Analysis of biofouling effect on the fatigue life and energy performance of wave energy converter system

This study presents an analysis of a wave energy converter (WEC) system consisting of a buoy, a mooring system, and a power cable connected to a hub. The design service life of the investigated WEC is 25 years. During this long period, its service life will be influenced by marine biofouling due to the biological activity in the water. The purpose of this study is to investigate the effect of biofouling with regard to the energy performance of the WEC and the fatigue lives of the mooring lines and power cable. The marine biofouling is modelled by an increase in the masses and drag coefficient of the mooring lines and power cable. Coupled response analysis using the DNV-GL software SESAM was conducted to simulate hydrodynamic and structural response of the WEC system. Energy performance analyses and stress-based rainflow counting fatigue calculations were performed separately using an in-house code. The results show that, for a WEC system which has been deployed for 25 years, biofouling can reduce the total power absorption by up to 10% and decrease the fatigue life of the mooring lines by approximately 20%

Ultimate limit state analysis of a double-hull tanker subjected to biaxial bending in intact and collision-damaged conditions

This study presents a comparison between nonlinear finite element analysis (FEA) and the Smith method of Fujikubo et al. (2012). The objective was to compare the accuracy and computation effort of the two methods for a double-hull tanker under biaxial bending and various ship conditions: intact hull structure, collision-damaged hull structure, newly built condition, and ship hull aged due to corrosion. The results for the non-corroded and intact ship hull structures showed good agreement between FEA, the Smith method and IACS CSR-H for vertical bending loading conditions. For all other bending load combinations, FEA always gave lower ultimate bending moments than the Smith method. The differences between the two methods were larger for the corroded and damaged ship hull structure than for other conditions. Results from ultimate strength analyses of the collision-damaged hull structures showed that both methods captured the expected asymmetric ultimate strength response due to asymmetric damage. A residual strength index calculation showed that the reduction was larger for the FEA than for the Smith method. A procedure is proposed that combines results of a few FEAs with the advantages of the Smith method to generate accurate biaxial bending load interaction curves for different ship conditions

Experimental and numerical investigation of a taut-moored wave energy converter: a validation of simulated mooring line forces

A reliable simulation model to calculate the motion and force responses of wave energy converters (WECs) is imperative to ensure the reliability and long-term performance of WEC systems; these aspects are fundamental to achieving full commercialisation of wave energy. A simulation model was developed and validated concerning the simulated WEC buoy motions in a previous study; this study validated the mooring force calculations for the same model. The example WEC system comprises a buoy, a power take-off (PTO) system, and a three-leg mooring system wherein each leg is divided into two taut segments joined by a submerged float. A 1:20 physical model was built and tested in the Deepwater Offshore Basin at Shanghai Jiao Tong University. Numerical models were developed to simulate the coupled hydrodynamic and structural responses of the WEC system, primarily using potential flow theory, the boundary element method, the finite element method, and the Morison equation. The simulated and measured axial force results at the top ends of the six mooring segments were compared; the results agreed best in the lower segments of each mooring leg and in the moorings on the downwind side because of the PTO system uncertainties and the uncalibrated damping in the numerical model. Nonetheless, the numerical model reasonably predicted the moorings’ accumulated fatigue damage, demonstrating that the model can be reliably used for mooring structural analyses. The study also used the validated numerical model to simulate a full-scale WEC system installed in Runde, Norway. A comparison of the results from the full-scale measurements and simulations shows that the numerical simulation model exhibited a good predictive capability for the mooring forces of the full-scale WEC system

Mechanical reliability analysis of flexible power cables for marine energy

Marine power cables connected to moving devices at sea may experience millions of load\ua0cycles per year, and thus they need to be flexible due to the movements of the cable and designed for\ua0mechanical loads. In this study, the focus is on the mechanical life of flexible low- and medium voltage\ua0power cables connecting devices to hubs. The reliability design method Variational Mode and Effect\ua0Analysis (VMEA) is applied, based on identifying and quantifying different types of uncertainty\ua0sources, including scatter, model and statistical uncertainties. It implements a load–strength approach\ua0that combines numerical simulations to assess the loads on the cable and experimental tests to\ua0assess the strength of the cable. The VMEA method is demonstrated for an evaluation of bending\ua0fatigue, and is found to be a useful tool to evaluate uncertainties in fatigue life for WEC (Wave Energy Converter) system cables during the design phase. The results give a firm foundation for the\ua0evaluation of safety against fatigue and are also helpful for identifying weak spots in the reliability\ua0assessment, thereby motivating actions in the improvement process. Uncertainties in terms of scatter,\ua0statistical uncertainty and model uncertainty are evaluated with respect to theWaveEL 3.0, a WEC\ua0designed by the companyWaves4Power, and deployed in Runde, Norway. A major contribution to\ua0the overall uncertainty is found to originate from the fatigue life model, both in terms of scatter and\ua0model uncertainty

Consequence analyses of collision-damaged ships - damage stability, structural adequacy and oil spills

A ship collision accident may pose a threat to human lives, the environment and material assets. A damaged ship can suffer from the loss of ship stability, reduced global structural strength, and the loss of the integrity of internal tanks carrying polluting liquids. This study presents a methodology as a framework that can be used to analyze the related consequences of ship-ship collision events using simulations and evaluations. The methodology includes nonlinear finite element analyses of the collision event, a METOCEAN data analysis module, damage stability simulations, analyses of the damaged ship’s ultimate strength and structural integrity, oil spill drift simulations, and finally, an evaluation of the three abovementioned consequences. A case study with a chemical tanker subjected to collision demonstrates the methodology. The collision event was assumed to occur in the Kattegat area (between Sweden and Denmark) at a ship route intersection with high ship traffic density

A comparison of two wave energy converters’ power performance and mooring fatigue characteristics – One WEC vs many WECs in a wave park with interaction effects

The production of renewable energy is key to satisfying the increasing demand for energy without further increasing pollution. Harnessing ocean energy from waves has attracted attention due to its high energy density. This study compares two generations of floating heaving point absorber WEC, WaveEL 3.0 and WaveEL 4.0, regarding their power performance and mooring line fatigue characteristics, which are essential in, e.g., LCoE calculations. The main differences between the two WECs are the principal dimensions and minor differences in their geometries. The DNV software SESAM was used for simulations and analyses of these WECs in terms of buoy heave motion resonances for maximising energy harvesting, motion characteristics, mooring line forces, fatigue of mooring lines, and hydrodynamic power production. The first part of the study presents results from simulations of unit WEC in the frequency domain and in the time domain for regular wave and irregular sea state conditions. A verification of the two WECs’ motion responses and axial mooring line forces is made against measurement data from a full-scale installation. In the second part of the study, the influence of interaction effects is investigated when the WECs are installed in wave parks. The wave park simulations used a fully-coupled non-linear method in SESAM that calculates the motions of the WECs and the mooring line forces simultaneously in the time domain. The amount of fatigue damage accumulated in the mooring lines was calculated using a relative tension-based fatigue analysis method and the rainflow counting method. Several factors that influence the power performance of the wave park and the accumulated fatigue damage of the mooring lines, for example, the WEC distance of the wave park, the sea state conditions, and the direction of incoming waves, are simulated and discussed. The study\u27s main conclusion is that WaveEL 4.0, which has a longer tube than WaveEL 3.0, absorbs more hydrodynamic energy due to larger heave motions and more efficient power production. At the same time, the accumulated fatigue damage in the moorings is lower compared to WaveEL 3.0 if the distance between the WECs in the wave park is not too short. Its motions in the horizontal plane are larger, which may require a larger distance between the WEC units in a wave park to avoid losing efficiency due to hydrodynamic interaction effects

Parametric study of the dynamic motions and mechanical characteristics of power cables for wave energy converters

A case study of a point-absorber wave energy converter (WEC) system is presented. The WEC system forms an array, with several WECs located around a central hub to which they are each connected by a short, free-hanging power cable. The objective of the study is to analyse the dynamic characteristics and estimate the fatigue life of the power cable which is not yet in use or available on the commercial market. Hence, a novel approach is adopted in the study considering that the power cable’s length is restricted by several factors (e.g., the clearances between the service vessel and seabed and the cable), and the cable is subject to motion and loading from the WEC and to environmental loads from waves and currents (i.e., dynamic cable). The power cable’s characteristics are assessed using a numerical model subjected to a parametric analysis, in which the environmental parameters and the cable’s design parameters are varied. The results of the numerical simulations are compared and discussed regarding the responses of the power cables, including dynamic motion, curvature, cross-sectional forces, and accumulated fatigue damage. The effects of environmental conditions on the long-term mechanical life spans of the power cables are also investigated. Important cable design parameters that result in a long power cable (fatigue) service life are identified, and the cable service life is predicted. This study contributes a methodology for the first-principle design of WEC cables that enables the prediction of cable fatigue life by considering environmental conditions and variations in cable design parameters

MODELLING GROWTH OF SMALL CRACKS IN A POLYCRYSTAL

Abstract In this contribution we study the microstructurally short crack propagation phase in a grain structure of a duplex stainless steel (DSS). The grains in the DSS are either austenitic or ferritic. In order to take the material microstructure into account in the simulations we generate models of the grain structure using the Voronoi polygonization algorithm. The material behavior of the grains is modelled by crystal plasticity, hence, the crystallographic directions of the grains are modelled explicitly and evolution equations for the plastic slip along each slip-system are defined. Furthermore, the crystal plasticity model is enhanced, in the spirit of the disclocation model proposed by Navarro and De Los Rios [1], to include a crack propagation model taking into account non-local effects of the dislocation density (via the accumulated plastic slip) along a slip direction within a grain. Finally, numerical results are presented which show the influence of material hardening and slip directions on the propagation of a crack in an austenitic grain

Reduction in ultimate strength capacity of corroded ships involved in collision accidents

Many studies in the literature present advanced numerical simulations of the assessment of the crashworthiness of marine structures. Most of these studies have not considered the effect of corrosion as an impairment of the struck structure’s resistance to the collision load and the consequence on the ship’s ultimate strength. The objective is to investigate the effects of sudden damage, and progressive deterioration due to corrosion, on the ultimate strength of a ship which has been collided by another vessel. Explicit finite element analyses (FEA) of collision scenarios are presented where factors are varied systematically in a parametric study, e.g. the vessels involved in the collision, and consideration of corroded ship structure elements and their material characteristics in the model. The striking ship is represented by a coastal tanker while the struck ship is either a RoPax ship, or, a coastal oil tanker vessel. The crashworthiness of the struck ships is quantified in terms of the shape and size of the damage opening in the side-shell structure, and the division of energy absorption between the striking and struck ships for the different collision simulations. The ultimate strength of the struck ship is calculated using the Smith method and the shape and size of the damage openings from the FEA. In conclusion, the study contributes to understanding of how corroded, collision-damaged ship structures suffer significantly from a reduction in crashworthiness and ultimate strength, how this should be considered and modelled using the finite element method and analysed further using the Smith method

Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb−1 of √ s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente