The development of floating offshore wind turbines presents a number of advantages over more conventional foundations, not least of which is the opportunity to exploit wind resources far offshore in deep water. Eize de Vries reports.
Until recently a typical state-of-the-art solution for securing offshore wind turbines was to place them on special foundations, which are either lowered onto a permanent position on the sea floor or rammed into the seabed. However, in addition to improving existing offshore foundation solutions and introducing new permanent foundation concepts, several innovative solutions are under development that are based on floating wind turbine foundation systems.
Advocates of floating wind turbine concepts point to a series of disadvantages with conventional offshore wind installations that feature a permanent seabed foundation, which do not affect their more mobile counterparts. These include demanding commissioning, service and repair, and retrofit operations that all have to be carried out offshore. These can be vulnerable to bad weather in conditions characterized by poor installation accessibility. Working offshore also implies high costs for installation upkeep, not least due to demanding sea travel and open sea transfers.
Another key issue is the substantial risk of extended installation downtime during high winds. Under these conditions dynamic turbine loads are at maximum, yields most favourable, and simultaneously the implications of a breakdown most severe due to difficult turbine access.
Decommissioning and compulsory removal of permanent foundations at the end of a wind farm’s operational service life is also widely considered to be an underestimated cost associated with fixed offshore machines, for which substantial sums must be set aside.
As an added advantage when compared with offshore installations put on permanent foundations, floating offshore wind turbines can be installed at sites with much greater water depths, far offshore. Locating wind turbines far offshore also results in reduced visual impact, less interference with bird migration, and increased power production due to stronger and more stable wind conditions.
Early movers
A joint co-operation agreement between power engineering giant Siemens Wind Power and energy company Hydro of Norway aims to develop a floating wind turbine. Based on Hydro’s Hywind concept the partners envisage a floating demonstration turbine offshore in 2009 near Karmøy, an island off the south-west coast of Norway for which Hydro has a licence. Siemens is to deliver the wind turbine for the proposed demonstration unit. while Hydro will apply its offshore platform expertise.
Hydro expects to apply the technology in future on sites located 90–180 km offshore and in water depths of up to 700 metres. Hydro’s design concept is similar to its technology used in oil rigs, which comprises a long, submerged floating concrete cylinder that is ballasted. However, this prominent feature makes the design unsuitable for shallower waters.
Marine engineering consultancy firm Sea of Solutions BV is based in the Netherlands and was founded in 2001. Key activities are the development of dedicated projects and products for operators, contractors and ship owners in the fields of marine exploration, construction and maritime transport. One of its latest products is a new offshore wind turbine foundation concept named the Floating to Fixed Wind Energy Concept (F2F). During the design stage, Sea of Solutions limited itself to a common North Sea water depth of about 25 metres for the initial market focus, but the design, best described as a ‘hybrid type foundation’ can be extended to deeper waters. The design comprises several interlinked sub-systems and components, including a long central steel column flanged underneath the actual wind turbine tubular steel tower that turns the combination into an elongated single vertical assembly. In addition to this primary foundation element, three long vertical steel pipes are equally spaced around the central column and welded to a support structure that is fitted to it. These three outer pipes act as flotation devices and also form the legs that fix the turbine semi-permanently to the seabed. These hollow structures are therefore equipped with a valve system that can be opened, closed and fine tuned on demand. Technical director at Sea of Solutions, Jeroen Lusthof, explains: ‘In floating transport mode the legs with internal concealed compartments are filled with air. This provides the necessary flotation capacity to the entire structure comprising both the foundation and top head (tower and nacelle). Individual units can be towed to their location in a wind farm, and once in position the valve system allows the pipes to fill with water. The added water ballast weight is sufficient to let the installation sink vertically until the legs firmly touch the seabed.’
Finally each leg is equipped with a suction bucket or suction anchor, which firmly fixes the foundation. A key advantage of suction anchors is that seabed preparation is not necessary, as the structure can be put exactly in a level position by adjusting the vacuum in each individual leg, says Lusthof. Re-floating and towing the machine inshore for maintenance, repairs and major overhauls, or for final disposal at the end of the operational lifetime, is relatively easily achieved by reversing the installation process.
Swapping replacement units
Lusthof points out that current offshore wind industry practice is to erect land turbines under complex maritime conditions with the aid of a dedicated jack-up barge aimed at creating a stable working platform for the operations offshore. The currently used dedicated installation equipment poses a risk of major delays in installation works when the jack-up barge encounters a breakdown, he says. Conversely, the floating concept enables the use of conventional, non-dedicated tugboats, which are readily available in any market. This limits possible downtime to a minimum.
Furthermore, current installation methods are almost, by definition, time-consuming and inefficient, a situation which also fully applies to offshore maintenance operations including major repairs and sometimes even complete retrofits.
In line with Sea of Solutions’ operational philosophy it is possible to remove one defective or malfunctioning unit from a given offshore wind plant, while the same transport vessel brings in a replacement unit. Indeed, a huge advantage of the F2F approach is the capability to interchange complete turbines quickly, which leaves cumulative wind farm power generating capacity almost fully intact all year around. ‘Long-term financial and other benefits are a substantial increase in wind farm total availability and overall project economics,’ says Lusthof.
Lusthof further explains that there were several key challenges to deal with during the design phase, which will initially concentrate on offshore wind installations in the 2–3 MW class. A primary difficulty is to ensure sufficient overall stability during sea transport. The huge dynamic loads and acceleration forces imposed upon the structure in floating mode are a second major challenge, still grossly underestimated by the wind industy. Says Lusthof: ‘Depending on wind turbine size, weight and other determining variables, the nacelle mass is usually concentrated at 60–100 metres above sea level. Under the influence of both wave movements and the top head mass in its elevated position, the entire floating installation oscillates continuously. These uncontrolled movements can potentially introduce premature material fatigue damage in critical parts such as the rotor blade foot, pitch bearings, yaw motors, and drive components – bearings, shafts and gears. Dynamic load constraints on a wind turbine top head during sea transport are not only valid for our F2F concept, but also occur during equipment transport on barges. A relative advantage of the F2F concept compared to transport by a conventional installation barge is that our floating structure is relatively open, which limits the impact of wave induced forces.’
The F2F design phase was completed in late 2007 and the Scandinavian certification institute, DNV, granted an ‘Approval in Principle’ for the concept. The next step is to build a prototype and Sea of Solutions is engaged in discussions with a major utility, says Lusthof. The company believes that a robust foundation system based on proven technology that is completely constructed, pre-assembled and commissioned inshore and which can also fully benefit from strong offshore winds, will prove to be economically viable. Especially since the F2F hybrid technology will effectively deal with the high costs typically associated with offshore maintenance.
WindSea
Norwegian engineering consultancy company Force Technology has over three decades experience in the design and maintenance of offshore structures and has gained extensive additional know-how in related fields like wind and sea wave topics and marine corrosion protection. Now, the company has developed a new patent-pending offshore wind technology. Known as the WindSea, this unmanned floating structure is self-orientating towards the wind and, the company says, has been designed with a dual focus on excellent dynamic response to wave and wind, along with safety and reliability.
The initial structure is calculated to accommodate three wind turbines of 3.2 MW each, but the WindSea concept is scaleable to 5 MW and substantially bigger in future, says company spokesperson Hans Jørgen Mikkelsen. ‘The WindSea lattice-type welded steel foundation structure features a 23 metre draught and towers founded on a structure 17 metres above sea level. One very important design criterion with regards to minimizing the materials fatigue of structural components due to cyclic loading, was stability. The WindSea deflection under normal operational conditions could be limited to one degree only. But even under a 100-year storm extreme load situation, the maximum deflection is less than 4.5o, and this is a condition when the turbines will be shut off anyway,’ says Mikkelsen.
The 3.2 MW wind turbine concept used for the design calculations features a 100-metre rotor diameter. The three rotors, each fitted in a fixed position on top of an inclined tubular steel tower but at two distinct hub heights, are situated on top of the fabricated structure. The two parallel upwind turbines in front always operate in ‘a line assembly,’ positioned perpendicular to the prevailing wind direction. Their internal spacing is 110 metres. The hub height of these front turbines is 60–80 metres while the third wind turbine, a downwind device, is located in the rear and features a hub height of about 100 metres. The rear rotor is deliberately placed higher in a bid to minimize unavoidable wake effect losses introduced by the two front rotors, a process that Mikkelsen describes as a straightforward cost–benefit analysis exercise. Model measurements in a wind tunnel are planned to commence during the course of 2008 and will show, among other things, whether or not it is advisable to raise the hub height of the downwind unit still further.
Furthermore, this rear rotor is situated on an airfoil shaped tower. Thus the assembly functionally acts like a tail-type wind directing system that specifically aims to continuously position all three rotors perpendicular to the prevailing wind direction and can therefore, to a certain degree, be compared to relatively straightforward mechanical wind directing systems that are commonly applied within micro-wind turbines. Larger turbines, in contrast, are fitted with an electric-mechanical yaw system comprising a yaw bearing and several yaw motors controlled by a wind direction sensor.
Anchors away
The installation has been designed to operate in water depths of about 35 metres minimum up to several hundreds of metres, says Mikkelsen. It is fixed to the seabed at the location with the aid of a normal anchoring system comprising a sea anchor and a cable that is fixed to the structure. An integral part of the WindSea solution is a central rotary mechanism integrated at platform level, eliminating the need for a conventional and maintenance prone yaw system for each individual turbine.
An essential part of the WindSea strategy is that the units are relatively easily removable for extensive repairs, Mikkelsen says, noting: ‘North Sea maintenance and repairs prove extremely costly. Hiring a jack-up barge for operations requiring a heavy crane can cost, for instance a100,000–a130,000 per day.’ The WindSea units can be towed to a sheltered harbour for comparable major repair and/or retrofit jobs for a fraction of these costs, says Mikkelsen.
Another beneficial design feature when planning larger wind farms comes from the robustness of the base structure, which allows for a central hub connection in one of the wind park units. Consequently, well-known and easy repairable technologies can be used here as well.
‘There is a lot of interest in our concept for deep water offshore locations like those typically found in Norwegian, Scottish, and Spanish waters. If everything goes according to plan, a prototype will be erected by 2011. But to achieve this timetable the dedicated co-operation of partners active in many different disciplines will be needed. We are therefore talking to a range of potential investors, from wind turbine suppliers to shipyards and financial companies alike about the next steps to take together,’ Mikkelsen concludes.
Blue H Technology
Netherlands-based industry newcomer Blue H Technology has developed a floating turbine system that, from a conceptual point of view, is rather different from existing solutions. In early February the company’s Chief Executive Neal Bastick said: ‘Blue H is now in the process of installing the world’s first deep-water floating prototype wind turbine. The test location is at 19.6 km off the southern Italian cost at a so-called Tricase site near Puglia, where the water depth is 108 metres.’
The prototype was built in Italy at a shipyard in the city of Brindisi. It is equipped with an 80 kW two-blade variable-speed WES18 mk1 wind turbine, supplied by Wind Energy Solutions (WES) of the Netherlands. Founded in 2003, WES specializes in small to medium-sized variable speed turbines rated from 2.5–250 kW, wind technology it acquired from the bankrupt former Lagerwey. However, commercial future projects will be equipped with Blue H’s in-house turbine technology, in line with long-term company strategy. In order to execute these plans the company acquired exclusive worldwide rights for offshore applications of the variable speed 1.5 MW Gamma 60 wind turbine technology. In addition to the 1.5 MW prototype, Blue H also acquired two additional prototypes from the owner. These machines were never erected and comprise a 2.5 MW generator. The innovative wind turbine concept itself features a 60 metre two-blade rotor and was developed with European Commission financial support during the late 1980s by Aeritalia of Italy and its partners. Compared to today’s standards, the Gamma 60 rotor diameter is modest, despite the typical strong winds offshore, but represented at that time a significant technological achievement. Among the innovative features are a teetering rotor hub and an unusual yaw-control system to curb output in the nominal power range. The functional basis of this output control technology sees the complete rotor gradually turn out of the wind as wind speed increases. The effective full rotor circle that initially faces the wind at low and medium speeds thereby gradually changes into an ellipse. As a result, the effective area is substantially reduced. A second contributing factor limiting power output in the upper range is the loss of aerodynamic efficiency as the angle of attack becomes less favourable. The power output control system bears a resemblance to mechanical systems with a folding tail, seen on numerous micro wind turbines. However, yaw motors are used in the Gamma 60 machine to operate the control system. This contrasts markedly with common state-of-the-art power output control technology in which the blade pitch angle is continuously adjusted in response to the output-related wind speed.
Among Blue H’s medium to long-term development plans are proposals to scale up the Gamma 60 from 2 MW to about 3.5 MW, including a larger rotor size. For the current 2 MW turbine, the rotor diameter will be enlarged to 80 metres from the third unit onwards.
Tension leg
Blue H’s floating wind turbine system is named the Submerged Deepwater Platform (SDP). The design principle has been derived from so-called tension-legged semi-submerged platforms, a proven technology successfully employed by the oil and gas industry for many years. The SDP foundation technology comprises four closely interlinked elements. The first element is a huge welded fabricated steel structure containing six separate airtight compartments, and six floating interlinked compartments that are open. The assembly serves as the SDP foundation counterweight. To each of the six sides of the structure, a heavy-duty chain is attached, the second main foundation element. In transport mode the floating counterweight is towed to its planned offshore destination. Here the open compartments are filled with gravel, and the additional mass sinks the structure to the seabed. The third element is the partly submerged upper part of a SDP-type system and is known as the platform. This steel structure with six interconnected hollow steel pipes provides the necessary buoyancy required during sea transport, as well as during offshore operation. The fourth element is the wind turbine itself, located on a tubular steel tower on the platform. Once the platform with the fully assembled turbine has arrived at its destination, the assembly is temporarily ballasted. This extra ballast forces the platform down into the water and creates the free play necessary to hook the six chains attached to the counterweight to the platform. When the extra ballast is removed, buoyancy creates upward force whereby the chains get tensioned to such an extent that it creates a ‘semi-stiff’ system between counterweight, chains and platform. This so-called tension-legged platform provides the required stability.
The tension-leg technology is described as a dynamic ‘mass-spring’ system in engineering terms. A key challenge is to ensure that sufficient stiffness can be maintained under all environmental conditions. Bastick says that Blue H looked into this challenge and found appropriate solutions to effectively deal with this. Once tests with the prototype prove successful, the Tricase site is to be expanded by twenty-five more units. This may become the first deepwater wind farm in the world with an installed capacity of 92 MW. For decommissioning, Blue H simply disconnects the platform from its anchorage and tows it to the shore, leaving the anchorage in situ.
I see no ships
It is still early days in the development of floating wind turbine foundations. However, in many respects the technology has been tried and tested by the offshore oil industry, giving its potential development a significant boost despite a number of challenges that remain to be overcome.
A series of clear advantages which are likely to confer financial benefits are certain to see more research in this exciting area of wind development. And, if successful, there may be no limits to those areas of the open sea available for offshore wind farm developments.
Eize de Vries is Wind Technology Correspondent for Renewable Energy World.
You can contact Elize de Vries at [email protected]
