Developing a New Floating Wind Turbine

Model Tests with a Novel Floating Wind Turbine Concept

Dr Ed Mackay & Prof. Lars Johanning, Offshore Renewable Energy Group

Dr Ed Mackay (Left) and Prof Lars Johanning (Right)

Floating offshore wind energy has been identified as being able to provide a significant contribution to meeting future renewable energy generation targets. Compared to traditional offshore wind turbines, which are fixed to the seabed, floating turbines can access deeper waters and areas with a higher wind resource. Current floating wind turbines are at the pre-commercial stage, with small arrays of up to five turbines being demonstrated. The cost of floating offshore wind turbines is currently significantly higher than fixed offshore wind. One of the main areas identified for reducing the cost of the structure is in the design of the platform. The platform must be designed to withstand large wave loads and keep the wind turbine as stable as possible. Large platform motions lead to reduced energy yield and increased loads on the wind turbine and drive train.

As part of the EPSRC funded RESIN project, the University of Exeter has been working with Dalian University of Technology (DUT) in China to investigate the use of porous materials in the floating platform for an offshore wind turbine, as a passive means of reducing platform motions. Porous materials are commonly used in offshore and coastal structures such as breakwaters or offshore oil platforms. As a wave passes through the porous material, energy is dissipated, reducing the wave height and wave-induced forces. The question posed by the RESIN project is: can porous materials be beneficial for floating offshore wind?

Examples of porous structures used in coastal and offshore engineering

The project has investigated this question using a combination of physical and numerical modelling. A range of analytical and numerical models have been developed [1-3] and validated against scale model tests in wave tanks. Two tests campaigns were conducted at the large wave flume at DUT in the summers of 2018 and 2019. The initial tests last year considered simple cases with flat porous plates with various porosities and hole sizes [4] and tests with fixed porous cylinders. These tests were used to validate the numerical predictions in a range of simple scenarios and gain an understanding of the effect of the porosity on the wave-induced loads.

 

A wave interacting with a fixed porous cylinder

Following the successful validation of the numerical models with simple fixed structures, a design was developed for a 1:50 scale model of a floating turbine, which could be tested with and without external porous columns. The model was tested at DUT this summer and further tests were conducted in the FlowWave tank at the University of Edinburgh this autumn. The test results showed that the motion response could be reduced by up to 40% in some sea states by adding a porous outer column to the platform. Work is ongoing to analyse the test results and optimise the design a platform using porous materials. However, initial results indicate that using porous materials in floating offshore wind turbines offers potential for reducing the loading on the turbine and mooring lines and improving energy capture.

1:50 scale model of a floating platform for an offshore wind turbine in various configurations. Left: inner column only. Middle: medium porous outer column. Right: Large porous outer column. The turbine rotor and nacelle are modelled as a lumped mass at the top of the tower.
The scale model installed at the FloWave tank at the Univeristy of Edinburgh

Thanks Ed!

To keep up to date with the Renewable Energy team, give them a follow on Twitter @Renewables_UoE 

For information on the Offshore Renewable Energy research group, check out their webpages.

References

  • Mackay EBL, Feichtner A, Smith R, Thies P, Johanning L. (2018) Verification of a Boundary Element Model for Wave Forces on Structures with Porous Elements, RENEW 2018, 3rd International Conference on Renewable Energies Offshore, Lisbon, Portugal, 8th – 10th Oct 2018.
  • Feichtner A, Mackay EBL, Tabor G, Thies P, Johanning L. (2019) Modelling Wave Interaction with Thin Porous Structures using OpenFOAM, 13th European Wave and Tidal Energy Conference, Napoli, Italy, 1st – 6th Sep 2019.
  • Mackay E, Johanning L, (2019). Comparison of Analytical and Numerical Solutions for Wave Interaction with a Vertical Porous Barrier. Ocean Engineering (submitted)
  • Mackay E, Johanning L, Ning D, Qiao D (2019). Numerical and experimental modelling of wave loads on thin porous sheets. Proc. ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering OMAE2019, 2019, pp. 1-10.

#ExeterMarine is an interdisciplinary group of marine related researchers with capabilities across the scientific, biological,  medical, engineering, humanities and social science fields.

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My #ExeterMarine Expedition: updates from the Indian Ocean

This is a series of updates from #ExeterMarine researcher, Dr Ines Lange, who is on a research expedition in the British Indian Ocean Territory with Professor Chris Perry, the Bertarelli Foundation and ZSL.

Hard at work in the Indian Ocean

03/05/2018 Somewhere in the Indian Ocean

We are in the Central Indian Ocean, sailing south on the “Grampian Frontier”. Prof. Chris Perry and myself, Dr. Ines Lange, from the Geography department are on a research cruise to study the coral reef ecosystems of the British Indian Ocean Territory (BIOT). The project is part of a large expedition funded by the Bertarelli Foundation and on board are twelve scientist, working on six different projects.
Chris and I are studying the carbonate budgets of coral reefs around islands of the Chagos archipelago. The “ReefBudget” method we use was developed by Chris and calculates how much carbonate is produced by corals and calcifying algae, and how much is eroded by grazing sea urchins and fish, as well as by internal bioeroders such as boring worms and microorganisms. The results provide a metric of reef “health” in terms of whether it is growing or eroding. On this trip we have two main goals:

1) To revisit sites that were surveyed in 2015, before the severe bleaching event that hit the Central Indian Ocean in 2016. We expect to see dramatically reduced rates of carbonate production due to low coral cover.
2) To investigate local rates of coral and calcifying algal growth as well as internal erosion rates by deploying experimental substrates.

On the two-day transit from the Maldives down to Chagos we are busy preparing material for the deployment of experimental substrates and discussing calculations for the model. Of course we also have to include a few safety exercises; it is very hot here so I would not mind going for a swim, but we ended up not abandoning the ship…

Stay tuned for our upcoming adventures underwater.

Survivors in the reef

News from the “Reef team” in the Indian Ocean. The sites in the Salomon and Peros Banhos Atolls we visited so far show a dramatically reduced coral cover due to the severe bleaching event in 2016, causing carbonate production rates to drop to about a third of the values in 2015. On the upside, there are many Porites and also some Acropora colonies that apparently survived the bleaching, and large numbers of small recruits of different genera. Especially in the understory of the reef structure we find many live encrusting corals. Also, the substrate is quite clean of macroalgae, thanks to the high abundance of grazing herbivorous fish. Calcareous algae covering the dead coral substrate continue to produce substantial amounts of carbonate which help “glue” the existing structure together and offer a great substrate for further coral recruitment. We therefore hope we can see a fast recovery of the once glorious reefs over the next years.

To investigate local bioerosion rates in the reefs we had a “fun” day sawing 1000s of blocks from dead Porites skeleton (well, it certainly felt like that, on my last count it was actually 28). Today we successfully deployed the substrates in the reef, where they will be settled by encrusting and bioeroding organisms (or eaten by parrotfish? Hope not …). The work days are long and it’s actually not always as sunny as you would imagine (see how we enjoy our surface interval in the rain?!). But the company is great, and encounters with curious turtles, dancing eagle rays and confused birds trying to land on our heads make every day a great adventure…

#ExeterMarine is an interdisciplinary group of marine related researchers with capabilities across the scientific, medical, engineering, humanities and social science fields. If you are interested in working with our researchers or students, contact Michael Hanley or visit our website!

Life on a Russian Icebreaker: ACE Maritime University

Author – Jen Lewis, PhD researcher

This blog originally featured on fabiogeography.com.

Before departure in Bremerhaven Germany

Last year (I can’t believe it’s been a whole year!) I was lucky enough to be awarded a scholarship to attend a one off opportunity, the ACE Maritime University. The scene for this month long ‘floating university’ was the Eastern Atlantic Ocean, aboard the Russian Polar Research Vessel Akademik Treshnikov.

On the 19th of November 2016, 49 students and 16 scientists from 20 different nationalities joined the ship in Bremerhaven, Germany for Leg 0 of the Antarctic Circumnavigation Expedition. Over 27 days we sailed down across the equator, to Cape Town, South Africa. ACE was a

privately funded expedition, that went on to circumnavigate the continent of Antarctica last summer, visiting the islands, measuring things like marine mammal populations, ocean acidification, carbon dioxide dynamics and marine plastic distribution. There were 22 different research projects, and if you want to read more about them then check out the ACE expedition website and blog.

The aim of the course was to bring together a global group of young researchers and introduce marine science as a cross-disciplinary field. Days were full of lectures on various principles of oceanography, or how to use different type of ocean monitoring instrumentation.

Releasing a radiosonde balloon to take atmospheric measurement

 

Deploying the CTD and water sampler

 

 

 

 

 

 

 

 

 

 

We even had homework every other day! We also took part in daily deck work with the different projects that were setting up for the Antarctic legs. This included things like assisting with the CTD deployment, processing data, collecting water samples and filtering them. Highlights included working with Florian and Yajuan to make a film about their research  investigating microbial and plankton communities that have big influences on primary production and the carbon cycle, seeing the different layers of biomass on the echosounder display, and also setting up and deploying a radiosonde balloon that measures the atmosphere.

Dolphins riding the bow waves

 

CTD profile from cruise data. Each graph shows information about temperature, salinity or oxygen from the surface down to 1000m depth. Samples start from near the Mediterranean (CTD001), over the equator (CTD10) and further south towards the African coast. You can see things like the high salinity Mediterranean outflow at the start, and oxygen minimum zones either side of the equator.

As part of a personal project, I was also filtering seawater samples from different depths from the CTD casts every day. These samples are being used to look at the difference in species diversity through the North to the South Atlantic, by looking for traces of DNA that has been left in the water by different species that are in the area at different depths – so watch this space!

#Research: Testing new mooring systems for #MarineRenewableEnergy

Author – Dr Tessa Gordelier

A new paper has been published by #ExeterMarine academics Prof. Lars Johanning, Dr Tessa Gordelier and Dr Philipp Thies in collaboration with the French research institute IFREMER (@Ifremer_en).

The mooring systems were put through their paces at the University of Exeter’s DMaC facility

Assessing the performance durability of elastomeric moorings: Assembly investigations enhanced by sub-component tests”  investigating novel mooring tether performance for offshore renewable applications.

The growing marine renewable energy sector is placing new demands on mooring systems; not only are they required to hold devices on station they must also provide the compliance required to harvest energy from the marine environment.  In response to this, several innovative mooring tethers have been proposed, with increased compliance and a degree of customisation of the stiffness profile.  Many of these novel systems utilise materials in a unique application within the challenging marine environment and their long term durability remains to be proven.

The novel mooring system underwent 6 months of sea trials on a mooring limb of the South West Mooring Test Facility

In response to this challenge, the work presented in this paper summarises a multifaceted investigation into the durability of a novel mooring tether with an elastomeric core.  Tether assembly testing is conducted before and after a 6 month sea deployment in addition to detailed laboratory investigations.  At a sub-component level, to represent the operational demands of the tether on the elastomer core, detailed material investigations review the effect of both marine exposure and repeated compression loading on key material properties. This work is the first of this type to be published and will be of interest to anyone utilising this material in other relevant applications.

Overall the results indicate an increase in both material and tether stiffness profile with use; this will affect both system dynamics and mooring loads.  If we are to realise the benefits of these novel mooring systems, this characterisation is crucial to ensure reliable and effective integration into offshore engineering projects.

#ExeterMarine is a interdisciplinary group of marine related researchers with capabilities across the scientific, medical, engineering, humanities and social science fields. If you are interested in working with our researchers or students, contact Michael Hanley or visit our website!