Regular waves onto a truncated circular column:a comparison of experiments and simulations

Accurate prediction of hydrodynamic forces on offshore structures is critical for safe and cost effective design of fixed and floating offshore structures exposed to a harsh environment. In the present paper, nonlinear interactions between regular waves and a single surface-piercing truncated circular column have been investigated using a frequency domain potential flow solver (DIFFRACT) and a full CFD solver in OpenFOAM for direct comparisons. Both the predicted free surface elevation around the column and the total force acting on the column have been analysed and compared with experimental data from MOERI. The degree of non-linearity and the contribution of each harmonic to the free surface run-up and wave forces have been examined, and evaluations of the accuracy and computational efficiency of the potential flow solver and the full CFD solver are provided and compared in the paper. Also of note are the local forms of the scattered waves around the column in numerical simulations, which are consistent with the Type-1 and Type-2 patterns identified in physical experiments at Imperial College

Numerical Study of a Three-float Wave Energy Converter - M4

Researchers at the University of Manchester have developed a moored, three-body line absorber M4 (shown in Fig.1 (a)) that can extract wave energy from various modes of relative motions (surge, heave and pitch) between the floating bodies. Experimental studies have shown that high crest capture widths of wave energy conversion can be achieved across a broad band of frequencies and the peak capture widths are greater than 25% of a wavelength in regular waves [1]. In the present project, numerical analyses for the wave energy converter M4 have been carried out using a frequency-domain potential-flow solver DIFFRACT [2] and a two-stage approach [3] has been used. In the first stage, the excitation forces, added mass and radiation damping of M4 are calculated. In the second stage, the motion equations are solved considering both hydrodynamic interactions and mechanical connections between the floating bodies of M4. Viscous effects have been included in the numerical study, and the power take-off system (PTO) is simplified as a linear rotational damper. Numerical results of relative rotations and moments at PTO have been compared with experimental measurements (e.g. Fig.1 (b) and (c)). Good agreements have been achieved and further investigations have been carried out for assessing the performance of the M4 device in multi-directional waves

Extreme wave elevations beneath offshore platforms, second order trapping, and the near flat form of the quadratic transfer functions

Extreme free surface elevations due to wave-structure interactions are investigated to second order using Quadratic Transfer Functions (QTFs). The near-trapping phenomenon for small arrays of closely spaced columns is studied for offshore applications, and the excitation of modes by linear and second order interactions is compared. A simple method for approximating near-trapped mode shapes is shown to give good results for both linear and second order excitation. Low frequency near-trapped mode shapes are shown to be very similar whether excited linearly or to second order. Approximating surface elevation sum QTF matrices as being flat perpendicular to the leading diagonal is investigated as a method for greatly reducing lengthy QTF calculations. The effect of this approximation on second order surface elevation calculations is assessed and shown to be reasonably small with realistic geometries for semi-submersible and tension-leg platforms