A comprehensive review and proposed architecture for offshore power system

Electrical Power and Energy Systems 111 (2019) 79–92

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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes

Review

A comprehensive review and proposed architecture for offshore power system

T

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Rodney Itikia,c, , Silvio Giuseppe Di Santoa, Cinthia Itikib, Madhav Manjrekarc, Badrul Hasan Chowdhuryc a

Department of Energy and Automation Engineering, University of São Paulo (USP), São Paulo, SP, Brazil Department of Telecommunications and Control Engineering, University of São Paulo (USP), São Paulo, SP, Brazil c Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, 9201 University City Blvd, Charlotte, NC 28223, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Offshore power system Generalized architecture Offshore environment Subsea Offshore wind power

This review describes the offshore power system (OffPS), a research area of electricity that—despite being deeply investigated in scattered fragments—has not been comprehensively conceptualized as an integral system of interrelated components, equipment, functions and subsystems. Part of this lack of systemic approach is due to the fact that this system normally does not subsist by itself. It depends on structural elements for protection and support. OffPS gathers all the characteristics of a system and has several configurations, subsystems, and functions. In many cases, OffPS is subjected to extreme forces imposed by offshore weather conditions. For this reason, it is duly enclosed, protected and supported by several offshore-resistant mechanical machines such as ships, submarines, offshore platforms, floating ports and power plants. The prevalence of OffPS has been geographically expanding in recent years and it is spreading over vast portions of the oceans. On a larger extent, it is evolving towards an offshore power grid, a network that connects countries and even entire continents. It is also adjusting to multi-agent power access, in what is called multi-terminal OffPS. In order to encompass such a variety of topologies and applications, a generalized architecture of OffPS is proposed. It establishes a basic framework for this review on the latest research advances in offshore power generation, transmission, distribution, consumption, energy storage, offshore intelligence and environment. Nuances between specific offshore power systems, influences of the surrounding environment, interactions between OffPS subsystems and other research areas are then described within this framework. The aim of this review is to facilitate a better systemic understanding of the major challenges and importance of offshore power system research in contemporary society.

1. Introduction Traditionally, power systems spread over onshore terrain to meet the demand of residential, commercial, and industrial consumers [1]. More recently, though, they have been impelled towards the sea and into the deep sea [2]. They have even been umbilically connected to offshore platforms and have provided power access points for powergeneration plants [3,4]. The oceans have vast resources of oil and gas, subsea minerals, commercially important fish species, water, and wide areas for wave, wind and solar energy harvesting. As a consequence, they have been increasingly disputed by nations, and explored as the new frontier of economic opportunities and development. In this context, offshore power systems have a critical role in supporting

electrification for the maritime economy and are also a valuable military asset in the ocean. Although recognized as systems, offshore power systems have been investigated by study cases involving only very specific research applications [5]. A model review and framework definition for offshore grid has been developed, based on system characteristics, categories and indicators in [4]. However, such a framework has only been applied locally, for the North Sea. It encompassed aspects of implementation, technology, transmission and generation. Nonetheless, offshore-power distribution, consumption, storage, intelligence and environment were not punctuated as components of the offshore power grid. Furthermore, power supply for offshore facilities and deep-water energy storage were not

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Corresponding author. E-mail addresses: [email protected] (R. Itiki), [email protected] (S.G. Di Santo), [email protected] (C. Itiki), [email protected] (M. Manjrekar), [email protected] (B.H. Chowdhury). https://doi.org/10.1016/j.ijepes.2019.04.008 Received 8 January 2019; Received in revised form 28 February 2019; Accepted 5 April 2019 0142-0615/ © 2019 Published by Elsevier Ltd.

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considered as relevant as the main functions of the grid. Similarly, a definition of the electrical grid architecture for offshore power systems has focused on power generation, high-voltage direct current transmission, and island operation in [6]. Nevertheless, it has not highlighted offshore power distribution, consumption, storage, intelligence and environment as integral parts of the offshore electrical grid. In both frameworks, [4,6], it was not clear if a point-to-point topology of an offshore power system would also be covered by the definition of electrical grid. In the literature, offshore power systems have not been approached by a strategic collection of all subsystems to build up a more global, generalized and comprehensive perspective, as it is proposed in this paper. This paper presents a review on the differences between onshore and offshore power systems as well as the interactions between subsystems in an offshore power system (OffPS). It also introduces an OffPS architecture that is generalized enough to provide a single framework to encompass several state-of-the-art research subsystems. In addition, this paper presents three important aspects: (a) the benefits of using a generalized OffPS architecture for systemic structuring of knowledge; (b) the authors’ perceptions about how evolved the OffPS subsystems are, based on research reported in the literature; and (c) a documentation of the challenges and opportunities for future research. The authors also envisage this work as a reference material for educational, professional and advanced research purposes, since it brings a comprehensive and systemic review of technologies and components of the OffPS, and includes some of the most relevant and updated reference citations for further deepening in this area. Section 2 highlights the specificities of offshore power systems for those who are more familiar with the onshore environment and their typical technologies. Section 3 introduces the generalized architecture of OffPS, subsystems and corresponding technologies. Section 4 classifies and analyses some research types under the framework of the generalized architecture of OffPS. Each research is then understood, regardless of the specific offshore technology or problem in focus, as being possibly categorized as a derivation of the framework brought by the generalized architecture of OffPS.

environment [9]. Engineering standards for electrical systems and equipment are exclusive for offshore environment [10–17]. Additional OffPS requirements are established by several organizations regulating issues related to maritime safety, security, and water pollution prevention, such as the International Maritime Organization (IMO), the International Seabed Authority (ISA), the International Association of Classification Societies (IACS) and government agencies from each nation. As a consequence, regulations of offshore authority are very distinct from onshore regulatory environment. Due to different environmental, technical and regulatory requirements, offshore businesses are normally set apart from onshore ones. This is one of the reasons for an exclusive scientific approach for OffPS. Offshore businesses need specialized human resources, processes, tools, equipment and assets that are not shared with those in the onshore environment. Almost the whole offshore business is structured for exclusive offshore environment and processes. As a consequence, it is reasonable to approach the OffPS investigation apart from the onshore power system. Consequently, a generalized architecture is required for a comprehensive understanding of all the distinct facets of OffPS.

2. Differences between onshore and offshore power systems

An OffPS can be envisioned from a generic and comprehensive perspective, and may include internal interactions between offshore subsystems and external interactions with the onshore power system. In Fig. 2, these interactions are represented by dashed lines for data transmission, and by continuous lines for power interaction. In the architecture illustrated in Fig. 2, the onshore power system has two distinct blocks. The onshore power and energy block represent the process in which electrical energy is generated, transmitted, stored, distributed and consumed in an onshore environment, and it is vastly studied on traditional technical literature [8,18]. The second block of the onshore system represents the intelligence dimension, and corresponds to all areas that depend on data, such as control, protection, automation, communication, and algorithms.

3. Generalized OffPS architecture A generalized OffPS architecture can be conceptualized by a breakdown of several interconnected offshore subsystems: power generation, transmission, distribution, consumption, energy storage, intelligence and environment. Each subsystem can be studied apart. However, generally speaking, most research papers cover more than one subsystem. This is due to the fact that subsystems depend on each other, in a symbiotic manner. Fig. 2 shows the generalized OffPS architecture and its interactions with external (onshore) and internal (offshore) subsystems. Interactions with external subsystems are described below, followed by a description of the OffPS subsystems. 3.1. External OffPS interactions

There is more than one meaning for the word offshore. As a consequence, we must clarify its usage in this review. The term offshore is associated to internal geographic limits. According to the International Convention for the Safety of Life at Sea [7], internal geographic limits are internal waters (except those in fresh waters such as pond, lakes and rivers), territorial waters, contiguous zone, exclusive economic zone, and international waters. For example, an electrical system floating in the water, laid-down on the deep subsea or even buried some meters under salty water is part of an OffPS, for the purposes of this review. Some of the concepts developed in this OffPS review are applied to rivers and lakes but our main focus is on sea, subsea, deep-sea, offshoreisland and maritime-water environments. OffPS have several similarities to power systems on land. However, technical constraints and risks are quite different. Fig. 1 illustrates some of the differences that determine the specific characteristics of robustness and reliability of power system components. A power system installed on a floating platform must withstand oscillatory forces from waves, humidity, and even high pressure and corrosion from subsea salty water. Overhead towers commonly employed in onshore high-voltage aerial transmission lines [8] are not suitable for installation in the offshore environment, due to adverse deep-sea conditions. Procedures for installation of subsea cables have no similarities to the procedures for installation of onshore overhead cables. Lightning strikes do not affect subsea transmission lines as in the onshore overhead ones. Thus, the subsea ecosystem, which interacts with the offshore power system, is totally different from this on onshore

3.2. OffPS subsystems OffPS internal interactions can be seen as an intricate network of data and power. In Fig. 2, data interactions are represented by dashed lines, while power interactions are represented by continuous lines. This conceptual network does not necessarily have a physical correspondence to actual data-system architectures or power cabling interconnections. The architecture was conceptualized to be sufficiently generalized, so that it would encompass the functionalities of a wide range of offshore systems investigated in the literature. The generalized architecture presented in Fig. 2 provides a reference framework for this review on OffPS research. Therefore, according to the generalized architecture, OffPS can be structured with seven interconnected subsystems: power generation, 80

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Fig. 1. Differences between onshore and offshore power systems.

environments, such as solar, wind, nuclear, logistic fuel and fossil-fuel sources. However, other sources can be more prevalent or exclusive of offshore environment, such as waves and ocean tides. Offshore power generation systems can be classified by the incoming energy source. Solar power [21,22], wind-power [23–25], and wave-power generations [26] have been investigated in fixed and floating structures. Wave [26] and nuclear power generations [27–33] have also been investigated in offshore structures. Other power-generation sources are fuel cells [34], ocean tides [35], fossil fuel [36], and hybrid sources [37,38]. Fig. 3 illustrates some technologies in place or under research in offshore environment. All these different energy sources demand specialized strategies of energy conversion. This specificity opens a vast field of research, development and multidisciplinary study about power generation systems dedicated to the offshore environment. Table 1 shows the characteristics of different offshore power generation technologies. Offshore power generation systems may also have interfaces with structural and auxiliary systems, such as subsea structures, fixed platforms, floating systems, and marine ecosystems. For the purposes of this review, such exogenous systems are considered offshore environment systems. This classification is attributed to them because these systems are neither directly nor intrinsically related to the main processes of power generation, transmission, distribution, consumption, energy storage and intelligence. On the framework of the generalized OffPS architecture, the research works—related to structural or auxiliary systems, and that are conducted by disciplines other than electrical power and energy—are included in an exclusive block called offshore environment system.

transmission, distribution, consumption, energy storage, offshore intelligence and environment. Power generation, transmission, distribution, consumption, and energy storage subsystems are intimately related to the path in which electric energy is generated, transmitted, stored, and converted to other forms of energy. Energy storage is also promoted to a relevant position in this architecture, due to recent advances and roles in addressing the intermittence of renewable power-generation sources such as wind, wave and solar power [19,20]. The offshore intelligence subsystem is related to the processing, transmission and use of all collected data from these subsystems, in order to control, protect and automate the OffPS. Offshore environment subsystems are related to the interactions with surrounding elements. The most obvious interactions are those with the weather and ocean subsystems—hurricanes, tsunamis and destructive waves, for example. But the less apparent artificial environments also play relevant interactions with OffPS. Some examples of artificial environment subsystems are the vibration-isolation subsystem for generators on floating offshore platforms and the hull of a submarine that encloses the entire OffPS against extreme pressure from deep sea. These artificial metal-structure subsystems provide low-resistance ground terminals, path and voltage reference for the OffPS grounding subsystem. The following sections will present a more detailed description of each subsystem. 3.2.1. Offshore power generation Offshore power generation is the conversion process from an energy source into electricity exclusive in the offshore environment. Most of the energy sources are equally present in the onshore and offshore

Fig. 2. Generalized architecture of an offshore power system and its interactions. 81

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Fig. 3. Offshore power generation.

oil is not a renewable and sustainable power source. As a consequence, other alternative fuels such as hydrogen and natural gas are being investigated [42]. The concept of injecting the flue gases released by a floating thermal-power plant into a carbon capture and storage scheme showed encouraging cost-assessment results [43].

3.2.1.1. Offshore wind power. Offshore wind power is an onshorederivative technology that can be understood from different perspectives: historic technical evolution, manufacturing market share, costs, software simulation tools, economic policies and geographic expansion [23,24]. This clean technology is increasingly expanding towards deeper and more distant offshore sites, with the aim of harvesting better wind potential and also substituting fossil-fuel onshore generation [25,39]. But gas-fired generation must not be neglected in some countries as a bridging transition until grid-scale electricity storage takes relevant place in the future [40].

3.2.1.6. Offshore floating nuclear plants (OFNP). Offshore Floating Nuclear Plants (OFNP) are designed in such a way that they may be assembled in offshore floating platforms, barges or ships. OFNP are an object of ongoing research and development by a few countries. These countries have both traditional expertise in nuclear reactor technology and strong government support for the development of OFNP’s critical components: small- and medium-size nuclear reactors and offshore floating platforms [28]. From the not so vast recent literature on OFNP, one may infer that many technological developments are not disclosed to the scientific community, due to patent rights and profit-generation potential for companies. Russian OFNP are in an advanced stage of development [30,31]. Akademik Lomonosov is a floating nuclear power plant, capable of generating 70 MW of electric power and heat. Current research is being carried out to integrate a similar floating nuclear plant into a combinedcycle power unit fed by gas, in order to increase power-generation efficiency from the steam-turbine unit but deeper feasibility study is still required [44]. Chinese OFNP are also in an advanced stage of research and development. However, they are still in an incipient construction phase [32]. Other countries are also in OFNP research and development stages. In the United States of America, two reactors—one with 300 MW and another with 1100 MW—have been developed for OFNP. These American OFNP are enclosed by a floating cylindrical-shape hull platform for oceanic waters [27]. Risk and cost analysis was carried out, and, the estimated cost for a floating nuclear-power plant would be 1.5 to 2.0 times the price of an equivalent onshore plant [29]. In the Republic of Korea, a conceptual design of an offshore nuclearpower plant has been proposed. It has been based on a transportable gravity-based structure made of steel-reinforced concrete to be laid down on shallow waters of the seabed [45]. In France, manufacturers and the academic community have developed a 160 MW subsea nuclear-power plant [33]. One of the advantages of subsea plants is the inherent protection against tsunamis, similarly to floating nuclear-power plants. Besides it, subsea nuclearpower plants have additional advantages, such as a better protection against extreme wind and wave conditions, and very limited exposure to accidental ship collisions, intentional sabotage and surface attacks [33].

3.2.1.2. Ocean tide, wave and thermal-energy power. Ocean tide, wave and thermal-energy technologies brought a multitude of creative and innovative assets to generate power either by converting energy from water tide, from wave and even from sun’s heat [26,35]. 3.2.1.3. Offshore solar power. Offshore solar power can be harvested by concentrating solar collectors and photovoltaic (PV) cells [22]. Offshore solar power generation plants have been investigated. They revolve the entire floating platform vertically, in order to achieve irradiance maximization on collectors. This process removes the need of motorized horizontal-rotation collectors. Furthermore, the sea water in the offshore environment is an abundant cooling resource that increases the efficiency of the solar-heated steam thermodynamic cycle [21]. PV solar-power generation plants can be classified into two basic types of in-water installation: floating and offshore. For the purposes of this review, PV floating installation only designates structures floating in onshore fresh waters, such as ponds, reservoirs and lakes. Floating structures in salty waters are considered in offshore installation. The offshore environment is frequently given by extreme weather conditions, such as high-intensity winds, waves, water currents, and chemically corrosive salty waters. As a consequence, offshore PV solar-power plants require additional robustness specifications for such conditions, similarly to offshore wind-power plants. Several kinds of offshore PV solar-power plants are already operational in some countries [22]. Furthermore, for tropical and subtropical zones, offshore application of thin-film PV has been investigated and has proven to be economically competitive [41]. 3.2.1.4. Fuel cells. Fuel cells convert logistic fuels into electricity. Some examples of logistic fuels are hydrogen, diesel, natural gas, methanol, dimethyl ether and ammonia. According to Biert and colleagues (2016), the power generated from fuel cells has some maritime applications such as submarine and ship propulsion [34]. 3.2.1.5. Fossil-fuel power. Fossil-fuel power ships are power-generation plants assembled in ships. Most of the power-ship fleet is driven by oil combustion [36]. Oil is a major and traditional energy source in the maritime ship industry, for its high energy-storage capacity. However,

3.2.1.7. Hybrid power. Hybrid sources have been investigated to explore the synergy between two or more complementary power82

Electrical Power and Energy Systems 111 (2019) 79–92 Combustion, thermodynamics and kinetic-electric conversion Non-renewable, Non-sustainable Constant or modulating

Fig. 4. Offshore power transmission.

Kinetic-electric conversion Renewable, Sustainable Intermittent

generation profiles, in order to smooth and increase power output, and to cut costs by sharing common infrastructure and support services. Schemes with wind and wave have been reviewed [37]. Other hybrid alternatives, such as PV-compressed air storage-hydraulic turbines, have been studied [38].

Photo-electric conversion Renewable, Sustainable Intermittent, Periodic

Kinetic-electric conversion Renewable, Sustainable Intermittent

Nuclear reaction, thermodynamics and kinetic-electric conversion Non-renewable, Sustainable Constant

3.2.2. Offshore power transmission High-voltage transmission systems in offshore environment can be implemented by alternating and direct current technologies. High-voltage alternating current (HVAC) technologies include subsea and lowfrequency transmission, as well as umbilical and floating cables for short distances [39]. High-voltage direct current (HVDC) technologies include modular multilevel converters and subsea cables for long distances [2,46]. Fig. 4 shows different technologies utilized or under investigation for offshore power transmission subsystems. Regarding power transmission, another major difference between onshore and offshore power systems should be noted. In onshore power systems, power may be cost effectively transmitted by non-insulated metallic conductors. On the other hand, in the offshore context, phase conductors are necessarily insulated. This is due to the low-resistance property of salt water. Whenever salt water is used as an electrical return conductor, precautionary safety measures to the surrounding life environment are mandatory. In fact, some HVDC transmission systems allow the return current to flow through deep-sea cathodes and seabed ground [9,46]. Subsea-cable technology is a key component for offshore power transmission. Highly-advanced subsea-cable manufacturing techniques were developed many decades ago, by subsea cable manufacturers. However, only recently a deeper understanding on cable dynamic modeling has been achieved [47]. Table 2 shows the characteristics of different offshore power transmission technologies. HVDC transmission has been a proven and prevailing technology for subsea transmission, due to HVAC limitations in connecting long-distance offshore wind farms to the onshore power grid. Nonetheless, some research has been conducted for 16.7 Hz transmission frequency, in order to stretch HVAC circuit length to 80–180 km, with no need of costly offshore HVDC/AC converter stations [39]. Offshore HVDC power transmission technology is being adopted for the expansion of offshore grid. According to Pierri and colleagues (2017), it may evolve organically to an HVDC grid, and ultimately to the interconnection of EU-UK, North Africa and Middle East by means of a super-grid [2].

Principle Energy source type Profile

Wind Solar

Table 1 Characteristics of offshore power generation technologies.

Nuclear

Tidal/Wave

Fossil fuel

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3.2.3. Offshore power distribution Distribution systems in offshore environment are implemented considering if the area is wet or dry. Aerial installation is used on dry areas of ships or platforms [16]. On wet areas, the distribution systems 83

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Table 2 Characteristics of offshore power transmission technologies. HVAC at standardized frequency

HVAC at low frequency

HVDC

Frequency Power electronics equipment

50 Hz or 60 Hz No need of converters

0 Hz Converters in onshore and offshore stations

Onshore and offshore AC grids synchrony Distance in subsea installation

Synchronous

< 50 Hz (e.g. 16.7 Hz) Standard frequency to low frequency converter in the onshore station Asynchronous 50–200 km [39]

More than 50 km or 80 km depending on the HVDC manufacturer

Less than 50 km or 80 km depending on the HVDC manufacturer

Asynchronous

Fig. 5. Offshore power distribution and consumption, . adapted from [48,49,52]

oil and gas drilling [57], and submersible electrical pumps for oil recovery [50,58]. Besides them, the transportation activity consumes power in the propulsion of marine crafts [59] and accommodation vessels [60]. Partnerships between governments, technology industries and academia are important for the initial establishment of a subsea mining industry. They are strategically crucial for obtaining control over subsea resources and patents on new exploration technologies. Leading organizations and research institutes are bearing initial development costs in search of technical and market dominance while assessing possible impact on marine ecosystems and biodiversity, for future global expansion and technology exportation [56,61–63]. Such developments on the subsea mining industry will result in a corresponding increase in the offshore power consumption. Subsea oil and gas drilling, in particular for ultra-deep waters, also demand power. Teleoperated robotic machines—such as remotely operated vehicles, autonomous underwater vehicles, unmanned drilling and production platforms, under-water welding robots and manipulators—are some examples of offshore power consumption [57]. A comprehensive study on typical electrical data -rated power, power profile, nominal voltage, and motor drives for subsea machines - is still missing. This study would facilitate further electrical power studies on upstream or seabed subsystems of OffPS. Deep-water oil and gas reservoirs experience a natural production drop as they get mature. Sometimes, the production is artificially boosted by water injection. The resulting increment in oil recovery is implemented through submersible electrical pumps controlled by subsea variable-speed drives. Some power-system topologies require power transformers and main power distribution units installed in subsea environment. Such subsea power distribution systems require a high level of reliability, due to the high costs and time consumption needed for any repair and maintenance procedure of subsea equipment

can be implemented by medium or low voltage equipment and cabling. For medium voltage AC, subsea and umbilical cables are used, as well as subsea transformers and drives [48,49]. For low voltage AC, umbilical and submarine cables are usual options [50]. For low and medium voltage DC, submarine cables and subsea power converters can be used [51]. Fig. 5 illustrates different components of the offshore power distribution and consumption subsystems. A seabed power-distribution system enclosed by a metallic pressure vessel was developed [48,52]. An electrical power-distribution system, with step-down transformers, switchgears and other auxiliary systems was constructed. It was laid down directly on the seabed, which resulted in significant weight reduction, compared to installing them on a dry topside area of an offshore platform. Subsea power-distribution technology has been the object of an intense development from manufacturers. However, most of the technical literature related to this technology is still in the databases of patent office [52–54]. An exception to this rule is the work by Monsen et al. (2014) that casts some light on the recent advances and future works on subsea-power technology. Their work also presents tests for subsea operation of an oil and gas power-distribution and consumption system [49]. 3.2.4. Offshore power consumption OffPS aims at providing power supply for consumers in activities ranging from economic and social needs to military applications. Some of these power-consumption activities are important because they are the driving forces behind the expansion of OffPS. Fig. 5 exemplifies some of these activities. 3.2.4.1. Economic-activity consumption. There are several examples of power consumption from traditional and emergent economic activities. Extraction activities include subsea mining machines [55,56], subsea 84

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[50,58]. Increasing participation of offshore power consumption is seen in electric ships propulsion. Some of the advantages of electric over mechanical propulsion are the generally higher efficiency in fuel consumption, smaller nitrogen-oxide emissions, reduced maintenance, and lower radiated noise. Some of the drawbacks are the high losses on power converters, the poor fuel consumption in certain operational conditions, the higher cavitation risk on fixed pitch propellers, and the potential for power instability under fault conditions. Power distribution in offshore environment is carried out in AC or DC voltage, in an all-electric scheme or also in a hybrid scheme that supports mechanical propulsion [59]. 3.2.4.2. Social-activity consumption. Other references show power consumption for social purposes, such as hospital ships [64]; cruise ships [65]; water desalinization on floating island [66]; floating cities in very large floating structures [67]. Shore-to-ship power distribution technology, also known as cold ironing, may support the power consumption of traditional fossil-fuel driven ships, by providing power supply from the onshore power grid. Hospital and cruise ships can switch off their internal diesel generators and be fed by a medium-voltage circuit from a port terminal [64,65]. Traditional solutions—such as static converters—as well as innovative ones—such as rotating converters with static VAR compensators—reveal the wide range of shore-to-ship technologies [68]. Stuyfzand and colleagues (2005) conceptualized a floating island for seawater desalination powered by renewable energy [66]. Lamas-Pardo and colleagues (2015) reviewed visionary projects of very large floating structures, which could host hotels, cities, and airports. They pointed out that if these social structures are constructed and fed by onshore power, they will become major offshore power consumption assets [67].

Fig. 6. Flowchart of offshore energy storage, its interactions and components, . adapted from [19]

3.2.4.3. Military-activity consumption. Offshore power consumption in military applications includes the electric propulsion of warships and submarines [69], and electric-powered weapons such as electromagnetic launchers and free-electron lasers [70]. In ships, such weapons are a major source of AC power oscillation, which can be minimized by a proper coordination from energy storage devices based on mechanical inertia [70]. AC voltage and frequency oscillations on the AC side of the ship’s power system may also introduce some disturbances on the vital loads of the DC side. As a consequence, a real-time load management of the ship’s DC power system becomes necessary. A proposed solution is based on the optimal regulation of individual DC loads and on the coordination of pulsed and controllable loads [71].

3.2.5.1. Power-to-power process. In this review, offshore energy storage is carried out by a “power-to-power” process. A “power-to-power” process converts electricity into an energy-storage mean, and converts it back into electricity, as needed. There are three means of energy storage: mechanical, such as compressed air; electrochemical, such as hydrogen; and electrical, such as batteries. Table 3 shows the different characteristics of three energy storage technologies. Compressed-air energy storage (CAES) relies on an onshore electrical compressor to pressurize air and a turbine to expand the compressed air. The air-expansion rotates an offshore electrical generator. Offshore CAES utilizing underground formations—such as sub-sea tanks or even depleted oil reservoirs—is seen as a promising large-scale energy-storage technology, in association with offshore wind-power generation. They may serve as a countermeasure for power intermittency [3]. Flexible bags are also reexamined as an alternative CAES mean [72]. A hydrogen storage system is composed by an electrolyzer, a storage tank and a power generator, such as a hydrogen cell or a hydrogenfueled gas turbine. Hydrogen is technically and economically suitable for storing variable renewable energy, such as wind power and photovoltaic systems, for long periods of time. Consequently, it is also a countermeasure for controlling the intermittency of renewable-power output [73]. Storage systems based on batteries are a technical-standard requirement for OffPS. The reason for this requirement is that batteries provide an uninterruptible power supply for critical safety-related loads [7,13]. Batteries also play a major role as energy-storage components in independent electrical-propulsion systems for submarines [69]. Besides it, hybrid schemes using batteries were also studied for ships. Some associated benefits were reduction of power fluctuation in engines,

3.2.5. Offshore energy storage Energy storage is ubiquitous in onshore applications. Storage technologies are diverse and are classified by either the energy-conversion process or the energy-storage process [19]. The same energy-storage classification system will be applied to offshore power systems. However, minor conceptual adjustments will be performed. Offshore energy-storage process can be classified as Power-to-Gas or Power-to-Power. Fig. 6 shows the offshore energy storage subsystem with interactions with power generation and transmission subsystems. Power-toPower is the energy storage for later retrieval as power. Power-to-Gas is power consumption for gas production. In this example, the electrolyzer converts power into hydrogen gas, which is stored, and dispatched for dual purposes: for usage as a final product or for power retrieval, whenever demanded. An electrolyzer and associated gas storage exclusively dedicated to the production of hydrogen as a final product, without power retrieval functionality, would be otherwise characterized as power consumption subsystem. The following subsections give more details of these two processes. 85

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Table 3 Characteristics of offshore energy storage technologies.

Principle Applications Limitations Energy release

CAES

Hydrogen storage

Batteries

Compressed air in subsea bags Research Not commercially common Air turbine

Chemical element in pressurized tank Submarines, research Rare in civil applications Hydrogen combustion turbine or fuel cells

Chemical elements in enclosure for dry indoor area Offshore platforms, submarines, and ships Autonomy Chemical reaction

have been accepted in 30% of the authorizations, instead of metallic cables. This fact raises another concern. The return current on subsea beds and electrodes may cause an environmental impact on aquatic biota. There could be an increase in electrolytic corrosion, due to the stray current that flows on the buried metallic piping, near the subsea electrical circuit [9]. There are mainly two methods that have been used to assess the interactions between OffPS and the environment: environmental impact assessment (EIA) and the ecosystem services assessment (ESA). The EIA applied to offshore wind power plants consists in establishing a cause and effect relationship between potential risks of human activities or power system components to the offshore living beings and nature elements (air and water), in its many ways of physical interaction (noise, vibration, electro-magnetic field, air pollution, and contaminant). This method can be applied to the phase of construction, operations and decommissioning [77]. The other method, ESA, is a standardized method for environment assessment based on the Common International Classification System. This system of the European Environment Agency establishes a classification system with a very comprehensive universe of benefits that humans can take from the nature. Whenever applied to offshore wind farms projects, ESA is not effective in providing the impact quantification of each ecosystem service due to the absence of standard indicators [79]. However, ESA provides a more enhanced framework for environmental assessment as compared to the EIA because it is not restricted only to the evaluation of harmful effects caused by human activities but it also considers the positive impacts to the integrated socio-ecological system. According to Hooper and colleagues, ESA is effective in the assessment of both negative and positive impacts, and of local benefits of an offshore wind farm [79]. There are also methods that assess the impact on marine biota caused by radiation-leakage from onshore nuclear-power plants into sea water [80]. However, neither offshore-floating nor submerged nuclearpower plants have been environmentally assessed yet. Floating nuclearpower plants, and potential effects of radiation leakage on aquatic environment either have not been properly investigated, or the results of such investigation have not been made available [81]. The impact of offshore power-consumption activities, such as deepseabed mining exploration, have been vastly investigated. Furthermore, the footprints that power-consumption activities leave on the environment and the framework for environmental management are described in the literature [82,83].

shaving of consumption peak, back up for diesel-generator failure, and increasing fuel efficiency. The later benefit was obtained by switching off generators during inefficient partial-load regimes [59]. 3.2.5.2. Power-to-gas process. Power-to-gas is an alternative process for energy storage. For the purposes of the generalized architecture of OffPS, energy stored in gas form must be retrievable into electricity. Otherwise this process would just lead to electrical power consumption. For this reason, hydrogen production in wind farms—which is obtained by electrolysis and associated to electricity retrieval in the power system—should be more properly classified as a “power-to-power” process whose storage is based in chemical mean: hydrogen [73]. On the other hand, hydrogen production without power retrieval is considered just power consumption. Literature points out the high costs of offshore hydrogen production, offshore wind power generation and hydrogen liquefaction or compression. It also indicates the absence of carbon tax to support the technology, and the high costs and losses of marine transportation of liquefied hydrogen from offshore to onshore hydrogen terminals. There is also a lack of information about adequate locations of salt offshore regions for hydrogen storage and there are competing solutions such as onshore hydrogen cavern storage. Additionally, the low efficiency of hydrogen re-electrification (below 40%) in case of power retrieval and the low price of hydrogen in the market seem to be the major factors against the offshore hydrogen technology [75,76]. 3.2.6. Offshore environment and multidiscipline subsystem OffPS has intricate bidirectional interactions with offshore-environment elements. Interactions may be originated from either natural or artificial environment elements—such as marine animals, excessive temperature, vibration, explosive gas, corrosion or even sabotage. On the opposite direction, OffPS poses risks to the environment elements. 3.2.6.1. Impact of OffPS on the environment. Some OffPS interactions can be harmful for the safety of the surrounding environment, by means of electric shock, arc flash and explosive-gas ignition in oil and gas facilities. Offshore power systems cause an impact on natural and social environments such as marine and biological ecosystems [77,78,9] and socio-ecological systems [79]. Sea water may be polluted by radiation from offshore nuclear-power plants [80,81] and by subsea mining from offshore power-consumption activities [82,83]. The impact of offshore wind-power plant construction, operation and decommission on natural and social environments has been investigated [77]. Submerged jackets of wind-power turbines are fixed on the seafloor and provide physical support to the formation of artificial reefs on their basins. Since fishing is usually prohibited in close proximity to offshore wind farms, due to risks of ship collision, offshore wind power also provides shelter for marine-life development [78]. OffPS can also mitigate the effects of extreme weather on offshore environment. For example, the presence of offshore wind turbines reduces the wind speed from hurricanes [84]. The impact of HVDC transmission systems on offshore environment has also been assessed. Similar to HVDC in onshore power systems, the return current through electrodes has also been authorized by localgovernment regulatory agencies. In subsea cable links, sea electrodes

3.2.6.2. Impact of the environment on OffPS. Besides the impact of OffPS on the environment, the impact of the environment on OffPS should also be studied. The impact from the environment may be of social nature — the public perception on OffPS risks and benefits may strongly influence the approval for an OffPS expansion. The community engagement on different levels of political openness may define the social approval of licenses for the construction of a new OffPS. Relative levels of concern with the perceived risks, benefits, scores, weight and effectiveness of mitigative actions should be measured. These data on the collective social consensus should be submitted to analysis on complex decisions. Energy infrastructure proponents should consider interdisciplinary and risk estimates, 86

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automation, control and intelligent electronic devices of power equipment for power generation, transmission, distribution, consumption, and energy storage, must be included in the offshore intelligence category. Each OffPS component has its own subsystem box on the generalized OffPS architecture. Offshore intelligence encompasses many different technologies, such as control strategies for offshore wave power generation [90]; control strategies and protection schemes for high-voltage direct-current transmission systems in offshore environment [91,92]; wind and wave forecast systems [93]; protocols for OffPS automation, control and protection [94]; and smart-ship control systems [74]. In the control technology of offshore wave power generation, the most innovative functionality of the control strategy is to regulate the active power, reactive power, current, voltage, or a combination of electrical variables, so that the connection of the power generation output would be compliant with the power quality requirements of the external grid. This control technology actuates in the circuity of the power electronics converter, which transforms the alternating asynchronous voltage from the electrical generator into the required AC or DC voltage waveform to the external transmission or distribution system. It is also possible to improve the power capture from ocean waves, through advanced control strategies on mechanical actuators such as valves or rotor blades [9]. Despite these strategies to keep the quality of the generated power, it would be impossible to maintain output power generation without an adequate minimum intake of wave energy. The extreme intermittency status of renewable energy imposes operational limits for control strategies, not only in wave but also in solar, wind, and tidal energies. In the control and protection technologies for HVDC systems, the contribution of the control strategy is the similarity with the power electronics converter on the offshore wave power generation. However, the HVDC control strategy has a breakthrough difference—it is also focused on distributing responsibilities between two or more highvoltage converters and HVDC circuit breakers, for the protection and the coordinated transfer of power from the entire HVDC transmission system to the external onshore power grid [91,92]. The limitation of this technology is the complexity of coordinating the performance of multi-terminal HVDC systems, despite the research efforts in investigating new control strategies [91]. Numerical wave models have been applied to predict offshore wind and wave power in each geographical area of the globe, some days in advance [93]. This kind of intelligence information may be used in the following way. Power generation stations can use the information provided by the offshore environment intelligence, and preventively increase nonrenewable generation. In this way, the imminent shortage of offshore wind and wave power generation may be compensated in advance. So, it becomes clear that OffPS control strategies support the smart side of power grids [90,95,96]. Offshore intelligence systems are also present in offshore wind power plants. Intelligent electronic devices (IED) measure electrical variables by instrument transformers on the live part of the power system. These IED are linked to servers by means of a data bus. Windpower industry and standardization committees have proposed protocols that support interoperability between devices from different manufacturers, such as the IEC 61850 on a substation-level control. Besides it, the IEC 61400-25 protocol—exclusively designed for wind power plants—has logical nodes dedicated to meteorological information recording and to motion sensing of the offshore wind-turbine foundation. At a higher level, the IEC 61970 protocol provides information regarding the Energy Management System, which is in charge of power-generation dispatching to the power grid [94]. Offshore intelligence is also relevant in smart ships, which manage their internal OffPS by means of several control strategies. These strategies depend on the propulsion-system type (mechanical, electric or hybrid), on the power supply (AC or DC), and on the existence or absence of energy storage systems [74].

including public and stakeholdeŕs perceptions. This comprehensive approach would allow participatory voices, on the energy-related decision process [85]. The impact of the environment on OffPS may be also of natural causes—waves cause pitching motion on floating wind-power platforms, fluctuation on power output [86], changes in the dynamic position of offshore marine crafts [59]; and extreme weather conditions such as on hurricanes, thunderstorm, tropical cyclones, and tsunami should also be considered [87,88]. Floating offshore structures, and the associated OffPS, are sensitive to weather elements and positioning on the environment. As a consequence, specific design criteria are applicable to offshore wind farms. A comprehensive range of sensors—for wind speed and direction, water current monitoring, heading gyroscope, motion reference sensors, and global positioning system—is available to monitor the offshore environment [89]. Weather measurement data feed different types of controllers that act on wind-turbine blade pitch and generator-torque settings. Control systems aim at reducing electrical-power output fluctuations and excessive pitch motion that could harm the mechanical stability of the floating wind-power platform [86]. Extreme weather conditions—tropical cyclones, hurricanes, typhoons, excessive wind speed, and turbulence—affect not only the design of power plants as described above, but also the functioning of such plants. An obvious example is the offshore wind power, which is intrinsically impacted by weather conditions on the environment. Weather can also restrict operations, trigger shutdowns, cause electrical-power disturbances, and even induce structural and mechanical collapse [12,87,88]. Several innovative technological developments are addressing and mitigating these extreme-weather risks on wind-power farms [90]. 3.2.6.3. Bidirectional interaction. In short, the interaction between offshore environment and OffPS is bidirectional. The offshore environment impacts the OffPS by determining, for example, the power demand of ship-positioning thrusters of the floating structure, by modulating the power generation of an offshore wind plant, and even by determining the OffPS shutdown [60,59,86]. On the other hand, components of an OffPS interfere in the offshore environment, for example, when offshore wind-turbine slows down extreme wind speed of hurricanes [84]. This bidirectional relation between offshore environment and OffPS components supports the idea that offshore environment should be considered as an integral part of the OffPS. This concept is indicated on the generalized architecture, described in Fig. 2. There is a vast range of subsystems in charge of monitoring the weather and warning agents in offshore environment — data acquisition, intelligent processing and information distribution. Since these subsystems improve safety and security on offshore environment, they may be considered as parts of the offshore intelligence subsystem in the generalized architecture proposed in this review. They are detailed on the next subsection. 3.2.7. Offshore intelligence In this review, the offshore intelligence subsystem involves the data processing to support, control, optimize, protect, manage and monitor other OffPS subsystems —power generation, transmission, distribution, consumption, energy storage, and the environment. This list-based definition of offshore intelligence was intentionally adopted in this review because it allows the inclusion of future intelligence roles on OffPS, not yet foreseen by the current state-of–theart intelligence. But everything not related to data and information should be conveniently excluded from the definition of intelligence. For example, high-voltage components of power-processing equipment for power generation, transmission, distribution, consumption, and energy storage do not contain any intelligence by themselves and therefore are not part of offshore intelligence subsystems. On the other hand, 87

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derived from the generalized architecture of OffPS shown in Fig. 2. Interactions between subsystems are apparent. Power interaction occurs through energy flow. The power harvested from wind kinetic energy follows a certain transmission and distribution path towards the final power consumption, in a one-way path. In the same architecture, the OffPS energy-storage subsystem can also absorb part of the generated power and deliver it to power consumption later. As a consequence, the interaction between the generation and storage subsystems is bidirectional. High-voltage AC or DC subsea transmission subsystems allow also a power-flow interaction between subsystems connected to both ends. Sometimes, energy storage may exceed power consumption. Whenever it occurs, power is exported to the onshore power grid through the transmission subsystem. On the other hand, a power deficit in OffPS could be solved, for example, by importing power from the onshore power grid. As a consequence, the transmission subsystem allows bidirectional power interaction between both ends [97,39,46]. Data interaction between subsystems occurs by means of communication, control, protection, monitoring and management functions. As a result, this OffPS intelligence interaction demands a communication infrastructure that includes protocols, networks, algorithms, controllers, and intelligent devices [94].

The breakthrough on smart ships control technology is the optimization of advanced control strategies. Multiple strategies are used so that ships may adapt their performance under variable environmental constraints (discontinuous sea wave regime and speed limitation to avoid cavitation on the propellers blades), human intervention (changes in the setting of the reference ship speed and in the starting command of a pulsating load), and optimization of objective functions (minimization of emissions and fuel consumption, and maximization of performance on acceleration maneuvers). As more electrical devices are used on ships—such as power AC/DC converters and energy storage for the propellers and auxiliary loads—more opportunities emerge for optimization of performance by innovative combined control strategies, as pointed out by Geerstma and colleagues [74]. 4. Interactions between OffPS subsystems Interactions between internal OffPS subsystems, onshore power systems and external intelligence are maintained by flow of electrons, data and information. The seven OffPS subsystems—power generation, transmission, distribution, consumption, energy storage, intelligence and environment—can be sorted in several combinatory configurations of OffPS. The environment subsystem always exists on offshore power systems that require physical structures to float on sea surface, to stay submerged or even to be left buried on seabed. The intelligence subsystem is also ubiquitous on OffPS, since all power-related subsystems require at least some sort of elementary electrical control, measurement, monitoring, protection, automation or energy management equipment. Assuming that environment and intelligence subsystems are always present on OffPS, the other five subsystems may be present or not. Theoretically, these subsystems could be rearranged among themselves, according to 32 possible combinations. The application of these theoretic sub-systemic architecture to three practical combinatory examples can be implemented by OffPS type characterization. The initials G, T, D, C, S, I, E represent the presence of the respective subsystems—Generation, Transmission, Distribution, Consumption, Storage, Intelligence and Environment—in the OffPS.

4.2. GDCSIE-type OffPS An OffPS with all subsystems except transmission subsystem is called GDCSIE-type. It is typically present in vessels. This GDCSIE-type OffPS has onboard generation and energy storage such as those in allelectric ships, smart ships, nuclear-powered aircraft carriers and submarines [98,59,99]. Fig. 8 illustrates the architecture of a GDCSIE-type OffPS, derived from the generalized architecture of OffPS shown in Fig. 2. A shipboard hybrid-power system simulation of an offshore support vessel resulted in a 15% reduction of fuel consumption and it was less harmful to the environment [98]. The vessel model had diesel generation, medium-voltage DC-power distribution, AC-power consumption by main electric propulsion, battery-based energy storage, and intelligence-algorithm for efficiency optimization. Similar shipboard hybrid-power systems, with different intelligence subsystem have also been investigated and resulted in promising outcomes [100]. Those systems had an interactive algorithm based on particle swarm optimization method and a fuzzy mechanism. They resulted in lower greenhouse emissions and operational costs. This intelligence algorithm allowed power and data interaction between electrical drives, batteries and electrical propulsion systems [100]. From the perspective of the generalized OffPS architecture shown on Fig. 2, the GDCSIE-type OffPS is not complete, because the transmission subsystem does not exist. Besides it, the power interaction between OffPS and onshore power system is not present during normal transit of the vessel. However, the generalized architecture is still valid

4.1. GTDCSIE-type OffPS A complete OffPS with all the seven subsystems is called GTDCSIEtype. An example of GTDCSIE-type OffPS is an offshore wind-power plant connected to an onshore power grid. A large-scale complete OffPS in the North Sea was proposed by Spro and colleagues (2015) [3]. Fig. 7 illustrates the architecture of a complete GTDCSIE-type OffPS,

Fig. 7. GTDCSIE-type OffPS.

Fig. 8. GDCSIE-type OffPS. 88

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The generalized architecture of OffPS introduces the concept that environmental interactions are relevant to the research and development of a power system. This concept is important to account for the effects of frequent and extreme weather events associated to climate change. A traditional single-line diagram does not show the surrounding context, interactions and intelligence level of an OffPS. In certain aspects, the generalized architecture of OffPS challenges the traditional electrical mindset by incorporating environment as a technical issue for electrical power systems engineering. Furthermore, the environment is an important subsystem since it provides the primary input energy for most of the offshore renewable power generation. 6. Emergent topics for future research Fig. 9. GTDCIE-type OffPS.

The benefits of installing several wind power farms in the offshore environment to slow down the high-speed hurricanes can be a potential topic for future research. In this case, the direct integration of numerical weather prediction modeling with electrical power flow simulation may be the next challenge for power system studies. In this sense, research on OffPS with high potential for positive impact in the environment is valuable for the humanity. The advances in intelligence systems have the potential to give total decision autonomy over OffPS operation, control, and protection to intelligent electronic machines and devices in the future. The offshore environment imposes many hardships in manpower and high costs for life-support systems, for example, in offshore structures such as oil drilling platforms, submarines, aircraft carriers and cargo ships. Additionally, the technical literature on intelligence systems for control of OffPS from remote distributed control systems located on onshore bases is also scarce. Future research may find great opportunities for innovative findings in the application of intelligence to reduce manpower and costs in offshore structures. The authors also observed a knowledge gap in the technical literature concerning the identification and quantification of intermittent, pulsating and constant loads profile for each type of offshore power consumer, which it is essential for OffPS sizing, simulation and power system analysis.

and it encompasses all the remaining subsystems and interactions. 4.3. GTDCIE-type OffPS An OffPS with all subsystems except storage subsystem is called GTDCIE-type. In GTDCIE-type OffPS, energy storage is not active. This OffPS type is typically studied in the interconnection with several offshore power assets such as HVDC transmission system, renewable power, floating power plants, power ships, and power consumers [101]. One example of the GTDCIE-type OffPS is a hybrid OffPS developed by Sanchez and colleagues (2017) [102]. Fig. 9 illustrates the architecture of a GTDCIE-type OffPS of a hybrid OffPS, derived from the generalized architecture of OffPS shown in Fig. 2. The variable-speed drive regulates the non-essential power consumption by following the intermittent wind-power availability. Effects of low-frequency oscillations in power generation, rotor speed, AC voltage, frequency and damping ratio are observed. A strategy to smooth the power profile during wind-power shortages must be adopted. The limiting factor in this strategy is the intelligence subsystem, more specifically the communication delay between the windpower control and the speed-drive control [102].

7. Conclusion 5. Advantages and disadvantages of the generalized architecture of OffPS

The ocean environment has vast resources of oil, gas, rare minerals, and wind power. As a consequence, the control over such resources is increasingly being disputed by several countries. However, the exploration of such resources demands significant investment in production assets, and respective military border vigilance for their protection and control. Despite the costs, OffPS plays a crucial role in powering all the demand of economic, social, and military activities in the offshore environment. In such a context, this review proposed an OffPS generalized architecture to provide a systemic and organized perspective of such a vast area that has been studied in scattered fragments. Recent research and technology advances on offshore power generation, transmission, distribution, energy storage, intelligence and environment were systemically reviewed under the conceptual framework of the generalized architecture. The main points of each subsystem are listed below. Offshore power generation is under intense development in the areas of offshore wind power. Furthermore, nuclear power seems to be the preferred technology for military applications due to its fuel autonomy. Offshore power transmission has experienced a rapid expansion of long-distance HVDC systems that interconnect offshore wind power plants to onshore grids. The subsea integration of isolated power grids between continents and the support to renewable power in the offshore environment (also known as offshore multi-terminal power grid) are very promising research areas. In offshore power distribution, the development of electrical

One of the advantages of the generalized architecture of OffPS is to expedite the understanding of the scope of a research. This architecture is useful for summarizing the main directions of a paper in a very compact space. It could also be used as a graphic summary in papers. The generalized architecture of OffPS is comprehensive enough to accommodate totally different types of offshore structures—such as wind farms, electric ships and hybrid generation plants—in the same conceptual framework with few customizations. Another advantage of the generalized architecture of OffPS is that it can support the structuring of new research. The researcher can select a combination of 7 or fewer subsystems of the generalized architecture, go to the review of each subsystem, choose one or more technologies described in the subsection and its associated literature references, and finally build the research scope with the customized architecture derived from the generalized architecture of OffPS. By doing this, different parts (subsystems) of the generalized architecture can be considered a single study, already integrated and conceptualized as a single OffPS, ready for further simulation and study. This is a practical and simple step-by-step recipe for investigative research, suggested by this review paper and the generalized architecture. A limitation of the generalized architecture of OffPS is that it is not useful for structuring components, for example, a new type of transformer for offshore applications and its intrinsic individual (not systemic) performance. 89

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equipment for subsea environment has unveiled new possible applications in subsea oil drilling. Offshore power consumption is associated to increasing economic, social, and military activities. Offshore energy storage uses not only traditional technologies originally developed for onshore installation but also specific developments for offshore installation such as CAES. Offshore intelligence has an important role in weather monitoring for OffPS. Floating structures can capsize, and fixed structures can be flooded by extreme high waves, imposing great damage to the integrity of the OffPS. Offshore harvesting of wind power depends on control loops and weather-variable monitoring. Offshore environment may determine performance or even cause damage to the integrity of the OffPS. On the other hand, OffPS may have a harmful effect on the ecosystem. It is worth to note that interactions between OffPS subsystems occur not only by means of power flow but also by data, weather, and environmental impact. The OffPS generalized architecture encompasses a more global, comprehensive and systemic assessment of the interactions between internal and external subsystems. It is not confined to the traditional investigation of electrical power flow and electrical variables. Since it is not limited to the electrical interactions between subsystems, the OffPS generalized architecture allows a more comprehensive understanding of the physical, social, biological phenomena associated to OffPS and its surrounding multidisciplinary environment. This review takes benefit of the completeness of the proposed OffPS generalized architecture.

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Acknowledgment

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This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The authors also thank the UNC Coastal Studies Institute for supporting this work.

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Appendix A. Supplementary material [25]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ijepes.2019.04.008.

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