Ireland's seabed territory, covering 880,000km2, is a vast maritime resource which to date has been underexploited. In addition, Ireland's long coastline is coming under increasing threat due to the rise in sea levels and increased storminess.
Development in our maritime space - whether through the construction of renewable energy devices, aquaculture cages, breakwaters or coastal protection structures - often requires scaled model testing in a laboratory setting. Such testing has the purpose of de-risking technologies/structures prior to moving to the very unforgiving open sea environment.
Lir's wave/current tanks and electrical test infrastructure allow scaled testing of marine structures and offshore renewable energy technologies along with their integration to the grid.
Incorporated in the MaREI Centre and located in the €20.5 million purpose-built Beaufort Building in Ringaskiddy, Co Cork, the 2,600m2 facility forms part of Ireland’s network of test infrastructures, which also includes quarter-scale testing at the Galway Bay Marine and Renewable Energy Test Site and full-scale testing at the Atlantic Marine Energy Test Site.
The significant state investment received has provided Lir with a world-class testing capacity together with the most experienced technical team currently operating in this space. Lir is supported by the Sustainable Energy Authority of Ireland (SEAI) and Science Foundation Ireland (SFI).
Lir NOTF facilities
The newly built facilities at Lir comprises the four test tanks, extensive electrical test infrastructure and mechanical and instrumentation workshops.
Deep Ocean Basin (35m x 12m x 3m deep): Used for the testing of larger-scale devices (circa 1:15) this basin has a moveable floor plate that allows the water depth to be adjusted up to a maximum depth of 3m and can generate waves with heights greater than 1m.
Ocean Basin (25m x 18m x 1m deep): Used for testing medium scale models (circa 1:50), this facility is now upgraded to make it the most advanced tank of this scale worldwide. It also has a movable floor with a maximum water depth of 2.5m and can generate waves up to 0.3m in height.
Wave and current flume (28m x 3m x 1m deep): A multi-purpose facility with the capability of running separate and combined unidirectional wave and current tests. It has a variable depth wave paddle system that can generate waves of up to 0.3m in height, three thrusters for generating current speeds of greater than 1 m/s, and a towing carriage that can operate at speeds up to 1.5 m/s.
Wavewatch Flume (15m x 0.75m x 0.75m): A small wave tank primarily for teaching and training purposes.
Electrical Test Rigs and Microgrid: A variety of linear and rotary rigs capable of testing power take-off systems of offshore renewable energy (ORE) technologies in order to determine power quality and grid integration.
Wave tank testing at Lir NOTF
Testing in the wave tanks in Lir requires precise scaling of the technologies and structures so that they correctly represent the prototype structures and enable design decisions to be made. Generally Froude Scaling laws are used as gravity forces are dominant in wave testing.
As an example, if a model had a scale factor of 49 and it was required to represent an ocean wave of a height of 4.9m and period 7s, then by Froude scaling the model height would be 0.1m (4.9/49) and the period would be 1s (7/490.5). Tests are normally undertaken for a variety of wave conditions that represent the real environment in which the structure will be placed.
For coastal structures, it is normally a set of extreme wave conditions and water levels that represent return periods ranging between one in one year to one in 200 years. For wave energy technologies, tests are undertaken for operational sea states when the device is in production mode and extreme sea states where the device is non-operational.
The information required from model testing can vary significantly depending on the client's requirements. For coastal structures and breakwaters, the client is normally interested in whether the armour units will stay in place or the crest level is high enough to prevent excessive overtopping, while for offshore renewable energy the determination of power production levels are required along with device dynamics and the structural and mooring loads.
The picture, right, shows testing of the pier development in Doolin, Co Clare. This structure consisted of X-bloc armour units placed at a slope of 1:2.5. Generally single layer armour units such as X-bloc and accropode require steeper slope placement (1:1.33 - 1:1.5) to maximise stability from both interlocking and gravity.
Satisfying the wave reflection and stability criterion
The 1:2.5 slope was required to reduce wave reflections, which were the dominant criteria for the design of this breakwater. A 1:40 Froude scale model was designed, constructed and subsequently tested at Lir under varying wave conditions and water levels in order to satisfy the wave reflection and stability criterion.
The results showed that the breakwater design performed satisfactorily in terms of armour unit stability and while overtopping was seen to be significant in the most extreme cases the reflections were less than the required value of 40 per cent of the incident wave heights.
The second example of tank testing shown is testing of a 1:36 scale platform of a floating wind platform for an 8MW wind turbine. This platform was developed by Aerodyn Energie Systeme GmbH, which is a German wind turbine design company. The company is an industry leader, having designed circa 10 per cent of the wind turbines installed globally.
For this testing, they teamed up with an Irish company called Blu Wind Power Ltd. The design concept was tested to understand stability characteristics and motions under a variety of incident wave conditions. The wind thrust on the platform due to the operation of the turbine is modelled using a controllable ducted fan. An important consideration for floating wind turbines is for platform motions to be minimised in order to keep the nacelle accelerations below specified threshold limits.
For the model shown, towing testing, ballasting operations as well as wave testing for operational conditions and survival conditions were carried out. The testing led to advancement of the platform to a higher Technology Readiness Level (TRL).
Lir NOTF contribution to ORE development
Lir has a 40-year history in tank testing and is one of the leading testing facilities in Europe particularly for ORE development and can be credited with being the first facility to implement a gauged system for ORE technology development and in influencing its adoption at both the European and International level.
The stages progress from small-scale tank tests (TRL1-3) to intermediate tests (TRL4-6) and finally full size sea trials (TRL 7-9). Lir provides test infrastructure and support services up to TRL4 and is therefore the first port of call for developers and the technical advice and support they receive at this stage clearly impacts their journey through the TRL pathway thereafter.
In addition, Lir is unique in that it combines both tank testing and electrical test infrastructure. This allows the facility to address both the hydrodynamic and electrical design of ORE devices. This is an important advancement as the two disciplines are inextricably linked with, for instance, the power take-off arrangement impacting significantly on how the ORE device interacts with wave conditions.
Lir Electrical Laboratory also has the capability of emulating a grid connection for ORE devices, which is used to investigate how it will respond to grid integration. This allows Lir to offer research capability to developers from wave to wire.
It is estimated that in excess of 70 different wave energy technology types and five floating wind platforms have been tested at Lir and only a small percentage of these have moved to open sea test sites.
This indicates the technological and financial challenges that the sector faces in the pathway to commercialisation. As testing forms such an integral part of the development process, the MaREI Centre and Lir are co-ordinating efforts at a European level, through the H2020 MaRINET2 and MARINERG-I projects, to improve and standardise testing methods. Standardised high-quality testing will provide the industry with the best chance of success which would unlock Ireland's vast offshore renewable energy resource.
Author: Dr Jimmy Murphy, lead technical engineer. Dr Murphy has more than 20 years’ experience working on consultancy and research projects related to coastal engineering and marine renewable energy. This work primarily involves examining the impact of engineering works on the physical marine environment through the use of field measurements, numerical modelling and physical modelling techniques. He has undertaken projects at many locations around Ireland in relation to coastal erosion and the construction of new piers/harbours and marinas. He also lectures in the School of Engineering at UCC on the subjects of environmental hydrodynamics and harbour and coastal engineering. He has a number of publications in the areas of coastal engineering and renewable energy.