Wave Diffraction Forces Research Articles

Comparison of wave diffraction forces on a surface-piercing body for various free-surface grid update schemes

This study analyzed the difference in wave diffraction force of a surface piercing body according to the grid update scheme of free surface nodes in the time integration process. A two-dimensional fully nonlinear numerical wave tank based on potential flow theory and boundary element method was used for numerical analysis. The MEL (mixed Eulerian–Lagrangian) method was used for free surface node treatment. This method was divided into a semi-Lagrangian approach and a full Lagrangian approach (material node approach).The Runge–Kutta 4th-order integration method was used to integrate the free surface boundary condition temporally. In the internal sub-time step of the time integration process, the frozen coefficient scheme and the full update scheme were used according to the node update scheme, respectively. The vertical diffraction forces of each frequency component for node update schemes were compared. The difference in the 2nd-order vertical forces according to wave frequency, wave steepness, time step interval, and node spacing was investigated. A more precise time-domain numerical analysis method was proposed for high-order external forces in the wave-floating body interaction.

Understanding the fluid–structure interaction from wave diffraction forces on CALM buoys: numerical and analytical solutions

ABSTRACT This research fills the gap in understanding fluid–structure interaction (FSI) from wave diffraction forces on CALM buoys and cylindrical structures, based on the hydrodynamics with connections. Recently, there is an increased application of (un)loading marine hoses for Catenary Anchor Leg Moorings (CALM) buoy systems in the offshore industry due to the need for more flexible marine structures that are cost-saving, easier to install, and service. However, different operational issues challenge these hoses, like during hose disconnection. Also, the fluid behaviour was investigated based on the analytical and numerical models. The numerical modelling involves the boundary element method (BEM) and Orcaflex line theory. Hydrodynamic analysis is conducted on the disconnection-induced load response of marine bonded hoses during normal operation and accidental operation under irregular waves. A comparative study on hose performance during normal operation and accidental operation is also presented. Results of statistical analysis on CALM buoy system shows good motion characteristics.

Global performances of a semi-submersible 5MW wind-turbine including second-order wave-diffraction effects

The global performance of the 5 MW OC4 semisubmersible floating wind turbine in random waves was numerically simulated by using the turbine-floater-mooring fully coupled and time-domain dynamic analysis program FAST-CHARM3D. There have been many papers regarding floating offshore wind turbines but the effects of second-order wave-body interactions on their global performance have rarely been studied. The second-order wave forces are actually small compared to the first-order wave forces, but its effect cannot be ignored when the natural frequencies of a floating system are outside the wave-frequency range. In the case of semi-submersible platform, second-order difference-frequency wave-diffraction forces and moments become important since surge/sway and pitch/roll natural frequencies are lower than those of typical incident waves. The computational effort related to the full second-order diffraction calculation is typically very heavy, so in many cases, the simplified approach called Newman\'s approximation or first-order-wave-force-only are used. However, it needs to be justified against more complete solutions with full QTF (quadratic transfer function), which is a main subject of the present study. The numerically simulated results for the 5 MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model test results by Technip/NREL/UMaine. The predicted motions and mooring tensions for two white-noise input-wave spectra agree well against the measure values. In this paper, the numerical static-offset and free-decay tests are also conducted to verify the system stiffness, damping, and natural frequencies against the experimental results. They also agree well to verify that the dynamic system modeling is correct to the details. The performance of the simplified approaches instead of using the full QTF are also tested.

Hydrodynamic Analysis of Oil Storage Vessels for National Strategic Petroleum Reserve

This paper expatiates hydrodynamic time-domain analysis on strategic oil storage vessels in free floating condition or with dolphin-fender mooring system by means of AQWA numerical software. The results indicate that motion responses of the oil storage vessel with dolphin-fender mooring have improved significantly. The time-history of wave diffraction forces in incident wave direction are basically consistent between the free floating and moored condition. However, in vertical direction of incident wave, the diffraction wave force of the oil storage vessel in free floating condition is obviously different from that with mooring system, which is the result of the coupling interaction between dolphin and vessels. Mooring forces include fender reaction and cable tension, as for which dominate, it depend on evocable motion responses about incident wave direction.

Scaled Boundary FEM Model for Interaction of Short-Crested Waves with a Concentric Porous Cylindrical Structure

This paper describes the development of an efficient scaled boundary finite-element model (FEM) for the simulation of short-crested wave interaction with a concentric porous cylindrical structure. By weakening the governing differential equation in the circumferential direction, the SBFEM is able to solve analytically the weakened equation in the radial direction. Only the cylinder boundary on the circumference of the exterior porous cylinder is discretized with curved surface finite elements, while a complete analytical representation is obtained for the radial differential equation. Comparisons of the numerical results on wave diffraction forces and surface wave elevations at the cylinder to available analytical solutions demonstrate that excellent accuracy can be achieved by the SBFEM with a very small number of surface finite elements. The influence of varying the wave parameters as well as the system configuration on the system hydrodynamics, including the wave force, wave run-up, and diffracted wave contour is examined and extensive results on them are presented. This parametric study will help determine the various hydrodynamic effects of a concentric porous cylindrical structure.

Linear and nonlinear irregular waves and forces in a numerical wave tank

A time-domain higher-order boundary element scheme was utilized to simulate the linear and nonlinear irregular waves and diffractions due to a structure. Upon the second-order irregular waves with four Airy wave components being fed through the inflow boundary, the fully nonlinear boundary problem was solved in a time-marching scheme. The open boundary was modeled by combining an absorbing beach and the stretching technique. The proposed numerical scheme was verified by simulating the linear regular and irregular waves. The scheme was further applied to compute the linear and nonlinear irregular wave diffraction forces acting on a vertical truncated circular cylinder. The nonlinear results were also verified by checking the accuracy of the nonlinear simulation.

Second-order wave diffraction forces on a vertical circular cylinder due to short-crested waves

A complete solution is presented in closed-form for the velocity potential, up to the second-order of wave amplitude, of the non-linear short-crested waves being diffracted by a vertical cylinder. The solution is found to consist of the forced waves generated from the nonhomogeneous free surface boundary condition and the scattered waves due to the presence of the cylinder. Non-uniform radiation conditions suggested by Kriebel [(1990), Nonlinear wave interaction with a vertical circular cylinder.—I. Diffraction theory. Ocean Engng 17, 345–377] are imposed at the second order to ensure the forced waves propagate towards infinity. The solution is expressed in the form of series, each term of which contains singular integrals to be evaluated numerically. With the adoption of the Hankel transform as well as other numerical techniques, the numerical convergence of the calculation of the singular integrals and the summation of the series can be speeded up; a presentation of the numerical results becomes possible. From our preliminary numerical results, the second-harmonic terms resulting from self-interaction of incident waves are found to play a very important role in determining the total second-order wave-induced forces. Our quantitative results also show that second-order forces induced by short-crested waves on a vertical circular cylinder can be quite large; the contribution of the second-order forces toward the total wave-induced forces is important and therefore should be taken into consideration in offshore engineering design and operation processes.

Second order wave diffraction forces and runup by finite—infinite element method

This paper presents a finite-infinite element method for solving the second order wave diffraction problem. The formulation of the problem is based on the inhomogeneous far field condition and its corresponding higher order asymptotic solutions for the second order diffracted potential suggested by Li 9 , and follows the finite-infinite element method as used by Lau and Ji 18 . The interpolation in the infinite element along the radial direction has a far field decay rate compatible diffraction by a vertical surface-piercing circular cylinder. Numerical solutions of the second order diffracted potential, satisfying all the governing and boundary conditions as well as the inhomogeneous far field condition, are obtainable with the method. Numerical tests of the method give very good results of wave forces and runup profiles when compared with those published by other researchers.

Experimental Studies on the Non-linear Hydrodynamic Forces Acting on Bodies Oscillating in Low Frequencies

Hydrodynamic forces acting on bodies with rectangular and circular sections were measured. Nonlinear forces were found to exist. Radiation forces on bodies forced to sway in simple and in combined two-harmonic oscillations at low and high frequencies were measured. Hydrodynamic coefficients obtained from the simple harmonies differed from the two-harmonic oscillations. Wave diffraction forces on bodies oscillating at low frequencies were also measured and found to be different from those acting on still bodies. These differences are considered as nonlinear hydrodynamic forces and have good correlation with the increases in the modified Keulegan- Carpenter number proposed in this paper for the case of combined oscillations.