Sustainability assessment of a conceptual multipurpose offshore platform in the South China Sea

Abstract

An offshore multipurpose floating platform (MPFP) combines different marine technologies to serve industry needs using one infrastructure; the aim of an MPFP is enlarging the synthesis benefits and reducing the negative impacts. Ocean thermal energy conversion (OTEC) in particular has attracted significant attention for its great potential and low environmental risk. This research demonstrates the system design of a conceptual MPFP in the South China Sea, evaluating its economic and environmental sustainability using an inclusive index. The system is based on a modular floating structure with a designed lifetime of 50 years. Tuna aquaculture, microalgae cultivation and processing, and the OTEC energy infrastructure are integrated to increase the profitability of the applications. We adopted a high-yield photobioreactor microalgae cultivation system and a low-cost barge-type floating structure combined with a semisubmergible to reduce the required area and cost of the floating structure and improve the sustainability of the system. The inclusive impact index “Triple-I-light (III_light)” was calculated to evaluate the environmental sustainability and economic feasibility of the floating system. The result shows that the new system becomes environmentally neutral (EF = BC) at a lifetime of 11.5 years, showing sustainability (III_light ≤ 0) at a lifetime of 20 years. The proposed system can produce fish with no external energy or feed supply. An autonomous system, such as the one proposed here, is considered very effective when it comes to utilizing the ocean and contributing to a sustainable society.

1 Introduction

1.1 Background

An offshore multipurpose floating platform (MPFP) combines different marine technologies to serve industry needs within one infrastructure, aiming at enlarging the synthesis benefits and reducing negative impacts. By integrating multiple offshore technologies, such as offshore renewable energy, aquaculture, leisure, and transport, an MPFP could have many significant benefits in terms of economic efficiency, spatial planning optimization, and environmental risk avoidance (Abhinav et al., 2020).

On the one hand, an offshore MPFP will bring economic synthesis benefits. For example, the electricity generated by marine energy offshore will be transmitted by cable or converted into other energy carriers and transported to neighboring land masses. The transportation process requires additional equipment construction and transportation costs, resulting in higher initial and operating costs than land-based energy products. These redundant costs limit the commercial application of marine energy, which is an urgent problem to solve when developing offshore energy (Griffiths et al., 2023). If offshore energy is combined with other offshore systems that have economic benefits, the economic performance might be improved.

On the other hand, it could also achieve higher ecology benefits. For example, when ocean thermal energy conversion (OTEC), which generates electricity by utilizing deep ocean water, has attracted significant attention for its large potential and low environmental risk. OTEC situated on a floating structure has a comparatively lower land use and impact than land-based installation and could be combined with other offshore technologies to reach higher ecology and economy synthesis benefits because areas where deep ocean water flows upward from the deep ocean to the surface have been observed to have high primary productivity and fish production (Wang & Tabeta, 2017). In addition, because of the broader area, higher solar radiation, and lower land-use price than a traditional land-based cultivation system, offshore microalgae cultivation systems have also become a practical choice when it comes to investment efficiency. Therefore, if the offshore microalgae cultivation technology and OTEC could be integrated, the fresh water and abundant nutrients from the OTEC might be utilized internally in this multipurpose system.

In recent years, the EU has promoted the conceptual design, demonstration, and construction of an offshore MPFP. The TROPOS project (Modular Multiuse Deep Water Offshore Platform Harnessing and Servicing Mediterranean, Subtropical, and Tropical Marine and Maritime Resources) integrated a range of functions from the transport, energy, aquaculture, and leisure sectors. Here, the Green and Blue Concept, that is, the aquaculture module, was integrated with renewable energy modules, including OTEC and offshore wind power, focusing on transport and deep water solutions utilizing floating energy-related platforms (Buck & Langan, 2017). Another EU-funded project, MERMAID, also developed concepts for the next generation of offshore platforms that can be used for multiple purposes, including energy extraction, aquaculture, and platform-related transport (Rasenberg et al., 2013).

2 Research gaps

Because few cases of offshore MPFP have been commercialized, there is little research discussing their environmental impact. Even though offshore renewable energy and aquaculture have matured as separate technologies, the environmental impacts when these sectors are combined are poorly understood and almost entirely based on theoretical projections. The TROPOS project studied the effects of combining offshore renewable energy with aquaculture in the MPFP, concluding that the multipurpose approach had the advantage of integrating diverse activities in the same location when compared with the single approach (Abhinav et al., 2020). Golman et al. (2014) also summarized the environmental impacts of the TROPOS platform following an environmental impact assessment (EIA) procedure, identifying some environmental synergies among different sectors. Zanuttigh et al. (2016) listed the common environmental benefits and risks of the MERMAID platform. The benefits include refuge for wild fisheries species and operational constraints such as increased biofouling, while the risks include internal environmental interactions between the biota and different types of foundation and material. However, none of these studies have performed a quantified analysis on the environmental impact.

There are also few previous studies that have conducted an economic analysis of the MPFP, so high uncertainty exists in this area. An economic analysis of the TROPOS project was done, and the capital and operational expenditures were estimated by applying simple linear programming techniques, namely IO analysis (TROPOS, 2015). Klijnstra et al. (2017) stated that, even though new jobs and revenue streams would bring socio-economic benefits to MPFP projects, potential risks might also exist, such as the conflict between fishery communities and the platform operation, and economic loss from the collision and corrosion.

The MPFP has some benefits that the single-purpose platform does not have. On the one hand, it can increase the benefit from different functions and improve both economic and environmental performance. On the other hand, it can reduce environmental impacts through synergies among single impacts. However, developing an offshore MPFP may result in both positive and negative impacts, many of which remain unknown, given the emerging nature of this field. Therefore, an integrated analysis should be carried out to ensure the sustainability and feasibility of an MPFP.

2.1 Aim of this research

In the present research, we propose a system design and clarify the sustainability performance of a conceptual MPFP. The system is based on a modular floating structure in which tuna aquaculture, microalgae cultivation and processing, and OTEC energy infrastructure are integrated for profitable applications. We also evaluate economic and environmental sustainability using the inclusive index to ensure that the scenario is ecologically valuable and feasible when OTEC is combined with other technologies.

3 Conceptual system design

3.1 Base system

Duan (2019) proposed a modular offshore floating structure located in the East China Sea, where the surface seawater temperature is relatively high and tuna aquaculture is possible. This system would be energy independent by including offshore renewable energy infrastructure in offshore microalgae fuel production systems and marine aquaculture. The system was based on a modular floating structure where tuna aquaculture, microalgae cultivation and processing, and OTEC energy infrastructure were integrated for profitable applications. The system examined the feasibility of a floating platform carrying multiple functions, aiming at more efficient utilization of marine resources, benefits to the sustainability of local society, solutions to the world food shortage problem, and contributions to climate change mitigation.

The system included five main components, as shown in Fig. 1. The first component was a floating structure that could provide enough space and hold the most activities of the multipurpose system in the open sea. The second was the fish aquaculture component, which could produce fish stock and create economic benefits for this system. Microalgae cultivation subsystems and microalgae processing subsystems were two more important components. Microalgae stock can be cultivated and crude algal oil extracted from the platform. The biomass residual after extraction can be processed into the fish feed and used for fish aquaculture. The last component was the energy infrastructure, making the platform energy independent.

Fig. 1

System diagram of the conceptual multipurpose platform (Duan, 2019)

The floating structure size was originally designed to be 1000 m × 1000 m. The total area of the microalgae cultivation ponds was 72 ha, the dry weight of the expected annual microalgae production was about 6.89 × 10³ tons, and the annual algal fuel yield was expected to be 1.96 × n10³ tons. About 2.30 × 10⁴ tons of tuna feed could be processed using the residual biomass during microalgae extraction, and when the feed conversion ratio of farmed tuna was from 7 to 10, about 2,400 to 3,400 tons of tuna could be obtained. We modified this system to improve its economic and environmental sustainability.

3.2 Floating platform

The floating structure type adopted in the original system (Duan, 2019) is the semisubmersible (semisub) type with a size of 1000 m × 1000 m and a weight of 2,200,000 tons. In the modified system, a new barge-type floating structure will be used instead of the previous floating structure to reduce the cost and materials. Here, the floating structure of the next-generation floating offshore wind power generation system “HIBIKI” (NEDO, 2018) is considered (Fig. 2). This new type of floating structure that is used in offshore renewable energy utilization is light weight and low cost.

Fig. 2

Barge-type floating structure (NEDO, 2018)

The weight of one 51 m × 51 m barge-type floating structure is about 3,100 tons. Considering the hole in the center of the structure, the available area of one floating structure is about 1,875 m². Therefore, about 134 barge-type floating structures are required for 25 ha of microalgae cultivation area. In this case, the actual area of the cultivation area is about 33.5 ha. In the center part of the modified system, where there are energy plants, microalgae processing infrastructure, and a management facility, a semisub type with the size of 250 m × 250 m is used. For the surrounding part, which is for microalgae cultivation and fish aquaculture, the barge-type structures are connected around the center part. The size of the modified system is 650 m × 650 m, and the area is 42.25 ha.

3.3 Microalgae cultivation and processing subsystems

One of the main purposes of developing this system is to utilize biological energy sources and produce next-generation bioenergy offshore. Algal fuel, which is carbon neutral, serves as an effective tool for dealing with the fuel crisis and global warming. It has been reported that microalgal systems have many merits compared with other biological feedstocks for biofuel production (Demirbas et al., 2011, Sathya et al., 2023: Sreekumar and Nandagopal, 2016). Initially, much less land is required to reach a much higher growth rate when cultivating microalgae than other agricultural crops, aquatic species, or biofuel feedstocks. Most microalgae species could grow into double their biomass within one day (Moazami et al., 2011). A previous statistical investigation revealed that the land requirement of algae cultivation is reduced by 49 or 132 times compared with rapeseed or soybean crops, respectively, and here, the oil content of algal species is 30% (Chisti, 2007). The second advantage is the ease of cultivating microalgae. Microalgae can grow in extreme conditions using wastewater with low economic value (Chisti, 2013). Moreover, there will be a variety of biological derivatives with high economic value during the algal oil production process, which might be applied to many biotechnology fields, such as cosmetics, health care products, and pharmaceuticals (Rosenberg et al., 2008).

Suspended microalgae cultivation is mainly divided into two modes: open raceway pond and closed photobioreactor (Sreekumar et al., 2016). Open raceway pond cultivation is a common method for commercial production of a few microalgae, such as Spirulina and Chlorella, but the development of open raceway ponds is greatly restricted because of the fact that many microalgae have no or weak antibacterial ability. Tubular photobioreactor cultivation can control the culture conditions well, making it more suitable for microalgae growth, and can be cultivated all year round. However, microalgae easily to stick to the tubular wall, which affects the yield. In addition, the light can hardly reach the middle of the pipeline. The advantages and disadvantages of the two cultivation modes are shown in Table 1.

Table 1 The advantages and disadvantages of two microalgae cultivation modes (Zheng et al., 2009)

The open raceway pond was adopted in the original system (Duan, 2019), while the bioreactor cultivation system is used in the present modified system. Even though the investment necessary to build and operate open systems is lower compared with closed photobioreactors, photobioreactors could reduce freshwater losses and diminish the risks of contamination, hence limiting the value of the biomass, especially for the cosmetic and food markets.

In addition, closed systems of photobioreactors allow for operation at higher cell concentrations and increase the number of algal species that can be cultivated outdoors. In the original system, the microalgae productivity using the open raceway pond was 38 g/(m² d), and the cultivation area was 75 ha. By adopting the tubular photobioreactor cultivation system, the microalgae productivity could reach 560 g/(m³ d), and only about 25 ha of the cultivation area is required when the plot ratio is 0.2. This change could significantly reduce the platform area. There is an algal oil extraction facility and fish feed production facility in the microalgae processing subsystem. The process flow of the entire processing system is shown in Fig. 3.

Fig. 3

The process flow in the microalgae processing system

Within the algal oil extraction facility, the efficiency of algal oil extraction could reach up to 95%, which is comparatively high. Generally, the dry weight that microalgae biomass harvested in an open raceway pond is only about 0.1%. After the dewatering process, the concentration could rise to about 8% through sedimentation. Then, the heating and centrifugation process will further dewater the biomass. Finally, by adding hexane solvent into the concentrated biomass, algal crude oil with high purity could be extracted. Most of the hexane will be recycled after the chemical extraction, which could reduce material loss and operating costs (Arashida, 2012; ANL et al., 2012; Kato & Arashida, 2013).

After the algal oil extraction process, the residual biomass, which is mostly occupied by protein, will also be produced. This substance is rich in a variety of nutrients that come from microalgae, containing not only vegetarian vitamins and minerals, but also animal unsaturated fatty acids. This feature has made it suitable for producing fish feed and replacing fishmeal and fish oil. In the fish feed production facility, the biomass will be further processed into fish feed, which will be utilized directly in the fish aquaculture system to reduce the operating cost and transportation cost because fishmeal is not required to be purchased and transported from land to the offshore platform (Duan, 2015).

It is essential to select the proper microalgae species in the microalgae cultivation and processing system because different kinds of microalgae might have totally different properties, such as biological features and lipid content. Table 2 shows the properties of different microalgae species, which have comparatively mature cultivating and extracting technology in bioenergy developing businesses in Japan and America (Milano et al., 2016; Sajjadi et al., 2018; Tiwari & Kiran, 2018). In this conceptual system, Euglena, which is a single cell microalgae species, is chosen as the cultivation object.

Table 2 Properties of different microalgae species for biofuel production (Milano et al., 2016; Sajjadi et al., 2018; Tiwari & Kiran, 2018)

There are three reasons for choosing Euglena. First, even though the lipid content of Euglena is not the highest compared with other microalgae species with commercialized applications, its fuel products have the highest economic value because the algal oil extracted from Euglena is light oil that could be used to produce jet fuel, which could be sold at 5.15–6.41 dollars per gallon. Using Euglena as the cultivation stock could ensure as much economic benefits as possible. According to Table 2, the cell structure of Euglena is a cell membrane, while most microalgae species have cell wall structures. Thanks to its cell structure, it is more digestible for fish to eat the byproduct fish feed in the Euglena extracting process. Also, Euglena protein contains 59 types of nutrients, so the residual biomass can produce fish feed and provide enough nutrients. Because there is fish aquaculture infrastructure on the platform, those byproducts could be used directly on the platform. Another important reason for choosing Euglena is that Japan has many research experiences in developing biofuel of Euglena by NEDO (2015) and JST (2017). Japan has developed not only the technology to increase the lipid content of Euglena, but has also worked on the fundamental technologies for Euglena biofuel production. In addition, there was also the empirical Euglena cultivation project showing that Euglena could be cultivated at a lower cost and with less carbon.

3.4 Tuna aquaculture subsystem

The tuna aquaculture subsystem consists of cages, feeding, and harvesting facilities. The system adopted the design of Marino-Forum 21 (2013) and is the same as the original system (Duan, 2019). The floating/sinking fish preserve culturing system is used to avoid severe weather conditions in the open sea, such as typhoons and storms. When waves are expected to reach a height of more than 3 m, the system can sink below the sea surface and temporarily avoid the risk. This has an expected role in other scenarios, such as red tide avoidance, the inhibition of net adhering organisms, excessive sun exposure prevention, and theft prevention. The shape of the tuna fish cage is round, with a diameter of 50 m.

3.5 Energy infrastructure

An OTEC infrastructure is designed on the MPFP to provide sufficient electricity for all the activities and to make this system energy independent. This generating process needs to take in deep seawater that is then discharged to the sea surface; this might have positive effects on the microalgae cultivation subsystem and tuna aquaculture subsystem because there are abundant nutrients in the discharged deep seawater, which may result in ocean fertilization.

Because we have adopted the photobioreactor in the present modified system, more electricity is required in the new system, and the OTEC scale should also be reconsidered. The electricity consumption of the open raceway pond cultivation system is about 1 W/m², while the required electricity of the photobioreactor is at least 50 times more than that of the open raceway pond, so the unit energy consumption for microalgae cultivation in the modified system could be calculated as 50 W/m². The other energy estimations are shown in Table 3. When the net power availed of gross power generated is 80% and the capacity factor of OTEC is 95% (IRENA, 2014), the estimated OTEC scale of the new modified system will be 12.5 MW.

Table 3 Energy estimation and OTEC scale of the modified system

3.6 System arrangement

The layout of the modified system is shown in Fig. 4. The platform size has been reduced from 1,000 to 650 m. Similar to the original system, the modified system also consists of five main components: the floating structure, the tuna aquaculture subsystem, the microalgae cultivation subsystem, the microalgae processing subsystem, and the OTEC facility. In the center of the platform is the production and management infrastructure, including the microalgae processing facility and living area for staff. In addition, there are rescue facilities, transportation infrastructure, and energy and other utilities at the edge area, which is 50 m wide.

Fig. 4

Layout of the modified system on the deck

Meanwhile, the system outputs, including Euglena algal oil and tuna stocks, are the same as the original one. Nevertheless, there are several modified points compared with the original, aiming to cut costs and reduce the environmental impacts of the system. The differences between the two systems are shown in Table 4.

Table 4 The differences between the original system and the modified system

3.7 Site selection

The applicable area is suggested to be in the East China Sea; however, a more thorough study on site selection must be carried out to ensure a practical site. When selecting sites, the present research has focused on higher economic efficiency and feasibility. Considering basic conditions, including OTEC resource availability, microalgae cultivation conditions, and tuna aquaculture conditions, the South China Sea (SCS) could be a feasible sea area to develop an MPFP integrating OTEC, microalgae, and fish aquaculture, and the sea area around Woody Island is another potential site.

In terms of OTEC availability, the SCS has the highest energy density and richest resources of ocean thermal energy in China’s offshore and adjacent sea areas. For example, the surface water temperature near Woody Island has an average value of over 28 °C, which shows great potential for OTEC development. In addition, in a spot 9.52 km away from Woody Island, the water depth suddenly reaches more than 1,000 m. This site may be the perfect place to set a floating platform with OTEC. In terms of microalgae cultivation conditions, the present research has adopted the microalgae biomass productivity model (Soerensen & Weinstein, 2008) to estimate the annual yield of algal oil based on the climate characteristics and water resource conditions in all coastal areas of China. The regional characteristics of microalgae growth and potential for developing microalgae biodiesel in China have been analyzed, with the results showing that the theoretical microalgae oil yield in Hainan Province, which covers a large area of the SCS, is the highest. In addition, it may also be a good place for tuna aquaculture because there are abundant resources of tuna fishery in the SCS. The stable climatic conditions, well-developed basic infrastructures, and convenient transportation are also the main reasons for choosing Woody Island as the target siting for platform construction.

4 Inclusive index evaluation

4.1 Method

An improvement of the system design has been considered, and the inclusive index has been calculated to evaluate the sustainability of the original and modified MPFP. The Inclusive Impact Index “Triple-I” (III) (Otsuka et al., 2018) is adopted here. It is a metric developed to assess the environmental sustainability and economic feasibility of ocean utilization technologies, which are defined as Eq. (1), where EF, BC, ER, C, B, and HR represent ecological footprint, biocapacity, ecological risk, cost, benefit, and human risk, and α, γ, and β represent the conversion factors from ER to EF, HR to C, and from economic value to environmental value, respectively.

$${\text{III}} = \left[ {\left( {EF - BC} \right) + \alpha ER} \right] + \gamma \left[ {\left( {C - B} \right) + \beta HR} \right]$$

(1)

However, in practical use, ecological risk and human risk are calculated in terms of probability, and the accuracies are sometimes lower than the other terms. Thus, we use a simpler index “Triple-I-light” (III_light) that is defined in Eq. (2). Sustainable technologies or systems will have negative values when using III_light.

$${\text{III}}_{{{\text{light}}}} { = }\left( {EF - BC} \right) + \gamma \left( {C - B} \right)$$

(2)

The ratio of EF to GDP of Japan in 2012, 1.27 (gha/million yen), is applied as the conversion factor γ.

4.2 Ecological footprint (EF)

In the present study, the EF is based on life cycle CO₂ emissions. The amount of CO₂ emissions will be converted into the forest area, A_forest = 0.19 ha/t-CO₂, which means that 0.19 ha forest is required to absorb 1 ton of CO₂. Following this approach, the total EF (gha) of the multipurpose offshore system could be obtained by Eq. (3):

$$EF = f_{{{\text{forest}}}} A_{{{\text{forest}}}} \left( {\frac{{E_{RM} + E_{B} + E_{S} }}{n} + E_{{{\text{OTEC}}}} + E_{{{\text{Microalgae}}}} + E_{{{\text{Tuna}}}} } \right)$$

(3)

where E_RM, E_B, and E_S are the CO₂ emissions associated with raw material production, building, and installation and scrapping, respectively, while E_OTEC, E_Microalgae, and E_Tuna stand for the CO₂ emissions associated with the operation of OTEC, microalgae cultivation, and tuna aquaculture, respectively. In addition, f_forest = 1.26 gha/ha represents the equivalence factor of the forest.

The estimated EF for the original and modified systems is shown in Tables 5 and 6, respectively. In these results, n is the lifetime of the system. When the initial EF and capital cost are divided by n, the average values over each year during its lifetime are obtained. For the original system, the floating structure contributes to the high amount of CO₂ emissions and EF value. When n = 50, the EF associated with the floating structure is 1.28 × 10⁴ gha, which takes up about 70% of the total EF of the system. We can see that, in the modified system, the floating structure still contributes the most to the CO₂ emissions and EF value. At a design lifetime of 50 years, EF associated with the floating structure is 3.36 × 10³ gha, which takes up about 36% of the total EF of the system.

Table 5 The CO₂ emissions and ecological footprint of the original system (n: life year of the system)

Table 6 The CO₂ emissions and ecological footprint of the modified system (n: life year of the system)

4.3 Biocapacity (BC)

The biocapacity of the offshore multipurpose system could be accounted for by biofuel production, tuna production, and ocean fertilization effects.

The first is CO₂ emission avoidance through replacing the traditional fossil fuel by producing algal biofuel. This could be done using the annual biofuel production amount and unit CO₂ emission of fossil fuel, which is shown in Eq. (4). The estimated biofuel outputs of the original and modified systems are the same.

$$BC = f_{forest} \; \times \;A_{forest} \; \times \;Annual \;biofuel \;production\; \times \;Unit\;{\text{ CO}}_{{2}} \;emission \;of\; fossil \;fuel$$

(4)

BC changes brought about by the tuna production can be calculated using two methods. One is to use the replacement of land-based animal proteins. It needs a productive area of at least 3.8 gha to produce 1 ton of animal protein on land (Duan, 2015). However, not all tuna production is animal protein because the fish bones of tuna could not be taken as human food. Thus, the results using this method may have been slightly overestimated. Another estimation method is to use the replacement of tuna catch in natural conditions, as in Eq. (5). It is reported that 12.6 tons of CO₂ would be emitted to catch 1 ton of tuna in the fishery. Based on the data, CO₂ emission reductions could be estimated to be 30,240 tons. This result shows a less significant effect than when replacing animal protein produced on land, but it is more practical.

$$BC\, =\, f_{{{\text{forest}}}} \times A_{{{\text{forest}}}} \times {\text{Annual Tuna Production}} \times {\text{ Unit CO}}_{2} {\text{ emission to catch tuna in natural condition}}$$

(5)

The discharged deep ocean water (DOW), which is rich in nutrients, may have an ocean fertilization effect on the sea area. The method used to estimate BC changes by enhancing marine primary production follows the approach taken in the earlier study (Otsuka, 2011). The enhancement of marine primary production by artificial upwelling leads to an increase in biocapacity, and the biocapacity change could be obtained by Eqs. (6)–(8)

$$P_{P} = \alpha_{CN} \;M_{C} \;N_{DOW} \;Q_{DOW}$$

(6)

$$P_{F} = P_{P} K_{U}^{{TL_{U} }}$$

(7)

$$BC = f_{sea} \frac{{P_{F} }}{{Y_{{F_{0} }} }}$$

(8)

In these equations, the parameters are as follows:α_CN: The C/N ratio of the phytoplankton, α_CN = 106/16 (the Redfield ratio)

M_C: The atomic weight of carbon, M_C = 12 g/mol.

N_DOW: The concentration of dissolved inorganic nitrogen in DOW.

Q_DOW: Intake volume of DOW.

K_U: Ecological efficiency, K_U = 0.2

TL_U: Trophic level, TL_U = 1.5

Y_F0: The average annual fish production in the productive sea area.

Initially, the annual primary production P_P (t-C/y) is expressed by Eq. (6). Then, using the P_P value, in the last step, we obtain the annual fish production because of artificial upwelling P_F (t-C/y), which can be estimated by Eq. (7). Finally, through the multiple equivalence factor f_sea and ratio of P_F to the average annual fish production Y_F0 in the productive sea area, as in Eq. (8), the biocapacity can be obtained.

The estimated biocapacity (BC) for the original and modified systems is shown in Table 7. For the original system, the BC change brought by the tuna aquaculture is the greatest, accounting for about 58% of the total BC. This indicates the good performance of introducing fish aquaculture to the offshore MPFP. For the modified system, the OTEC and intake water amount also bring a significant increase in the ocean fertilization effects. Here, the BC change by the OTEC facility is the most, accounting for about 70% of the total BC.

Table 7 The biocapacity change of the original and modified systems

4.4 Cost (C)

The cost of the multipurpose system mainly consists of the capital cost and annual operation cost. In terms of the capital cost, the floating structure, OTEC facility, tuna aquaculture facility, and microalgae cultivation infrastructures need to be considered. The annual operation cost includes fertilizers and consumable items during microalgae cultivation operation, tuna aquaculture operation, maintenance of the platform, which could be estimated as 5% of the capital cost excluding floating structure, and the labor cost.

The floating platform consists of the mooring system and floating structure, which is constituted by the semisub center part and barge-type surrounding part.

Basically, the cost estimation methods of mooring and anchor (Castro-Santos et al., 2016) follow Eqs. (9) and (10), respectively.

$$COST_{{{\text{MOOR}}}} = \left[ {\left( {p_{{{\text{MOOR}}}} \times L_{{{\text{MOOR}}}} } \right) \times C_{{{\text{MOOR}}}} } \right] \times LP \times NA$$

(9)

$$COST_{ANC} = m_{ANC} \times C_{ANC} \times LP \times NA$$

(10)

Here, the cost of the mooring depends on the unit mass of the mooring chain p_MOOR (kg/m), the length of the mooring L_MOOR (m), the unit cost C_MOOR (yen/kg), the number of mooring lines LP, and the number of devices NA. Similar to mooring, the cost of the anchor depends on the weight of the anchor m_ANC (kg), the unit cost C_ANC (yen/kg), LP, and NA.

The cost estimation of the semisub center part of the floating platform can be obtained using the data of the previous design of Takenaka Corporation (2014), while the cost of the surrounding barge-type structure is estimated as well. The barge-type structure we adopted is the floating structure part of the 3 MW offshore wind power plant “HIBIKI.” According to the cost data of the barge-type wind power generation system in a previous paper (Suzuki, 2009), the capital cost of the barge-type offshore wind power generation system is about 350 thousand yen/kW, while the general offshore wind power generation systems cost is 500 thousand yen/kW. It is a low-cost system, and the cost of the floating structure is about 34% of the total cost. Therefore, we could estimate the cost of the floating structure part of one “HIBIKI” to be about 3.57 × 10⁸ yen. Because 134 barge-type floating structures will be used in our modified system, the cost could also be estimated. Following the above approach, the cost of the floating platform set at a water depth of 800 m is estimated. For the modified system, about 75% of the capital cost could be cut, mainly because of the area reduction and cost-cutting effect of the barge-type structure.

According to Muralidharan (2012), the relation between plant size and unit cost of OTEC follows Eq. (11). In addition, this trend line also indicates a relationship between the costs of systems at different plant sizes, as Eq. (12). Here, a = 3.99 × 10⁴, b = 0.35.

$$Unit {\text{Cos}} t = a \times Size^{ - b}$$

(11)

$${\text{Unit}}\,\, {\text{Cost}}\,\,B = {\text{Unit\,\, Cost}} \,A \times \left( {\frac{{{\text{Size}}\,\, B}}{{{\text{Size}} \,\,A}}} \right)^{ - b}$$

(12)

Therefore, using the cost data of a known previous OTEC system, the capital cost could be acquired. Because the present size is about 9 times larger than the previous one, the unit cost would be about half of it; therefore, the total capital cost is only about 4.25 times more than before.

In the modified system design, the photobioreactor cultivation system is used instead of the open raceway pond. To estimate the cost of the photobioreactor, we have used the economic data of previous research (Banerjee and S. Ramaswamy, 2019) and then used Eq. (13) (Acién et al., 2012) to acquire the cultivation costs at the size of 25 ha.

$${\text{Cost}}\;B = {\text{Cost}}\;A \left( {\frac{{{\text{Size}}\;B}}{{{\text{Size}}\;A}}} \right)^{0.85}$$

(13)

Therefore, based on the known case and this formula, the result of the photobioreactor cost is acquired. The energy consumption is not considered here as the operating cost because it will be provided independently by the OTEC system on the platform. Because we have only considered the nutrients and maintenance cost in the operation, while the total microalgae biomass is set to the same quantity as the original one, the operating cost does not change much compared with the former design.

The estimated results of the costs and benefits of the original and modified systems are shown in Tables 8 and 9, respectively. We can confirm the high cost of the floating structure, which contributes the most to the system’s initial costs.

Table 8 Estimated costs of the original system

Table 9 Estimated costs of the modified system

4.5 Benefit (B) and other items

The benefits of the original and modified systems are the same when considering the same outputs, as shown in Table 10. We can confirm the positive performance of the tuna aquaculture system because it contributes to the majority of the system’s economic benefits.

Table 10 Estimated benefits of the original and modified systems

4.6 Results and discussions

The sustainability evaluation of the original and modified systems has been conducted. The comparison of the calculated components and Triple-I indicators for the original and modified systems at the design lifetime (50 years) is shown in Table 11.

Table 11 The III_light result at the design lifetime of the original and modified systems

For the original system, the positive value indicates unsustainable performance. In addition, the annual cost outweighs the benefit, which indicates that the system is also unprofitable. Because the value of EF and costs determine the unsustainability of a system, although a floating structure contributes the most to the two indicators, it should be improved to reach higher sustainability. Figure 5 shows the lifetime dependence of III_light of the original system. For the total system, the value of III_light is always larger than 0 until the lifetime reaches 190 years, which means that the conceptual system is unsustainable when its lifetime is smaller than 190 years. Obviously, in practical use, because of equipment depreciation and technological progress, it is impossible for the system to operate so long.

Fig. 5

The lifetime dependence of III_light for the original system

For the modified system, it is sustainable at a design lifetime of 50 years. The estimated lifetime dependence of III_light for the modified system is shown in Fig. 6. The modified system becomes environmentally neutral (EF = BC) at a lifetime of 11.5 years, showing sustainability (III_light ≤ 0) at a lifetime of 20 years. Furthermore, after modification, the EF associated with the floating structure is reduced from 70 to 26% of the total EF, and the costs of the floating structure are reduced from 98 to 24% of the total cost. The results show a significant improvement in the sustainability of this conceptual system. The high-yield microalgae cultivation system and low-cost floating structure significantly reduce the EF and cost, and the biocapacity is increased largely by the enlarged OTEC scale.

Fig. 6

The lifetime dependence of III_light for the modified system

However, the annual costs are still slightly higher than the benefits, meaning that the system is unprofitable as well. Some measures relating to the product output should be considered to improve economic performance, such as producing other microalgae products with a high value.

In addition, in the evaluation of the modified system, the BC change by the ocean fertilization effects of DOW accounts for about 70% of the total BC and has a large influence on the Triple-I result of the system. Nevertheless, this term has been calculated following the method of precious research, which only provided a simple estimation that assumed a perfect condition, so the real situation might not be so ideal. Therefore, a more precise and reliable estimation should be introduced to this part to verify the biocapacity change by ocean fertilization effects and avoid overestimation.

5 Conclusion

The present research has improved the conceptual system design of an offshore MPFP and selected a proper site for it before then evaluating the economic and environmental sustainability of the original and modified systems using the inclusive index and ensuring the sustainability of the system modification. The conceptual system design is based on a modular floating structure with a design lifetime of 50 years. Tuna aquaculture, microalgae cultivation and processing, and OTEC energy infrastructure are integrated here for profitable applications in the SCS, where 90% of China’s ocean thermal energy resource is distributed and the microalgae growth rate is high. Because of the high cost and large raw material requirements of large-scale floating structures, however, the inclusive performance of the previous system is relatively poor. Therefore, we have adopted a high-yield photobioreactor microalgae cultivation system, instead of the open pond system, and a low-cost barge-type floating structure, instead of a semisub structure, to reduce the required area and cost of the floating structure and improve the sustainability of the system. The Triple-I-light (III_light) has been calculated to evaluate the environmental sustainability and economic feasibility of the floating system. The results show that the new system becomes environmentally neutral (EF = BC) at a lifetime of 11.5 years and shows sustainability (III_light ≤ 0) at a lifetime of 20 years.

The utilization of marine resources, such as renewable energy and foods, is expected to significantly contribute to the realization of a sustainable society. Although MFPF is one of the promising approaches for effective utilization of marine resources, the sustainability of the system depends a great deal on the design. The combination of OTEC, microalgae production, and aquaculture could provide good economic performance. From the viewpoint of seafood supply, whose demand is globally increasing, realizing a sustainable offshore aquaculture system is also significant. The critical issues for large-scale aquaculture are costs and environmental impacts because of feed and energy supply. The proposed system can produce fish with no external energy or feed supply. An autonomous system, such as the one proposed here, is considered very effective when it comes to utilizing the ocean and contributing to a sustainable society.

However, the annual costs are still larger than the benefits of the modified system, which indicate that the system is still not profitable; therefore, some other measures relating to the economic profits should be considered in the future if better economic performance is expected. Because there are few commercialized cases of the offshore multipurpose system for reference, the veracity of some items in the cost estimation still can be verified and improved.