Minesto Holyhead Deep - Non-grid connected DG500

Deep Green Deployment in StrangfordLough

MRE Project Site

Title: Minesto Holyhead Deep - Non-grid connected DG500

Status:

In Operation

Project Manager:

Minesto

Tech Developer:

Minesto

Info Last Updated:

May 5, 2025

Contact:

Jennifer Fox
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PRIMRE Link:

Projects Database

Website:

External Link

Details

Device Name:

Deep Green Tidal Kite DG500

Technology:

Marine Energy, Tidal

Installed Capacity:

0.5 MW

Country:

United Kingdom

Water Depth:

65-91 m

Description

Minesto have developed a tidal stream energy device known as a Deep Green Utility (DGU) unit. The device resembles an underwater kite made up of a wing with a small turbine attached to the underside, tethered to a foundation fixed to the seabed. The device can also be attached to a barge or similar structure and operate in an “upside down” mode. The 0.5 MW unit (DG500) deployed between July and November 2018 was attached to a self-contained Micro Grid System (MGS) buoy, which handled and analysed electricity generated.

The DGU unit moves through the water column in a figure-of-eight, sweeping an area of approximately 100-140 m wide and taking advantage of hydrodynamic lift created by the wing as the current flows past. This allows the device to operate in areas of relatively low current velocity. The DG500 unit is 12 m (wing span) by 3.3 m, weighs between 10 and 16 tonnes and has a generating capacity of 0.5 MW.

The Holyhead Deep low-flow tidal stream project aims to put Minesto on track for commercialisation of their Deep Green technology. Developing Holyhead Deep into a grid-connected commercial tidal energy array will be carried out in phases. In Phase 1 a single non-grid connected 0.5 MW Deep Green demonstrator device (DG500) was installed at the site, the purpose of which was to prove functionality and power production performance. Commissioning of the DG500 demonstrator was initiated in June 2018 and completed in November 2018. The DG500 unit will be redeployed for further sea trials in summer 2019.

Following successful deployment and testing of a single DG500, Minesto’s ambition is to install further Deep Green devices in phases, gradually expanding the site to a commercial demonstration array of up to 10MW installed capacity. In the first instance term, in addition to further sea trials of the DG500 unit, Minesto plan to install and operate an addition uprated (at least 0.75 MW) DGU.

Minesto’s long term plan is to further expand the commercial roll-out of its Deep Green technology in Holyhead Deep by taking steps to increase the planned installed capacity of the tidal energy array to 80 MW. Minesto intends to develop the Holyhead Deep site in phases as part of a deploy-and-monitor approach. This gradual expansion would see the Deep Green technology progressing from demonstrator to full industrial roll-out in North Wales.

Minesto has been awarded a new patent for its Deep Green technology to enhance its efficiency and scalability. In addition to project developments, Minesto signed an MoU with Schneider Electric to enhance grid compatibility testing for future large-scale deployments of the Deep Green technology.

Export Cables: The electricity produced by the DG500 is transmitted directly to a self-contained Micro Grid System buoy via a subsea umbilical cable where it is monitored and then dissipated. Electricity produced by the DGU will be transmitted via a subsea umbilical cable running from the foundation to the surface MGS buoy.

Vessel Spread:

Activity	Vessel Type	Indicative Operating Period
Installation
Deploying pin piled tripod, monopile, mud mat or GBS.	DP vessel or underslung barge (depending on mooringstructure solution).	Maximum of five days.
Installation of MGS buoy	Anchor handler tug or multicat.	Approximately 1 week.
Transport of MGS buoy to site.	Tug(s).	1 day per transfer, but multiple transfers expected.
Transport DGU unit to site.	LARS support vessel designed specifically for purpose.	1 day per transfer, but multiple transfers expected. Could be daily for the last fewweeks of installation/first few weeks of operation, dropping to around 1 per week thereafter.
Deployment of an ROV to attach tether to foundation, if not handled from barge deck).	Support vessel with ROV capabilities.	Less than 1 week. May also be required intermittently throughout field testing and calibration.
Operation
Field testing and calibration of DGU unit and hosting of electricity meter.	Barge (or similar such surface platform).	Throughout the 5 year testing period, although the barge is highly unlikely to be present consistently throughout the 5 year period due to constraints imposed by weather.
Retrieval, deployment and routine maintenance of DGU unit.	LARS support vessel designed specifically for purpose.	Throughout the 5 year testing period.
Unplanned maintenance of DGU power plant.	Service vessel, occasional requirement for DP vessel.	Approximately 6 days per year.

Location

The deployment site is located in the southern corner of Holyhead Deep, off the northwest coast of Wales. The area was carefully selected to maintain separation from shipping lanes and to minimise the impact on other sea users.

Licensing Information

In April 2017 Minesto received a Marine Licence (ref. ORML1618) under the Marine Works (Environmental Impact Assessment) Regulations 2007, for the initial 0.5 MW phase of their Holyhead Deep project. This was approved by the Natural Resources Wales Marine Licencing Team on behalf of the Welsh Ministers. The EIA Consent Decision can be viewed here.

The project has also secured additional funding through the Swedish Energy Agency to support the commercialization of the Deep Green technology.

Licencing Conditions:

The following project-specific conditions relevant to environmental monitoring and management were stated within the Marine Licence (ref. ORML1618):

8.3 The Licence Holder must submit an Environmental Monitoring Strategy (EMS) to NRW acting on behalf of the Licensing Authority for written approval at least 8 weeks prior to commencement of the works. No works may be undertaken prior to written agreement from NRW acting on behalf of the Licensing Authority.

The EMS must include, but may not be limited to:

Underwater noise monitoring;
Scour monitoring; and;
Marine mammal monitoring.

The Licence Holder must submit an Adaptive Environment Management Plan (AEMP) to NRW acting on behalf of the Licensing Authority for written approval at least 8 weeks prior to commencement of the works. No works may be undertaken prior to written agreement from NRW acting on behalf of the Licensing Authority.

The AEMP must include, but may not be limited to:

All mitigation measures outlined in the Environmental Statement;
Adaptive management
Operational management and mitigation; and
Marine mammal mitigation protocol.

Project Progress

In November 2013 Minesto submitted a scoping report to UK consenting authorities Marine Management Organisation (MMO) and Natural Resources Wales (NRW), asking for their scoping opinion for development of a 1.5 MW site in Holyhead Deep.

In spring 2018, Minesto installed and commissioned the DG500 site infrastructure consisting of the seabed foundation, tether, umbilical and a buoy containing a microgrid system (MGS). The MGS provided auxiliary power and communications to the DG500 device

In May 2018 a micro grid system (MGS) buoy was installed, which provided auxiliary power and communications to the DG500 device, as well as acting as a floating micro grid system to facilitate grid compatibility testing by handling and analysis electricity generated by the DG500 device.

Minesto initiated commissioning of its DG500 demonstrator device in June 2018, which consisted of two main phases. The first stage comprised a series of tests including verification of launch and recovery procedures, testing of each function of the control system, and finally operation of the DG500 in full figure-of-eight trajectories. The second stage involved electricity generation.

At the end of August 2018 Minesto unveiled its DG500 marine energy kite device after verifying the functionality of the Deep Green technology at utility scale. Minesto resumed further testing before the second commissioning phase to verify the power take-off system and electricity generation. The device generated electricity for the first time in early October.

The offshore commissioning and test programme of the DG500 device completed in November 2018.

In May 2019 the Welsh European Funding Office awarded 14.9 million of EU funding for the next phase of Minesto’s Holyhead Deep site. The investment will include:

Continued site development and operations at Minesto’s Holyhead Deep site
Enhancement of Minesto’s utility-scale product range;
Installation and operation of an additional , uprated (at least 50% higher than the current 0.5MW system) unit in Holyhead Deep;
Strengthened manufacturing and assembling capability;
Securing permits and consents for expanding the Holyhead Deep site towards a commercial 80MW tidal energy farm; and
Design of subsea infrastructure for the 80MW site development.

Minesto is seeking to submit consent applications and Environmental Statement for the 80MW later in 2019. The 0.5 MW non grid connected DG500 device was redeployed in August 2019 in Holyhead Deep site offshore North Wales, along with a hydrophone array to monitor cetacean activity around the device. In late 2021, the Swedish Energy Agency has awarded Minesto a SEK 5.8 million grant to the ongoing commercialisation of the Deep Green technology.

As of May 2025, Minesto continues testing, including a commitment to increase the installed capacity and expand the project with new units and collaborations.

Key Environmental Issues

The following potential impacts were deemed to be potentially significant during the EIA process: Collision risk for marine mammals and diving birds. Possible impacts on bottlenose dolphin were a particular concern, given the small population size of this species in Welsh inshore waters.

Mitigation Measures: The following key mitigation and best practice measures to be applied are outlined in the Environmental Statement:

All vessels associated with Project operations will comply with all relevant guidance (including IMO guidelines) regarding ballast water and transfer of Marine non-native species (MNNS)
Best practice measures to reduce the risk of and impact from vessel hydrocarbon and chemical spills (see ES for details). The mitigation measure detailed in the Hydrocarbon and chemical spills and Shipping and navigation sections of this table will minimise the risk and uncertainty associated with accidental spills to ensure that Project activities do not significantly impact the marine mammals in the Project area or wider region.
Development of an Environmental Monitoring Strategy.

Environmental Monitoring: Minesto Holyhead Deep - Non-grid connected DG500

Phase

Stressor

Receptor

Searches description, design & methods, results.

ID	Phase	Stressor & Receptor	Design and Methods	Results
6789	Baseline	Marine Mammals	Visual vessel based surveys Boat transects	Complete Harbour Porpoise: Of the 42 marine mammal sightings recorded by SEACAMS (2015) in the west Anglesey area, 39 were harbour porpoise. This corresponds to a rate of 1.88 sightings per hour. SEACAMS (2015) report that the spatial distribution of sightings of harbour porpoise appears to be relatively well spread across the survey area. Specifically within the DG Holyhead Deep site, from approximately 26 km of survey effort there was only one recording of harbour porpoise (detected acoustically), although further harbour porpoise sightings have been recorded close to the DG Holyhead Deep site (SEACAMS, 2015). Considering its status as the most common cetacean in the Irish Sea and considering the results of local surveys, it seems likely that the DG Holyhead Deep Project site is well-used by harbour porpoise, although relative densities suggest the site is not amongst the areas of highest importance in the Irish Sea Bottlenose Dolphin: Of the 42 marine mammal sightings recorded by SEACAMS (2015) in the west Anglesey area (Figure 11.2), only one was bottlenose dolphin. This corresponds to a rate of 0.05 sightings per hour. Specifically within the DG Holyhead Deep site, from the approximately 26 km of survey effort, no bottlenose dolphins were recorded and only a single sighting was recorded from the entire survey area (375 km of survey effort). This was in the far south-east of the survey area (SEACAMS, 2015). Considering the above, it seems extremely unlikely that the Project is located in an area of particular importance to bottlenose dolphins, but the small number of observations to the west of demonstrates that their presence cannot be ruled out. Common Dolphin: Of the 42 marine mammal sightings recorded by SEACAMS in the west Anglesey area, none were of common dolphin. Considering the apparent scarcity of common dolphins in the northern Irish Sea and off the coast of Anglesey, it seems likely that the DG Holyhead Deep Project is located in an area that is not of particular importance to this species. Risso’s Dolphin: Of the 42 marine mammal sightings recorded by SEACAMS in the west Anglesey area, none were of Risso’s dolphin. Considering the above, it is unlikely that the Project area is of particular importance to this species, though it is possible that they could use the area on occasion. Minke whale: Of the 42 marine mammal sightings recorded by SEACAMS in the west Anglesey area (Figure 11.2), none were of minke whale. Grey Seal: Of the 42 marine mammal sightings recorded by SEACAMS (2015) in the west Anglesey area (Figure 11.2), only 2 were of grey seal. This corresponds to a rate of 0.10 sightings per hour. Specifically within the DG Holyhead Deep site, the approximately 26 km of survey effort has not recorded any grey seal, although the two sightings from the 375 km of total survey effort have been recorded in the north of the survey area within which the DG Holyhead Deep sits Harbour Seal: The Irish Sea is not known to be an important area for harbour seal and animal densities here have been shown to be extremely low. Of the 42 marine mammal sightings recorded by SEACAMS (2015) in the west Anglesey area (Figure 11.2), none were of harbour seal.
6790	Baseline	Fish	Visual vessel based surveys Boat transects	Complete Basking Shark: Basking shark typically appear along the west coast of the UK from May to October as they follow plankton blooms. No sightings of basking shark were recorded in the Project area during the SEACAMS survey (SEACAMS, 2015).
6791	Baseline	Birds	Desk-based study N/A	Complete Common Guillemot: Based on previous survey work, and before adjusting for recent population change, the estimated at-sea densities of guillemots in the vicinity of the Project site was approx. 1 bird/km2 in the colony-attendance part of the breeding season (April to July), 3 birds/km2 in the post-breeding season (August and September) and slightly less than 1 bird/km2 in the non-breeding season (October to March) (NRP, 2016). The JNCC population trends analysis shows that since these surveys were undertaken (mostly in the late 1980s and early 1990s), breeding numbers in Wales have increased approximately three fold. It follows that at-sea densities, at least in the breeding season, will have risen similarly. Razorbill: Based on previous survey work, and before adjusting for any population change, the estimated at-sea densities of razorbills was approx. 0.3 bird/km2 in the colony-attendance part of the breeding season (April to July), 1.9 birds/km2 in the post-breeding season (August and September) and 0.3 bird/km2 in the non-breeding season (October to March). However, the JNCC population trends analysis shows that since these surveys were undertaken (mostly in the late 1980s and early 1990s), breeding numbers in Wales have increased by approximately 75%, with the main period of increase being in the late 1990s and before the Seabird 2000 census. It follows that at-sea densities, at least in the breeding season, will have risen similarly. Atlantic Puffin: Based on previous survey work, and before adjusting for recent population change, the estimated at-sea densities of puffins was approx. 0.3 bird/km2 in the colony-attendance part of the breeding season (April to August), and 0.2 bird/km2 in the non-breeding season (September to March) (NRP, 2016). Based on the population trend observed at Skomer Island (JNCC, 2014) it is reasonable to assume that at-sea densities of puffin off Wales in the breeding season are likely to have increased since the time when ESAS survey data were collected, mostly in the 1980s and 1990s. It is assumed that on average at-sea densities have increased by 50% since the surveys were undertaken. Northern Gannet: The Seabird 2000 census counted a total of 70,260 Apparently Occupied Sites (AOSs) at these colonies (Mitchell et al., 2007). Multiplying the number of AOSs by two, the size of the regional breeding population at the time of the Seabird 2000 census is estimated at 140,520 adults. Between 2004 and 2009 the numbers of gannets breeding at the Grassholm colony (by far the largest colony in the region) increased by approximately 22% (JNCC, 2014). The present day regional population size is assumed to be 25% greater than at the time of the Seabird 2000 census, at 175,650 adults. Based on previous survey work, and before adjusting for recent population change, the estimated at-sea densities of gannets was approx. 0.3 bird/km2 in the breeding season (April to September) and 0.15 bird/km2 in the nonbreeding season (October to March). Based on the population trend (JNCC, 2014) it is reasonable to assume that at-sea densities of gannet off Wales in the breeding season are likely to have increased since the time when ESAS survey data were collected, mostly in the 1980s and 1990s. It is assumed that on average at-sea densities have increased by 25% since the surveys were undertaken.
6792	Baseline	Invertebrates	Subtidal survey Benthic grabs and underwater camera survey (video and stills). Sampling stations for general seabed classification purposes were spread throughout the survey area to ensure representative coverage of all predicted habitats identified in the review of geophysical data. Where the geophysical data pointed towards the possible existence of Annex I habitats, specific targeted sampling stations were added. In addition, some reference stations were added outside of the core survey area to contextualise the other samples and to serve as control locations, should they be required for any future monitoring work.	Complete Infaunal Grabs: A total of 13,078 individuals from 318 taxa were identified in the 23 grabs samples taken in the survey area. Sampling stations within the project area tended to have higher abundance than elsewhere, but the highest abundance was recorded at Station 38 (outside the project area), where 2,140 individuals from 75 taxa were recorded. Infaunal communities were generally dominated by annelid worms and molluscs, followed by crustacea (primarily barnacles). At Stations 32 and 35, located close to shore, high numbers of molluscs were encountered, which was primarily attributable to two bivalve species – Nucula nitidosa and Abra alba. Elsewhere, annelids were by far the most abundant organisms. More than 46% of annelids were identified as Sabellaria spp., mostly Sabellaria spinulosa. Camera Survey: Camera survey images were analysed to classify habitat and identify visible fauna. The large majority of images showed a seabed of very coarse sediment, predominantly pebble and gravel, but with varying proportions of cobble, boulder, sand and shells of dead bivalves. In the project area, the seabed consisted mainly of pebble and gravel with sand and/or cobble at a few stations and a relatively small area supporting aggregations of S. spinulosa. The seabed outside the project area to the east consisted of coarser particles and small areas of exposed bedrock. Bedrock became more prevalent further to the east in the project area and was interspersed with areas of pebble and gravel as well as biogenic reef. Even further east, there were finer sediments including areas of predominantly sand, but also an area of pebble and gravel supporting encrusting growths of S. spinulosa and another area of exposed bedrock. Epifauna was variable but generally sparse (with a few exceptions) and was principally made up of scour tolerant taxa including various anemones, hydroids, erect bryozoa and epifaunal polychaetes.
6793	Baseline	Reptiles	Visual vessel based surveys Boat transects	Complete Marine Turtles: There are five species of marine turtle found in UK waters, green turtle Chelonia mydas, hawksbill turtle Eretmochelys imbricata, Kemp's ridley turtle Lepidochelys kempii, leatherback turtle Dermochelys coriacea and loggerhead turtle Caretta caretta. Of these, leatherback turtles are the most commonly recorded. No sightings of marine turtles were recorded in the Project area during the SEACAMS survey (SEACAMS, 2015).
6794	Baseline	Physical Environment	Geophysical survey Geophysical survey to provide information on bathymetry, shallow geology, seabed features and magnetic anomalies.	Complete The project area is located in the southern extent of the Holyhead Deep, with water depths ranging between 65 to 91 m, becoming deeper immediately north of the project area. The geophysical survey covered an area approximately 19.5 km long by 2.3 km wide at its wider section. The tidal resource in the area is understood well, through numerous modelling studies undertaken throughout Project design and the mean tidal power at the site is predicted to be between 0.8 and 1.2 kW/m2 (Atlas of UK Marine Renewable Energy Resources, 2008). The tidal asymmetry at Holyhead Deep favours sediment transport towards the north east. Sediment disposal modelling conducted by Potter (2014) found sediment to disperse widely across the local area, with 90% predicted to remain in suspension in low concentrations and <3% remained on the seabed after 15 days. Areas where mean bed shear stress is low, such as bays, accumulate sediment, most notably in the outer area of Holyhead port, which is evidenced by their annual requirement to dredge the outer harbour. The results suggest a highly dispersive and dynamic site.
6795	Baseline	Physical Environment	Sediments Multibeam and sidescan sonar	Complete Based on side-scan sonar mosaics and multibeam bathymetry data it was determined that the seabed across the project area comprises mainly sand and gravel, with several small irregular patches of megarippled sand and gravel, including boulders. This was corroborated by particle size analysis of grab samples obtained which revealed that two sediment types exist there – sandy gravel and muddy sandy gravel (according to British Geological Survey (BGS) classification)
6796	Baseline	Human Dimensions, Fisheries	Desk- and consultation-based study N/A	Complete Key commercial species: The most important fisheries active in the vicinity of the PDA are those for whelks, king scallops, lobsters, queen scallops, prawns and mussels. Whelks, lobsters and prawns are caught using pots and traps whilst king and queen scallops are targeted by dredgers and, in the case of queen scallops, beam trawlers. Potting and trapping fishery: Monetary Value Whelks have the highest economic value of any species landed from ICES rectangle 35E5 comprising 43% of the value and 50% of the liveweight of landings (average 2010-2014). Four of the most landed species (in terms of value) are shellfish, which contribute 96% of the value of landings and 99% of the liveweight of landings from ICES rectangle 35E5. Whelks are principally landed at Amlwch and Holyhead, although between 2010 and 2013 there were also landings at Cemaes Bay and Anglesey. Operating patterns and practices Whelks are principally targeted by full time static gear vessels setting pots. Whelk pots are made from a plastic container, one end of which is partially removed and partially covered with netting. The rest is perforated with 25-35 mm holes and about 25 cm of concrete is set in the bottom to weight the pot. Whelk pots are typically used on sandgravel substrates. Parlour pots, or creels, are used to catch lobster and crab. These are typically constructed from steel and netting material and they contain two chambers making it difficult for trapped animals to escape. Static gear used in this region of Wales involves attaching either several pots to a principal fishing line that is then attached to an anchor and a surface buoy; or alternatively in setting single pots. Local fishermen have indicated that lobster (and crab) potting activity currently takes place close to the shore. Dredging fishery: Monetary Value King scallops are the second most landed species in terms of value comprising 24% of total value whilst queen scallops are the second most landed species in terms of weight comprising 37% of landed weight. Operating patterns and practices King and queen scallops are both amongst the most landed species from ICES rectangle 35E5, both of which are targeted by dredges (though queen scallops can also be targeted by otter trawls or toothless dredges). For dredging, vessels tow one (astern) or two (either side) beams onto which a number of dredges are attached. The number of dredges used depends on vessel size, engine power and winch capacity. There is an exclusion zone for scallop dredgers within 1 nm (1.9 km) from the coast. This is due to the Scallop Fishing (Wales) Order, 2010, which excludes certain areas from scallop dredging. In addition to this, vessels greater than 12 m in length are restricted to only fishing beyond 6 nm (11.1 km) from shore and will therefore not be present in the project development area (PDA). Whilst scallop dredgers under 10 m in length do fish in Welsh waters, there is no evidence of them operating in the PDA area, based on FishMap Môn (2014) data and feedback from consultees.
6797	Post-Installation Monitoring	Collision Marine Mammals	2019: monitoring of cetacean activity around the non-grid connected DG500 unit Minesto are seeking to redeploy the DG500 device in late 2019 with a Passive Acoustic Monitoring (PAM) system (hydrophone array) to monitor cetacean activity around the unit.	Planned TBC
6798	Post-Installation Monitoring	Collision Marine Mammals	Minesto is committed to understanding the potential interactions between marine mammals and the Deep Green technology and intend to develop an adaptive monitoring strategy commensurate with the risks identified in the impact assessment. In all likelihood this will be achieved through collaborations with NRW and specialist research bodies and/or academic institutions, for example SEACAMS, so as to ensure the production of high-quality output that will be of significant value to future projects. The scope of the any potential monitoring plan would also be informed by monitoring of other tidal array projects as results of this monitoring becomes available.	Planned TBC
6799	Post-Installation Monitoring	Collision Birds	Minesto will work with the regulator and its advisory bodies to agree details of appropriate monitoring, including whether site specific monitoring at this Project site is appropriate. -	Planned TBC