Semi-Submersible Feasibility Study

What would it take to build a semi-submersible seastead community?

One promising seastead design based in existing technology is the semi-submersible – a very stable floating platform, most often used in the offshore drilling industry. Our engineering team, led by George Petrie, set out to determine the feasibility of this design for an early seastead platform in terms of costs and logistics, while factoring in the necessary amenities to sustain a small residential and commercial community. The study is part of our Floating City Project, but is not the only design under consideration. We are also partnering with Dutch aquatic urban design firm, DeltaSync, to engineer a novel architectural seastead design. We will release the design in the Fall of 2013.

The top-side residential and commercial space was designed by naval architect Lina Suarez, whose full report can be viewed here.



In order to produce realistic estimates for the seastead – the semi-submersible hull and deck structure – our director of engineering George Petrie rendered a hull design based on industry standards, and submitted it for bidding to several shipyards. As of May 2013, only one shipyard – located in Texas – returned a bid. While the figure quoted represents the single largest expense for a semi-submersible seastead, there were numerous other costs which needed to be estimated based on educated estimates, previous research, and quotes from maritime professionals in our network. Estimates for the structure and basic systems were based on existing technologies, at current market rates and employing standard installation protocols used in offshore industries.

Simultaneously, Petrie worked with a Naval Architecture intern, Lina Suarez, to produce a novel top-side design for residential and commercial units, which can be rearranged to fit the Institute’s vision of a modular seastead. In turn, the cost of these units was estimated to produce full plans and cost figures for a complete seasteading community. The specifications, estimates and visuals for the platform are one part of our Floating City Project, and are available to anyone looking to actualize this vision through collaboration with the Institute, potential customers, and outside sources of financing.


General Specifications

Number of units180Residential square footage105,600
Average residents per unit2Commercial square footage125,200
Total population360Common Area:
Number of decks5• Roof top area (sq. ft.)22,040
Number of cranes2• Floating pier (sq. ft.)17,500

Overall CapEx Estimate

Component               Cost Component              Cost
Hull and Deck Structure$115,492,000 Corner towers, common areas$1,250,000
Transport to Caribbean site$2,000,000 Roof top areas$2,750,000
Anchor Handling Tugs$560,000 Helideck$1,000,000
Fuel for Anchor Handling Tugs$140,000 Floating Pier$2,625,000
   Revolving Cranes$1,500,000
Mooring System[1]$15,000,000
Fuel System$1,000,000Sub-Total$186,597,000
Ballast System$1,000,000   
Fresh Water System$1,000,000Engineering 
Waste Treatment System$1,000,000 Pre-FEED$559,791
Navigation & Communication$1,000,000 FEED$1,865,970
Lifeboats & Lifesaving Equipment$1,000,000 Construction Support$5,597,910
Safety & Firefighting Systems[2]$1,000,000 Project Management$7,463,880
HVAC System$1,000,000 Contingency$18,659,700
Residential modules[3]$17,280,000 Supply/Transport Boat (optional)[4]$5,000,000
Deckhouse structure$15,000,000   
Gensets (two @ 1000-kW each)$1,000,000Total$225,744,251 
Cooling System (FW & SW)$1,000,000 
Power Distribution and Control$1,000,000 Per Sq. Ft. of interior space$978

[1] From Parametric Study p. 28

[2] Eight (8) 120-person enclosed lifeboats, $125,000 each, from

[3] 180 duplex modules at $96,000 each

[4] Capacity ~ 20 people, speed ~ 30 knots, docked at platform

Overall OpEx Estimate

ItemCapital CostAnnual % Maintenance CostAnnual $ Maintenance CostService Life (yrs)Sinking FundInterest RateOverride Default RateAnnualized  Cost
Maintenance & Inspection
Hull and Deck Structure$100,000,0001%$1,000,00040$386,0168%8%$1,386,016
Mooring system$15,000,0001%$150,00020$453,6395%$603,639
Fuel System$1,000,0003%$30,00020$30,2435%$60,243
Ballast System$1,000,0003%$30,00020$30,2435%$60,243
Fresh Water System$1,000,0003%$30,00020$30,2435%$60,243
Waste Treatment System$1,000,0003%$30,00020$30,2435%$60,243
Navigation & Communication$1,000,0001%$10,00020$30,2435%$40,243
Lifeboats & Lifesaving Equipment$1,000,0001%$10,00020$30,2435%$40,243
Safety & Firefighting Systems$1,000,0003%$30,00020$30,2435%$60,243
HVAC System$1,000,0003%$30,00020$30,2435%$60,243
Residential modules$17,280,0003%$518,40040$143,0475%$661,447
Deckhouse structure$15,000,0001%$150,00040$124,1725%$274,172
Gensets (two @ 1000-kW each)$1,000,0005%$50,00020$30,2435%$80,243
Cooling System (FW & SW)$1,000,0003%$30,00040$8,2785%$38,278
Power Distribution and Control$1,000,0001%$10,00040$8,2785%$18,278
Corner towers, common areas$1,250,0001%$12,50040$10,3485%$22,848
Roof top areas$2,750,0001%$27,50040$22,7655%$50,265
Floating Pier$2,625,0005%$131,25020$79,3875%$210,637
Revolving cranes$1,500,0001%$15,00040$12,4175%$27,417
Supply/Transport Boat (optional)$5,000,0005%$250,00020$151,2135%$401,213

Recurring Costs
Insurance1% of total construction$186,597,000$1,865,970
Licensed officers – 1st Officer2$48,000salary$96,000
Non-licensed crew – Bosun2$24,000salary$48,000
Non-licensed crew – Seaman4$18,000salary$72,000
Supply Boat – Charter -Trips per year  =>26$6,000cost/day1day/trip$156,000

Fuel Costs
Diesel Generators – Avg power, kW  =>5000.072gal/kW-hr8,760hours/year$4.00per gallon$1,261,440
Supply Boat – Charter -Trips per year  =>263,000gal/day1day/trip$3.00per gallon$234,000
Supply/Transport Boat (optional)104120gal/hour6hours/trip$3.50per gallon$262,080

Total square footage: 230,800

Annual maintenance per square foot: $36.22

Average apartment size: 640 sq. ft.

Monthly maintenance per unit: $1,932

Annual maintenance per unit: $23,181

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Recruiting Collaborators

If you believe you can help The Seasteading Institute actualize these plans, or provide us with a lower bid on any of the semi-submersible components, we would love to hear from you.