energies Review Concrete Support Structures for Offshore Wind Turbines: Current Status, Challenges, and Future Trends Alexandre Mathern 1,2,* , Christoph von der Haar 3 and Steffen Marx 4 1 Department of Architecture and Civil Engineering, Chalmers University of Technology, Sven Hultins Gata 6, SE-41296 Gothenburg, Sweden 2 Research and Innovation, NCC AB, Lilla Bomen 3c, SE-41104 Gothenburg, Sweden 3 grbv Ingenieure im Bauwesen GmbH & Co. KG, Expo Plaza 10, 30539 Hannover, Germany; [email protected] 4 Institute of Concrete Structures, Technische Universität Dresden, August-Bebel-Straße 30/30A, 01219 Dresden, Germany; [email protected] * Correspondence: [email protected] or [email protected] Abstract: Today’s offshore wind turbine support structures market is largely dominated by steel structures, since steel monopiles account for the vast majority of installations in the last decade and new types of multi-leg steel structures have been developed in recent years. However, as wind turbines become bigger, and potential sites for offshore wind farms are located in ever deeper waters and ever further from the shore, the conditions for the design, transport, and installation of support structures are changing. In light of these facts, this paper identifies and categorizes the challenges and future trends related to the use of concrete for support structures of future offshore wind projects. To do so, recent advances and technologies still under development for both bottom-fixed and floating concrete support structures have been reviewed. It was found that these new developments meet the Citation: Mathern, A.; von der Haar, challenges associated with the use of concrete support structures, as they will allow the production C.; Marx, S. Concrete Support costs to be lowered and transport and installation to be facilitated. New technologies for concrete Structures for Offshore Wind support structures used at medium and great water depths are also being developed and are expected Turbines: Current Status, Challenges, and Future Trends. Energies 2021, 14, to become more common in future offshore wind installations. Therefore, the new developments 1995. https://doi.org/10.3390/ identified in this paper show the likelihood of an increase in the use of concrete support structures in en14071995 future offshore wind farms. These developments also indicate that the complexity of future support structures will increase due to the development of hybrid structures combining steel and concrete. Academic Editor: These evolutions call for new knowledge and technical know-how in order to allow reliable structures Charalampos Baniotopoulos to be built and risk-free offshore installation to be executed. Received: 19 February 2021 Keywords: wind energy; offshore wind; support structures; foundations; concrete structures; trends Accepted: 26 March 2021 Published: 4 April 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in In the past two decades, offshore wind has emerged as a new source of renewable published maps and institutional affil- iations. energy. By the end of 2019, the cumulative capacity installed worldwide had reached 29.1 GW [1] (22.1 GW in Europe [2] and 7.0 GW in Asia, mostly in China [1,3]). The installed offshore wind power capacity is increasing rapidly. This is illustrated in Figure1, which shows the annual and cumulative offshore wind capacity installed between 2000 and 2019. According to forecasts, this trend is expected to continue in the coming decades, Copyright: © 2021 by the authors. in line with the targets set by many countries to decarbonize their economies. Hence, the Licensee MDPI, Basel, Switzerland. cumulative capacity of offshore wind plants in Europe could reach between 45 GW and This article is an open access article 100 GW by 2030 [4], and globally as much as 400 GW by 2045 [5]. In line with these forecasts, distributed under the terms and the European Union strategy for offshore wind recently set the objective of reaching 60 GW conditions of the Creative Commons Attribution (CC BY) license (https:// by 2030 and 300 GW by 2050, which are estimated to require investments of EUR 800 billion creativecommons.org/licenses/by/ until 2050 and make offshore wind an essential factor or the European Union’s climate 4.0/). neutrality target at the horizon of 2050 [6]. High offshore wind development objectives Energies 2021, 14, 1995. https://doi.org/10.3390/en14071995 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 31 Energies 2021, 14, 1995 2 of 31 climate neutrality target at the horizon of 2050 [6]. High offshore wind development ob- havejectives also have been also declared been declared in other in regions: other regi e.g.,ons: 40 GWe.g., in40 theGW UK in the by 2030UK by [7 ],2030 30–45 [7], GW 30–45 in JapanGW in by Japan 2040 by [8 ],2040 and [8], 86 GWand 86 in theGW US in bythe 2050 US by [9 ].2050 [9]. 4000 25,000 3500 20,000 3000 2500 15,000 2000 1500 10,000 1000 5000 500 Annual installed capcity [MW] 0 0 Cumulative installed capacity [MW] 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Year Figure 1. Cumulative and annual installed capacity ofof EuropeanEuropean offshoreoffshore windwind farmsfarms betweenbetween 2000 and2000 2019, and 2019, elaborated elaborated with datawith fromdata from [2,10, 11[2,10,11].]. The development of offshore wind power has been dependent on government fund- ing [12], [12], due to to the the higher higher cost cost of of offshore offshore wind wind installations installations compared compared to to that that of of installa- instal- lationstions of of onshore onshore wind wind and and other other sources sources of of el electricity.ectricity. Cost Cost reductions are required forfor this technology to become more competitivecompetitive with respect toto otherother energyenergy sourcessources andand toto contribute toto thethe high high targets targets set set by by many many countries countries in termsin terms of the of reductionthe reduction of greenhouse of green- gashouse emissions. gas emissions. Therefore, Therefore, the choice the of choice support of structuressupport structures and their production,and their production, transporta- tion,transportation, and installation and installation methods is methods particularly is pa important.rticularly important. Support structures Support structures account for ac- a largecount part—asfor a large much part—as as 20–40%—of much as the20–40%—of total capital the costs total ofcapital offshore costs wind of offshore farms [13 wind–16]. Indeed,farms [13–16]. it has been Indeed, reported it has thatbeen optimization reported that of optimization the support structureof the support presents structure the largest pre- cost-reductionsents the largest potential cost-reduction regarding potent investmential regarding costs investment [17]. costs [17]. Several different types of support structures, consisting of either steel or concrete Several different types of support structures, consisting of either steel or concrete (re- (reinforced or prestressed) have been used as support structures for offshore wind turbines. inforced or prestressed) have been used as support structures for offshore wind turbines. Gravity-based foundations made of concrete, similar to those used for onshore wind Gravity-based foundations made of concrete, similar to those used for onshore wind tur- turbines, were a commonly used solution in the very first offshore wind farms situated bines, were a commonly used solution in the very first offshore wind farms situated in in very shallow waters. These foundations have successively been supplanted by steel very shallow waters. These foundations have successively been supplanted by steel monopiles, which account for most of the support structures installed over the past two monopiles, which account for most of the support structures installed over the past two decades. decades. The rotor diameter and height of the wind turbines increase with their increasing The rotor diameter and height of the wind turbines increase with their increasing capacity (Figure2a). In addition, wind farms are nowadays located further from shore capacity (Figure 2a). In addition, wind farms are nowadays located further from shore and in deeper waters (Figure2c,d). As the conditions for offshore wind farms change, and in deeper waters (Figures 2c,d). As the conditions for offshore wind farms change, requirements for support structures also change. Support structures have been scaled up in orderrequirements to support for thesupport larger struct turbinesures inalso these change. water Support depths, structures and monopiles have withbeen diametersscaled up asin order large asto 8support m are now the larger used [ 12turbines]. in these water depths, and monopiles with diame- ters asThe large vast as majority 8 m are ofnow support used [12]. structures for offshore wind turbines are made of steel, and mostThe vast of those majority are monopiles. of support However,structures recentfor offshore studies wind highlight turbines the are potential made benefitsof steel, ofand using most concrete of those forare offshoremonopiles. structures However, in recent general studies [18] and highlight for offshore the potential wind turbines benefits inof using particular concrete [19– 22for], offshore since concrete structures structures in general have [18] lower and productionfor offshore costs,wind