[Report of] Specialist Committee V.4: ocean, wind and wave energy utilization

The committee's mandate was :Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and wind

Hydrodynamic modelling of marine renewable energy devices : a state of the art review

This paper reviews key issues in the physical and numerical modelling of marine renewable energy systems, including wave energy devices, current turbines, and offshore wind turbines. The paper starts with an overview of the types of devices considered, and introduces some key studies in marine renewable energy modelling research. The development of new International Towing Tank Conference (ITTC) guidelines for model testing these devices is placed in the context of guidelines developed or under development by other international bodies as well as via research projects. Some particular challenges are introduced in the experimental and numerical modelling and testing of these devices, including the simulation of Power-Take-Off systems (PTOs) for physical models of all devices, approaches for numerical modelling of devices, and the correct modelling of wind load on offshore wind turbines. Finally, issues related to the uncertainty in performance prediction from model test results are discussed.The paper is based on the report of the International Towing Tank Conference specialist committee on Hydrodynamic Modelling of Marine Renewable Energy Devices to the 27th ITTC held in Copenhagen, Denmark in 2014 (ITTC Specialist Committee on Hydrodynamic Modelling of Marine Renewable Energy Devices, 2014a. Final Report and Recommendations to the 27th ITTC Proc. 27th International Towing Tank Conference, Copehagen, Denmark, vol. 2, pp. 680–725)

Development of numerical wave power prediction tool offshore oscillating water column wave energy converter

Marine renewable energy sources are crucial alternatives for a sustainable development. The idea of generating electrical power from water waves has been realized for many years. In fact, waves are now considered as an ideal renewable energy source since a Wave Energy Converter (WEC) has no fuel cost and provides cleanly a high power density that is available most of the time. The third generation of WECs is intended to be installed offshore. This allows the device to harvest the great energy content of waves found in deep water and minimise the environmental impacts of the device. On the other hand, moving WECs to offshore locations will increase the initial and maintenance costs. So many types of device may be suggested for wave power extraction that the task of selecting a particular one is made complicated. Therefore modelling of different WECs allows the comparison between them and the selection of the optimum choice. Recent studies showed that the SparBuoy Oscillating Water Column (OWC) has the advantage of being simple, axi-symmetrical, and equally efficient at capturing energy from all directions, but its efficiency (capture factor) is affected significantly by the incident wave periods variation due to the dynamic coupling of the water column and the floating structure. The proper modelling of the device allows the optimization of the geometries and the Power Take-Off (PTO) mechanism in order to maximise the power absorbed. The main objective of this research is to develop experimentally validated numerical wave power prediction tool for offshore SparBuoy OWC WEC. The numerical tool should be able to predict the spar motions and the water column oscillations inside the structure, in addition to the estimation of the pneumatic power absorber and the evaluation of the device performance. Three uncoupled linear second order differential equations have been used to predict the spar surge, heave and pitch motions, where wave forces have been calculated analytically in frequency domain in inertia and diffraction regimes. Mooring system has been involved in surge motion only using static and quasi-static modelling approaches.;Finite element multi-static model have been developed using OrcaFlex to validate the analytical results. Single Degree of Freedom (DOF) mechanical oscillation model has been presented to simulate the water column oscillations inside captive cylindrical OWC where PTO damping and stiffness due to air compressibility inside the pneumatic chamber have been taken into account linearly. Later on, nonlinearity due to large waves has been investigated. Linearized frequency domain model based on classical perturbation theory and nonlinear model where wave forces are calculated in time domain have been proposed. Furthermore, nonlinearity due to damping forces has been considered. First, iterative procedure has been used to optimise the linear and quadratic damping coefficients in frequency domain. Then, another model has been provided where equivalent viscous damping coefficients are calculated in time domain by taking into consideration the instant oscillation amplitude. Finally the nonlinear effects due to air compressibility inside the OWC chamber has been considered in a time domain model which include the water column oscillations amplitudes. Two different dynamic models have been implemented to describe floating OWC and will be referred to in the text as simplified 2DOF model and Szumko model. Both models considered two translational modes of motions in heave direction. Simplified 2DOF model has been solved analytically in frequency domain due to its simplicity, while numerical solutions in time domain have been provided for both models using Matlab. Different approaches have been adopted to modify both models in order to obtain a satisfactory agreement between the predicted and measured results. A floating platform consists of four similar SparBuoy OWC WECs rigidly attached together by trusses where spars are located at the corners have been tested experimentally. Numerical model has been developed to predict the platform motions. Finally the experimental results have been compared to those obtained from the modelling of single SparBuoy OWC.Marine renewable energy sources are crucial alternatives for a sustainable development. The idea of generating electrical power from water waves has been realized for many years. In fact, waves are now considered as an ideal renewable energy source since a Wave Energy Converter (WEC) has no fuel cost and provides cleanly a high power density that is available most of the time. The third generation of WECs is intended to be installed offshore. This allows the device to harvest the great energy content of waves found in deep water and minimise the environmental impacts of the device. On the other hand, moving WECs to offshore locations will increase the initial and maintenance costs. So many types of device may be suggested for wave power extraction that the task of selecting a particular one is made complicated. Therefore modelling of different WECs allows the comparison between them and the selection of the optimum choice. Recent studies showed that the SparBuoy Oscillating Water Column (OWC) has the advantage of being simple, axi-symmetrical, and equally efficient at capturing energy from all directions, but its efficiency (capture factor) is affected significantly by the incident wave periods variation due to the dynamic coupling of the water column and the floating structure. The proper modelling of the device allows the optimization of the geometries and the Power Take-Off (PTO) mechanism in order to maximise the power absorbed. The main objective of this research is to develop experimentally validated numerical wave power prediction tool for offshore SparBuoy OWC WEC. The numerical tool should be able to predict the spar motions and the water column oscillations inside the structure, in addition to the estimation of the pneumatic power absorber and the evaluation of the device performance. Three uncoupled linear second order differential equations have been used to predict the spar surge, heave and pitch motions, where wave forces have been calculated analytically in frequency domain in inertia and diffraction regimes. Mooring system has been involved in surge motion only using static and quasi-static modelling approaches.;Finite element multi-static model have been developed using OrcaFlex to validate the analytical results. Single Degree of Freedom (DOF) mechanical oscillation model has been presented to simulate the water column oscillations inside captive cylindrical OWC where PTO damping and stiffness due to air compressibility inside the pneumatic chamber have been taken into account linearly. Later on, nonlinearity due to large waves has been investigated. Linearized frequency domain model based on classical perturbation theory and nonlinear model where wave forces are calculated in time domain have been proposed. Furthermore, nonlinearity due to damping forces has been considered. First, iterative procedure has been used to optimise the linear and quadratic damping coefficients in frequency domain. Then, another model has been provided where equivalent viscous damping coefficients are calculated in time domain by taking into consideration the instant oscillation amplitude. Finally the nonlinear effects due to air compressibility inside the OWC chamber has been considered in a time domain model which include the water column oscillations amplitudes. Two different dynamic models have been implemented to describe floating OWC and will be referred to in the text as simplified 2DOF model and Szumko model. Both models considered two translational modes of motions in heave direction. Simplified 2DOF model has been solved analytically in frequency domain due to its simplicity, while numerical solutions in time domain have been provided for both models using Matlab. Different approaches have been adopted to modify both models in order to obtain a satisfactory agreement between the predicted and measured results. A floating platform consists of four similar SparBuoy OWC WECs rigidly attached together by trusses where spars are located at the corners have been tested experimentally. Numerical model has been developed to predict the platform motions. Finally the experimental results have been compared to those obtained from the modelling of single SparBuoy OWC

Power Performance and Response Analysis of a Semi-Submersible Wind Turbine Combined with Wave Energy Converters in Intermediate and Deep Water

Renewable energies are the forefront against environmental pollution and the leading technology in sustainable energy sources. Power produced by wind energy is a well established technology. There is a lot of space for expansion though, especially at offshore environment. Floating wind turbines can take advantage of the abundant wind energy available far out at the oceans. Increasing the power production of such structures and ensuring the efficient and safe position keeping of them in every depth is crucial. Two concepts that aim to tackle down these two issues are proposed. A combination of wind and wave energy converters and an intermediate water depth mooring arrangement. The installation of wave energy converters (WECs) at the floating wind turbine base will reduce the levelized cost of energy (LCoE). The use of the same power cables and mooring arrangement to deploy the WECs can prove beneficial financially wise. Furthermore, it can act as a boost to the development of wave energy in general. The transient depth between shallow and deep is a challenging field for the traditional mooring arrangements. A cost effective solution will help the deployment of floating wind turbines in this intermediate water depth fields. A novel concept of combining a floating wind turbine with WECs is proposed. A semi submersible floater is used to support a 5MW wind turbine. Three flap typed WECs are deployed on the pontoons of the floater. A two-point type absorber called Torus is installed at the central column. The proposed concept is named STFC. A thorough analysis of the natural periods, regular and irregular wave tests is performed to evaluate the effect of the WECs to the floater’s behavior. Small to no effect is observed across all degrees of freedom except the reduction in pitch period due to increased hydrostatic stiffness. The irregular wave tests indicate that the absorbed power amplitude operator (RAO) of Torus has a wider excitation range than the flaps. The floater heave RAO is affected by the addition of Torus and the floater surge and pitch RAOs are affected by the additions of flaps. Irregular waves tests are carried out to evaluate the influence of the added WECs to the motions of the combined concept and the total power performance. The results indicate that the WECs have little to no effect to the motions of the structure except the expected reduction in pitch motions. This is preliminary indication that no changes should be made to the mooring due to the addition of WECs. The wind turbine power performance is not affected by the addition of WECs. There is small reduction in pitch standard deviation though, in loading condition EC3 that leads to slightly better wind quality. The Torus performance is satisfactory as it accounts for 9% of the total STFC power production. The total three flaps account only for 14% of the Torus power production thus their power performance is not satisfactory. The flaps rotation is out of phase with the wave excitation force so there is room for improvement if active damping and stiffness control is added. Following, the STFC is moved to intermediate water depth z = -50m and a new hybrid mooring design is proposed. A brief explanation of the intermediate water mooring challenges is given and the basic design criteria are established. A step to step designing process is presented. The combination of studded chain, buoys, and clump weights is proposed. The name of the concept is CCCB. A restoring force test is carried out and the linearity of the restoring force response is verified. Irregular wave tests are carried out and the data indicate that the new mooring design is a feasible design. The maximum floater offset is restrained to 14% of the water depth. The pretension of the mooring system proves to be a significant factor for the total performance of the mooring arrangement. The maximum mooring line tension is kept within safe limits throughout the whole offset range. CCCB is compared with similar concepts and proves itself stiffer mainly because of its increased pretension. Points of interest are defined across the mooring line and spectra analysis is performed to evaluate the distribution of response frequencies across the chain length. The role of buoys as dampers of motions is established. CCCB utilization factor indicates that this mooring design can be used also in larger structures and there is room for cost reduction measures

Optimal strategies of deployment of far offshore co-located wind-wave energy farms

[EN] The most profitable offshore energy resources are usually found away from the coast. Nevertheless, the accessibility and grid integration in those areas are more complicated. To avoid this problematic, large scale hydrogen production is being promoted for far offshore applications. The main objective of this paper is to analyze the ability of wave energy converters to maximize hydrogen production in hybrid wind and wave far offshore farms. To that end, wind and wave resource data are obtained from ERA5 for different locations in the Atlantic ocean and a Maximum Covariance Analysis is proposed for the selection of the most representative locations. Furthermore, the suitability of different sized wave energy converters for auxiliary hydrogen production in the far offshore wind farms is also analysed. On that account, the hydrodynamic parameters of the oscillating bodies are obtained via simulations with a Boundary Element Method based code and their operation is modelled using the software tool Matlab. The combination of both methodologies enables to perform a realistic assessment of the contribution of the wave energy converters to the hydrogen generation of an hybrid energy farm, especially during those periods when the wind turbines would be stopped due to the variability of the wind. The obtained results show a considerable hydrogen generation capacity of the wave energy converters, up to 6.28% of the wind based generation, which could remarkably improve the efficiency of the far offshore farm and bring important economical profit. Wave energy converters are observed to be most profitable in those farms with low covariance between wind and waves, where the disconnection times of the wind turbines are prone to be more prolonged but the wave energy is still usable. In such cases, a maximum of 101.12 h of equivalent rated production of the wind turbine has been calculated to be recovered by the wave energy converters.This paper is part of project PID2020-116153RB-I00 funded by MCIN/AEI/ 10.13039/501100011033. Authors also acknowledge financial support by the University of the Basque Country under the contract (UPV/EHU, GIU20/008)

Renewable Energy in Marine Environment

The effects of human-caused global warming are obvious, requiring new strategies and approaches. The concept of business-as-usual is now no longer beneficial. Extraction of renewable energy in marine environments represents a viable solution and an important path for the future. These huge renewable energy resources in seas and oceans can be harvested, including wind, tide, and waves. Despite the initial difficulties related mostly to the elevated operational risks in the harsh marine environment, newly developed technologies are economically effective or promising. Simultaneously, many challenges remain to be faced. These are the main issues targeted by the present book, which is associated with the Special Issue of Energies Journal entitled “Renewable Energy in Marine Environment”. Papers on innovative technical developments, reviews, case studies, and analytics, as well as assessments, and papers from different disciplines that are relevant to the topic are included. From this perspective, we hope that the results presented are of interest to for scientists and those in related fields such as energy and marine environments, as well as for a wider audience

Performance Analysis on the Use of Oscillating Water Column in Barge-Based Floating Offshore Wind Turbines

Undesired motions in Floating Offshore Wind Turbines (FOWT) lead to reduction of system efficiency, the system’s lifespan, wind and wave energy mitigation and increment of stress on the system and maintenance costs. In this article, a new barge platform structure for a FOWT has been proposed with the objective of reducing these undesired platform motions. The newly proposed barge structure aims to reduce the tower displacements and platform’s oscillations, particularly in rotational movements. This is achieved by installing Oscillating Water Columns (OWC) within the barge to oppose the oscillatory motion of the waves. Response Amplitude Operator (RAO) is used to predict the motions of the system exposed to different wave frequencies. From the RAOs analysis, the system’s performance has been evaluated for representative regular wave periods. Simulations using numerical tools show the positive impact of the added OWCs on the system’s stability. The results prove that the proposed platform presents better performance by decreasing the oscillations for the given range of wave frequencies, compared to the traditional barge platform.This work was supported in part by the Basque Government, through project IT1207-19 and by the MCIU/MINECO through the projects RTI2018-094902-B-C21 and RTI2018-094902-B-C22 (MCIU/AEI/FEDER, UE)

Committee V.4 - Offshore Renewable Energy

This is the author accepted manuscript. The final version is available from CRC Press via the DOI in this recordProceedings of the 19th International Ship and Offshore Structures Congress, Cascais, Portugal, 7 - 10 September 201