Floating wind has four main platform types: semi-submersible, spar, tension-leg platform (TLP) and barge. About 80% of adopted and planned projects use semi-subs, yet Japan’s only commercial floating farm, Goto, is a spar. What separates them is the water depth where the turbine goes on, and the port behind it. Yard, port and lending plans all depend on that choice.
The deepest operating floating farm, Hywind Tampen, is a spar, and so is Goto. Japan’s next two demonstrators chose semi-subs.
A semi-sub can take its turbine in a port about 15 m deep. A spar needs deep, calm water, and at 15 MW its draft exceeds 100 m. One Japanese developer group argues such sea areas are scarce in Japan and Asia.
NEDO’s cost model puts semi-sub CAPEX at 1.2 times spar CAPEX. Japan’s port study estimates 19 to 32 hectares for a floating base port, against the 8 hectares most base ports offer.
👉 Background: floating offshore wind in Japan
The Four Types: How Each Stays Stable and Where the Turbine Is Installed
A floating platform has to keep the turbine working while wind and waves move it. Each type manages motion differently, and each sets a different installation sequence.
| Type (concepts at TRL 6 or above) | Stability | Where the turbine goes on | Examples |
|---|---|---|---|
| Semi-submersible (8) | Several columns spread across the water surface | At the quay, then towed out complete | WindFloat Atlantic, Kincardine. In Japan, the Fukushima demonstrations (removed) |
| Spar (4) | Long cylinder with ballast low down | In deep, calm water after upending | Hywind Scotland, Hywind Tampen, Goto |
| TLP (1) | Taut tendons pull the hull down against its buoyancy | Varies by project, with precise tendon work on the seabed | Provence Grand Large. In Japan, under development by Obayashi |
| Barge (2) | Wide, flat hull | At the quay, then towed out complete | Floatgen, Hibiki (NEDO demonstration) |
The counts come from a UK assessment by OpenWater Renewables (June 2026). TRL is a technology readiness scale, and TRL 6 means a machine of at least 1 MW is operating offshore. Examples come mainly from a Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT) paper.
Semi-submersible: built and fitted out in port
Columns and submerged members form a wide structure. Its shallow draft lets the turbine go on at the quay, and the finished unit is towed to site. The trade-off is a large, complex hull with more steel.
Spar: the type Japan actually operates
A long cylinder carries ballast low down, which keeps the center of gravity low. The structure is simple, but the draft is deep. The hull travels horizontally, is upended in deep and calm water, and takes its turbine there.
TLP: a small footprint on the seabed
Tendons under tension connect the hull to the seabed and keep vertical motion small. Because the lines do not spread, the occupied sea area stays small. Tendon installation and control demand high precision.
Barge: simple to build, harder to keep still
A flat-bottomed, wide hull is easy to fabricate with ordinary shipbuilding equipment. It relies on its waterplane for stability, so waves affect it more, and it needs measures to limit motion.
Why Semi-Subs Hold About 80%: The Turbine Can Go On in 15 m of Water
A September 2025 NEDO paper says about 80% of adopted and planned projects use semi-subs, because the type is the most technically mature. It also says their cost disadvantages make it hard to say they will stay dominant.
The clearest explanation of Japan’s choice comes from Marubeni Offshore Wind Development, which leads the Akita demonstrator. Its business strategy paper sees most of the resource in deep water: about 1,000 GW in Japan at 200 to 1,000 m, against about 400 GW at 100 to 200 m and about 200 GW at 50 to 100 m, assuming wind speeds of 7 m/s or more.
Turbines, however, are basically installed in port, at about 15 m. The same paper says calm sea areas deeper than 100 m are limited, and more limited in Japan and Asia than in Europe, which has fjords. That is why the group focuses on semi-subs. This is a developer’s view, and the Japanese government has not selected a type.
Both demonstrators in the Green Innovation Fund’s second phase use semi-subs:
- South Akita: about 400 m deep, two 12 to 15 MW turbines, with operation planned for October 2029.
- Aichi: one 12 to 15 MW turbine on a steel and concrete hybrid semi-sub.
MLIT’s port study also uses a base case of 200 m depth, 1 GW, 15 MW turbines and a steel semi-sub, citing the global trend. It treats the other types as variants.
👉 Japan’s Floating Offshore Wind Demonstration Phase 2
Japan’s Only Commercial Farm Is a Spar: What 16.8 MW at Goto Shows
Goto started operating on 5 January 2026 on a hybrid spar with a steel upper section and a concrete lower section. It has eight 2.1 MW turbines, a 76 m draft and a site depth of about 100 m.
The developer’s plan set out this sequence:
- The hull is built lying on its side at Fukue port, which spreads the weight on a quay with limited bearing capacity.
- Ironworks and shipyards in Nagasaki Prefecture make the steel sections.
- The hull is carried to calm water north of Kabashima, upended, and fitted with the turbine there.
- The finished unit is towed to site and connected to mooring lines already laid.
Local yards and a nearby calm sea area made this work. NEDO’s report says a spar’s draft passes 100 m at 15 MW. In our view, the Goto method cannot simply be scaled to large turbines. Finding calm water deep enough becomes the next condition.
👉 Goto City Floating Offshore Wind Project
Cost and Ports: 1.2 Times the CAPEX, and 19 to 32 Hectares
Easier installation has a price. In NEDO’s 2024 cost model, a baseline floating farm costs JPY 688,000/kW on semi-subs and JPY 570,000/kW on spars, a ratio of 1.2. These are model values at the 2023 exchange rate of JPY 174 to the pound, and they are not comparable with auction prices.
Ports are affected too. MLIT’s study estimates a floating base port needs 19 to 32 hectares, depending on conditions. Most designated base ports offer 8 hectares, and the ministry is studying how several ports could share the work.
👉 Japan’s Floating Wind Base Ports Cannot Work Alone
The UK assessment puts the spread at about 15% across the types it compares. With a concrete barge at 100, a concrete semi-sub sits at 100 and a steel semi-sub at 105 to 115. That is an approximate figure for a hypothetical North Sea project, spars are not in the comparison, and it cannot be applied to Japanese costs. In our view, the gap moves with conditions, so price alone cannot yet settle the choice of type.
Maturity: 117 Concepts Tracked, None at TRL 8
The OpenWater assessment, commissioned by the UK’s Technology Leadership Board, is useful because it applies one method across the field. In February 2026 its database held 117 floating concepts and six hybrids, which are bottom-fixed designs adapted for deep water.
OpenWater defines TRL 8 and 9 against a commercial wind farm of 100 MW or more. On that definition, nothing has reached TRL 8. The highest level is TRL 7. Most concepts sit at TRL 3 or 4, 27 are at TRL 5, and 15 have reached TRL 6 or above.
A separate list of 15 concepts was shortlisted for a hypothetical North Sea project (100 to 150 m, 15 MW turbines, 750 MW). Only three of those are at TRL 6 or 7. The report recommends choosing a concept at TRL 7 or above by final investment decision. These are OpenWater’s own conclusions, and it is not an official industry rating.
Japan’s position stands out. Three of the 15 concepts at TRL 6 or above came from the Fukushima demonstration: JMU’s Advanced Spar, Mitsui’s Compact Semi-sub and MHI’s V-shaped Semi-sub. All have been removed and their development has ended. Japan holds part of the world’s demonstrated experience, but no longer in equipment that is running.
Lenders test the debt service coverage ratio (DSCR), which shows how many times cash flow covers debt payments in the tightest year. That rests on the energy yield forecast. In our view, a type without large-turbine operating data has to carry a wider range in that forecast. Goto’s record is a 2.1 MW spar, which does not directly support a 15 MW semi-sub. When weighing a type, check which machine and which sea area the data comes from.
A related constraint is often misread. The assessment holds hybrids to about 80 m because no jack-up vessel can install a turbine deeper. Floating units are towed out, so that ceiling does not apply to them.
The Next Candidates in Japan: TLP, Concrete and Offshore Assembly
Japanese development is also targeting the semi-sub’s weak points. NEDO’s five next-generation projects, announced in August 2026, include:
- TLP: in May 2026, Obayashi received Approval in Principle from ClassNK for a TLP support structure with a steel and concrete hybrid design. It estimates floater construction cost 25% below a steel semi-sub. The next goal is a sea trial with a 1 MW class turbine.
- Concrete floaters: Taisei and partners will test fabrication for the 15 MW class.
- Offshore turbine work: TEPCO Renewable Power and partners will test installing and repairing turbines without disconnecting the moorings.
Offshore construction methods are moving on a separate track. MLIT selected seven research projects in July 2026, which shows the installation layer needs as much preparation as the hull.
👉 Japan Funds Floating Wind Installation R&D: Seven MLIT Projects
Mooring costs are covered in a separate article.
👉 Floating Offshore Wind Mooring Costs: What a 50% Load Reduction Actually Moves
Choosing a platform means choosing which port, which sea area and which yard the project will depend on.
The semi-sub reconciles deep sites with shallow ports, at the price of a large hull and a large yard. The spar may cost less to build but needs deep, calm water. The TLP limits the occupied area but still has its record to establish. None of them wins until the site and the port are specified.
Japan has picked semi-subs for its next demonstrators, while its commercial record is in a spar. No operating data yet connects the two, and South Akita is not due to start until October 2029. Consider what the project can show lenders about large turbines, and exercise caution if a plan fixes a type before securing a port that suits it. As MLIT’s port study takes shape, the real cost gap between types should become easier to read.
Related DeepWind Articles
- Floating Offshore Wind in Japan: Market Structure, Costs, and Policy Framework
- Japan’s Floating Wind Base Ports Cannot Work Alone
- From Demonstration to Commercialization: Floating Offshore Wind Case Studies
- Post-2030 Floating Offshore Wind: Technology Trends and Market Outlook
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