Assessment of Multi-Use Offshore Platforms: Structure Classification

sustainability

Review Assessment of Multi-Use Oﬀshore Platforms: Structure Classiﬁcation and Design Challenges

Walid M. Nassar * , Olimpo Anaya-Lara, Khaled H. Ahmed, David Campos-Gaona and Mohamed Elgenedy

Department of Electronic & Electrical Engineering, Faculty of Engineering, University of Strathclyde, Royal College Building, 204 George St, Glasgow G1 1XW, UK; [email protected] (O.A.-L.); [email protected] (K.H.A.); [email protected] (D.C.-G.); [email protected] (M.E.) * Correspondence: [email protected]

Received: 19 January 2020; Accepted: 26 February 2020; Published: 1 March 2020

Abstract: As the world continues to experience problems including a lack of seafood and high energy demands, this paper provides an assessment for integrated multi-use offshore platforms (MUPs) as a step towards exploiting open seawater in a sustainable way to harvest food and energy. The paper begins with background about MUPs, including information regarding what an MUP is and why it is used. The potential energy technologies that can be involved in an offshore platform are introduced while addressing similar applications all over the world. The paper presents the state of the art of MUP structures on the light of EU-funded programs. An MUP would have a positive impact on various marine activities such as tourism, aquaculture, transport, oil and gas and leisure. However, there are concerns about the negative impact of MUPs on the marine environment and ecosystem. Building an MUP with 100% renewable energy resources is still a challenge because a large storage capacity must be considered with a well-designed control system. However, marine bio-mass would play a vital role in reducing battery size and improving power supply reliability. Direct Current (DC) systems have never been considered for offshore platforms, but they could be a better alternative as a simpler control system that requires with lower costs, has lower distribution losses, and has an increased system efficiency, so studying the feasibility of using DC systems for MUPs is required.

Keywords: multi-use platform; aquaculture; oﬀshore; energy

1. Introduction Water covers 71% of the surface of the earth, and oceans hold around 96.5% of this water [1]. As a result of gradually increasing global warming emissions and increasing of populations all over the world, there is a high tendency for more sustainable activities to cut these emissions and provide food in a sustainable way. Oceans, as an alternative, have a lot of opportunities for the energy and food sectors. To exploit the ocean’s resources, we must go offshore and use platforms that are suitable for different kinds of activities. These platforms are known as multi-use platforms (MUPs) or multi-purpose platforms (MPPs). An MUP includes sustainable components and activities, which are discussed in detail in Section2, such as wind, solar, floating tidal, wave and ocean thermal energy. Additionally, aquaculture should be designed in a sustainable way as per the revised European Union Commission Fisheries Policy (CFP) and its strategic guidelines for the sustainable development of the European Union aquaculture, which are intended to guide the development of aquaculture in Europe such that “it can contribute to the overall objective of filling the gap between the European Union (Member Organization) consumption and production of seafood in a way that is environmentally, socially and economically sustainable.” These objectives exist in addition to other activities such as

Sustainability 2020, 12, 1860; doi:10.3390/su12051860 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 1860 2 of 23 tourism and fishing, all of which make the MUP a sustainable platform for harvesting food and energy with other activities. Offshore platforms are not a new concept—they existed as early as the 19th century. The first offshore platform was for oil production was well-drilled in California by 1897. The first design for an offshore platform was in 1869 by Thomas Rowland, but it was never built because it was unrealistic idea at that time [2]. Oil and gas are the only mature sectors which have experience of constructing platforms further offshore. Thus, platform studies of oil and gas represent a large base for other sectors such as energy and aquaculture in terms of floating platforms and subsea engineering. The objective of the present study was to review the proposed techniques and configurations of offshore platforms to give a complete view of the integrated offshore platforms in the EU. The paper is structured as follows: Section2 gives a general idea about MUPs and highlight their components such as aquaculture and potential energy resources. Section3 explores various structures of MUPs. Section4 explains the design methodology for o ffshore platforms. Section5 examines the electrical issues of MUP such as network configuration challenges. Sections6 and7 highlight the control of MUP grids and power quality challenges, respectively, while the last section concludes the study.

2. What is an Offshore Multi-Use Platform? An offshore area can be defined based on different criteria such as distance from shore, water depth, jurisdictional boundaries, and wave exposure [2]. This offshore area is far from shore where there is a lack of topographical features such as capes, headlands or islands [3]. Some studies [3,4] have provided a classification for marine sites based on sea state or wave energy spectra; see Table1. Class 1 refers to marine areas that are exposed to wave heights of less than 0.5 m, so the degree of exposure to strong waves is insignificant; these are normally the nearshore areas. The site class increases as it moves further from the shore, because it is more exposed to higher waves and more open ocean. When wave height goes above 3 m, sites are classified as extreme at site Class 5 and this is an offshore area. In between Class 1 and Class 5, there are various classes which used for classification of aquaculture cages techniques.

Table 1. Classiﬁcation of oﬀshore sites [4].

Location Class Wave Height Exposure Degree to Wave 5 Higher than 3 m Extreme (oﬀshore area) 1 Below 0.5 m Insigniﬁcant (Nearshore area)

An MUP could be defined as an area of sea or ocean that combines different activities such as aquaculture, tourism, transportation, oil production, and energy farms. The combination of these activities could be completely integrated into one platform (shared structure) or could just share a marine space (shared area); more focus on this comes later in Section3[ 5]. Bringing different activities together could potentially benefit each other by lowering installation and maintenance costs, increasing resource utilization, reducing the environmental impact, etc. [6]. To sum up, an MUP comprises different kinds of renewable energies based on site parameters, in addition to other activities such as aquaculture, maintenance service, and leisure, as shown in Figure1. The energy array in Figure1 involves many hybrid energy units because every unit could comprise a wind turbine, a wave converter, and a small solar farm. Figure1 illustrates the idea of an MUP, and, later, Section3 shows that there are many other structures with different concepts. One of the ideas of MUPs is to use a floating substation that is separated from the platform that could export energy to the shore or supply onsite loads via a main distribution board (MDB). The European Union (EU) oversees two big programs to back the concept of offshore MUPs. The Ocean of Tomorrow is a primary step to pave the road for the other bigger European project “Horizon 2020.” The main goal of these projects is to develop a multi-use platform (MUP) in order to Sustainability 2020, 12, 1860 3 of 23

extractSustainability energy 2020, from12, x FOR marine PEER resources,REVIEW as well as other uses such as aquaculture in the same area3 of or24 structure. Horizon 2020 is a more recent program and is considered to be a continuation of “The Ocean ofcosting Tomorrow.” 80 billion It iseuros the biggestover seven funded years program from 2014 by theto 2020. EU for Some research of the and Horizon innovation, 2020 projects ultimately are costingprovided 80 in billion Table euros2. The overOcean seven of Tomorrow years from project 2014 focused to 2020. strongly Some of on the technologies Horizon 2020 and projects innovation are providedissues for inmarine Table2 activities. The Ocean in ways of Tomorrow that do not project have focused negative strongly impacts on on technologies the marine andecosystem. innovation The issuesOcean forof Tomorrow marine activities continued in waysover the that period do not of have 2010 negativeto 2013, and impacts it comprised on the marine 31 projects ecosystem. under Thethe framework Ocean of Tomorrow program continued (FP7), as overper Table the period 2, which of 2010 shows to 2013, some and of itthese comprised projects. 31 projectsFor interested under thereaders, framework the first program book about (FP7), offsho asre per platforms Table2, which was recently shows publishe some ofd these by Koundouri projects. For[7]. interestedIt provides readers,an environmental the first book and aboutsocio-economic offshore platforms assessment was of recently multi-use published offshore by platforms. Koundouri [7]. It provides an environmental and socio-economic assessment of multi-use offshore platforms.

Figure 1. Schematic diagram of a potential multi-use platform (MUP).

Table 2. Ocean of Tomorrow and Horizon 2020 Projects.

Project: The Ocean of Tomorrow [8] EU Fund Status FP7-Ocean-2010 Arctic Climate Change Economy and Society € 10,978,468 Done Vector of Change in Oceans and Seas Marine Life € 12,484,835 Done Sub-seabed CO2 Storage: Impact on Marine Ecosystems € 10,500,000 Done FP7-Ocean-2011 Development of a Wind-Wave Power Open-Sea Platform € 4,525,934 Done Innovative Multi-Purpose Oﬀshore Platforms: Planning, Design € 5,483,411 Done and Operation Sustainability 2020, 12, 1860 4 of 23

Table 2. Cont.

Project: The Ocean of Tomorrow [8] EU Fund Status Modular Multi-Use Deep Water Offshore platform € 4,877,911 Done Marine Microbial Biodiversity, Bioinformatics and Biotechnology € 8,987,491 Done FP7-OCEAN-2012 Priority Environmental Contaminants in Seafood: Safety Assessment, € 3,999,874 Done Impact and Public Perception Integrated Biotechnological Solutions for Combating Marine Oil Spills € 8,996,599 Done Suppression of underwater Noise Induced by Cavitation € 2,999,972 Done Science and Technology Advancing Governance on Good € 999,733 Done Environmental Status FP7-OCEAN-2013 Marine Environmental In-Situ Assessment and Monitoring Tool € 5,434,221 Done Real-Time Monitoring of SEA Contaminants by an Autonomous € 5,751,459 Done Lab-On-A-Chip Biosensor Sensing Toxicants In Marine Waters Makes Sense Using Biosensors € 4,144,263 Done Marine Sensors for the 21st Century € 5,924,945 Done Low-Toxic, Cost-Efficient, Environment-Friendly Antifouling Materials € 7,447,584 Done Synergistic Fouling Control Technologies € 7,995,161 Done Logistic Efficiencies and Naval architecture for Wind Installations with € 9,986,231 Done Novel Developments Project: Horizon 2020 [9] United Multi-Use Offshore Platforms Demonstrators for Boosting Cost-Effective and Eco-Friendly Production in Sustainable € 11,399,118 End 2023 Marine Activities Lean Innovative Connected Vessels € 7,808,691 Done Functional Platform for Open Sea Farm Installations of the Blue € 9,854,077 End 2021 Growth Industry Multi-Use in European Seas € 1,987,603 Done Multiple-Uses of Space for Island Clean Autonomy € 9,834,521 End 2024 Multi-Use Affordable Standardized Floating Space@Sea € 7,629,927 End 2020 Marine Investment for the Blue Economy € 1,977,951 Done

2.1. Aquaculture Aquaculture one of the most promising sectors for offshore platforms because it is an increasingly important contributor to economic growth and global food supply. Aquaculture is the fastest-growing animal food-producing sector on the planet. While capture fisheries production decreased by 2.6% from 1992 to 2012, this reduction was compensated for by the increased global supply which rose from 15% to 42% over the same two decades. China is the biggest producer of aquaculture products in the world, with around 62% of the world’s fish and shellfish, while European aquaculture contributes less than 2% of the global aquaculture production in terms of weight [10]. This lack of growth in the EU’s aquaculture could be explained by its strict environmental regulation, and this may open the door for more environmentally friendly alternatives such as integrated offshore farms. Offshore fish farming has been recognized as an alternative option for increasing seafood, and there has been international attention on this issue since the 1990s [11]. In its attempt to improve the aquaculture situation in Europe, the EU issued this policy statement: “Fish cages should be moved further from Sustainability 2020, 12, 1860 5 of 23 the coast, and more research and development of offshore cage technology must be promoted to this end. Experience from outside the aquaculture sector, e.g., with oil platforms, may well feed into the aquaculture equipment sector, allowing for savings in the development costs of technologies” [12]. The relatively controlled and planned nature of aquaculture with respect to fishing guarantee, to a large extent, better social stability in coastal communities and achieves progress in job creation especially when integrated with other activities such as renewable energy,tourism, and conservation [13]. Moving to open-sea fish farming bears huge opportunities for this industry. The ocean has enough space for farms extensions, it has no or reduced conflict with user groups, and farms will be in a safe place because it is far from human sources of pollution. Using offshore farms would have a positive environmental impact while reducing the costal fish farms that have a negative impact on the environment. In addition, offshore farms will be in optimal environmental conditions for a wide variety of marine species [12]. Moving offshore would facilitate operations such as hydrolysing and thermalizing, which are highly important for salmon fish [13]. Offshore fish farms have the potential to develop and increase organic production [10]. At offshore sites, a greater water exchange makes it easy to remove farm waste and offers better salinity stability [14]. Furthermore, larger and deeper cages that are located further offshore would provide a safer environment for some European species, such as seabass and seabream [13]. Integrating aquaculture with offshore platforms including wind farms, aquatic sport centres, angling centres, and tourism facilities is a great potential business opportunity. Additionally, they would be very good academic centres for studying energy and aquatic animal lives. Having said that, there are environmental concerns regarding the moving fish farms offshore such as nutrient and chemical pollution, habitat damage, disease introduction, and the interbreeding of wild stock with escapees from farms; the productivity of a farm depends on location choice, and so spatial planning is required to ensure that farms have no impact on their surrounding ecosystems and to ensure the sustainable growth of offshore aquaculture. In addition, offshore farms can be exposed to wild predators such as otters, sea lions, seals and birds. Additionally, there is a fear of local oxygen depletion due to the organic matter that is likely to fall to the seafloor during fed and unfed aquaculture operations. However, there are approaches to reduce such pollution. Lastly, there are higher costs for farm operations with offshore aquaculture. To sum up, aquaculture from land-based and nearshore fish farms have experience a lot of critique and have many challenges due to economic issues, political issues, environmental issues, and resource limitations [11]. Moving aquaculture offshore is urgently necessary, but there are concerns that need to be addressed to ensure the sustainable growth and to overcome administrative and regulatory barriers such as licenses, spatial planning, the use of water resources, multi-level governance, and competition at fish markets [10].

2.2. Potential Offshore Energy Resources Some studies have proposed the combining of marine energies as a better alternative instead of using a single source of energy [15–17]. They claim that many advantages could be achieved such as better liability systems, increased energy yields, smooth output power, and shared grid infrastructure. In addition, this option is environmentally friendly when compared with the independent installation of energies [17]. The costs of construction and maintenance could be significantly reduced via the use of shared resources such as foundations, logistics, operations, and maintenance. For example, the MWh generated from wave converters is still more expensive than their counterparts from other renewable sources and conventional sources of energy when constructed individually [18]. The main ocean energy technologies are the wave, tidal and ocean thermal energy conversion (OTEC). Wind and solar energies are not marine-based, but they could be implemented offshore at different scales. Today, ocean energy from various marine energy resources is developed to commercial scale with a generation capacity of 0.5–17 GW under construction. Tidal range energy predominates all forms of ocean energy in terms of installed capacity, with a rate of around 99% [19]. The following Sustainability 2020, 12, 1860 6 of 23 sub-sections explore potential renewable sources of energy that could be implemented on offshore platforms such as wind, wave, solar, tidal, ocean thermal energy conversion, and biomass. • Wind Energy Converter There are two main categories of wind turbines: horizontal and vertical axis. The three bladed horizontal axis wind turbine (HAWT) is the most popular wind turbine that is used offshore and onshore in a commercial way [20]. On the other hand, vertical axis wind turbines (VAWTs) have the advantages of simpler structures that make them a better option for floating turbines and cut costs of foundations and quicker response for changing wind direction. Moreover, VAWTs offer lower noise levels, which makes them suitable for MUPs. They have simpler control system as there is no need for pitch control, and their simple stricter means that any required maintenance is easier to perform [20]. Many recent studies have developed VAWTs [21–23]. In this regard, the authors of [23] proposed a new unusual design for a wind turbine that could reduce the cost by 65%. • Wave Energy Converter Abundant energy could be harvested from the ocean, along with wind energy. The existing wave turbines can be divided into two main groups, the first of which is direct action turbines that directly convert hydrodynamic energy into electrical energy. The indirect group does the same function indirectly. The first group of turbines has a simpler structure and, as such, is more reliable and less costly [23]. It is worth mentioning that the first open sea testing facility at Orkney Island in Scotland (in operation since 2003) has a wave testing site. It comprises five berths of 2.2 MW power capacity. Another grid-connected wave hub in South West England includes four separate berths, each with a capacity of 4–5 MW [24]. Recently, the NEMOS team developed a wave energy converter to be installed offshore. This converter has an 8 2 m-sized floater and a structure that is 16 m long. The team × is currently working on the installation of a large-scale prototype in the North Sea that could generate enough energy for several households. The standalone floating design of this converter makes it ideal for offshore installation with an MUP [25]. • Tidal Energy Converter There are many ways to classify tidal turbines. The most common category is based on conversion technology. The authors of [26] explored the most popular types of tidal turbines based on conversion technology. Other studies, such as [27–29], have classified hydrokinetic turbines in more detail. Real projects include the Skerries Tidal Energy Array with 10 MW in Wales, which has been in operation since 2015, and the Irish Open Hydro Tidal Energy Array project, which is bigger at 100 MW and is expected to be in commercial operation by 2020. Moreover, the first open sea testing facility on Orkney Island in Scotland has a tidal testing site. It is located near Eday at water depths of 12 and 50 m on an area of 2 3.5 km. It has eight berths of 5 MW power capacity [24]. Tidal energy is included in this × study with expectations that the floating version of the tidal turbine (SR2000) will be developed for offshore installation in the future, though such a version is limited to a water depth of 25 m so far. • Floating Solar Farms Floating Photovoltaic (PV) panels have been proposed under the Modular Multi-use Deep Water Offshore Platform Harnessing and Servicing Mediterranean, Subtropical and Tropical Marine and Maritime Resources (TROPOS) project as potential offshore energy sources. Floating PV panels have proven themselves as an available technology that is already used in onshore lakes, reservoirs and ponds. There is a floating PV farm at Far Niente Winery, USA. There is a floating PV farm under construction in Ichihara city in Japan with a power capacity of 13.4 MW [30]. However, due to the harsh marine environment, there are no PV structures installed offshore in open water. PV panels should be qualified to withstand humidity, salinity, sea spray, corrosion, and fouling in the open seas. The use of PV panels in a sea environment is still very limited, and, so far, they have only been used on boats or as hybrids with a wind turbine in a pilot project [31]. Sustainability 2020, 12, 1860 7 of 23

• Ocean Thermal Energy Converter (OTEC)

The extraction of energy by using an OTEC is based on the thermodynamic Rankine cycle which is used in steam power plant [32]. For an OTEC to be a feasible source of energy, a minimum temperature difference between the warm water and water at 1000 m deep should be at least 20 ◦C[33]. For this reason, harvesting this energy is specifically available in tropical areas. An OTEC is classified based on the location of the plant or the thermodynamic cycle used. For location-based units, there are three kinds of plants: floating, self-mounted and land-based. In terms of the thermodynamic cycle, there are three main types: open-cycle, closed-cycle, and hybrid-cycle. For interested readers, details about all types have been given in the ocean engineering handbook. Electricity and fresh water are the main outputs of a large OTEC plant, but various by-products could be harvested such as air-conditioning and aquaculture [32]. One 50 MW hybrid cycle plant could provide the daily water needs for a small community with 300,000 people. In addition, deep water is 28 times richer in inorganic nutrients such as nitrates, silicates, and phosphates, which could be used in a commercial way for sea farming.

• Marine Biomass Energy

Conventional technologies that are used for extracting biofuels are based on animal oils, vegetables, starch, and sugar, but these methods have been widely criticized because they consume food resources. For this reason, marine algae have appeared on the horizon as more environmentally sustainable and friendly feedstock because they do not compete with food resources, they save freshwater, and they could be grown by wastewater. Algal feedstock could be used to obtain energy and non-energy products. Many biofuels could be extracted from algae such as biodiesel, bioethanol, biogas, and bio-jet fuels [34]. Bharathiraja et al. [35] concluded that biofuels from marine algae are still not economically feasible, an issue which comes back to the high costs of the operation, cultivation, processing, and separation of biofuels. However, integrating algae farms in the oﬀshore platform proposed under this study would improve the technology and make it available at reasonable costs. In addition, depending on algae, biofuels that are used as potential storage can provide better reliability for oﬀshore islands and can avoid the use of large capacities of batteries.

3. Offshore MUP Structures Combining marine resources of energy (wind, wave, tidal, floating solar farms, algae biomass, and ocean thermal energy conversion) with the different activities mentioned above could be fulfilled in very different ways and concepts. Various structures have been proposed under the funded projects of the EU. These projects are Innovative Multi-purpose offshore platforms: planning, Design and operation (MERMAID), Development of a wind-wave power open-sea platform equipped for hydrogen generation with support for multiple users of energy (H2Ocean) and TROPOS. Offshore wind energy appears strongly in all structures because it is an already developed and mature energy. Different wave converters share these structures to promote wave energy, which still at an early stage of development. The combination of wave and offshore wind energy to generate electricity is a recent topic, and few studies have tackled this issue. Most works in this regard have been done by EU-funded research projects, as mentioned earlier. Khrisanov et al. [23] highlighted the advantages of using the hybrid wave/wind power system; they found that hybrid floating wind and wave power systems are promising directions for more harnessing ocean energies. There are two main concepts for hybridizing these two sources of energy: mechanical and electrical combination. The first is combining wind and wave turbines in a mechanical complex system, and the resulting rotation moment of both turbines is used to drive the generator’s rotor. Unfortunately, this kind of system suffers from less reliability and increased costs [23]. Thus, this kind of system has not been used, and an electrical hybrid system has been proposed. The electrical system depends on the electrical combination between the wind and wave converters, i.e., each converter has its own generator and the output power of both are combined via a power electronic converter. SustainabilitySustainability 2020 2020, 12,, 12x FOR, x FOR PEER PEER REVIEW REVIEW 8 of8 24of 24 an anelectrical electrical hybrid hybrid system system has has been been proposed. proposed. The The electrical electrical system system depends depends on on the the electrical electrical combinationcombination between between the the wind wind and and wave wave converters, converters, i.e., i.e., each each converter converter has has its itsown own generator generator and and Sustainability 2020, 12, 1860 8 of 23 thethe output output power power of ofboth both are are combined combined via via a power a power electronic electronic converter. converter. ThereThere is isa generala general classification classification for for the the offshore offshore structures structures in interms terms of offoundation foundation type type as asa a functionfunctionThere of iswaterof a water general depth: depth: classification Fixed Fixed structures structures for the are o ffare constrshore constr structuresucteducted in shallowin in shallow terms water of water foundation with with water water type depth depth as a less function less than than of50 50 waterm, m, and and depth: floating floating Fixed structures structures structures exist exist are in constructed inwater water at ata depth ina depth shallow of oflarger waterlarger than withthan 50 water 50m m[24]. depth[24]. An An MUP less MUP than could could 50 m,be be andclassifiedclassified floating based based structures on on its itstechnological exist technological in water atbasis, abasis, depth its itsrelative of relative larger location thanlocation 50 to m tothe [24 the ].shoreline, Anshoreline, MUP or could orits itswater be water classified depth. depth. basedHowever,However, on its this technologicalthis study study classifies classifies basis, itsplatforms relativeplatforms locationto toth reeth toree categories the categories shoreline, based based or its on wateron the the depth.connectivity connectivity However, among among this studyactivitiesactivities classifies to toco-located co-located platforms systems, systems, to three combin categoriescombined edstructures, basedstructures, on and the and connectivityisland island structures. structures. among activities to co-located systems, combined structures, and island structures. 3.1.3.1. Co-Located Co-Located System System 3.1. Co-Located System WindWind farms farms and and wave wave arrays arrays share share the the same same ma marinerine area, area, maintenance, maintenance, operation operation equipment, equipment, activities,activities,Wind grid farms grid connections, andconnections, wave arraysetc., etc., th ought sharethought thethey they same do do not marine not share share area, foundation foundation maintenance,s (Sees (See Figure operation Figure 2). 2).This equipment, This kind kind of of activities,combinationcombination grid is connections,proposedis proposed for for etc.,th eth early thoughte early stage stage they of ofdevelopment. do development. not share foundations (See Figure2). This kind of combination is proposed for the early stage of development. 3.2.3.2. Combined Combined Structure Structure 3.2. Combined Structure TheThe idea idea of ofthis this structure structure is ishighlighted highlighted under under the the MERMAID MERMAID project. project. In Inthese these structures, structures, differentdifferentThe ideaenergy energy of thisconverters converters structure share is share highlighted the the same same foundati under foundati theon MERMAIDon and and connections, connections, project. and In and these everything everything structures, is sharedis di sharedfferent as as energya unita unit (see converters (see Figures Figures share3–15). 3–15). theSome Some same loads loads foundation such such as asaquacu and aquacu connections,lturelture and and algae andalgae farms everything farms could could isbe sharedbeattached attached as to a unittothat that (seeunit,unit, Figuresas asproposed proposed3–15). in Some thein the TROPOS loads TROPOS such project project as aquaculture (Satellite (Satellite Un Un andit; seeit; algae see Figure Figure farms 4). 4).The could The foundation foundation be attached of thisof to this that structure structure unit, ascouldcould proposed be bebottom-fixed bottom-fixed in the TROPOS or orfloating projectfloating as (Satellite asper per Figure Figure Unit; 3a,b, see3a,b, respectively Figure respectively4). The [17]. foundation [17]. of this structure could be bottom-fixed or floating as per Figure3a,b, respectively [17].

CollectorCollector

To Shore Sub-sea cables To Shore Sub-sea cables FigureFigure 2. Co-locatedCo-located2. Co-located independent independent array. array.

(a)( a) (b)( b)

FigureFigure 3.3. Hybrid 3. Hybrid system. system. (a) (a) (a)Fixed Fixed bottom. bottom. (b) (b )(b) Floating Floating Floating system system system [24]. [24 [24].].

• • •Satellite SatelliteSatellite Unit Unit Unit Structure Structure Structure

TheThe TROPOSTROPOS TROPOS project project project proposed proposed proposed what what iswhat called is iscalled acalled ﬂoating a floatinga satellitefloating satellite unitsatellite (see unit Figureunit (see (see4 ),Figure which Figure combines4), 4),which which windcombinescombines turbines, wind windPV turbines, turbines, solar panels, PV PV solar solar and panels, anpanels, aquaculture and and an anaqua aqua breedingcultureculture ﬁshbreeding breeding facility fish withfish faci faci anlitylity algae with with farman analgae attachedalgae farm farm toattached itattached [36]. to itto [36]. it [36].

• Poseidon Wave/Wind Structure

The platform proposed under the MERMAID project that was developed by the Poseidon Floating Power Company with a ﬂoating foundation and a combination of wind and wave converters, as shown in Figure5[24]. Sustainability 2020, 12, x FOR PEER REVIEW 9 of 24 Sustainability 2020,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 9 of 24

• Poseidon Wave/Wind Structure • Poseidon Wave/Wind Structure The platform proposed under the MERMAID project that was developed by the Poseidon SustainabilityThe platform2020, 12, 1860 proposed under the MERMAID project that was developed by the Poseidon9 of 23 Floating Power Company with a floating foundation and a combination of wind and wave Floating Power Company with a floating foundation and a combination of wind and wave converters, as shown in Figure 5 [24]. converters,• Two as Wave shown One in Wind Figure Structure 5 [24]. • Two Wave One Wind Structure • Two Wave One Wind Structure This structure was proposed by the MERMAID project and developed by Ocean Wave and Wind This structure was proposed by the MERMAID project and developed by Ocean Wave and Wind EnergyThis Company structure withwas proposed a ﬁxed foundation. by the MERMAID It combines project a and wind developed converter by with Ocean dragon Wave andand Wind point Energy Company with a fixed foundation. It combines a wind converter with dragon and point Energyabsorber Company wave converters, with a fixed as shown foundation. in Figure It6 [combines24]. a wind converter with dragon and point absorber wave converters, as shown in Figure 6 [24]. absorber wave converters, as shown in Figure 6 [24].

Figure 4. Satellite unit proposed in green and blue concept [36]. Figure 4. SatelliteSatellite unit unit proposed in green and blue concept [36]. [36].

Figure 5. Poseidon wave/wind energy platform [24]. Figure 5. PoseidonPoseidon wave/wind wave/wind energy platform [24]. [24].

Figure 6. Two wave structures and one wind structure [24]. Figure 6. TwoTwo wave structures and one wind structure [[24].24].

Sustainability 2020, 12, 1860 10 of 23 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 24 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 24 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 24

Figure 7. TwoTwo wind Power structure [[24].24]. Figure 7. Two wind Power structure [24]. • • W2PowerW2Power Structure Structure • •This W2Power was proposed Structure under the MERMAID project and was developed by the Pelagic Power This was proposed under the MERMAID project and was developed by the Pelagic Power Company with a semi-submersible floating platform. It combines two wind turbines with a wave CompanyThis was with proposed a semi-submersible under the floatingMERMAID platform. project It and combines was developed two wind by turbines the Pelagic with aPower wave converter (point absorber type), as shown in Figure 7 [24]. Companyconverter (pointwith a absorber semi-submersible type), as shown floating in Figureplatform.7[24 It]. combines two wind turbines with a wave converter (point absorber type), as shown in Figure 7 [24]. • • WaveWave Treader Treader • Wave Treader This Wave was was proposed Treader proposed under under the the MERMAID MERMAID project project and andwas developed was developed by the byGreen the Ocean Green Energy Ocean Company. It has fixed piles and combines a wind turbine with a wave converter, as shown in Figure EnergyThis Company. was proposed It has under fixed the piles MERMAID and combines project a windand was turbine developed with a by wave the Green converter, Ocean as Energy shown 8 [24]. Company.in Figure8[ 24It has]. fixed piles and combines a wind turbineine withwith aa wavewave converter,converter, asas shownshown inin FigureFigure 8 [24].• Seagen W-Shape • Seagen W-Shape • •This Seagen was developed W-Shape by the Seagen Company. It has a fixed pile (monopole) and combines a wind This was developed by the Seagen Company. It has a fixed pile (monopole) and combines a wind turbineThis with was a developed tidal converter, by the as Seagen shown Company. in Figure It9 [24].has a fixed pile (monopole) and combines a wind turbineThis with was a developed tidal converter, by the as Seagen shown Company. in Figure9 It[24 ha].s a fixed pile (monopole) and combines a wind turbineturbine withwith aa tidaltidal converter,converter, asas shownshown inin FigureFigure 99 [24].[24].

Figure 8. Wave Treader Structurer [24]. FigureFigure 8. WaveWave Treader StructurerStructurer [[24].24].

Figure 9. Seagen W structure [24]. Figure 9. Seagen W structure [24]. • WEGA: Hybrid Coupling (WindFigure 9.turbineSeagen + WWEC) structure Structure [24]. • WEGA: Hybrid Coupling (Wind turbine + WEC) Structure • •This WEGA:WEGA: was developed Hybrid Hybrid Coupling Coupling by Sea for (Wind (Wind Life turbineLda. turbine It has+ +WEC) WEC) a fixed Structure Structure foundation and combines a wind turbine with a Wave Energy Converter (WEC), and other uses could be added, as shown in Figure 10 [24]. This was developed by Sea for Life Lda. It has a fixed foundation and combines a wind turbine with a Wave Energy Converter (WEC), and other uses could be added, as shown in Figure 10 [24]. Sustainability 2020, 12, 1860 11 of 23

This was developed by Sea for Life Lda. It has a ﬁxed foundation and combines a wind turbine withSustainability a Wave 2020 Energy, 12, x FOR Converter PEER REVIEW (WEC), and other uses could be added, as shown in Figure 10[24 11 ]. of 24 Sustainability 2020, 12, x FOR PEER REVIEW 11 of 24

Figure 10. WEGA Hybrid Coupling (Wind turbine + WEC) structure [24]. Figure 10. WEGA Hybrid Coupling (Wind turbine + WEC)WEC) structure structure [24]. [24].

Figure 11. Triangular structure [[24].24]. Figure 11. Triangular structure [24]. • • Triangular Structure • TriangularTriangular StructureStructure ThisThisstructurewasproposedanddevelopedundertheMarinaplatformproject.This structurestructure waswas proposedproposed andand developeddeveloped underunder ththee MarinaMarina platformplatform Ithasasemi-submersible project.project. ItIt hashas aa semi-semi- submersiblefloatingsubmersible foundation floatingfloating and foundationfoundation combines aandand wind combinescombines turbine with aa wiwi andnd wave turbineturbine converter, withwith aa as wavewave shown converter,converter, in Figure as11as [ shown24shown]. inin FigureFigure 1111 [24].[24]. • Three Branches Wave/Wind Structure • • ThreeThree BranchesBranches Wave/WindWave/Wind StructureStructure This structure was also developed and tested under the Marina platform project. It has a semi- ThisThis structurestructure waswas alsoalso developeddeveloped andand testedtested underunder thethe MarinaMarina platformplatform project.project. ItIt hashas aa semi-semi- submersiblesubmersible floating floating platform platform and and combines combines a awind wind turbine turbine with with a wave a wave converter converter (Flabs (Flabs type), type), as submersibleas shown in Figurefloating 12 platform[24]. and combines a wind turbine with a wave converter (Flabs type), as shownshown inin FigureFigure 1212 [24].[24]. • • Spar Floating Buoy Structure • SparSpar FloatingFloating BuoyBuoy StructureStructure AsAs with with the the previous previous two two structures, structures, this this stru structurestructurecture was was developed developed and and tested tested under under the the Marina Marina platformplatform project.project.project. It ItIt has hashas a a spara sparspar floating floatingfloating buoy buoybuoy and andand combines combinescombines a wind aa windwind turbine turbineturbine with with awith wave aa wave converterwave converterconverter (point (pointabsorber(point absorberabsorber type), astype),type), shown asas shownshown in Figure inin FigureFigure13[24]. 1313 [24].[24]. •• Cantabria Platform Structure • CantabriaCantabria Platform Platform Structure Structure ThisThis structurestructure waswas developeddeveloped andand testedtested inin aa laboralaboratorytory atat thethe CantabriaCantabria sitesite toto validatevalidate thethe finalfinal This structure was developed and tested in a laboratory at the Cantabria site to validate the design.design. TheThe semi-submersiblesemi-submersible floatingfloating platformplatform hashas threethree oscillatingoscillating waterwater columnscolumns (OWCs)(OWCs) final design. The semi-submersible floating platform has three oscillating water columns (OWCs) constructedconstructed ofof twotwo wavewave convertersconverters andand oneone windwind tuturbine,rbine, asas shownshown inin FigureFigure 14.14. TheThe powerpower capacitycapacity ofof thethe proposedproposed floatingfloating structurestructure isis 88 MWMW (5(5 MWMW fromfrom thethe windwind turbineturbine andand threethree OWCsOWCs withwith 11501150 KWKW each)each) [24].[24]. Sustainability 2020, 12, 1860 12 of 23 constructed of two wave converters and one wind turbine, as shown in Figure 14. The power capacity of the proposed floating structure is 8 MW (5 MW from the wind turbine and three OWCs with 1150Sustainability KW each) 2020, [1224, x]. FOR PEER REVIEW 12 of 24

Figure 12. Three branches wave/wind structure [24]. Figure 12. Three branches wave/wind wave/wind structure [[24].24].

Figure 13. Figure 13. Spar floating ﬂoating buoy structure [[24].24]. • Hoxicon Platforms • Hoxicon Platforms This was developed by the Hoxicon Hoxicon Company. Company. Ther Theree are various shapes of of the the floating ﬂoating platform, platform, which only combinescombines windwind turbines,turbines, asas shownshown inin FigureFigure 1515[ 24[24].].

Sustainability 2020, 12, 1860 13 of 23 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 24 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 24

Figure 14. Platform at Cantabria, MERMAID project [24]. Figure 14. Platform at Cantabria, MERMAID project [[24].24].

Figure 15. Floating structure that combines wind turbines [24]. Figure 15. Floating structure that combines wind turbines [[24].24]. 3.3. Island Structure 3.3. Island Structure The concept of an island structure is becoming very clear under the TROPOS project via the The conceptconcept ofof an an island island structure structure is becomingis becoming very very clear clear under under the TROPOSthe TROPOS project project via the via three the three proposed island configurations, which are the sustainable service hub island, the green and proposedthree proposed island configurations,island configurations, which are which the sustainable are the sustainable service hub service island, hub the island, green and the bluegreen island, and blue island, and the leisure island. TROPOS has aimed to develop a floating MUP to adapt to deep andblue theisland, leisure and island. the leisure TROPOS island. has TROPOS aimed to has develop aimed a to floating develop MUP a floating to adapt MUP to deep to adapt waters to deep with waters with focus on Mediterranean, tropical and sub-tropical areas [37]. Basically, the four main focuswaters on with Mediterranean, focus on Mediterranean, tropical and sub-tropicaltropical and areassub-tropical [37]. Basically, areas [37]. the Basically, four main the sectors four tomain be sectors to be integrated into these islands are those of transport, energy, aquaculture and leisure integratedsectors to be into integrated these islands into are these those islands of transport, are those energy, of transport, aquaculture energy, and leisureaquaculture (TEAL). and For leisure more (TEAL). For more information on two kinds of islands—artificial and floating islands—interested information(TEAL). For onmore two information kinds of islands—artificial on two kinds andof islands—artificial floating islands—interested and floating readers islands—interested can look at [17]. readers can look at [17]. readers can look at [17]. • • SustainableSustainable Service Service Hub Hub Island Island • Sustainable Service Hub Island This is a floating offshore platform of an industrial nature, as it includes a lot of cranes and This is a floatingfloating ooffshoreffshore platformplatform of an industrialindustrial nature, as it includes a lot of cranes and workshops. This concept focuses mainly on energy and transport issues, though it still has leisure workshops. This This concept focuses mainly on energy and transport issues, though it still has leisure activities and aquaculture (see Figure 16). It includes a large floating offshore port with repair and activities and aquaculture (see Figure 1616).). It It includes a large floating floating offshore offshore port with repair and maintenance facilities for large ships. Lifting capabilities have been proposed for workshop activities maintenance facilities for large ships. Lifting capa capabilitiesbilities have been proposed for workshop activities and for material storage and handling. The deliverable D4.3 “Complete Design Specification of 3 and for materialmaterial storagestorage andand handling.handling. The deliver deliverableable D4.3 “Complete Design Specification Specification of 3 References TROPOS systems” explores all required elements on this platform [36]. References TROPOS systems” exploresexplores allall requiredrequired elementselements onon thisthis platformplatform [[36].36]. Based on this proposed platform, TROPOS has had big influences on the transport, energy and Based on this proposed platform, TROPOS has hadhad big influencesinfluences on the transport, energy and aquaculture sectors in terms of reducing the operation and maintenance costs that are related to these aquaculture sectors in terms of reducing the operationoperation and maintenance costs that are related to these sectors. The implementation of other energy sources such as solar (photovoltaic or thermal), OTECs, sectors. The implementation of other energy source sourcess such as solar (photovoltaic or or thermal), OTECs, OTECs, and marine energies within the same platform can all act to reduce related costs and increase the and marine energies within the same platform can all act to reduce related costs and increase the reliability of the power system. This platform could serve the transport and mining industries by reliability of the powerpower system.system. This platform could serve the transport and mining industries by providing them with fuel, electrical energy, food and freshwater. providing themthem withwith fuel,fuel, electricalelectrical energy,energy, foodfood andand freshwater.freshwater.

Sustainability 2020, 12, 1860 14 of 23

Sustainability 2020, 12, x FOR PEER REVIEW 14 of 24

Figure 16. Conﬁguration of the industrial complex concept [38].

Within this configuration, energy production is comprised huge renewable sources of large wave and wind farms. Wind turbines could be integrated with the platform itself or with floating turbines. The idea of this configuration is that the generated energy supplies all facilities at the platform, while the excess energy is used to produce gases and liquid fuels that are stored as energy storage to be used as a fuel for fuel cells or even internal combustion engines. There have been few case studies for this concept, which are used to perform maintenance operations for the existing offshore wind farms. Horns Rev2 was established in the Danish North Sea to provide service for a 209 MW wind farm. Nordsee Ost, Dan Tysk and MittlePlate are other three case studies which were constructed in the German North Sea for maintenance purposes as well [39]. Aquaculture in this platform is limited because it conflicts with the other considered facilities, such as workshops and material handling. Thus, when possible floating cages could be used for aquaculture in different locations, such as areas between wind turbines and areas that are close to the platform. In this case, feeding operation could be managed via the platform itself or from independent floating silos among the cages [38]. It is worth mentioning that this configuration and the green and blue configuration are designed to be grid-connected. However, grid-connected offshore floating wind turbines and arrays have yet to be developed due to a lack of experience. An Edinburgh University report [40] suggested that a mobile floating offshore substation could be used until a floating substation is proved. • Green and Blue Island • ThisGreen concept and Blueis mainly Island focused on physical and biological ocean resources to extract food and energy [36]. An offshore wind farm, a potential wave energy farm, and an OTEC, if applicable with This concept is mainly focused on physical and biological ocean resources to extract food and aquaculture installation, are considered to be the main contents of this configuration (see Figure 17). energy [36]. An offshore wind farm, a potential wave energy farm, and an OTEC, if applicable with It has been proposed that aquaculture that is integrated into the same floating platform of other aquaculture installation, are considered to be the main contents of this configuration (see Figure 17). activities is called a shared structure. Other than the previous configuration, this one completely It has been proposed that aquaculture that is integrated into the same floating platform of other avoids industrial activities that might jeopardize the aquaculture sector. There are two main sub- activities is called a shared structure. Other than the previous configuration, this one completely avoids concepts for this configuration: One is wind and wave plus aquaculture, and the other is aquaculture industrial activities that might jeopardize the aquaculture sector. There are two main sub-concepts with an OTEC [38]. Configuration details about the above two sub-concepts and other configurations for this configuration: One is wind and wave plus aquaculture, and the other is aquaculture with are available in [38,41] and the energy island website [42]. an OTEC [38]. Configuration details about the above two sub-concepts and other configurations are available in [38,41] and the energy island website [42]. Sustainability 2020, 12, 1860 15 of 23

Sustainability 2020, 12, x FOR PEER REVIEW 15 of 24

Figure 17. Green and blue platform conﬁguration [38].

This configuration comprises two main structures: the central unit and the floating module. The central unit involves various sections such as the crew’s accommodation, a rescue system, communication, electrical units, a fish processing plant with all required areas, a unit for exporting Figure 2. Green and blue platform configuration [38]. aquaculture, and laboratories for aquaculture facilities. On the other hand, the floating unit has areas for twenty-footThis equivalentconfiguration unit comprises (TEU) two of storage main structures: and satellite the central spare unit parts, and the as wellfloating as berthingmodule. The capability for an ocentralffshore unit supply involves vessel. various sections such as the crew’s accommodation, a rescue system, communication, electrical units, a fish processing plant with all required areas, a unit for exporting The green and blue concept focuses on algae as a source of energy by converting biomass to aquaculture, and laboratories for aquaculture facilities. On the other hand, the floating unit has areas energy.for Algae twenty-foot could beequivalent used as unit biomass (TEU) toof harveststorage and energy satellite and spare non-energy parts, as products.well as berthing From algae, energycapability products for such an offshore as biodiesel, supply biogas,vessel. bioethanol, and bio-jet fuels are produced. On the other hand, algaeThe produce green and non-energy blue concept products focuses suchon algae as carbohydrates,as a source of energy pigments, by converting proteins, biomass biomaterials, to and bioproductsenergy. Algae [34]. could The algaebe used farm as biomass is attached to harvest to a satellite energy unitand non-energy (see Figure products.4). This unitFrom has algae, two wind energy products such as biodiesel, biogas, bioethanol, and bio-jet fuels are produced. On the other turbines which are integrated into a floating structure with a fish cage and a floating algae farm [36]. It is hand, algae produce non-energy products such as carbohydrates, pigments, proteins, biomaterials, worth mentioningand •bioproductsLeisure that I sland[34]. the The diff algaeerent farm kinds is attached of generators, to a satellite integrated unit (see into Figure this 4). configuration, This unit has two would be operated based on the fuels that are produced from the algae farm to get a fully sustainable system. windThis turbines platform which is are in integratedrelatively shallowinto a floating water structurenear the withcoast, a comparedfish cage and to thea floating other algaepreviously farm [36].mentioned It is worth configurations. mentioning It that inclu thedes different different kinds modules: of generators, a diving ceintegratedntre, an aquaculture into this configuration, structure, a • Leisure Island wouldwater besports operated centre, based and on an the underwaterfuels that are obse producrvatoryed from to thewatch algae the farm marine to get aenvironment fully sustainable and system.aquaculture around the site (see Figure 18). The floating module in this configuration is a bit different This platform is in relatively shallow water near the coast, compared to the other previously from• theLeisure other two Island configurations because it has a PV plant with storage and a substation to provide mentionedelectricity configurations. to the central Itmodule includes when di requiredfferent [36]. modules: a diving centre, an aquaculture structure, a water sportsThis centre, platform and is an in underwaterrelatively shallow observatory water near to watchthe coast, the compared marine environment to the other previously and aquaculture mentioned configurations. It includes different modules: a diving centre, an aquaculture structure, a around the site (see Figure 18). The floating module in this configuration is a bit different from the water sports centre, and an underwater observatory to watch the marine environment and other twoaquaculture configurations around the because site (see it Figure has a 18). PV The plant floating with module storage in and this aconfiguration substation is to a providebit different electricity to the centralfrom the module other two when configurations required because [36]. it has a PV plant with storage and a substation to provide electricity to the central module when required [36].

Figure 18. Leisure island configuration [38].

Figure 18. Leisure island configuration [38]. Figure 18. Leisure island conﬁguration [38]. Sustainability 2020, 12, 1860 16 of 23

The island should be self-sufficient in terms of all kinds of energy demands: electrical power, hot water, and air conditioning inside the buildings. For this purpose, solar energy—either PV or thermal with storage—is extensively integrated into the island’s architecture, and wind turbines are integrated into this configuration. In other words, the energy demand for this configuration is met by wind and solar energy [38]. Though an MUP can provide sustainable and economical solutions for the problem of a lack of seafood and higher prices of offshore energy, it should be designed in a way to avoid or reduce negative environmental and ecological impacts. Some stakeholders have concerns about living marine environments and habitats that could be affected by foundations of MUPs and cages [5]. Additionally, there are concerns about MUPs conflicting with other marine activities such as transport, tourism, fishing, entrance to marine ports, and wildlife and birds area protection. Marine litter is another problem that could be increased with commercialized MUPs. Plastic alone (around 60–80% of marine litter) was estimated to exist as 275 million metric tons (mt) in 2010, and this quantity could be increased by increasing the number of MUPs. Marine litter has a negative impact on human health, marine environments, marine ecosystems, marine industries, and marine species, and this leads to negative economic impacts. Another challenge for the energy system of MUPs is that they should use 100% renewable energy resources to avoid releasing CO2, the use of which leads to ocean acidification, which has a negative impact on marine ecosystems [43].

4. Offshore MUP Design Methodology Designing offshore platform requires the assessment of the project site from different sides: technical, economic, social and environmental. Thus, involving all relevant stakeholders at an early stage of development is required because the design of such installations depends on experts’ judgement from different sectors. Barbara et al. [44] proposed a methodology consisting of four phases for the purpose of design of an offshore platform: the pre-screening phase, the preliminary design of the single-use platform, the ranking phase, and, lastly, the preliminary design for the selected multi-use platform, as shown in Figure 19. The pre-screening phase examines the platform components (energy sources, aquaculture, marine service hub, and leisure island) based on the site conditions in terms of wind speed, yearly wave power, tidal range, and potential fish production. The outcome of this phase is to define the various uses that are integrated onto the offshore platform at a specific site. The preliminary design phase chooses the most suitable energy converters and applicable fish farms based on the site assessment that was accomplished in the first phase and took legal constraints into account [44]. The third phase is a ranking step to give a score for each component in the platform based on different aspects such as the technology development level in terms of reliability and performance, the installation and maintenance costs of various elements as a function of system mechanical complexity and water depth, and potential risks in terms of pollution, power take-off failure, geotechnical failure, and structure modularity. Then, the assigned scores for each module are combined and alternatives are weighted based on the score of each combined module. The outcome of this phase is the determination of the best scheme for an offshore platform at a specific site. The last phase is the preliminary design of the selected scheme, taking the interaction and the conflict between different modules on the platform into account, in addition to the optimization of spatial planning [44]. For example, the proposed service hub platform in the TROPOS project has a conflict between the aquaculture cages and the service activities make surrounding water not pure enough for growing fish. Sustainability 2020, 12, 1860 17 of 23

Sustainability 2020, 12, x FOR PEER REVIEW 17 of 24

• Preliminary assessment of energy resources and taking into account physical water parameters. • Determine the applicable resources based on thresholds values for each resource. (I) Prescreening • Then, identify feasible uses for the proposed platform. Phase • Design the system for the selected energy resources and the potential aquaculture. • Select the device components required based on the design. • Production estimation and layout for a single-use platform. (II) Preliminary • Identify the legal constraints. Design • Score the components individually. • Rank the various applicable platforms. • Identify the best option. (III) Ranking Phase • Specify the site of installation. • Estimate the MUP production and prepare layout for it. • Operation indication for the MUP. (IV) Design of MUP

Figure 19. Offshore platform methodology phase [44]. Figure 19. Offshore platform methodology phase [44]. 5. Offshore MUP Grid Configuration 5. Offshore MUP Grid Configuration This section highlights the electrical connection between different components of an offshore This section highlights the electrical connection between different components of an offshore platform.platform. It is It important is important to to di ffdiffererentiateentiate between between twotwo different different electrical electrical layouts: layouts: the thehybrid hybrid energy energy resourcesresources layout layout and and the the MUP MUP local local grid grid layout. layout. HybridHybrid energy energy resources resources represent represent the thegeneration generation stationstation or the or the supply supply to to the the MUP MUP locallocal grid. This This supply supply depends depends on different on diff erentmarine marine energies, energies, as as presentedpresented in in Section Section3 ,3, of of the the combined combined structures.structures. To To the the authors’ authors’ knowledge, knowledge, the theelectrical electrical connectionconnection between between such such structures structures has has not notbeen been reportedreported in in the the literature literature of MUPs. of MUPs. For this For reason, this reason, this paperthis paper proposes proposes the the layout layout of of wind wind energy energy farmsfarms to to be be applied applied for for hybrid hybrid energy energy resources resources that that considersconsiders a structure a structure asone as one unit, unit, as as shown shown in in Figure Figure 2020.. A A cluster cluster within within the the wind wind farm farm could could include include n wind turbines, but under this study, it combines n structures when a structure could have various n wind turbines, but under this study, it combines n structures when a structure could have various turbines. Figure 20 shows the connection levels, starting from the structure level, through the cluster turbines. Figure 20 shows the connection levels, starting from the structure level, through the cluster and array level, to the node level, and ending with the substation level. A substation could supply and arraythe offshore level, toplatform the node or export level, energy, and ending after adding with the extra substation equipment, level. to the A substationshore in the couldcase of supplylarge- the offshorescale platform energy farms. or export energy, after adding extra equipment, to the shore in the case of large-scale energy farms.A node is a single collecting point within an array [45]. An array connection could be a proper Aalternative node is awhen single multiple collecting devices point are within connected an array together [45]. in An an arrayarray. connection There are different could be array a proper alternativeschemes when that multipleare chosen devices based on are geotechnical connected conditions together in and an resource array. There characteristics. are different The array size of schemes an that arearray chosen is limited based by onthe geotechnicalacceptable voltage conditions drop along and cables resource and characteristics.the array’s maximum The sizecapacity of an [24]. array is limited by the acceptable voltage drop along cables and the array’s maximum capacity [24]. An offshore MUP local network has been explored under the TROPOS project for the three different island configurations presented in Section 3.3. Two Alternating Current (AC) low voltage levels have been introduced: 400 VAC, which supply loads for sections such as those of acclimatization, a refrigeration system, a lubricant system, a sewage system and a rescue system, and 230 VAC, which supplies sections such as those of the aquaculture and algae systems, illumination, restaurant, hotel, and battery charger, in addition to a 24 VDC line for DC loads (see Figure 21). The daily load profile of the three island scenarios is provided in [36]. SustainabilitySustainability 20202020, 12, 12, x, FOR 1860 PEER REVIEW 18 18 of of 2423 Sustainability 2020, 12, x FOR PEER REVIEW 18 of 24

Utility MUP local Layout in Figure 21 Utility MUP local Grid GridLayout in Figure 21

Grid Grid

Node 1 to Node n Node 1 to Node n

Array 1 Array n Array 1 Array n

Cluster 1 Cluster m Cluster 1 Cluster m Hybrid Energy Farm Layout Structure 1 Structure k Hybrid Energy Farm Layout Structure 1 Structure k

Figure 20. Configuration of hybrid marine farm supplies an MUP’s local grid. Figure 20.Figure Configuration 20. Conﬁguration of hybrid of marine hybrid farm marine supplies farm supplies an MUP’s an local MUP’s grid. local grid.

FigureFigure 21. 21. SingleSingle line line diagram diagram for for an an MUP’s MUP’s local local network. network. Figure 21. Single line diagram for an MUP’s local network. Sustainability 2020, 12, x FOR PEER REVIEW 19 of 24

An offshore MUP local network has been explored under the TROPOS project for the three different island configurations presented in Section 3.3. Two Alternating Current (AC) low voltage levels have been introduced: 400 VAC, which supply loads for sections such as those of acclimatization, a refrigeration system, a lubricant system, a sewage system and a rescue system, and 230 VAC, which supplies sections such as those of the aquaculture and algae systems, illumination, restaurant, hotel, and battery charger, in addition to a 24 VDC line for DC loads (see Figure 21). The daily load profile of the three island scenarios is provided in [36]. Sustainability 2020, 12, 1860 19 of 23 It is worth mentioning that the MUP local network presented in Figure 21 was modified from that of the TROPOS project. An MUP local grid of an island structure under TROPOS is supplied by generatorIt is worth sets and mentioning from the thatutility the grid. MUP This local co networknception presented does not inmake Figure sense, 21 wasbecause modified MUPs from are thatdesigned of the for TROPOS offshore project. areas and An to MUP operate local in grid sust ofainable an island way. structure For this underreason, TROPOS Figure 21 is shows supplied an byMUP generator that was sets modified and from to be the supplied utility grid. from This a hy conceptionbrid marine does farm not via make a floating sense, substation. because MUPs are designed for offshore areas and to operate in sustainable way. For this reason, Figure 21 shows an MUP that6. Offshor was modifiede MUP Grid to be Contro suppliedl from a hybrid marine farm via a floating substation. Using on large synchronous generators in a conventional grid makes an MUP’s control system 6. Offshore MUP Grid Control simpler with respect to isolated grids. For example, a synchronous generator changes its output powerUsing in response on large to synchronous load change generatorswithout the in need a conventional for any control grid or makes communication an MUP’s control links [46]. system On simplerthe other with hand, respect an MUP to isolated grid has grids. a completely For example, differe a synchronousnt nature from generator that of changes a conventional its output grid, power as inthe response former basically to load changedepends without on a collection the need of forinverters, any control synchronous or communication generators, and links asynchronous [46]. On the othergenerators hand, [24]. an MUPAn MUP grid control has a completely system is required different for nature the fromregulation that of of a frequency conventional and grid,voltage, as the as formerwell as basicallyfor controlling depends load on sharing a collection among of included inverters, mi synchronouscro-resources. generators, In addition, and it is asynchronous necessary to generatorsresynchronize [24 ].an AnMUP MUP network control with system the main is required grid and for power the regulation flow control of frequencybetween the and two voltage, grids as[47]. well as for controlling load sharing among included micro-resources. In addition, it is necessary to resynchronizeSome studies an MUP [46–49] network have withproposed the main a hierarchical grid and power control flow strategy control with between three thecontrol two gridslevels [ 47for]. controllingSome studiesa microgrid [46–49 by] haveconsidering proposed the a islanded hierarchical mode, control the grid-connected strategy with three mode, control or connections levels for controllingwith other amicrogrids; microgrid bysee considering Figure 22. theHierarchical islanded mode, control the includes grid-connected three control mode, orlevels: connections a local withcontroller, other microgrids;a central controller, see Figure 22and. Hierarchical a supervisory control controller includes threefrom control the lower levels: to a localhigher controller, levels, arespectively. central controller, and a supervisory controller from the lower to higher levels, respectively.

Figure 22. Microgrid Hierarchical Control level [46].

There are two kinds of local controllers: a micro source controller (MC) and a load controller (LC). An MC performs some local functions, such as controlling the voltage and frequency of microgrid in transient conditions, and it follows a Central Controller (CC) when connected to the grid. Both MC and CC are used to optimize the active and reactive power of the microsource and track the load after islanding operation. An LC is installed at the load side in order to manipulate the load via a central controller for load management [48]. There are various strategies for implementing Sustainability 2020, 12, 1860 20 of 23 local controllers, and these were well-presented in [46]. Frequency and voltage changes could occur with a local controller even during steady-state. Thus, a central controller is used in order to acompensate local controller for this even deviation. during steady-state. However, controllers Thus, a central at this controller level are is useddesigned in order with to slower compensate response for thistime deviation. than local However,controllers. controllers This is the at slowest this level control are designed level which with manages slower the response flow of time power than among local controllers.microgrid and This utility is the grid slowest in order control to ac levelhieve which optimal manages economic the flowoperation of power [46]. among microgrid and utility grid in order to achieve optimal economic operation [46]. 7. Offshore MUP Grid Challenges 7. Offshore MUP Grid Challenges A local MUP network is very similar to a microgrid in terms of grid contents. The microgrid has a distributionA local MUP system, network micro-sources, is very similar loads, to a microgridand a control in terms system. of grid An contents. MUP grid The is microgrid expected has to acomprise distribution similar system, components, micro-sources, and loads,it could and be a controloperated system. in an An islanded MUP grid or isgrid-connected expected to comprise mode, similarsimilarly components, to a microgrid. and it Moreover, could be operated an MUP in annetwork islanded is orbased grid-connected on many distributed mode, similarly generators to a microgrid. (wind, Moreover,solar, wave) an that MUP are network unreliable is based sources on many of energy. distributed Thus, generators an MUP network (wind, solar, is anticipated wave) that to are face unreliable similar sourceschallenges of energy. as a microgrid. Thus, an MUP network is anticipated to face similar challenges as a microgrid. GenerallyGenerally speaking,speaking, a utilityutility powerpower gridgrid basicallybasically dependsdepends onon veryvery largelarge generatorgenerator capacitiescapacities thatthat makemake itit stablestable eveneven withwith bigbig disturbancesdisturbances occur.occur. The situation is different different with an MUP grid, whichwhich dependsdepends onon manymany micro-sourcesmicro-sources toto supplysupply itsits fluctuatingfluctuating power.power. DueDue toto thethe individualindividual naturenature ofof thethe ooffshoreffshore grid,grid, oneone ofof thethe challengeschallenges itit couldcould faceface isis powerpower qualityquality problems.problems. TheThe authorsauthors ofof [[50]50] identifiedidentified thesethese problemsproblems asas shownshown inin FigureFigure 23 23..

Power Quality Problems

Transient Unbalanced Harmonics Fluctuation Stability volatge

FigureFigure 23.23. Power quality problems in the ooffshoreﬀshore locallocal grid.grid.

AnAn ooffshoreffshore grid grid depends depends mainly mainly on renewableon renewable sources sources of energy, of energy, such assuch solar as or solar wind, or that wind, already that havealready a low have degree a low of degree controllability of controllability due to their due output to their depending output depending on the availability on the availability of resources. of Thisresources. could This result could in the result output in the power output of power renewable of renewable energy systems energy systems being unsmoothed. being unsmoothed. To avoid To this,avoid Rashad this, Rashad [50] proposed [50] proposed a fuzzy a fuzzy logic controllerlogic controller as a wayas a forway mitigating for mitigating the fluctuationthe fluctuation of the of outputthe output power power of the of wind the turbine.wind turbine. Harmonics Harmonic coulds arise could due arise to electronic due to electronic interfacing interfacing and nonlinear and loadsnonlinear [50]. loads Harmonic [50]. Harmonic frequencies frequencies have a negative have a impactnegative on impact grid components, on grid components, and this leadsand this to reducingleads to reducing a system’s a system’s lifetime, lifetime, efficiency efficiency and reliability and reliability [51]. The [51]. harmonic The harmonic current current problem problem can be solvedcan be viasolved a series via activea series power active filter power (SAPF) filter [52 ](SAPF) or a unified [52] or power a unified quality power conditioner quality (UPQC) conditioner [53]. Transient(UPQC) [53]. stability Transient is another stability challenge is another for MUP challeng grids,e for as itMUP depends grids, on as small it depends inertia on generators small inertia that couldgenerators suffer that from could even suffer small from disturbances. even small For distur this issue,bances. battery For this storage issue, has battery been storage used to has support been microgridsused to support during transientmicrogrids conditions, during andtransient this strategy conditions, has shown and betterthis strategy performance has [shown54]. An better MUP gridperformance could su ff[54].er from An MUP an unbalanced grid could phase suffer voltage from an problem unbalanced because phase it has voltage many problem single-phase because loads it thathas many are unbalanced single-phase in nature loads [that50]. are Various unbalanced control schemesin nature have [50]. been Various presented control for schemes voltage have balancing been inpresented a microgrid for voltage [49,50]. balancing in a microgrid [49,50].

8.8. Conclusion TheThe EUEU highlyhighly support support the the idea idea of of MUPs MUPs for for harvesting harvesting food food and and energy energy in a sustainablein a sustainable way overway twoover big two projects: big projects: the Ocean the of Ocean Tomorrow of Tomorrow and Horizon and 2020. Horizon An MUP 2020. would An beMUP a good would opportunity be a good for the aquaculture sector because moving to open sea would provide enough space for fish farms, and they would be safe from human sources of pollution. In addition, the organic production of fish would be developed and increased. Various marine activities such as offshore energy, tourism, transport, and fisheries, would be positively influenced by MUPs, as many activities could be integrated together to get benefit from each other. Workshops being integrated onto MUPs would have a positive impact on transport, energy and aquaculture sectors, as the operation and maintenance costs of these sectors would be reduced. However, exploiting oceans and seas in a sustainable way still an environmental Sustainability 2020, 12, 1860 21 of 23 and ecological challenge. There are concerns about the living marine environment and the habitats that could be negatively impacted by the foundation of MUP and fish cages. An MUP would conflict with other marine activities such as entrances to marine port and wildlife and birds area protection. Additionally, there is concern regarding increasing marine litter and ocean acidification. This study had proposed classifications for offshore structures based on connectivity among different energy converters and activities of co-located systems, combined structures, and island structures. The island structure represents state-of-the-art offshore structures that have been proposed under the TROPOS project. The design methodology of MUPs has been explored. The pre-screening phase of this methodology is highly important for figuring out various activities that should be integrated into MUP-based on-site assessments for extracting the maximum benefit. There is scarce literature about electrical grid configurations and control under MUPs. This paper has introduced a configuration of offshore local networks that in line with the control systems that are based on isolated microgrid literature for the above reason. Future studies of electrical local networks of MUPs should address challenges such as space limitation, the high costs of system components and installation, and a lack of available backup sources because MUPs are far offshore and use critical loads, such as aquaculture cages. A DC system has never been considered for offshore platforms, but such systems could be better alternatives when a simpler control system, lower costs and distribution losses, and increased system efficiency are required, so studying the feasibility of using DC systems for multi-use platforms is an open research area. In addition, algae biofuels would play a vital role because potential energy storage provides better reliability for offshore power systems and avoids using large capacities of batteries.

Author Contributions: Conceptualization, W.M.N. and O.A.-L.; Data curation, K.H.A.; Writing—Original Draft Preparation, W.M.N.; Writing—Review and Editing, M.E., D.C.-G.; Supervision O.A.-L.; All authors read and approved the revised manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Ross Mackay for his efforts in proofreading of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References

1. School, W.S. How Much Water is There on Earth? USGS Science for a Changing World: 2020. Available online: https://www.usgs.gov/special-topic/water-science-school/science/where-earths-water?qt-science_ center_objects=0#qt-science_center_objects (accessed on 28 February 2020). 2. AOGHS. American Oil and Gas History Society. AOGHS, 2017. Available online: https://aoghs.org/offshore- history/offshore-rig-patent-2/ (accessed on 28 February 2020). 3. Ryan, J.; Maguire, D.; Mills, G. Farming the Deep Blue. Available online: http://www.bim.ie/media/bim/ content/downloads/Farming,the,Deep,Blue.pdf (accessed on 28 February 2020). 4. Shainee, M.; Ellingsen, H.; Leira, B.J.; Fredheim, A. Design theory in offshore fish cage designing. Aquaculture 2013, 134–141. [CrossRef] 5. Stuiver, M.; Soma, K.; Koundouri, P.; Burg, S.V.d.; Gerritsen, A.; Harkamp, T.; Dalsgaard, N.; Zagonari, F.; Guanche, R.; Schouten, J.-J.; et al. The Governance of Multi-Use Platforms at Sea for Energy Production and Aquaculture: Challenges for Policy Makers in European Seas. Sustainability 2016, 8, 333. [CrossRef] 6. Bard, J. Multi-use Platform Concepts for the Oceans of Tomorrow. Available online: https://www. icoe-conference.com/publication/multi-use-platform-concepts-for-the-oceans-of-tomorrow/ (accessed on 28 February 2020). 7. Koundouri, P. (Ed.) The Ocean of Tomorrow; Springer: Berlin, Germany, 2017. 8. European Commission. The Ocean of Tomorrow Projects (2010–2013); European Commission: Brussels, Belgium, 2014. 9. European Commission. Horizon 2020 Projects. 2020. Available online: https://ec.europa.eu/programmes/ horizon2020/en/h2020-sections-projects (accessed on 28 February 2020). 10. Nielsen, R.; Motova, A. The Economic Performance of the EU Aquaculture Sector (STECF 14-18); European Commission: Brussels, Belgium, 2014. Sustainability 2020, 12, 1860 22 of 23

11. Burnell, G.; Allan, G. New Technologies in Aquaculture; WoodHead Publishing Limited: Cambridge, UK, 2009. 12. Commission of the European Communities. A Strategy for the Sustainable Development of European Aquaculture; Commission of the European Communities: Brussels, Belgium, 2002. 13. Clarke, R.; Bostock, J. Regional Review on Status and Trends in Aquaculture Development in Europe–2015; FAO: Rome, Italy, 2015. 14. Subasinghe, R.P.; Curry, D.; McGladdery, S.E.; Bartley, D. Recent Technological Innovations in Aquaculture. Available online: http://www.fao.org/tempref/docrep/fao/005/y4490E/y4490E05.pdf (accessed on 15 February 2020). 15. Pérez, C.; Iglesias, G. Integration of Wave Energy Converters and Offshore Windmills. In Proceedings of the 4th International Conference on Ocean Energy, Dublin, Ireland, 17 October 2012; Available online: https://www.researchgate.net/publication/264082313_Integration_of_Wave_Energy_Converters_ and_Offshore_Windmills (accessed on 15 February 2020). 16. Perez-Collazo, C.; Jakobsen, M.M.; Buckland, H. Synergies for a Wave-Wind Energy Concept. In Proceedings of the European Offshore Wind Energy Conference, Frankfurt, Germany; Available online: https://www. researchgate.net/publication/264082615_SYNERGIES_FOR_A_WAVE-WIND_ENERGY_CONCEPT (accessed on 15 February 2020). 17. Pérez-Collazo, N.C.; Greaves, D.; Iglesias, G. A Review of Combined Wave and Offshore Wind Energy. Renew. Sustain. Energy Rev. 2015, 42, 141–153. [CrossRef] 18. Astariz, S.; Iglesias, G. The Economics of Wave Energy: A Review. Renew. Sustain. Energy Rev. 2015, 45, 397–408. [CrossRef] 19. Council, W.E. World Energy Resources Marine Energy|2016; World Energy Council, 2016; Available online: http://large.stanford.edu/courses/2018/ph240/rogers2/docs/wec-2016.pdf (accessed on 15 February 2020). 20. Tjiu, W.; Sopian, K.; Ruslan, M.H.; Mat, S.; Marnoto, T. Darrieus vertical axis wind turbine for power generation II: Challenges in HAWT and the opportunity of multi-megawatt Darrieus VAWT development. Renew. Energy 2015, 75, 560–571. [CrossRef] 21. Tjiu, W.; Sopian, K.; Ruslan, M.H.; Mat, S.; Marnoto, T. Darrieus vertical axis wind turbine for power generation I: Assessment of Darrieus VAWT configurations. Renew. Energy 2015, 75, 50–67. [CrossRef] 22. Liu, W.; Xiao, Q. Investigation on Darrieus Type Straight Blade Vertical Axis Wind Turbine with Flexible Blade. ElSevier 2015, 110, 339–356. [CrossRef] 23. Khrisanov, V.I.; Dmitriev, B.F. Marine Electrical Power Industry with Renewable Energy Carriers. Part 1. Wind and Wave Turbines of Offshore Power Plants. Elektrotekhnika 2016, 7, 49–57. [CrossRef] 24. Martínez, J.S.; Guanche, R.; Belloti, G.; Cecioni, C.; Cantu, M.; Franceschi, G.; Suffredini, R. Integration of Energy Converters in Multi-Use Offshore Platforms; FP7 Mermaid EU, 2015; Available online: http://www.vliz.be/projects/mermaidproject/docmanager/public/index.php?dir=Deliverables%2F& download=D3_4_Integration_of_energy_converters_in_multi_use_offshore_platforms.pdf (accessed on 28 February 2020). 25. Team, N. The NEMOS Wave Energy Converter. NEMOS Company, 2020. Available online: https: //www.nemos.org/waveenergy (accessed on 20 February 2020). 26. Koca, K.; Kortenhaus, A.; Holtz, H.; Angelelli, E.; Zanuttigh, B.; Sarmiento, J.; Armesto, J.A.; Guanche, R.; Losada, L.; Kirca, O.; et al. Energy Converters; Mermaid EU Project, 2013; Available online: http://www.vliz.be/projects/mermaidproject/docmanager/public/index.php?dir=Deliverables%2F& download=D3_3_Report_on_energy_concerters_PU.pdf (accessed on 28 February 2020). 27. Khan, M.J.; Bhuyan, G.; Iqbal, M.T.; Quaicoe, J.E. Hydrokinetic Energy Conversion Systems and Assessment of Horizontal and Vertical Axis Turbines for River and Tidal Applications: A Technology Status Review. Appl. Energy 2009, 86, 1823–1835. [CrossRef] 28. Kaygusuz, K.; Güney, M.S. Hydrokinetic Energy Conversion Systems: A Technology Status Review. Renew. Sustain. Energy Rev. 2010, 14, 2996–3004. 29. Eshra, N.M.; Abdelnaby, M.E. Study of Application of Small Hydropower for Nile River in Egypt. Middle East J. Appl. Sci. 2014, 4, 1118–1129. 30. Technology, P. Kyocera Floating Solar Power Plant, Chiba Prefecture. Available online: https://www.power- technology.com/projects/kyocera-floating-solar-power-plant-chiba-prefecture/ (accessed on 28 February 2020). 31. Hellenic Center for Marine Research. D4.5 Aquaculture Pilot Scale Report; Tropos EU, 2015; Available online: http://www.troposplatform.eu/content/download/593/3012/version/1/file/150130_TRP_WP4_ECD_ D4.5.pdf (accessed on 28 February 2020). Sustainability 2020, 12, 1860 23 of 23

32. Dhanak, M.R.; Xiros, N.I. Handbook of Ocean Engineering; Springer Nature: London, UK, 2016. 33. Edinburgh, U.o. D2.3 Sample Locations and Setups for Further Design; Tropos EU: Edinburgh, UK, 2013; Available online: http://www.troposplatform.eu/content/download/382/2093/version/1/file/130114_TRP_ WP2_ECD_D2.3+-+Sample+locations+and+setups+for+further+design.pdf (accessed on 28 February 2020). 34. Trivedi, J.; Aila, M.; Bangwal, D.P.; Kaul, S.; Garg, M.O. Algae based biorefinery-How to make sense? Renew. Sustain. Energy Rev. 2015, 47, 295–307. [CrossRef] 35. Bharathiraja, B.; Chakravarthy, M.; Kumar, R.R.; Yogendran, D.; Yuvaraj, D.; Jayamuthunagai, J.; Kumar, R.P.; Palani, S. Aquatic biomass (algae) as a future feedstock for bio-refineries: A review on cultivation, processing and products. Renew. Sustain. Energy Rev. 2015, 47, 634–653. [CrossRef] 36. Madrid, U.P.d. D4.3 Complete Design Specification of 3 Reference TROPOS Systems; Tropos EU: Madrid, Spain, 2015. 37. Team, T.P. TROPOS Project. EU. Available online: http://www.troposplatform.eu/tropos-european- collaborative-project (accessed on 15 February 2020). 38. Quevedo, E.; Cartón, M.; Delory, E.; Castro, A.; Bard, J.; Lara, J.d.; Jeffrey, H.; Ingram, D.; Papandroulakis, N.; Anastasiadis, P.; et al. Multi-Use Offshore Platform Configurations in the Scope of the FP7 TROPOS Project. In Proceedings of the OCEANS, Bergen, Norway, 10–14 June 2013; MTS/IEEE: Bergen, Norway, 2013. 39. Edinburgh, T.U.o. D5.7 Technical Strategy for the Deployment and Exploitation of Multi-use Marine Platforms; Tropos EU: Edinburgh, UK, 2014. 40. Edinburgh, T.U.o. D5.8 Overall Deployment Strategy for the Deployment and Exploitation of Multi-use Marine Platforms; Tropos EU: Edinburgh, UK, 2014. 41. Team, T.p. D2.1 Report on Methodology, Eveluation Matrix setup, and Overall Design Constraints; Tropos Project EU: Edinburgh, UK, 2012. 42. School of Engineering Sciences, U.o.S., UK. Energy Island; School of Engineering Sciences, Univ. of Southampton: Southsampton, UK, 2018. 43. Nunes, P.A.L.D.; Svensson, L.E.; Markandya, A. Handbook on the Economics and Management of Sustainable Oceans; Edward Elgar: Cheltenham, UK, 2017; p. 624. 44. Zanuttigh, B.; Angelelli, E.; Kortenhaus, A.; Koca, K.; Krontira, Y.; Koundouri, P. A Methodology for Multi-Criteria Design of Multi-Use Offshore Platforms for Marine Renewable Energy Harvesting. Renew. Energy 2016, 85, 1271–1289. [CrossRef] 45. IWES, F.I. D3.5 Integrated Concept Offshore Platform System Design; Tropos EU Project: Glasgow, UK, 2014. 46. Nikos. Microgrids Architectures and Control; IEEE: Noida, India, 2014. 47. Bidram, A.; Davoudi, A. Hierarchical Structure of Microgrids Control System. Trans. smart GRID 2012, 3, 1963–1976. [CrossRef] 48. Antonis, G.T.; Hatziargyriou, D.N. Centralized Control for Optimizing Microgrids Operation. In Proceedings of the Power and Energy Society General Meeting, Detroit, MI, USA, 24–28 July 2011; Available online: https://ieeexplore.ieee.org/document/4454000 (accessed on 15 February 2020). 49. Savaghebi, M.; Jalilian, A.; Vasquez, J.C.; Guerrero, J.M. Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid. Trans. Smart GRID 2012, 3, 797–807. [CrossRef] 50. Kamel, R.M. New inverter control for balancing standalone micro-grid phase voltages: A review on MG power quality improvement. Renew. Sustain. Energy Rev. 2016, 63, 520–532. [CrossRef] 51. President, D.I.I.P.L. Cover Story—Energy Efficiency: Role of Harmonics Standards in Reducing Power Quality Problems in the Grid. Bus. Insights Glob. 2017. 52. Gao, X.; Li, L.; Chen, W. Power Quality Improvement for Mircrogrid in Islanded Mode. Procedia Eng. 2011, 23, 174–179. 53. Rasheed, A.; Rao, G.K. Improvement of Power Quality for Microgrid using Fuzzy based UPQC Controller. Indian J. Sci. Technol. 2015, 8. Available online: http://www.indjst.org/index.php/indjst/article/view/70751 (accessed on 14 February 2020). [CrossRef] 54. Kamel, R.; Kermanshahi, B. Enhancement of Micro-Grid Dynamic Performance Subsequent to Islanding Process Using Storage Batteries. Iran. J. Sci. Technol. Trans. B Eng. 2010, 34, 605–618.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).