International Marine Energy Journal

Experimental measurements of two elastic taut-slack mooring configurations for the multi-float M4 WEC

2025-01-07T11:19:22+00:00

Moorings are a vital and often problematic component of any floating offshore renewable energy system, whether for wind or wave energy conversion, and here we consider the M4 multi-float wave energy converter (WEC) system in a 122 configuration. Previous experimental work has shown elastic cables to reduce extreme snap loads by a factor of about 6 when considering single cables between the bow float and a mooring buoy (hawser), and between the mooring buoy and the bed. Here, we compare results for two alternative configurations designed to reduce the mooring footprint as well as extreme snap loads. Both systems are taut-slack with configuration 1 consisting of a single mooring buoy and configuration 2 with a dual buoy set-up with one submerged in an attempt to reduce loads further. The configurations are experimentally tested in irregular (JONSWAP) wave conditions up to limiting steepness, with run times of 35 minutes at 1:40 scale or about 3.5 hours full scale. Through statistical analysis it is concluded that both mooring configurations offer similar advantages in terms of reduction of peak loads, and display very similar load and motion (hence power) statistics. Configuration 2 displays slightly larger maximum forces yet these occur at significantly lower frequencies (near to the surge natural frequency) with reduced response at the wave frequencies. This demonstrates that subtle changes to mooring line configuration can be used to affect the frequency, and hence number, of loading cycles considerably without affecting power performance.

An Early Design Phase Method for Characterizing and Comparing Wave Energy Converter Archetypes

2025-01-10T03:26:37+00:00

Wave energy converters (WECs) hold promise for powering blue economy applications, but their success relies on their ability to survive in the highly energetic ocean climate. Existing approaches for understanding WEC functional benefits often occur during detailed design, which can lead to crucial functional decisions based on archetype selection rather than design goals. This paper presents a methodology that leverages functional decomposition for identifying high-level overlaps between WEC archetypes during early design stages. Functional decomposition is a method for analyzing complex systems and tracking energy, information, and material flows. We apply this method to seven fully submerged WEC archetypes. This process provides helpful information for understanding the necessary functions of a submerged WEC. The methodology and case study results will help communities advance their efforts to combat climate change by selecting the best device for their application and location in the blue economy. Better identifying functional overlaps between WEC archetypes will help researchers and developers generate effective designs that build resilient coastal communities.

Enabling the Ocean Internet of Things with Renewable Marine Energy

2025-01-09T10:29:38+00:00

Marine renewable energy can play an integral role in reducing the cost and complexity of collecting data from the oceans and enhancing its exploitation. The processing and transmission of data at sea may be modelled as an 'internet of things' (IoT) application. The 'thing' is any offshore device which can collect and process data in situ, while the 'internet' represents the medium for transmitting data. IoT is differentiated from other internet-based communication paradigms by the constraints on the system. These constraints are typically limited energy budgets and computing power, intermittent and low-bandwidth connectivity, and limited physical access. While land-based IoT applications are already well served by a wide selection of hardware and software components, the needs of offshore IoT applications are not well served. Utilizing marine energy can advance the adoption of IoT at sea by providing in-situ energy generation, whilst also benefitting from the same technology. The Sustainable Energy Authority of Ireland funded 'BlueBox' project aims to overcome barriers to entry for applying IoT technologies to offshore sensing by developing Ocean IoT (OIoT) hardware and software solutions. Features of the BlueBox system include modular offshore-focussed hardware, no-code configuration and control of peripherals (such as sensors and actuators), duplex transmission of data using multiple media, serverless cloud server architecture, and an edge computing framework. This paper presents an overview of BlueBox, tank tests for system validation of a prototype wave-energy powered ocean-observing platform, and a discussion of future applications of the technology.

ITSASDRONE, an autonomous marine surface drone for fish monitoring around wave energy devices

2025-01-09T12:20:28+00:00

One of the primary foundations of the EU Blue Growth policy is the development of ocean energy. While the technological development of devices is advancing quickly, little is known about their possible environmental implications. The SafeWAVE initiative aims to increase understanding of potential environmental effects from wave energy projects in the coastal waters of Portugal, Spain, and France. The aim of this work was to monitor the interaction between the WEC and the fish community in the Spanish study area (BiMEP). However, the device to be monitored had to be replaced by a floating laboratory (HarshLab) in the same area, since the WEC suffered a series of unforeseen events that made its use impossible. The monitoring was carried out using an autonomous vehicle (ITSASDRONE) equipped with a scientific echosounder, which recorded acoustic data that provided information on the abundance of fish in the area. Schools of unidentified small pelagic fish were observed distributed throughout the water column, predominantly near the bottom in the area of the device. The acoustic sensors showed a relatively high abundance in the BiMEP area, generally equal to or higher than in the access route from Armintza harbor. However, these results are preliminary and should be considered as baseline information. Future studies are needed to further investigate the relationship between WECs and fish aggregation.

Investigating the impact of multi-rotor structure shadowing on tidal stream turbine performance

2025-01-09T11:59:56+00:00

As the tidal stream energy sector develops, reducing the Levelised Cost of Energy (LCOE) is essential to sustain commercialisation. Modular multi-rotor foundations, with bi-directional turbines, reduce offshore operational complexity through smaller turbine diameters and lift weights, in turn reducing the device Operational Expenditure (OpEx). With the introduction of modular, multirotor foundations, the wake-induced impacts that these structures have on turbine performance must be investigated to better estimate energy yield, loading, and fatigue life. This study sets the scene for investigating the relationship between the turbulent wake generated by a modular ballast weighted foundation and 2-bladed Horizontal Axis Tidal Turbine (HATT) motivated by the HydroWing multirotor device concept. The presented work aims to determine the broader magnitude and severity of the loads and establish a transparent and well-defined methodology to be followed with further high-fidelity modelling. Initially, a transient RANS Computational Fluid Dynamics (CFD) simulation environment with a sliding mesh is configured and validated against experimental data. A turbine in freestream isolation is simulated as a benchmark case with the modular foundation sequentially introduced to analyse the impact of the structure. Key findings suggest that operating turbines downstream of the multi-rotor foundation could cause a ≈20% fluctuation in CT loading at a 1.82 Hz frequency resulting in a mean CP reduction of ≈9% over a revolution.

Effects of projected wave climate changes on the sizing and performance of OWCs: a focus on the Atlantic North African and European coastal waters

2025-01-09T16:32:11+00:00

Accurate estimates of the annual energy production achievable by a given wave energy converter are essential for a robust assessment of the associated levelized cost of energy, a key factor in investment decision-making. Inaccurate productivity estimates can arise - among other factors - from uncertainties in evaluating the available wave energy resource. The Climate Data Store of the Copernicus Climate Change Service delivers projections of the wave climate along the 20 m bathymetric contours of the whole European coastlines, covering the period 2040-2100, under two Representative Concentration Pathway scenarios
(RCP4.5 and RCP8.5). This work addresses the effect of such long-term wave climate changes on the optimal sizing and performances of an Oscillating Water Column wave energy converter intended for installation along the North African and European Atlantic coastline. The capture width ratio of the device under different wave conditions is computed using an empirical model capable of predicting the device performance with acceptable accuracy and limited computational time. The results show that the optimal geometry of the OWC varies significantly in the different geographical locations and that the long-term changes in the wave energy resource could cause a slight modification of the optimal geometry in each potential installation site.

Test rig for submerged transmissions in wave energy converters as a development tool for dynamic sealing systems

2025-05-16T15:11:40+01:00

A submerged transmission, fitted with a dynamic sealing system, in a wave energy converter (WEC) serves the purpose of transmitting the force, absorbed by a wave activated body, to an encapsulated power take-off (PTO) system, while preventing seawater from entering the capsule. Dry generator operation is generally a prerequisite for attaining long technical service life. Little attention seems to be devoted in publications to the study of dynamic sealing systems in WECs, and to test rigs for experimental verification and/or evaluation of the ability/performance of existing dynamic sealing systems in a controlled laboratory environment. This paper begins by presenting some of our earlier research within the focus area of dynamic sealing systems, incl. design considerations and typical operating conditions. This part also presents the 1^st laboratory test rig, used for verifying the sealing ability of the piston rod mechanical lead-through design in the 1^st and 2^nd full-scale experimental WEC prototype from Uppsala University. In 2021 project DynSSWE (Dynamic Sealing Systems for Wave Energy) was initiated. Drawing from experience, the project includes development of a new test rig, representing a tool for further development of dynamic sealing systems. This paper introduces steps in the design and development process of that new test rig, enabling accelerated long-term test runs with a setup of multiple piston rod specimens. The test specimens’ will be surface treated differently with the aim of improving the prospects of a long maintenance free service life. Since the new test rig is in the design stage, seal testing results are not yet reported. The presented work is funded by the Swedish energy agency with the aim of improving subsystem performance in wave energy devices.

A Methodology to capture the single blade loads on a cross-flow tidal turbine flume model

2025-05-27T16:32:31+01:00

The OPTIDE project aims to improve the efficiency and durability of hydrokinetic cross-flow tidal turbines (CFTT). CFTT are attractive for the exploitation of tidal energy, because of high area-based power densities of such turbine arrays, a simple design and the ability to operate under varying flow conditions.
Nevertheless, the efficiency of single CFTT is lower relative to the most commonly used axial turbine type. Furthermore the life time is often limited due to alternating and pulsating stresses.
A promising approach to overcome these drawbacks is intracycle blade pitching. In this case the angle of attack is continuously adjusted individually for each blade during the rotation.
The project aims to explore the influence of active blade pitching on CFTTs and to optimize it by numerical and experimental means.
Therefore, a lab-scaled three-bladed experimental turbine with embedded pitch actuators is developed.
The turbine is equipped with multiple full-bridges of strain gauges for the detection of tangential and radial components of the blade loads, from which the structural stress is calculated subsequently.
To ensure the turbines mechanical durability, weakly coupled fluid-solid-interaction (FSI) simulations have been performed.
Additionally, the FSI simulations show that a setup with strain gauges on a single blade support only allows for the measurement of the radial blade load component, as the measurements are distorted by pitching moments and secondary force paths.
A revised instrumentation setup is developed and presented. The FSI simulations are adapted, to prove the suitability of the revised setup.

Hydrodynamic Response of Mocean Wave Energy Converter in Extreme Waves

2025-01-10T11:41:19+00:00

The design of moored floating wave energy converters (WECs) must take into account extreme responses and mooring line loads in order to ensure their survival and continued wave power generation in the ocean environment. This study focuses on Mocean Energy’s Blue Horizon C1 hinged raft WEC and aims to provide a better understanding of its hydrodynamic characteristics in survival wave conditions. To achieve this, a physical model study was conducted on a Froude scale of 1 in 50 at the FloWave Ocean Energy Research Facility, University of Edinburgh. The experiments involved the use of NewWaves focusing of crest and trough at the model hinge location, as well as long crested irregular waves. Motion responses of the fore and aft bodies of the WEC were measured using a Qualisys camera, and single component load cells were used to measure the forces in the 3-point catenary mooring lines. The hydrodynamic characteristics of the WEC were evaluated in terms of response amplitude operators and non-dimensional mooring line loads. Results indicate that the fore and aft bodies of the WEC exhibit similar motion responses, except for the pitch motion. The aft body has a pitch response 2 to 3 times higher than the fore body. Concerning the moorings, the wave load on the mooring line in line with the wave direction was found to be higher than the other two mooring lines which were arranged at an angle to the wave direction. The results will provide insight into the behaviour of the Mocean device in survival wave conditions and will aid with the determination of appropriate design parameters for optimal performance and survival in the ocean environment.

Verification and validation of blade-resolved viscous-flow tidal turbine simulations

2025-05-27T16:38:32+01:00

Tidal turbines are a renewable energy source on the rise. The exceptional predictability of tidal currents contributes to a high reliability of this technology, which represents a key advantage in the endeavor to become a major contributor to the energy mix. To foster the development and to support the design process of tidal turbines, reliable numerical modeling techniques are required. This paper presents verification and validation work performed within the framework of the Supergen ORE Tidal Turbine Benchmarking Study. Viscous-flow CFD code ReFRESCO is used to conduct blade-resolved simulations of the towing tank experiments. In a first approximation, a steady-state frozen-rotor approach is chosen. A transition model, Gamma-ReTheta, is employed to predict the flow state transition on the turbine blades. In the process, the sensitivity to input turbulence quantities is highlighted. The numerical uncertainty is estimated based on mesh refinements. Finally, a conclusion is drawn to which accuracy the presented numerical models can predict the outcome of the experiments.

On tidal array layout sensitivity to regional hydrodynamics representation

2025-05-27T16:38:41+01:00

Hydrodynamic models are required to predict the power produced by a tidal array and the impact on the surrounding environment. The influence of common model inputs to layout optimisation are investigated herein. This is achieved using a shallow water equation based tidal array modelling framework, Thetis, coupled with a low cost analytical wake model (FLORIS) that allows for rapid assessment of the impact of small changes in hydrodynamic results on array micro-siting. The sensitivity of
array optimisation at an intermediate development point (43 turbines) is interrogated through both artificial flow field manipulation and variation of inputs pertinent to optimisation. A small margin exists in which an optimised layout performs efficiently for a deviation in flow prediction accuracy. However, incorrect flow predictions by a range sensitive to model inputs led to a ≈5% variation in array efficiency relative to a control case. The sensitivity of flow field variance on energy yield and layout are substantial. Comparing arrays sited using different bathymetry resolution models leads to a discrepancy on average of almost 2% to average array power. Arrays sited for different mesh resolution and friction representation also changes exceeding 0.85%. For array developers and the future of this nascent industry, acquisition of reliable bathymetry data coupled with repeated calibration of array models is critical for accurate array power and efficiency.

On the design of a small scale tidal converter for long time deployment at sea

2025-05-27T16:38:51+01:00

This paper presents the design of a scale model of a kite-like tidal converter, GEMSTAR, intended for long-term deployment at sea. The main objective of this experiment is to develop a digital twin with integrated fault detection and isolation capabilities to improve the reliability and performance of the system. A fully functional 1:10 scale model of GEMSTAR was designed based on extensive measurements of tidal currents at the planned deployment site. Several locations were investigated to determine the most suitable flow profiles for energy generation. This study outlines the key challenges for operational functionality and describes the selection of critical physical parameters to be monitored during deployment using onboard sensors.

Observations from structural testing of full-scale tidal turbine blades

2025-01-20T14:26:06+00:00

In recent years, tidal energy has emerged as a potential key player in future energy security, as it provides a reliable, predictable and dependable source of renewable energy, where, in 2022, the cumulative installed capacity of tidal stream energy in Europe, since 2010, reached 30.2 MW. As tidal energy strives towards commercial viability, optimisation of structural components, along with their de-risking through structural testing, has become more prevalent. Full-scale structural testing of tidal turbine blades provides a mechanism to ensure the blades can withstand the high operational loads when deployed, in a controlled laboratory environment. In recent years, this type of testing has been used to de-risk prototype blades in advance of operational trials. However, a limited number of these tests have been performed globally. Therefore, in this paper, observations during the structural (static, dynamic and fatigue) testing of 5 full-scale tidal turbine blades are presented and discussed. The length of these blades range from 2-8 metres, for devices of 70kW to 2MW. A case study of a large blade from a 2MW floating tidal turbine has been used to illustrate some of the results obtained from the structural testing. The experience gained from these structural testing programmes highlighted a number of best practices that could be introduced to the next revision of both the IEC 62600-3:2020 test specification and the DNV-ST-0164 standard.