speed cameras, are analyzed using a processing and analysis procedure based on the correlation algorithm to evaluate their capture factors (CF). The CF variations of the first BHF-WEC with d exhibit a sinusoidal wave form with wavelength of λ/2. The combined CF of the two in-tandem BHF-WECs is always greater than 1.0, indicating that the wave energy outside the frontal area is drawn via diffraction. It reaches the maximum at around d = 14λ/16 with λ/4 apart from the minimum at around d = 9λ/16 owing to the underlying mechanism of the Bragg Reflection.

Abstracts:In this work we present the inter-comparison of wave hindcasts using third generation models WAM and WAVEWATCH (WWIII) for the North Indian Ocean at a 1° × 1° (lat × long) grid resolutions and we show a first assessment of their relative performance by inter-comparing the model results to observational data at selected points in the Arabian Sea and Bay of Bengal. WAM and WWIII inter-comparison studies are carried out for the year 2000 and for the period June 2008 to June 2009. Overall, the inter-comparison shows that both wave models are rather skillful in predicting the integral wave parameters; with lesser PE of the range 8.9–26.7% using WAM than WWIII. It is also quite evident that the WWIII model has a tendency to overestimate mean wave periods, while the opposite is true for WAM model. Further, the validation results using altimeter measurements are quite promising in the Arabian Sea and Bay of Bengal. The study further suggests that, it would be wise to use long-term measurements both in deep and coastal waters of North Indian Ocean to validate and inter-compare WAM and WWIII further, and they may also be coupled with SWAN for the nearshore waters.

Abstracts:Health monitoring of mooring lines is essential to ensure the safe performance of floating structures during the service life. In the literature and offshore industries, damage diagnosis of mooring lines is based on fatigue analysis by considering rope behavior. Mostly, this type of diagnosis is accomplished by the results, obtained from the simulation model of mooring system. Further, one of the important factors in modeling is applying uncertainties in the simulation model. In this paper, due to the complex behavior of mooring lines, a new design of Radial Basis Function (RBF) neural network is proposed for damage diagnosis. Also, the modeling method is based on Rod theory and Finite Element Method (FEM). In the proposed modeling process, for improving the accuracy of the modeling, boundary conditions uncertainty are applied using Submatrix Solution Procedure (SSP). Additionally, round-off error is removed by SSP. Finally, the proposed modeling and diagnosis are investigated experimentally. The obtained results showed that proposed RBF has better performance compared with conventional one and other well-known methods in the literature.

Abstracts:Vortex-induced motions (VIM) is becoming a noteworthy issue for column-stabilized floating platforms, mainly due to its substantial fatigue damage to risers and mooring system. The VIM of deep-draft semi-submersible is more complex than single column floaters because of the wake interference between columns, as well as the considerable yaw motions. In the present work, a numerical approach for simulating VIM of deep-draft semi-submersible is proposed. Specifically, detached-eddy simulation is used for turbulence modeling and dynamic overset grid technique is used for moving objects. Simulations for stationary drag and VIM of a model-scale paired-column semi-submersible are conducted with the proposed approach. The numerical results are compared with experimental data. Transverse, in-line and yaw motions are allowed during VIM simulations and are further analyzed in frequency domain by Fast Fourier Transform (FFT). Different VIM characteristics are observed at different current velocities. The work done by each component of the structure is also discussed. Flow visualizations are presented for better understanding of the wake interferences during VIM. The accuracy and reliability of the current numerical approach is assessed.

Abstracts:Ships in congested waterways are more prone to collision than open sea conditions. Although in most waterways, the overall accident probability is known, their spatial distribution, even for the most dangerous waterways in the world, is not commonly available as a detailed map. Existing solutions distribute the ships along predetermined routes which distorts the actual spatial distribution.

Abstracts:Slow strain rate tensile (SSRT) test in a simulated device was employed to investigate the effect of SO2 on stress corrosion cracking (SCC) behavior of E690 steel in SO2-polluted marine atmosphere. Results revealed that SO2 can greatly enhance the SCC susceptibility of E690 steel in marine atmosphere and the SCC mechanism in this environment is a combination of anodic dissolution (AD) and hydrogen embrittlement (HE). The increase of SCC susceptibility was attributed to the formation of a compact rust layer on steel surface that SCC microcracks can initiate from the bottom of cracks in the rust layer consequently. Moreover, hydrogen evolution was greatly enhanced with the increase of SO2 content, and SCC susceptibility increased rapidly as a result.

Abstracts:Flexible risers are being required to be installed in a water depth of over 3000 m for fewer remaining easy-to-access oil fields nowadays. Their innermost carcass layers are designed for external pressure resistance since the hydrostatic pressure at such a water depth may cause the collapse failure of flexible risers. Determining a critical collapse pressure for the carcass is of great importance to the whole structural safety of flexible risers. However, the complexity of the carcass profile always makes FE analysis computational intensive. To overcome that problem, the treatment of the interlocked carcass as an equivalent layer is adopted by researchers to accelerate the anti-collapse analyses. This paper presents an equivalent layer method to enable that treatment, which obtains the equivalent properties for the layer through strain energy and membrane stiffness equivalences. The strain energy of the carcass was obtained through FE models and then used in a derived equation set to calculate the geometric and material properties for the equivalent layer. After all the equivalent properties have been determined, the FE model of the equivalent layer was developed to predict the critical pressure of the carcass. The result of prediction was compared with that of the full 3D carcass model as well as the equivalent models that built based on other existing equivalent methods, which showed that the proposed equivalent layer method performs better on predicting the critical pressure of the carcass.