Axis Energy Projects – Innovative Tension Leg Buoy Technology
Axis Energy Projects has developed a novel Tension Leg Buoy for floating offshore turbines. The solution makes it possible to keep the turbines vertical and stable even in extreme weather conditions.
The EU foresees 60 GW of offshore wind by 2030 and 300 GW by 2050. Europe currently has an offshore wind capacity of 12 GW, and floating wind technology continues to develop rapidly. As most installations will need to be in deeper water where traditional bottom-fixed turbines are not viable, this creates huge opportunities for the floating wind sector. According to the Offshore Renewable Energy Report, floating wind could account for up to a third of all offshore capacity by 2050.
From oil and gas to renewable energy
Axis Energy Projects, consisting of a team of engineers and project managers, provides a vast palette of services to the floating production industry, from concept development to project delivery. Since 2015, the Aberdeen-based company has developed a mooring system for offshore floating wind, which is based on technology they originally developed for the floating production and subsea industry over twenty years ago. Since then, Axis has completed successful model testing and received two national awards for its Tension Leg Buoy (TLB) technology, the Rushlight Wind Power Award and the Rushlight Natural Energy Award. “We have used our experience and technical knowledge of working in the offshore oil and gas sector to design the TLB for use in the floating offshore wind sector”, John Baross, Founder and Managing Director at Axis, explains to Periscope.
The TLB provides controlled stability, roll and pitch for wind turbines, keeping the turbine stable even in extreme weather conditions. The system consists of a subsurface buoy that supports the wind turbine, which is fixed by tensioned tethers to a modular gravity base foundation on the seabed. The Leg Buoy can be deployed from depths of 50 metres and up to hundreds of metres, with optimal water depth considered to be at around 100 metres.
But how does their technology compare to other offshore wind solutions? “Most companies engaged in floating wind offer catenary solutions that rely on floating buoyant structures to provide stability, and what they have in common is movement in all six degrees of freedom. These solutions also require a lot of steel. With tension leg technology, you can control roll, pitch and heave, so it allows you to provide great stability for turbines”, Baross says. He also explains that their concept minimises the weight of the steel structure and fabrication costs while maximising turbine stability. There are also several environmental benefits, such as a relatively small seabed footprint and no need for piling or jack-up vessels. Potentially, the TLB concept could reduce foundation costs by 25-30 per cent, and reduce energy costs from floating wind by 10-15 per cent.
Proven stability even in harsh weather conditions
In the development of the TLB, Axis has been supported by Periscope’s partner the Offshore Renewable Energy Catapult and the University of Strathclyde. The latter undertook all of the initial stability analyses, and tested a 10 MW turbine with an overall hight of 200 metres in a North Sea Block 9 100 Year Storm Condition. The results showed that the TLB system will remain stable under this condition, with only minor motion while in operation. A 2018 study by the Department of Naval Architecture at Strathclyde University concluded that the TLB technology, compared with other floating offshore wind concepts, “demonstrates better stability performance, flexibility in installation and transportation, reduced requirement for water depth, lowered tether/tendon tension fluctuation with free flooded central column and possibility of integrating a wave energy device to form a multi-energy complimentary system.”
In April 2020, Axis completed successful model testing at the FloWave Ocean Energy Research Facility of the University of Edinburgh, simulating a 10 MW turbine in a North Sea 100-year storm scenario, with hurricane force winds and maximum wave height of 21 metres. Even in this scenario, the turbine tower remained vertical with an impressive maximum pitch and row of only half a degree. The University of Edinburgh hence confirmed the results of the stability analysis of Strathclyde University.
What are the next steps?
Baross notes that they have been attracting much attention and support for their floating wind concept, which have been of great importance. The development and commercialisation of the TLB received grant and advisory support from the UK’s Department of Business, Energy & Industry Strategy (BEIS) through its Energy Entrepreneurs’ Fund. The company is also supported by Scottish Enterprise and the Oil & Gas Technology Centre (OGTC). The TLB has not reached the Technology Readiness Level (TRL9) yet, but a prototype full-scale demonstrator is scheduled to be installed on the UK Continental Shelf early 2023.
Due the big drive to decarbonise oil and gas production, the first step for Axis could be to use their technology to provide power to oil and gas installations, and then offshore wind farms. “The Covid-19 pandemic has created a big emphasis to keep moving forward, and could help accelerate the decarbonisation efforts of the oil and gas sector”, adds Bernie Morrison, Commercial Director at Axis. The primary market will likely be Europe, but as floating wind is gaining a stronger hold globally, markets like Asia and the US also provide vast opportunities. Moreover, Axis is also looking into the possibility of integrating a wave energy device to their TLB concept.
Images © Axis