Wave energy converter offers lower-cost subsea power option

Feb. 1, 2022
Trials will resume later this year on Mocean Energy’s Blue X wave energy machine following five months of tests at sea offshore Orkney, northern Scotland, in 2021.

Trials will resume later this year on Mocean Energy’s Blue X wave energy machine following five months of tests at sea offshore Orkney, northern Scotland, in 2021. The development, supported by £3.3-milion ($4.5-million) funding from Wave Energy Scotland, could potentially provide reliable and relatively low-cost power to subsea production equipment, inspection ROVs and AUVs and other applications, for offshore oil and gas and wind. The consortium involved in the program includes one of the UK North Sea’s leading producers, Harbour Energy, and Baker Hughes.   

Mocean Energy’s founders are Managing Director Cameron McNatt and Technical Director Chris Retzler. McNatt, an American with a background in naval architecture and software development, came to Britain to study for a PhD in wave energy at Edinburgh University. There he met Retzler, who had been Principal Scientist for local company Pelamis Wave Power, a developer of a wave energy converter system which had entered administration in 2014.

They decided to form their own company, Mocean Energy, headquartered in Edinburgh and with a base in Aberdeen, to industrialize their ideas for wave energy and to take advantage of financial support on offer from Wave Energy Scotland, the Net Zero Technology Centre (NZTC), the European Union, and the US Department of Energy. According to Netzler: “Our plan was to use software and numerical optimization to design wave energy converters, with a particular focus on the geometry. The algorithms we developed spawned new shapes not previously identified for wave energy that generated greater movement in waves, potentially producing up to 10 times more power than earlier designs of the same size. These designs evolved into Blue X and the other products we are currently working on, Blue Star and Blue Horizon.” Recently, Commercial Director Ian Crossland joined the company, bringing a background in oil and gas and new technology start-ups.    

Mocean Energy (Mocean) started its engagement with the North Sea oil and gas sector in 2019 after being selected for the TechX business accelerator program in Aberdeen, organized by the NZTC (formerly the Oil & Gas Technology Centre). This led to an introduction to E&P independent Chrysaor, since merged with Premier Oil to form Harbour Energy. “Chrysaor’s management saw the value in using renewable energy to provide power to remote subsea tiebacks and equipment in place of subsea umbilicals, which are typically capex-intensive, slow to deploy, and for permanent installation,” McNatt explained.

“In parallel, we identified the benefits that subsea batteries could provide to our system, which led to connections with Aberdeen-based EC-OG. We also saw that wave energy coupled with battery storage could be an ideal solution for residential subsea robots such as AUVs, offering the potential for big reductions in the cost and CO2 footprint of subsea IRM. That led to us reaching out to Modus, which was doing some work on residential AUVs. And the final piece of the jigsaw was Baker Hughes, which joined the project via its existing relationship with EC-OG.” Baker Hughes has stated that it sees potential for Blue X to power and communicate with its future all-electric subsea production systems.

The Renewables for Subsea Power (RSP) project brings together the Blue X wave energy converter, EC-OG’s Halo subsea battery system, and Modus’ residential hybrid autonomous underwater vehicle (HAUV). Blue X and Halo will direct power to subsea control modules provided by Baker Hughes and the HAUV, and the set-up will also enable remote ‘over the horizon’ (direct line of sight) operation of the subsea equipment.

According to McNatt, while each individual technology developer collectively forms a piece of the puzzle, “what operators such as Harbour Energy are seeking are complete, fully-integrated and field-proven solutions. For developers, the project gives each individual company the chance to prove that its technology is low-risk and can therefore raise its Technology Readiness Level. Baker Hughes is a natural fit: aside from the subsea control modules that will be powered and remotely controlled via the RSP system, the company contributes system integration expertise. Ultimately, it will most likely be Tier 1 engineering groups that deliver the integrated systems to offshore clients.”

McNatt is also now a member of the NZTC’s panel advising the offshore sector on how to deliver on Net Zero targets for North Sea operations. “Scotland’s government sees a massive opportunity for wave energy to supply up to 10% of the global renewable energy mix by 2050. For Wave Energy Scotland (WES), the prime focus is to develop and foster commercial wave technologies in Scotland, potentially creating thousands of jobs in Scotland and elsewhere. At the same time, WES and Mocean also recognize that the pathway for wave energy will involve the transfer of skills from other industries such as offshore oil and gas, and this in turn can help the sector undertake its transition to a lower carbon future.”

Wave diving

The company first developed Blue X as a conceptual design in 2018 as part of its WES-funded WEC (Wave Energy Converter) Stage 2 project. The main considerations were to maximize power production through the geometry of the wave energy machine; and to develop a solution that would be both reliable – a simple hinge design driving a generator – and capable of operating in extreme seas over long periods of time. 

“Blue X has a sloped forward geometric feature that we call a ‘wave channel’,” McNatt explained. “This improves power performance, but it also ensures the bow of the device always remains under water, diving through waves like a swimmer in the surf zone. This is completely different to how ships handle large waves, where they are lifted from the water and slam (in a particularly bad load case) on re-entry.” Through the relative rotation of its hulls around the hinge, the machine converts energy in ocean waves to mechanical power: this drives a generator outputting ‘wild’ three-phase electrical power, managed by power electronics and stored in DC batteries onboard the Blue X system. That energy is used by onboard systems and exported through an umbilical downline to a subsea energy storage system, which distributes it to subsea equipment.

“Energy storage is essential to providing reliable and consistent power to equipment above and below the water line. Although wave energy is variable, like all renewable power sources, when combined with energy storage it can ensure availability of power to the application. Essentially, we are creating an offshore renewable ‘micro-grid’. With subsea batteries more energy can be stored (not being limited by weight and space constraints), thereby providing a level of redundancy.”

Mocean has identified a range of potential applications for offshore oil and gas:

  • Umbilical and subsea electrical failures can take down production, leading to lost revenue. Blue X can be deployed rapidly as a back-up generator to bring production back online.
  • New subsea tiebacks or tieback expansions: The system can provide low-cost, reliable and low-environmental impact power, rather than running a long, expensive, capex-intensive, permanent umbilical.
  • A low-cost, reliable solution for retrofits of subsea equipment such as sensors and subsea pumps to existing fields where there is no readily available power supply.
  • Subsea IRM. Residential ROVs and AUVs allow for subsea inspection, repair and maintenance without the need for a host vessel. However, these technologies are limited by their battery capacity, whereas Blue X could theoretically allow for IRM subsea campaigns of unlimited duration.  The same benefit applies to offshore wind, for time-intensive inspections of subsea facilities such as cables and foundations, and above-water infrastructure, such as turbine blades using airborne drones.
  • Marine domain awareness – monitoring activity in the local offshore environment. The system could be equipped with cameras and radar to monitor incursions by fishing or other vessels into exclusive zones around the offshore facilities. 

Early testing scope

Last year Mocean and its partners performed the first open-water trials in Orkney at the Scapa Flow test site, operated by the European Marine Energy Centre (EMEC). EMEC has a lease of the seabed and its support eases the process for an equipment developer to secure consent for testing. Scapa Flow is one of the world’s largest natural harbors and therefore experiences only locally wind-generated waves, as opposed to ocean waves.

The 20-m (65.6-ft) long, 38-metric ton (42-ton) wave energy prototype was fabricated by AJS in eastern Scotland, with Supply Design responsible for the power electronics and electrical distribution systems. Blackfish provided structural and mechanical design services, with the University of Edinburgh designing the C-Gen generator, built by Fountain Design. “We selected the C-Gen since there were no off-the-shelf generators suited to the types of motions experienced by the power train,” McNatt said. “Also, the university developed C-Gen specifically to support wave energy applications, in part via WES’ Scotland Power Take Off project.”

“We tested the prototype at Scapa Flow for five months last year between June and November, proving a variety of operations – towing, installation, removal, maintenance, access at sea; reliable power production – 5 kW average power and 30 kW safe peak powers; and survival behavior – diving through steep waves (in sea states of up to 2.3 maximum wave height), safe hinge angle motions, and electrical system safe operational modes. Solar panels were included as part of a test of offshore solar systems because for future products, we intend to combine solar into our wave energy converter products to provide complementary renewable power. While solar does not provide a large amount of power, it offers the benefit of providing power when waves are not available. We were also able to relay commands from shore and compare the results of actual power production with our numerical predictions.”

Currently the RSP project partners are constructing and testing onshore the various subsea equipment items that will be deployed offshore for trials later this year, again at Scapa Flow. To date they have proved the communications system, while commissioning of the power system should be completed in the next couple of months.  For the coming trials, Blue X will be connected to a Saab Sabertooth AUV with 30 kWh battery capacity, and with extensive technology and software developed and integrated by Modus. Although the exact offshore test program is yet to be determined, the AUV will likely perform various demonstration ‘over-the-horizon’ survey activities in the area where the equipment is deployed.

Following this year’s campaign, the hope is that Blue X will be qualified to TRL 7 or 8 on the DNV scale and then available for delivery to customers in 2023, McNatt said. “Depending on where it is deployed it will generate 3-6 kW of power on average over the course of a year. Its capacity to supply consistently available power will be governed by a variety of system specifications, including battery capacity. But system-specific design is straightforward and the subsea battery system is modular, and can therefore be expanded to suit the application.   

“Our plan is to deliver the technology primarily through ‘Power as a Service’, whereby we own and operate the equipment for customers, providing the power they require. The cost base for customers is then opex, which over short deployments is often preferable to a capex arrangement. Beyond that we are developing Blue Star, which will double (or more) the power capacity and will include solar panels to provide higher levels of consistently available power. We are also working on the Blue Horizon 250, which will be able to produce 50-100 kW or more of average power for large offshore infrastructure, including platforms, vessel recharging, and aquaculture.

“For Blue X, we are targeting two years between maintenance intervals while for the next-generation Blue Star, the goal is five years between maintenance. Offshore work is one of the costliest aspects of the technology, so it is key to reduce the amount of work needed and therefore bring down costs. Some maintenance activities can be conducted offshore by going onboard the device if seas states allow. But others will require retrieval of the device to shore ahead of a future redeployment. Blue X is designed to be swiftly connected and disconnected and then towed at a reasonable speed, using low-cost multi-cat vessels. These are operations that have already been proven during the 2021 test campaign.”