top of page

Buoyant Production Technology's
Floating Offshore Substation (FOSS)

This whitepaper outlines the key challenges and technology developments that need to be tackled to unlock the potential of commercial scale floating offshore wind (FLOW). It also outlines a key technical solution developed by Buoyant Production Technologies (BPT) that has the potential to close the gap between small scale FLOW demonstrator projects and commercial scale FLOW developments.

BPT have developed a floating offshore substation (FOSS) design with a particular focus on the UK market. FOSS have been identified as being crucial technology that requires development unlock the potential of commercial scale FLOW projects around the globe.


It is clear that a global energy transition is required to achieve a net-zero energy system by 2050 to limit global warming to 1.5 degrees. Achieving a net zero future is more difficult than ever with recent economic, energy security and supply chain challenges increasing the risk to investors and policy makers. However, it is clear that major players across the industry are continuing to push the energy transition forwards.

DNV predict that by 2050 82% of the world’s grid connected electricity will be generated from renewable sources, with 69% from variable renewables, including 33% from wind. Floating offshore wind is a rapidly developing market sector that will contribute greatly to the energy mix of the future (1). 

Floating Offshore Wind Opportunity

It is expected that FLOW will not reach commercialisation until 2030, however after this point the International Energy Agency’s (IEA) roadmap predicts that 70GW will be deployed each year between 2031 and 2050.

Floating offshore wind (FLOW) provides enormous opportunities due to the access to abundant wind resources in deep water and the increased flexibility in site selection. 80% of the world’s offshore wind resource potential lies in waters deeper than 60m. In many regions around the world even locations near to shore are in deep water making fixed bottom installations an unrealistic option. 


FLOW is anticipated to generate 15% of all offshore wind energy by 2050 contributing 264 GW globally. The global FLOW pipeline has doubled in the last 12 months, topping 240GW according to the Global Wind Energy Council (GWEC) (2).


UK Floating Offshore Wind Sector  

FLOW will play a crucial role in decarbonising the UK energy mix; ensuring the UK meets its energy security and net zero targets. 

The UK has the greatest FLOW project pipeline anywhere in the world (37GW) and the UK government has set a target of installing 5GW of floating wind capacity by 2030. The UK is also home to the world’s first commercial FLOW windfarm, Hywind. The recent ScotWind leasing round alone involved 14 floating project with the potential to produce 17 GW of clean energy.

The latest report by the Floating Offshore Wind Taskforce (3) contained a series of recommendations to upgrade UK port infrastructure that if implemented could unlock the opportunity to see 34GW of floating offshore wind installed by 2040. In this scenario the sector could provide tens of thousands of jobs and high levels of investment into the UK. 

Many believe that the UK was slow to act on fixed bottom wind, leading to investment and development lagging behind other European nations. 

The early adoption of floating offshore wind in the UK and the current project pipeline has positioned the UK as a global leader with a significant market share. However, the UK government and energy sector must move at pace to ensure the UK remains a leader in cutting edge floating technology for decades to come. 

The UK is in a unique position due to the existing expertise developed over years of floating projects in the North Sea and the relative ease of access to deep water locations with high average wind speeds. It is already clear that significant learnings for the global market will come from the development of the UK FLOW sector. 

FLOW has the potential to deliver £43.6bn in UK gross value add (GVA) by 2050, creating more than 29,000 jobs in the process. One key investment opportunity is the development of UK port facilities. If the UK to is to achieve its floating offshore wind targets substantial investment into UK port infrastructure will be required. According to Renewable UK every £1 invested in UK port infrastructure will generate approximately £3.4 - £4.4 of value added to the economy by 2040 (3).

FLOW Commercialisation Challenges 

To achieve the global and UK growth of FLOW numerous challenges will need to be overcome. 

BPT have identified three main challenges that will need to be addressed in order to lower the levelized costs of FLOW and enable the large-scale commercialisation of FLOW in the near future. 

Supply chain and port infrastructure 

Turbine Components

The GWEC predicts that from 2027 Europe’s offshore wind turbine nacelle assembly capacity will struggle to cope with the growth in demand expected in Europe. To meet the projected demand for this region by 2030 the existing capacity will need to double. Challenges are also expected around the supply of key turbine components such as gearboxes, generators and bearings. China currently has a 70% market share of these key components which may cause supply chain issues if the US and EU impose tighter trade policies. 

Port Infrastucture

A report by the UK Floating offshore wind task force (3) recommended that £4 billion needed to be invested into key UK port locations to support the growth of floating offshore wind. Large scale assembly areas capable of providing the facilities required to lift and assemble hubs up to 150m above the floating hull will be required. In addition to assembly areas the UK will need to increase its steel manufacturing capacity again this will require high levels of investment. 

Legislation and Policy  

In order to achieve the growth in the offshore wind sector required to meet net zero and climate target set huge amounts of investment will be required. The industry has faced challenging market conditions in recent years, emerging from the COVID-19 pandemic and the Ukraine invasion. This has led to low profitability and a high volatility within the industry. 

Government and policy makers will play a key role in creating an environment with the stability and incentives required to ensure investment in the offshore wind sector. 

According to a report by Catapult into consenting they estimate that the overall process from pre-application to final determination of all necessary consents is estimated to take from between 3-5 years. The consenting process is crucial, and Catapult outlined numerous opportunities that the consenting process could present. However, it is key that the consenting process is streamlined, and the time required for the decision making is reduced to ensure that the process does not hinder but helps support the rate of growth of the offshore wind sector.

Technology development 

The design of floating offshore wind turbines has been a hot topic for many years, with hundreds of different design concepts and several prototypes tested in the ocean environment. 

However, floating turbine itself is only a single part of the overall offshore wind farm and the process of transferring wind power into available electrical power on land. 

Supporting infrastructure such as the national grid and interface between offshore energy sources and the grid will need to be upgraded to enable the increased power produced to reach end users efficiently. 

A report by the national grid outlined that by 2030 we will need to build over 5 times more new transmission lines (overhead and underground) than we’ve built in the last 30 years to support the electrification of the UK energy mix. 

Other areas of supporting infrastructure include workboats, heavy lift cranes and installation vessels. Again, the numbers of these types of vessels will dramatically need to increase over next decade to support the growth of the offshore wind sector. 

As floating wind progresses over the coming years and wind farms move into deeper waters further from shore, floating offshore substations will be required to ensure the efficient transfer of energy back to shore. 

Floating substations will be a key building block of commercial scale floating offshore windfarms; however, substations have not received the same level of attention as the floating turbines themselves. 

BPT has developed a patented floating substation design which aims to solve this challenge and provide one of the missing links within the floating offshore wind sector. 

Floating Offshore Substation (FOSS)

Offshore substations are crucial for ensuring the efficient transmission of power to shore by stepping up the voltage. 

Fixed bottom wind farms already rely offshore substation on fixed jacket structure platforms as shown below. Siemens Energy have developed an Offshore Transformer Module (OTM) which has been widely used across North Sea wind farm developments. The OTM transforms the output voltage of wind turbines from 66 kV to the transmission to shore voltage of 220kV.


Three OTMs have been used on the Moray East Offshore Windfarm project and have the combined capacity to deliver 950MW of energy. 

As offshore wind progresses into deeper waters in the hunt for higher and more constant winds fixed bottom supports are not feasible. The design of floating offshore wind turbines has been a hot topic for many years, with hundreds of different design concepts and several prototypes tested in the ocean environment. 

Commercial size floating offshore windfarms will require substations just like current fixed bottom windfarms. However, the development of this key part of the floating offshore wind has been drastically lagging behind with only one current floating offshore substation (FOSS) in operation based in Japan.

Within a DNV article titled “Floating Substations: the next challenge on the path to commercial scale floating windfarms” they state “now that the first commercial floating windfarms are about to enter the development phase the industry needs to pay attention to this critical element (FOSS)”. 

FOSS present a host of challenges mainly driven by the requirement for minimal motions. Substation equipment is highly sensitive to motions so any FOSS design must ensure motions are within acceptable limits. 

The conventional floater design that presents the best motions performance is a deep draught spar buoy. However conventional deep draught spar buoys have a number of disadvantages that would limit its suitability to the UK market. Spar’s are launched horizontally before being towed to a sheltered very deep-water location such as a Norwegian fjord. They are then inverted and ballasted down vertically before the topside’s payload is lifted on and installed. This whole operation is highly costly and can only be performed in a limited number of locations. Conventional spar buoy designs would be incompatible with UK facilities and would drive up the cost of FOSS to UK developers. 

The challenges presented by FOSS and the limitations of conventional Spar buoys formed the motivation for BPT FOSS design. 

The mission statement was to design a FOSS with comparable motions to a conventional deep draft spar buoy that can be simply fabricated and launched in the UK with the topsides installed at the quayside to limit installation costs. 

BPT believe that the design will provide the missing link required by the UK floating offshore sector on the route to commercialisation and rapid growth needed to decarbonise the UK energy mix. 

BPT Patented Hull Form


BPT has been developing their patented floating technology buoy for five years. The technology provides a floating platform suitable for a range of payloads, providing minimal motions comparable with conventional spar buoys in some of the harshest environments such as the North Sea


BPT have been working with industrial partner Siemens energy optimising the design to support a Siemens Energy developed Offshore Transformer Module (OTM). 

In 2021 BPT was awarded a grant from the government department for Business, Energy, and Industrial strategy (BEIS) to demonstrate that BPT’s patented hull design is suitable for fabrication at a large range of facilities within the UK and offers the opportunity to maximise local continent in UK offshore wind projects by using the hull substructure to support substation equipment. 

As part of this grant BPT further developed the buoy, performing model tests and detailed numerical analysis. 

Screenshot 2023-11-13 124037.png
Screenshot 2023-11-13 130734.png

The lower buoy is subdivided into eight watertight ballast tanks which are initially buoyant providing a very shallow draft at launch. After launch the soft tanks are pumped full of seawater, taking the buoy from the shallow launch draft to the deep operational draft. 

The OTM topsides will be installed onto the buoy at the quayside removing the need for an expensive and challenging offshore lift.
The technology has been designed to comply with relevant rules and regulations. The structure has been designed against DNV-OS-C101 (structural design for offshore units), while the stability has been assessed against DNV-OS-C301 (Stability of cylindrical units) and the mooring design against DNV-OS-E301 (position mooring). 

A FOSS solution for the UK market

The BPT FOSS design provides a solution to each of the key challenges highlighted within the previous section. 


Minimal motions within acceptable limits governed by substation equipment. 


Simple modular fabrication that could be performed at UK yards


Shallow launch draft with topsides installed. 

The BPT FOSS offers minimal motions comparable with deep draught spar units and is well suited to supporting substation equipment which are sensitive to motions. Results show that the floating substation would experience peak pitch angles of 9.9 degrees with lateral accelerations of 1.94m/s when faced with an extreme 100 YPR North Sea Environment (16.5m significant wave height). These results exceed required targets and show the design has comparable motions with conventional deep spar buoy.

The BPT FOSS has been specifically designed to ensure a simple modular fabrication. This has been achieved through the structural design and designation of modules; with both the lower buoy and upper column being further subdivided into modules and blocks. 

The hull is constructed separately from the substation topsides, which would later be mated to the hull. By splitting the hull and topsides construction, the design provides the ability to maximise regional local content by fabricating the hull close to the wind development, whilst the more complex substation is fabricated elsewhere in the UK.

In the UK, where fabricator quaysides are rarely located in deepwater the ability to launch the hull in shallow water is a significant advantage, increasing the number of fabricators able to construct and launch the FOSS throughout the UK. The floating substation can be launched, and topsides installed in water depths of 5-6m.
The topsides can be installed at the quayside prior to the buoy being towed into deeper water for the ballasting operation. 

BEIS Project - Development

During the recent development of the BPT FOSS funded by the BEIS grant the two following objectives where set; 

  1. Show that the BPT FOSS provided motions suitable for an OTM topsides. 

  2. Outline how the BPT FOSS provided an opportunity for UK fabrication and launch. 

These objectives were achieved through a set of model test campaigns supported by the Wolfson Unit at the University of Southampton and a detailed UK fabricator compatibility study. 

Model Test Campaign

1:55th scale model tests have been conducted at the University of Southampton, Wolfson Unit.

Good Pic to show size.jpg

Model tests allowed the buoys linear and quadratic damping to be calibrated and numerical models validated.

Decay Tests

Decay tests consist of setting up the buoy in a free-floating equilibrium condition (i.e. free of any mooring system or restraining devices).  The buoy is then displaced in one singe degree of freedom. After being displaced the buoy is left to move freely until it comes back to its equilibrium position and the motions of the unit recorded.

Decay tests allows the following: 

  • Assessment of pitch and heave natural periods; and

  • Calculation of an equivalent damping using the logarithmic decrement method.

A total of 66 decay tests were performed, comprising of 27 heave decays tests and 39 pitch decay tests. The quality of the data was considered as very good. Very low noise on the record were experienced which allowed for a clean processing of the data.

The results from the decays tests allowed the period (natural period) of the structure to be calculated. The observed periods were found to be near identical to the values calculated numerically. The match between the numerical values and model test values provides confidence that the model mechanical properties were as specified. 

Irregular Sea Tests

The buoy was moored in the tank with a 3-points mooring system representing a catenary mooring system. The irregular sea tests consist of subjecting the model test buoy to an irregular wave train which can be characterised by its significant height (Hs), its peak period (Tp) and its peakedness factor γ. The irregular sea tests allow the validation of the numerical simulation by ensuring that the buoy behaviour in the wave tank is similar to the numerically calculated responses.

During the irregular sea tests, the model behaved as expected, showing motions amplitude within the same order of magnitude calculated numerically. Numerical simulations were performed using damping calibrated after the results of the decay tests, aiming to reproduce the motions observed in the tow tank.

Screenshot 2023-11-13 142137_edited.jpg

The correlations between the numerical model with in-house calibrated damping and the model tests observation show that the software used by BPT, is able, once properly calibrated, to reproduce with a certain level of confidence the motion experienced by a real structure.

Results show the BPT FOSS has excellent motions performance with heel angles of 0.7 degrees and lateral accelerations of 0.29m/s2 in normal day conditions and 6.75 degrees heel with 1.45m/s2 in the 1-year storm conditions (11.6m Hs and 15.6s Tp). 

Marine Operations  

The ballasting operation was performed at model scale, simulating the process of sequentially flooding lower buoy ballast tanks.

Heave Plate

Motions analysis involved an investigation into the impact of a heave plate attached to the keel plate of the FOSS. The heave plate was found to provide an increase in heave damping and hence an improvement in the motions of the unit. 

UK Fabricator compatibility study 

During the fabricator screening 20 UK fabricators were researched and their suitability to fabricated and launch the BPT FOSS determined. 


Firstly, a set of fabrication and launch facility requirements where set based of key features of the BTP FOSS. 
On the fabrication side the following points were considered;

Experience fabricating steel structures of this size. 

The fabricator should have a proven track record of fabricating large steel structures of this size. 

Size of fabrication workshops. 

The BPT FOSS lower buoy section has a footprint of 62.5m diameter. A yard will require fabrication halls that are able to fit full or sub-assemblies of the BPT FOSS. 

Fabrication equipment

The single curvature steel plates required for the hull will require plate rolling equipment and experience. The symmetrical cylindrical body is suited for automatic welding, to increase production efficiency. The heaviest list expected during construction is when lifting the topsides payload onto the hull, this requires a lifting capacity of 1,300t. 

Launch facilities/constraints were governed by; 

Drydock breadth or quayside length of 64.5m. 

Overhead cranage or access for mobile crane to be positioned near to quayside/dry dock. 

Depth over sills or quayside depth of 5.5m 

Access to deepwater (Ensure channel to deepwater is wide enough and there are no issues around air draft). 






Once an initial screening exercise had been performed BPT and Siemens energy completed a fabricator engagement process comprising engagement with UK fabricators via virtual meetings and submission of a questionnaire covering the fabrication and launch requirements outlined above. 

The screening and subsequent fabricator engagement highlighted three organisations the facilities required to fabricate and launch the BPT FOSS.

  • Harland and Wolff;

The questionnaire indicates that all but the Harland and Wolff Appledore yard would be capable of constructing, assembling, and launching the FOSS hull and topsides for an AC solution. Harland and Wolff Belfast can provide dry-dock facilities for the unit construction and launch, whereas the other sites would be able to offer quayside facilities for the launch of the unit. All of the yards employ automation within their fabrication facilities, with the level of automation varying by location, no single yard provides the complete automation package

  • GE Group;

The questionnaire indicates that Nigg has the fabrication facilities available to construct the entire FOSS in the AC configuration. The dry dock and quaysides are sufficiently equipped for launch of the unit, providing project flexibility. It should be noted that Nigg has limited automated fabrication facilities, and therefore the fabrication cost may not be as competitive as a more modern yard. 

  • Smulders;

The Smulders questionnaire responses indicate that Smulders Newcastle facility has sufficient quayside length and depth for launch of the FOSS, and a clear route with sufficient air draught for tow to the North Sea. With sufficient craneage for the final assembly of the unit, the Newcastle facility could be considered for final assembly of the FOSS.  Being part of a wider group of fabricators, it is feasible for sub-assemblies to be constructed off-site and shipped to Newcastle for final assembly.

Study conclusions

The fabricator screening study shows that there are various UK sites with the facilities capable of fabricating and launching the complete FOSS. The majority of fabricators in the UK are not large, and the FOSS hull is a significant construction undertaking for these facilities.  Adopting a consortium approach or sharing construction within a group of companies may be a suitable means of accessing schedule advantages or efficiency savings.  

An area of concern identified from the study was a relatively low level of automation available in UK fabrication facilities.  As a simple, cylindrical structure, the FOSS design lends itself to automated plate rolling and welding.  


Floating offshore wind will be crucial to achieve the next zero targets that have been set and ensuring a future that limits global warming to 1.5 degrees. Floating offshore wind provides enormous opportunities to access the untapped potential of deepwater offshore wind energy around the globe. The UK is currently in a fantastic position as global leaders in the floating offshore wind space, however it must act quicky to address the challenges to ensure that the current project pipeline can be realised.

The BPT floating offshore substation design provides a solution to one of the key challenges facing the sector as offshore windfarms move into deeper waters. As with fixed bottom offshore wind, substations will be crucial to scale windfarms and ensure they are economically viable. 

UK content has been a key driving factor in the BPT FOSS design. The simple modular construction and a shallow launch draft opens up the opportunity to fabricate and launch the FOSS at multiple sites across the UK. 

bottom of page