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Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2017;140(3):031101-031101-10. doi:10.1115/1.4038500.

The interaction of surface gravity waves with horizontal pitching plate for actively control waves is investigated based on the linearized theory of water waves. The three-dimensional (3D) problem is formulated for the submerged plate pitching about its middle point and the other plate is considered to be floating above the submerged plate. The submerged plate's thickness is considered negligible in comparison with the water depth and wavelength of the incident wave. The study is carried out using the matched eigenfunction expansion method and the analytical solution is developed for the interaction of the surface gravity waves with horizontal submerged structure. The performance is analyzed for both impermeable and porous submerged pitching plate. The numerical results for the reflection coefficient, transmission coefficient, and free-surface deflection are computed and analyzed. The study is carried to find the optimal value of the length and depth of the submerged plate at which the dissipation of the incident wave energy is observed. The reduction the wave transformation due to the pitching of the plate with the change in angle of incidence is also analyzed. The present study will be helpful in the analysis of proper functioning of submerged pitching plate to control wave motion for the protection of offshore structures.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(3):031102-031102-11. doi:10.1115/1.4038501.

Motion responses of moored very large floating structures (VLFSs) in coastal regions are remarkably influenced by shallow water, seabed topography, and mooring system, which were given particular focus in this paper. A three-dimensional (3D) numerical model of a moored semisubmersible single module (SMOD) was described, and time domain simulated and experimentally validated. A catenary-taut-hybrid mooring system was adopted considering coastal space limitations. Large-scale catenary mooring lines were deployed on the deep water side, while taut chains were used on the shore side to decrease the anchor radius. Although the mooring system may induce a stiffness difference between the two sides, the effectiveness of the mooring system was demonstrated by time-domain simulation and model tests. The moored semisubmersible SMOD in shallow water exhibits significant low frequency characteristics. Water depth, asymmetric stiffness, and bottom topography effects were investigated by a series of sensitivity studies. The results show that these factors play an important role in motion responses of the moored SMOD, which can further conduce to better understandings on the hydrodynamic of the semisubmersible-type VLFSs.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;140(3):031301-031301-8. doi:10.1115/1.4038396.

A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes results in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin's (2001, “On the Piston and Sloshing Modes in Moonpools,” J. Fluid Mech., 430, pp. 27–50.) theoretical and Fukuda's (1977, “Behavior of Water in Vertical Well With Bottom Opening of Ship and Its Effects on Ship-Motion,” J. Soc. Nav. Archit. Jpn., 1977(141), pp. 107–122.) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross section. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and nonzero Froude number especially in the presence of waves.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2017;140(3):031801-031801-11. doi:10.1115/1.4038349.

Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semisubmerged circular cylinder. The 2D simulations are carried out by solving Navier–Stokes equations discretized by finite volume method. Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank (NWT). Validation studies have been performed by comparing the numerical results of free surface waves past the cylinder with the published experimental and numerical data. The present numerical results are in good agreement with both the experimental and the other numerical results in terms of hydrodynamic forces and free surface elevation. Subsequently, the effects of the wave height and the wavelength on wave–structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semisubmerged cylinder at different wave heights and the wavelengths. The averaged and maximum vertical wave forces on the cylinder increase with the increasing wave height. The numerical results for the irregular waves are compared with those induced by the regular waves in terms of the maximum and averaged vertical wave forces. When the significant wave height and the spectral peak period of the irregular waves are equal to the wave height and the wave period of the regular waves, the maximum vertical wave force induced by the irregular waves is larger than that induced by the regular waves, meanwhile, the average vertical wave forces have the contrary relationship.

Topics: Waves , Simulation , Cylinders
Commentary by Dr. Valentin Fuster

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