Wind energy: offshore

Offshore wind energy shares many characteristics with the more familiar onshore wind energy.  Compared to onshore wind energy, which today is a commercial technology with global application, offshore wind is an earlier stage technology. However significant expansion is expected in the near future and initial full scale offshore wind farms have been built and are operational in Europe and China. Costs for offshore wind energy are still higher than onshore wind, however the benefits of developing sites offshore (including greater available resource and reduced planning issues) may make this form of wind energy preferable in many countries in the future. Global wind energy capacity has been growing very rapidly over the past decade. In 2009, newly installed capacity was more than 34GW of which 0.6GW was offshore. Total offshore installed capacity was roughly 2.1GW at the end of 2009 (GWEC, 2010).

Wind energy is actually a form of solar energy; the temperature differences caused by the sun shining on the earth act, along with other factors, to cause large bodies of air, winds, to move across the face of the planet.
The conversion of the kinetic energy in these winds into electrical power is known as wind energy.  There are a number of ways in which this conversion can be done; however after a period of experimentation and development beginning primarily in the seventies one design has come to dominate the market. This is known as the horizontal axis wind turbine (HAWT) with its archetypal three-bladed rotor as seen in the figure above.
A large wind turbine primarily consists of a main supporting tower upon which sits a nacelle (the structure containing the mechanical to electrical conversion equipment).  Extending from the nacelle is the large rotor (three blades attached to a central hub) that acts to turn a main shaft, which in turn drives a gearbox and subsequently an electrical generator (Fig 1). In addition to this there will be a control system, an emergency brake (to shut down the turbine in the event of a major fault) and various other ancillary systems that act to maintain or monitor the wind turbine.

                                                 Figure 1: Cut-away view of a typical wind turbine (source ZF, 2010)

Modern offshore wind turbines have main towers that are typically 50 to 100 metres high supporting rotors with a similar range of diameters; however larger machines up to 130m are also operational. The key difference between onshore and offshore designs is obviously related to the type of foundations required.  While onshore designs can use a conventional concrete foundation, offshore designs must use foundations which are suitable to the subsea environment and can be installed offshore. Depending on the seafloor geology this may include (Fig 2):

  • simple percussive hammering of a long tubular structure/monopile into the seabed in water depths of up to 25 metres,
  • drilling of a hole in harder strata and grouting of a monopile,
  • concrete gravity bases that rely on the weight of the foundation (possibly in combination with some suction between the sand and foundation) to provide support in depths up to 30m
  • tripod/jacket type structures that use a number (often 3) smaller piles to provide support in deeper water in the order of 30 to 40 metres.

                                          Figure 2: Generic foundation types for offshore wind (source: Züblin,2010)

There is also R&D work occurring in the field of floating wind turbines in order to allow for the utilisation of deeper areas of the sea.  These vary in design but in general some kind of counterweight or structure beneath the surface of the ocean acts to hold the turbine upright and a conventional mooring system is used to stop drift of the device.  The most notable of these designs is the Hywind full-scale pilot project that was installed in 2009 (Statoil, 2009).
The export of power from an offshore wind turbine is done by subsea cables. For larger distances to shore it can be beneficial to additionally install a substation offshore in order to step up the voltage (or possibly rectify it to DC) in order to lower losses during export.
Inside the tower there is a mechanism that ensures that the nacelle/rotor faces into the wind (i.e. is yawed correctly) to give maximum generation and maintain symmetric loads on the three blades and drive shaft. Generally the three blades are constructed from composites which provide a relatively high strength (required due to the large bending moments they experience) whilst maintaining a low weight and size given their length. Modern designs have a relatively low rotational speed in the order of 10 revolutions per minute (partly due to the desire to keep noise levels low) and thus typically require a gearbox to increase the speed of the drive shaft to match the rated generator speed. While most wind turbines use gearboxes (or indirect drive systems) there also exist direct drive configurations whereby the generator is coupled directly to the slow moving rotor.  These types of designs do not require gearboxes and thus avoid the reliability issues that have been known to trouble certain gearbox designs. However the larger generator size that is required in order to obtain the correct generation frequency faces its own challenges in regards to construction and cost.
Even for indirect systems, differences exist in the type of generator that can be used.  Older designs tend to be classed as ‘fixed speed’ meaning that the rotor always rotates at the same speed under all wind conditions.  For a number of reasons many modern turbines use generators that allow for variable speed generation whereby the rotational speed is optimised to the incoming wind speed and the generator provides output at a range of frequencies.  The resulting fluctuations in voltage and frequency are corrected by power electronics in order to provide electricity suitable for export to the grid. The advantages of this approach include reduced harmful torque fluctuations into the gearbox, increased conversion efficiency, the ability to continue operation during a grid disturbance and the ability to provide reactive power.  These last two are increasingly being demanded of wind parks by transmission system operators.  Further improvements to the level of energy capture are obtained by most modern turbines by changing the angle of the blades.  This ‘variable pitch’ system rotates the blades about their own axes so that for changing wind conditions the optimum efficiency is achieved. The system also acts to control the turbine, angling the blades ‘out of the wind’ during periods of high wind speed to prevent damage and providing the primary method for disabling the device.
In spite of continuing advances in turbine technology, there is an inherent physical limit as to the amount of energy in the wind that can be extracted.  A theoretically perfect (yet infeasible to construct) wind turbine could only ever extract 59 percent of the available energy, also known as the Lanchester-Betz limit. Modern turbines reach a conversion efficiency of approximately 50 percent, close to this theoretical limit and very close to the practical limit that is imposed by the drag of the blades. Nevertheless there is a significant body of ongoing global R&D into construction methods/materials for larger turbines, conversion efficiency refinements, lower cost components and improved reliability. Reliability and maintenance is a key concern for offshore wind installations due to the considerable additional cost of offshore interventions and the often significant down-time that can be caused while waiting for suitable weather windows.

Feasibility of technology and operational necessities : 

The first requirement when considering the possibility for wind energy is the identification of a suitable site that has a high level of resource; i.e. it is windy. More specifically it should be windy at the height above the ground at which the rotor will be situated. The surface friction of the earth’s surface, local topology and surface cover means that wind speeds are lower near the ground than they are higher up. Offshore wind regimes have the advantage of having considerably higher levels of wind and more consistency from moment to moment; this makes offshore wind turbines significantly more efficient per MW of installed capacity than their onshore counterparts.
In order to quantify the different levels of resource they are grouped into classes based on the yearly average energy available ranging from 1 to 7; with class 1 having low wind levels and class 7 having very high.  Many studies of global resource levels consider that for a site to be economically feasible it should have a resource/wind level of higher than class 3; corresponding to an average wind speed of more than 7 metres per second at 50m height (or approximately 5.6m/s at 10m height). However in any given region it is understandably preferable to develop those sites that have the highest level of resource first as these will provide the best economic return.

Figure 3: Wind power density over global oceans for winter (top panel) and summer (lower panel) in the Northern Hemisphere. Red and white colors indicate high energy is available, while blue color reflects lower energy. Source: NASA/JPL, 2008

Even at good sites there will be many times when a wind turbine is operating below its rated power (its nameplate or nominal capacity) or producing no power at all because of a lack of wind.  This means that although a turbine may be rated at, for example, 2MW it will produce a certain percentage of the theoretical power it could have produced had it operated continuously; this percentage is the capacity factor. For onshore wind turbines this capacity factor varies between roughly 20 to 30 percent while for offshore sites developed to date in Europe capacity factors of 35 to 45 percent (IPCC, 2010).
Specialised software programs can be used to identify potential sites that could be suitable for wind energy development. These programmes take historical wind data from any local monitoring stations and lower resolution wind data from satellites to interpolate the estimated wind resource across an area or region of interest. The software attempts to take into account the local topology and sea surface friction/roughness to estimate the available resource. This type of software is useful to identify possible sites, but is typically insufficient in regards to data reliability to allow a project to proceed, actual measurements of the wind speed must be taken at the proposed site.
Because the wind speed is constantly changing it is necessary to take measurements over an extended period of time in order to gain a good understanding of the available resource and also to provide assurance of returns to investors such as banks.

Until now the vast majority of wind turbines have been situated onshore. However increasingly there are plans in many industrial countries to develop offshore wind farms at scale. Although the issues of interference with onshore land users are avoided by siting wind farms at sea there are also planning issues with sea users that must be checked and resolved.
In addition to requiring an appropriate/economically-feasible level of wind, any site for a wind farm must also account for both certain technical requirements and also the needs of a number of possible stakeholders including:
  • Access to grid infrastructure at the exit point of the subsea cable for power export
  • Suitable water depth
  • Suitable sea floor geology
  • Clashes with other sea functions and stakeholders such as fisheries, oil & gas platforms, subsea piping and cables, sand extraction activities, military zones and shipping lanes.
  • Vicinity to protected nature areas and bird migration routes
  • Possibility of radar interference (sometimes raised as an issue due to neighbouring airports or military facilities)
As mentioned above, specialist software is typically used to map these constraints and determine possible locations for the installation of offshore wind turbines.

Technical Requirements
The technical capability required to support the deployment of large offshore wind energy largely depends on the level of localisation that is desired in the long term. The fabrication and assembly of modern wind turbines has become a highly technical industry dominated by a relatively small number of global players. In certain countries that foresee a high level of wind energy installation there may be potential for local fabrication of certain components in partnership, under licence or behalf of an international company, for example blades or towers which have relatively lower technical demands than the drive-train conversion equipment. Equally, there is potential for other more technical aspects of the fabrication to be done locally should the demand and skill set allow it. 
It is likely that early deployments in countries that do not have a large experience with wind energy will be with imported components. Furthermore the specialised nature of offshore wind installation and the associated offshore activities and vessels means that much of this expertise is likely to be imported as well, at least until a viable scale of industry was established within a region to justify the localisation of these skills.
Ongoing maintenance of offshore wind turbines requires a relatively high level of experience and familiarity with the equipment and is often carried out by the OEM supplier during the initial years while the devices are under warranty, but could then pass into the hands of local teams who have received the necessary training. It should be noted that all interventions must be by boat; or possibly helicopter in the future, depending on the distance to shore, the sea conditions, the type of turbine and the kind of maintenance being carried out.

Grid integration
Grid integration of wind energy has been a topic of substantial discussion, which is why it is discussed separately here. The primary perceived problem with wind energy is related to the intermittency of supply. The variability of wind on any given day, week or month means that the amount of power that is produced can change accordingly. In the short term wind levels and thus power generation can be estimated/forecast from meteorological reports with a reasonable degree of accuracy. However this does not solve the issue that on days when there is little or no wind, an alternate form of generation is required, requiring additional backup capacity that would not be needed for traditional base-load power stations.
To date there have been a large number of studies of the integration of wind energy into electricity networks. IPCC (2010) provides a good summary of the related literature which broadly concludes that at levels of penetration of up to 20 percent of supply the effects of variability and associated costs are relatively low but not insignificant. The report also discusses the issue of transmission costs which are more of a problem for wind compared to traditional power stations due to the often large geographic distribution and remote nature of the resource. Again these costs are found to be moderate but not insignificant.  It is worth noting that the integration studies mentioned, mostly consider large electrical systems in industrialised countries; in smaller or less developed countries where the electrical generation system is less diversified and extensive, the effects/costs of wind variability are not so clear and would need to be studied further, should high levels of wind penetration be planned.

There is considerable experience in a number of countries with permitting and planning frameworks in relation to offshore wind parks but less so in most developing countries.
  • A sea bed lease for the area of interest for deployment of the wind park.  This is typically a lease from the national government who would control the continental shelf around the country.  In a number of European countries that are more advanced in planning offshore wind deployments these lease have been allocated through a competitive tendering process.
  • Appropriate environmental permits by conducting an environmental impact assessment (EIA) that can take between 1-2 years depending on the level of baseline data demanded by the permitting authority and the sensitivity of the area.
  • Appropriate consultation outcomes with any relevant stakeholders or other sea users.
  • Grid connection agreement / power purchase agreement with the relevant body to ensure distribution and a market for the resulting electricity.
Social Acceptance
It can generally be observed that the level of acceptance of wind parks offshore is high. The chief concern with onshore wind farms has been one of visual amenity; however when wind turbines are situated some kilometres or ten’s of kilometres offshore there is typically an almost negligible visual effect, in fact many offshore parks will not be visible from shore due to their distance.

Status of the technology and its future market potential 
Offshore wind energy is entering a phase of large scale market deployment that is starting in Europe. In 2009, 0.6GW of new capacity was installed offshore and total offshore capacity was roughly 2.1GW at the end of 2009 (GWEC, 2010).  There is significant variation in the forecasts for offshore wind levels but within Europe, the most advanced market, offshore wind is expected to make up almost 18 percent of all installed wind energy by 2020 based on proposed and planned projects (EWEA, 2009) (Fig 4).

                              Figure 4: Forecast of European onshore and offshore generation (Source: EWEA, 2009)

The forecast beyond 2020 continues to be strong with many countries still having a low penetration of wind energy but excellent levels of resource. Total offshore world resource estimates vary widely due to different assumptions on the available sea surface for installation, acceptable water depths, economic distances from shore and also the available wind resource.  IPCC (2010) summarise literature with a range of 96,000 to 627,000TWh of technical resource or approximately 30 to 200TW of installed capacity for both onshore and offshore with the majority of this located offshore. In the short term much of the growth in offshore wind energy is likely to come from industrialised countries due to the additional cost of offshore wind versus onshore, and specialised skills/equipment required. However in the longer term, all countries are likely to face constraint on land use and should the resource and economics allow it, offshore wind will become an attractive alternative to onshore wind power.

Contribution of the technology to protection of the environment 
Although wind energy has a net positive impact on climate change mitigation (see below) local environmental impacts must also be considered. The most well publicised potential issue is the impact that wind turbines can have on bird and bat populations due to collisions.  IPCC (2010) provides a good summary of the specific studies that have looked at the number of fatalities of these species.  There is a strong argument that the number of recorded fatalities, while site specific, is relatively low compared to other anthropogenic causes of bird and bat deaths such as cars, collisions with buildings, feral cats and transmission lines.
In terms of other ecological effects related to the installation, there is a summary of the limited available literature in IPCC (2010) noting that there are both negative and positive environmental impacts which are likely to be moderate in both instances. They note, however, that further research is required to understand the effect of large scale offshore wind deployment.

The renewable nature of wind energy, the large available resource and the relatively advanced nature of the technology mean that it has the potential to make a significant contribution to climate change mitigation efforts. By acting to displace generation from thermal power plants offshore wind energy can prevent the emission of roughly 3,000 tonnes of CO2 per year per megawatt of installed offshore wind capacity (assuming it replaces coal and is located at a reasonable wind energy site).
Although there is some amount of carbon used in the manufacture of the devices, studies have shown that the payback period (the time it takes for the wind energy to offset the emissions associated with its fabrication and installation) is relatively low, typically in the order of 6 months or less (IPCC, 2010).

Financial requirements and costs 
“Though the cost of wind energy has declined significantly since the 1980s, in most regions of the world, policy measures are required to make wind energy economically attractive” (IPCC, 2010). This statement concerns onshore wind, but can be stated even more strongly for offshore wind which has higher costs. For modern turbines the levelised cost of electricity in 2009 (accounting for capital costs, lifetime O&M and typical financing costs) ranges between US$100 to 200/MWh at good to excellent sites (IPCC, 2010).  The site specific costs are influenced by the nature of the local wind resource, local capital costs (for example wind power capital costs are lower in China) and the financing arrangements for the specific project.
Offshore a significant proportion of the costs are associated with the turbine (Figure 5); however grid connection and civil/installation costs are considerably higher than for onshore wind due to the nature of operating in the offshore environment and larger fabrication cost for foundations.

                         Figure 5: Installed cost distribution for onshore and offshore wind power plants (IPCC, 2010)

Clean Development Mechanism market status 
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group]
Project developers of off-shore wind farms can use the same CDM methodology as for large-scale wind projects, which is ACM 2(Consolidated baseline methodology for grid-connected electricity generation from renewable sources). As of April 2011, there is only one registered off-shore wind project under the CDM, the Shanghai Dong Hai Bridge Offshore Wind Farm Project.
CDM projects based on wind represent 17.3% of all CDM projects in the pipeline. Recent years have shown a tendency towards a more widening geographical dispersal of CDM wind projects, indicating that countries other than India and China observe the CDM as a tool to support wind projects.

Figure 6: Overview of wind projects in the CDM (source: UNEP Risoe CDM/JI Pipeline Analysis and Database, February 1st 2010)

References : 
EWEA (European Wind Energy Association) 2009. Pure Power: Wind energy targets for 2020 and 2030, available from:
GWEC (Global Wind Energy Council), 2010. Global Wind 2009 Report. Global Wind Energy Council, Brussels, Belgium, 68 pp.
IPCC 2010. Special Report on Renewable Energy Sources and Climate Change Mitigation, In Press.
NASA/JPL 2008, Ocean Wind Power Maps Reveal Possible Wind Energy Sources, available from:
ZF, 2010. ZF Signs Contract to Supply Wind Turbine Gear Boxes, available from:

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