High-order mooring simulations for increased accuracy in wave energy applications

The prevailing simulation technique for floating wave energy converters (WECs) is to use the linear hydrodynamics in convolution form to solve for the motion of the floating structure. This allows for very fast time domain simulations with the possibility to include non-linear reaction forces from e.g. moorings and power take off (PTO). Although its accuracy for very large motion amplitudes is debated (Yu and Li 2015, Palm 2015), it is the best method available to make long-term simulations of WEC response. Brown and Mavrakos (1999) showed a big difference in mooring response depending on how the mooring cables of marine structures where modeled. As WECs are subjected to large motions in relatively shallow water, the differences between different methods are pronounced compared to traditional oil and gas structures, and the uncertainty in model choice is still to be properly quantified. We present a coupling between the open source code WECSim (NREL), and a finite element code for mooring cable dynamics that specializes in accurately capturing snap loads. The in-house mooring model is based on the discontinuous Galerkin method and provides very low numerical diffusion of load propagation. We also present a comparison between our mooring model and using the native, lumped mass mooring model of WECSim, which is a standard method for mooring dynamics. The effect of changing mooring model is evaluated in terms of structural WEC response, peak, mooring load and its potential impact on the fatigue damage

Dynamically Scaled Model Experiment of a Mooring Cable

The dynamic response of mooring cables for marine structures is scale-dependent, and perfect dynamic similitude between full-scale prototypes and small-scale physical model tests is difficult to achieve. The best possible scaling is here sought by means of a specific set of dimensionless parameters, and the model accuracy is also evaluated by two alternative sets of dimensionless parameters. A special feature of the presented experiment is that a chain was scaled to have correct propagation celerity for longitudinal elastic waves, thus providing perfect geometrical and dynamic scaling in vacuum, which is unique. The scaling error due to incorrect Reynolds number seemed to be of minor importance. The 33 m experimental chain could then be considered a scaled 76 mm stud chain with the length 1240 m, i.e., at the length scale of 1:37.6. Due to the correct elastic scale, the physical model was able to reproduce the effect of snatch loads giving rise to tensional shock waves propagating along the cable. The results from the experiment were used to validate the newly developed cable-dynamics code, MooDy, which utilises a discontinuous Galerkin FEM formulation. The validation of MooDy proved to be successful for the presented experiments. The experimental data is made available here for validation of other numerical codes by publishing digitised time series of two of the experiments

Assessment of Scale Effects, Viscous Forces and Induced Drag on a Point-AbsorbingWave Energy Converter by CFD Simulations

This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier–Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled, the response reduction is: (i) 3% due to the nonlinear mooring response and the Froude–Krylov force; (ii) 1–4% due to viscous forces; and (iii) 18–19% due to induced drag and non-linear added mass and radiation forces. The effect of the induced drag is shown to be largely scale-independent. It is caused by local pressure variations due to vortex generation below the body, which reduce the total pressure force on the hull.Euler simulations are shown to be scale-independent and the scale effects of the WEC are limited by the purely viscous contribution (1–4%) for the two waves studied. We recommend that experimental\ua0model scale test campaigns of WECs should be accompanied by RANS simulations, and the analysis complemented by scale-independent Euler simulations to quantify the scale-dependent part of the nonlinear effects

Coupled BEM/hp-FEM Modelling of Moored Floaters

A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hydrodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code (MoorDyn) that has an existing coupling to WEC-Sim. The two models are compared for a standard test case and the results are similar, giving confidence in the new WEC-Sim-MooDy coupling. Finally, the coupled model is validated using experimental data of a spread moored cylinder with good agreement

Датчики интегральной поглощенной дозы ионизирующего излучения на основе МОП-транзисторов

Определены требования к конструкции технологии изготовления р- и n-канальных МОП-транзисторов с толстым слоем оксида, предназначенных для применения в качестве интегральных дозиметров поглощенной дозы ионизирующего излучения.Визначено вимоги до конструкції та технології виготов лення р-канальних та n-канальних МОП-транзисторів із тоѕстим шаром оксиду, призначених для вжитку як інтегральні дозиметри поглинутої дози іонізуючого випромінення. Розроблено технологію створення радіаційно-чутливих МОП-транзисторів з товстим шаром оксиду в р-канальному и в n-канальному вариантах.The requirements to technology and design of p-channel and n-channel MOS transistors with a thick oxide layer designed for use in the capacity of integral dosimeters of absorbed dose of ionizing radiation are defined. The technology of radiation-sensitive MOS transistors with a thick oxide in the p-channel and n-channel version is created

Limitation of loading and unloading operations in harbours due to ship motion

Motions of moored ships in harbours are excited by winds, currents, waves, seiches, tides, passing ships, and cargo handling. This lecture will describe the properties of these excitations as well as the mechanics of the motion response of the ships. Firstly, however, the recommended allowed limits of motions for safe working conditions of various ship types, from pleasure boats to fishing vessels and large bulk carriers, will be described. Such recommendations were presented by PIANC (1995) and the Nordic Council (1986). Following this, a theoretical study of the motion of a ferry at berth in the Port of Visby will be presented, which contains some novel ideas. Lastly, results and animations from some ongoing developments at Chalmers University of Technology on wave computations over large areas with complex topography will be presented