Design Improvement and Sustainability Assessmentof a Conceptual Multi-purpose Offshore Platform

Peng Yixing 47216641
Department of Ocean Technology, Policy, and Environment
Supervisor: Shigeru TABETA

1. Introduction
2. Improvement of Multi-purpose
Offshore Platform

CONTENTS

3. Inclusive Index Evaluation
4. Assessment Results
5. Comparison with conventional
aquaculture system
6. Conclusion

1 Introduction

Introduction: Research background
Ocean Thermal Energy Conversion (OTEC)

OTEC plant situatedon a semi-subplatform(Lockheed, 2014)

Multi-purpose offshore platforms
TROPOS (EU-FP7project, 2012-2015)
Integrated a range offunctions from thetransport, energy (OTEC),
aquaculture and leisuresectors.

Japan's self-sufficiency rate for edible
seafoodIntroduction: Previous study
Inclusive Impact Indicator (III) (IMPACT Research Group, 2006)
A metric developed to assess environmental sustainability and economic feasibility of ocean utilization
technologies.

!!!!"#$% = ($% − '() + +(( − ')!!!!"#$%∗$% + +(='( + +'EF: Ecological Footprint
BC: Biocapacity
C: Cost
B: Benefit
γ: Environmental Economic coefficient（EFregional/GDPregional）

IIIlight <0: SustainableIIIlight >0: Unsustainable
IIIlight* <1: Sustainable
IIIlight* >1: UnsustainableIntroduction: Previous study
System component• This system can develop marine
renewable energy for productive activities.
• The platform not dependent on electricity
and energy from the land.
• Microalgae cultivation can help reduce
CO2 emissions.
• It can increase the production ofaquaculture fish and improve the rate offish self-sufficiency in Japan.
• But the system is unsustainable.

Conceptual Multi-purpose Offshore Platform (Duan, 2019)

Introduction: Previous study
The differences between the original system and the modified system (Chen, 2020)
Platform size
Microalgae cultivation system
Floating structure
OTEC scale

The original system
1000m×1000m
Open raceway pond
Semi-sub type2MW

The modified system
650m×650m
Photobioreactor
Barge type (surrounding)Semi-sub (center)
12.5MW• Chen reduced the cost and the impact of the system on the environment.
• The system became sustainable with modifications, but it is unprofitable.
• In the inclusive index, changes in Biocapacity due to ocean fertilization may beoverestimated, and only the environmental impact of CO2 emissions is considered.Introduction: Previous study
The operation days and rate of OTEC
platform in the target regions• In subtropical areas, the seasonal surface sea
water temperature limits the operation days of thesystem, thus affecting its economic feasibility.
• If the platform is implemented in the Ogasawara
sea area, it will increase employment
opportunities in the Ogasawara Islands andimprove the utilization of Japanese marine
resources.

Site assessment of multi-purpose offshore platform with OTEC
(Ding, 2022)

Introduction: Objectives• Improve the conceptual system design of the offshore multi-purpose platform
to increase the economic benefit and ecological value of the multi-purpose
platform;• Consider more reasonable evaluations for Ecological Footprint andBiocapacity in the inclusive index;• Assess economic and environmental sustainability using the inclusive index.

2 Improvement of Multi-purpose
Offshore Platform

Improvement of Multi-purpose Offshore Platform: System
Production process in original system
Fish feed

Fish stock

Microalgae
Algal oil
Input

OutputPrice

Benefit1000 kg Microalgae

285kg Algal oil

74 Yen/kg (Duan, 2015)

21090 Yen

1000 kg Microalgae

513kg Sea bream

1500~2000 Yen/kg

769500~1026000 Yen(SAKAMA.inc)• Replacing biofuel production with fish feed production will generate more fish production, resulting inincreased economic benefits.
• Feed conversion ratio of tuna: 7~10 (Chen, 2020)

Feed conversion ratio of sea bream: 2.5 (Kallitsis, 2020)• The feed conversion ratio of seabream is lower than that of tuna. So changing aquaculture fish to sea
bream can increase fish production, resulting in the platform generating more economic benefits.

Improvement of Multi-purpose Offshore Platform: System
Modified system• Removing the algae oil production partallows more fish feed to be produced frommicroalgae, resulting in increased
aquaculture production.
• The aquaculture fish is changed from tunato sea bream.
• A component for hydrogen production fromexcess electricity is added.
• These improvements will create more
economic benefits.

Improvement of Multi-purpose Offshore Platform: System
Nutritional content of feed ingredient

Proximate composition of the no fishmeal
feed for sea bream

Study on the effectiveness of fishmeal-reduced
feeds for sea bream (Matsukura, 2016)
Potential of microalgae as a sustainable feed ingredient foraquaculture (Nagappan, 2021)• The nutritional composition of Euglena is very similar to that of the no fishmeal feeds forsea bream.
• With the addition of only some water, Euglena can be used entirely to produce fish feed.
• Fish feed production will increase, leading to an increase in aquaculture production.

Improvement of Multi-purpose Offshore Platform: Energy
Electricity consumption of the multi-purpose platform

Electricity consumption for microalgae cultivation 12500 kW
Electricity consumption for feed production
78.2 kW/t-feedElectricity consumption for fish aquaculture1060 kW/t-fish
Heat consumption
3.1 MJ/kg-algae
Electricity consumption for the residence of staffs 0.16 GWh• The minimum surface watertemperature in the Ogasawara
Islands is 20°C during the year
(Ding, 2022).

• At a surface water temperatureof 20°C, the 12.5MW OTEC
generation power is about
12.5MW OTEC annual electricity generation change in Ogasawara sea area
6800kw (Okinawa Prefectural
19600kw
30
Electricity
Government, 2014).
Generated electricity
demand• When the surface seawater
Excess electricity
temperature rises in summer,
12500kw
25the platform will generateElectricity shortage
excess electricity can be used
to produce hydrogen.
6800kw20161121181

241301361

Improvement of Multi-purpose Offshore Platform: Energy
Plan 1：Multi-energy complementary system with 6×2MW wind turbines*
Layout of the multi-purpose platform systemwith 6×2MW wind turbines
Electricity consumption of the systemwith 6×2MW wind turbines
Annual total electricity consumption

128 GWhOTEC Scale

12.5 MWWind turbines (6 units)

12 MW

Total annual electricity generation

155 GWh

Annual excess electricity

27 GWh* Vestas V80 2MW wind turbine (Noori, 2015)

Improvement of Multi-purpose Offshore Platform: Energy
Plan 2: Increase in the OTEC scale
Layout of the multi-purpose platform systemwith 23MW OTECThe OTEC scale must be increased toElectricity consumption of the systemwith 23MW OTEC

23MW to support the platform

Annual total electricity consumption

128 GWh

operation at 20°C surface seawater

OTEC Scale

23 MW

temperature.

Total annual electricity generation

189 GWh

Annual excess electricity

61 GWhImprovement of Multi-purpose Offshore Platform: Energy
Plan 3: Enlargement of the platform size
Layout of the multi-purpose platform systemwith enlarged platform size
Electricity consumption of the systemwith enlarged platform size
Basic electricity consumption

26250 kW

Annual total electricity consumption

258 GWhOTEC Scale

50 MW

Total annual electricity generation

410 GWh

Annual excess electricity

152 GWhImprovement of Multi-purpose Offshore Platform: Productions
The production scale of modified systems

Production scale of the original system

System with 6×2MW
wind turbines

Items

System with 23MW
OTEC

System with enlarged
platform size

Items

ValueUnits

Days of operation

266d/yFish feed

24000

t/y

Feed conversionratio (Tuna)

10

Fish stock (Tuna)

2400

t/y

Algal oil

1963t/yFish stock(Sea bream)

Benefit of Algaloil

145

M Yen

Hydrogen482 t/y1089 t/y2714 t/yBenefit of Tuna

6480

M Yen

Benefits of Hydrogen

578.4 M Yen

1306.8 M Yen

2356.8 M Yen

Days of operation

360 d/yFish feed

16467 t/y

25806 t/yFeed conversion ratio(Sea bream)

2.56587 t/yBenefits of Sea bream

10322 t/y

9880.5~13174 M Yen

15483~20644 M Yen

Original System

System with 6×2MW
wind turbines

System with 23MW
OTEC

System with enlarged
platform size

Total Cost (Million Yen)

8215

8489~8531

5662~8731

11024~11196

Total Benfit (Million Yen)

6625

10458.9~13752.4

11187.3~14480.8

18739.8~23900.8

Modified platforms are all profitable compared to the original platform.

3 Inclusive Index Evaluation

Inclusive Index Evaluation
Triple I: !!!!"#$% = ($% − '() + +(( − ')!!!!"#$%∗$% + +(='( + +'Ecological Footprint in the system,:;<=>% $?@ + $A + $B$% =(+ $CDEF +$G"HI + $@"J<;K!#K= +$L">$ ) + $%G-:;<=>%.
fforest: Equivalence factor for forest area, fforest = 1.26 gha/ha
Aforest: Estimated CO2 emissions are converted into a forest area, Aforest = 5.2 t-CO2/ha/year
ERM: CO2 emissions associated with raw-material production and transportation
EB: CO2 emissions associated with building
ES: CO2 emissions associated with scrapping
EOTEC: CO2 emissions associated with operation of OTEC
EWind: CO2 emissions associated with operation of wind turbines
EMicroalgae: CO2 emissions associated with operation of microalgae cultivation and processing
EFish: CO2 emissions associated with operation of fish aquaculture
EFW : Ecological footprint of aquaculture waste treatment
n: Number of years of the platform operation

Inclusive Index Evaluation
Ecological footprint of aquaculture waste treatment
One of the significant environmental impacts of aquaculture is the discharge of organic matter andnutrients. In coastal aquaculture systems, the ecological footprint of aquaculture waste treatment canbe calculated as:$%G = ,>=K ×

Carbon content in aquaculture waste?N!"#$%#&

fsea: Equivalence factor for marine area, fsea = 0.37 gha/haM!!"#$%#& : Primary productivity in the coastal area, M!!"#$%#& = 100 (g-C/m2/year)
Since the multi-purpose platform is an offshore aquaculture system, these aquaculture wastes areprobably carried by ocean currents. But the portion that is carried away is difficult to quantify. So it isassumed to be between zero and the ecological footprint of coastal aquaculture waste treatment.

Inclusive Index Evaluation
Biocapacity (BC) changes in the system
1. CO2 emissions avoidance through replacing the fossil fuel by producing hydrogen'(@"J<;K!#K= = ,:;<=>% ×-:;<=>% ×-..@AB 'CD,@EB ?FDG@HICD.×
Unit CO2 Emission of Fossil Fuel
2. Fish production that could replace the piscatorial sea bream catch'(L">$ = ,:;<=>% ×-:;<=>% ×-..@AB %CJℎ ?FDG@HICD.× Unit
CO2 emission to catch fish in natural conditions

Inclusive Index Evaluation
Biocapacity (BC) changes in the system
3. Enhancement of marine primary production (ocean fertilization) by nutrients supply in DOW
It can be assumed that all nutrients from deep
ocean water will lead to increases in primaryproduction wherever it is transported. The BCchanges can be calculated as:!( = #)* $) %+,- &+,!. = !( ' /0().123454673489 = *:17!.
+.(Assuming that fish productivity near OTEC issimilar to that in the upwelling area, theecological efficiency K is 0.2 and the trophic
level TL is 1.5. (Otsuka, 2008)
However, the nutrients brought by deep seawater
may be carried away by ocean currents, so thismethod may be overestimated.

Then assuming that the increased nutrients
contribute to only the primary production (ignore
the consequent effects on fish productivity), thenBC changes can be calculated as().123454673489 = *:17!(!()*+,-+.

The actual BC value is between these two results.
PP: The annual primary production PP (t-C/y)αCN: The C/N ratio of the phytoplankton, αCN=106/16 (the Redfield ratio)
Mc: The atomic weight of carbon, Mc = 12 g/mol
NDOW: The concentration of dissolved inorganic nitrogen in DOW (μM/ m3)
QDOW: Intake volume of DOW (m3/y);
PF: The annual fish production due to artificial upwelling (t-C/y)
YF0: The average annual fish production in the productive sea area''!"#$%#& : Primary productivity in the coastal area, ''!"#$%#& = 100 (g-C/m2/year)4 Assessment Results

Assessment Results
Calculated annual components of worst Triple I
25000

20000

15000

10000

5000

0-5000-10000-15000-20000-20000 -15000 -10000-5000
0
γ·CmaxEF maxγ·BminBC min

5000

10000

15000

20000

System with enlarged platform size
System with 23MW OTEC
System with 6×2MW wind turbines
Original system-25000

Original system
System with 6×2MW wind turbines

EF(gha)
11482
10645

BC(gha)878813129

Cost(M Yen)
8215
8531

Benefit(M Yen)
6625
10458.9

IIIlight
(gha)
4713
-4786

System with 23MW OTEC
System with enlarged platform size

10590
15520

13582
22034

8731
11196

11187.3
18739.8-6112-16095

IIIlight*
1.27
0.82
0.78
0.65

Assessment Results
Calculated annual componentsof optimal Triple I
80000 70000 60000 50000

40000

30000

20000

10000

0-10000 -20000-80000 -70000 -60000 -50000 -40000 -30000 -20000 -10000
γ·CminEF minγ·BmaxBC max010000

System with enlarged platform size
System with 23MW OTEC
System with 6×2MW wind turbines
Original system

Original system
System with 6×2MW wind turbines
System with 23MW OTEC
System with enlarged platform size

20000

EF(gha)BC(gha)Cost(M Yen)

Benefit(M Yen)

IIIlight(gha)IIIlight*

8427
8698
8640
12464

21762
26103
37452
73926

8215
8489
8662
11024

6625
13752.414480.823900.8-11315-24092-36202-77815

0.63
0.45
0.35
0.25

Assessment Results
Multi-energy complementary system with 6×2MW wind turbines
Lifetime dependence of maximal Triple I

Lifetime dependence of minimal Triple I5 Comparison with conventional
aquaculture system

Comparison with conventional aquaculture system
Calculated annual composition of Triple I for conventional aquaculture system, which produces the samesea bream production as the multi-purpose system with 23MW OTEC (Kallitsis, 2020)&(Kitazawa, 2008)
Term
Ecological Footprint

SourceTotal (EF)Fish aquaculture facility
Production of feed
Electricity
PackagingTransportation
Treatment of aquaculture waste

Biocapacity

Replacing the same production in fishery

Cost

Total (C)Fish aquaculture facility
Transportation
Feed
Electricity
MaintenanceLabor
Total (B)

BenefitEstimation/y51660(gha)712 (gha)/n48836 (gha)1112 (gha)702 (gha)
225 (gha)
1950(gha)12769(gha)4753(106 Yen)
17144(106 Yen)/n25 (106 Yen)
3112 (106 Yen)
215(106 Yen)
875 (106 Yen)
200(106 Yen)
13174(106 Yen)

Assessment Analysis
Comparison with conventional aquaculture system
Multi-purpose system with 23MW OTEC

Conventional aquaculture system

5050

Fish 6578t/y Hydrogen 1089t/y

Fish 6578t/y

Ecological Footprint (gha)8640~10590

51660

Biocapacity (gha)13582~37452

12769

Cost (Million Yen)8662~8731

4753

Benefit (Million Yen)

11187.3~14480.8

9880.5~13174

III light (gha)-6112~-36202

28196~32379

0.35~0.78

1.96~2.28

Lifetime (year)
Production

III light *• Conventional aquaculture system is more profitable (larger B-C), but the multi-purpose system
has a significant advantage in environmental impact.
• Because of the high initial investment cost of the multi-purpose system, a long lifetime is required
for the advantage to be realized.
• At least 16 years of lifetime in the worst case is required for the multi-purpose system to be better
than the conventional aquaculture system, while only 7 years is required in the optimal case.

6 Conclusion

Conclusion• Improved the system design of the offshore multi-purpose platform.
• Proposed a new calculation method of biocapacity caused by ocean fertilization andenvironmental impacts other than CO2 emissions.
• Assessed the sustainability performance of the original and modified systems using theInclusive Impact Index and confirmed the improved sustainability and economy of themodified systems.
• Compared the offshore multi-purpose system with conventional aquaculture systemand demonstrated the advantages of the multi-purpose system in the long lifetime.

Thanks for your attention!