Project

The technical challenge in designing a lasting structure and PTO is the key issue for any wave energy concept. The project aims at solving this main hindrance for wave energy converters. The intention with the project is to further develop a digital hydraulic PTO system with focus on the mechanical implementation.

It is also the intention of the project to broaden the knowledge to more than just a single absorber system. In reality, a WEC most often consists of several moving parts in the water, such as several floats which strongly interact hydrodynamically and possibly also mechanically. The focus in the project is, therefore, also on array effects.

The purpose of the project is to:

Perform numerical and small-scale laboratory array interaction tests with several
absorbers in an array consisting of multiple closely spaced point absorbers. Experimental tests will be performed at small scale in a wave basin on 5 existing experimental set-ups which are available at the Department of Civil Engineering, Aalborg University.

Further develop full-scale digital hydraulic PTO system. The use of better and
cheaper components, and including further measurements of forces and accelerations. This is achieved by upgrading, improving and further developing the digital hydraulic PTO-system, which is tested at the Department of Energy Technology, Aalborg University. Development will have a strong focus on the valve control strategies to reduce and minimize the loads on the structure by more intelligent switching of the valves, while maintaining a high power output.

Apply existing methods and do further development on fatigue analysis, reliability and risk assessment strategies for a digital hydraulic PTO. Tools like Fault Three Analysis (FTA)/Failure Mode and Effects Analysis (FMEA) will be used to define critical failure modes of the PTO system and their influence on the structural loads. The fatigue analysis of the structural parts is performed using existing probabilistic reliability methods.

Measure in reality prototype performance of the digital hydraulic PTO which
Wavestar plans on implementing at their demonstrator test machine at Hanstholm. Sensors for measuring forces, pressures and accelerations will be implemented in the demonstrator, thereby enabling real investigations on the behaviour of the system and making further development possible.

Investigate new control strategies to maximize the power output of a highly
efficient PTO while the minimization of structural loads is taken into account. Relevant parameters such as PTO efficiency, motion and force constraints are included in the design. The strategies are to be implemented in the wave basin and ultimately in Wavestar test machine at Hanstholm.

The Danish wave energy sector consists of several large floating and loosely anchored wave power plants. These plants require specially designed anchoring systems, as “standard” solutions (largely coming from the offshore oil and gas industry) are not designed for the conditions and specifications applicable to wave power plants. For these wave power plants, it is necessary to reduce the resulting anchoring and structural loads, which can be done by making the anchoring solutions more compliant. This will reduce the costs of the anchoring solution and the structure of the plant and thereby the overall costs of the plant and its produced energy, while making the systems more reliable.

The four plants selected to be part of this project are all at a stage of development where they have either completed, or are about to complete, testing of the plants at sea. The four plants are Floating Power Plant, Wave Dragon, Weptos and Leancon. They all require comparable anchoring solutions, as the plants are large, floating, loosely anchored structures operating in water depths of around 30 – 100 m at full commercial scale. This project investigates and compares different anchoring solutions that are useful for these wave power plants. The anchoring solutions are assessed step by step, in order to carry out a systematic and thorough evaluation. The project is organised in the following work packages:

- WP 1: Design practices and tools.
- WP 2: Anchoring solutions.
- WP 3: Preliminary design.
- WP 4: Full analysis.
- WP 5: Cost evaluation.
- WP 6: Selection and results.
- WP 7: Dissemination and project management.

Throughout the project, reports will be produced presenting the results of the selected studies and milestones according to the project Gantt chart. Each of them is crucial for the next step of the analysis and will thus be of great importance. The final results of this project are numerous. It will provide experience and insight into the development of anchoring solutions for all project partners. Furthermore, it will provide the developers with detailed analyses of the various anchoring solutions, and evaluate their prices and practical applicability. Aalborg University will build up experience and know-how in the field, which will enable them, and/or a possible spin-off company, to offer design services in the field to companies in the future. It is also expected to be significant cost and reliability benefits, in addition to having an effective anchoring solution, for the partner plants.

Description
100 kW EXOWAVE wave energy testing in Hanstholm.

Key results
• Design, build and demonstrate an Exowave wave energy converter (WEC) block at a 14-meter water depth in the Danish North Sea in conjunction with a hydro turbine driven electrical generator connected to the grid. The power generation would be +100 kW.
• Include learnings from EUDP1: numerical model verified by tank test (AAU) and CFD analysis (Delft University), feasibility study: wind and wave plant in very large scale, WEC detailed design and engineering, FAT and demonstration at DanWEC site.
• Assess the environmental impact and improve animal life by shaping the WEC foundation for fish breeding grounds.
• Life cycle analysis and include eco-friendly materials as waste materials from wind turbine blade waste materials.
• Assess supply chain in the North Sea region with special focus in Denmark and its raw material, production facilities, knowledge provider for fulfilling the aim above LOI target and support the Danish national energy target in 2030 and 2050. And to include the results in the design phase. The overall KPI here is to lower LCOE.
• TRL improve from 6 to 7

The overarching objective of VALID (Verification through Accelerated testing Leading to Improved wave energy Designs) is to de-risk the whole WEC design process in terms of components reliability and survivability by developing an integrated and open platform for the testing of critical components and subsystems, proposing novel testing procedures going beyond current testing practices. As a consequence, it will facilitate developers to take sound design decisions at early stages of technology developments.

Wave power is one of the most reliable resources for renewable energy utilisation. However, the development of high-performance wave energy converters (WECs) is a complex challenge and requires a solid framework of evaluation tools. The EU-funded VALID project will focus on developing and validating a new test rig platform and methodology for accelerated hybrid testing that can be used across the wave energy sector. By improving the reliability and survivability of the components and subsystems that form WECs, the project aims to establish a standard for future use.

Moorings of floating oil and gas (O&G) structures present surprisingly large failure rates. A top solution is a redundancy in the design. However, marine renewables cannot afford such redundancy in the mooring design to obtain a competitive levelised cost of energy (LCOE). The EU-funded ISLINGTON project will reduce uncertainties in the estimated fatigue damage of mooring cables due to soil-cable interaction in the touch-down zone (TDZ) and the economic cost for marine renewables. ISLINGTON will improve the numerical modelling of the cable-soil interaction in the TDZ for mooring cables, generate experimental data for mooring line trenching and perform a numerical investigation of the effect of trenching on the fatigue of mooring cables.

The Wave Star wave energy machine converts the energy of waves into electricity. Wave Star is under development and model tests are being carried out at Aalborg University. The tests are carried out with a view to optimising the efficiency in different wave conditions and mapping effects such as the orientation of the plant in relation to the waves. The tests are supplemented with computer calculations. The project is being carried out in collaboration with Wave Star Energy.