Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology



V.M.I. Haag Transport and installation of Offshore Wind Turbines.
Literature survey, Report 2003.TL.6789, Transport Engineering and Logistics.


Current onshore exploration of wind turbines is restricted due to transport considerations. A possible solution to harvest wind energy is the installation of wind energy converters at near or offshore locations.
The land-based development of the three bladed, upwind wind turbine configuration has evolved into two distinct directions for wind energy conversion: variable speed and direct drive.

The current status of technology for land-based turbines will be available for offshore wind turbines. Several pilot projects have already been built and the first commercial wind farms ar being commissioned off the cost of Denmark.
Although the offshore wind regime is well suited to provide even better conditions for wind energy conversion (low wind shear, less turbulence) the practical implications of offshore installation and operation are still very uncertain. In addition, offshore turbines will have to be able to withstand the maritime conditions and show (highly) improved reliability in comparison to the land-based concepts.

For the existing wind farms two methods of transport have been used to transport the components to the location: A longer-term objective should aim for an integrated design, where the foundation and the turbine are transported and installed as one piece. The installation procedure should at least be simplified and include a minimum of operations offshore. Comparison of several possible installation procedures shows a 'one-stop-shopping' concept to be the most effective method of installing offshore wind turbines. The possibility to transport and install a completed structure is the main focus for development.

Turbine availability is one of the most important parameters to be considered in the design of an offshore turbine. It connects directly to accessibility for maintenance and reliability.
Furthermore, maintenance and repair for offshore turbines will be far more expensive in comparison to land-based installations because of reduced serviceability.


Summary

Land-based

Virtually all wind turbines installed at present are based on one of three main wind turbine types:
  • Fixed speed with directly grid-coupled (asynchronous) squirrel cage induction generator
  • Variable speed with doubly fed induction generator
  • Variable speed based on a direct drive synchronous generator.
For the near future, wind turbine development will focus on the three bladed, upwind turbine configuration. The trend is towards advanced designs, especially for wind turbines in the megawatt class. Currently, the two trend-setting technological designs in this category (>1.5 MW) are variable pitch turbines with doubly fed induction generators, and direct drive systems. Both designs incorporate variable speed operation. Turbine material usage is and will continue to be dominated by steel, but opportunities exist for introducing aluminium or other lightweight composites, provided strength and fatigue requirements can be met. Increasing the use of offshore applications may partially offset this trend in favour of the use of composites.

Wind power is gaining rapid momentum in the world energy balance. With it, certain issues have to be addressed in order to upkeep reasonable power quality in sub transmission and distribution networks more or less dependent on wind power. Unless properly remedied, voltage fluctuations may otherwise proliferate and become a nuisance to grid users as well as operators.
No commercial turbine manufacturer uses high-voltage generators. There would be advantages in studying the technology and the costs of high-voltage generators (up to 35 kV) in volume production.

There is no indication that size will be limited in the near future by technical boundaries or transport considerations, although increasing component dimensions will add to project costs.
Although total assembly costs per turbine increase as the turbine sizes increase, it was found that the assembly costs per installed kW did not experience dramatic changes. The increased assembly effort associated with larger land-based turbines does not appear to increase faster than the power rating of the turbines.

Transport is limited by handling equipment and infrastructure, such as bridge height and allowable surface loads. Normal transportation by road or rail is possible for smaller components, for larger components it is becoming an increasing problem. Some manufacturers make use of standard containerized units for transport by sea, road and rail. Components can be fitted with corner fittings or lifting frames.
Most turbine components, .normal. and oversized, can be handled with conventional rigging and transport equipment. This includes specialized equipment used for general heavy lift and oversized cargo handling and transport. Road transport of oversized components is frequently carried out at night.


Offshore

There is a growing interest in the expansion of offshore exploration of wind energy. After several pilot projects, large-scale commercial wind farms are being constructed and several are already operational. So why go offshore?

An offshore wind environment will experience high wind speeds for longer periods of time than an onshore location. Local wind speeds are often significantly higher offshore than onshore because of the absence of obstructions. The temperature difference between the sea surface and the air above it is far smaller than the corresponding difference on land, particularly during the daytime. This means that the wind is less turbulent at sea than over land. This, in turn, will mean less turbulent winds and lower mechanical fatigue load. Thus longer lifetime for turbines located at sea rather than land can be achieved using the same turbine or components.
Another argument in favour of offshore wind power is the generally smooth surface of water. Wind shear above the surface of the sea is much lower then above land.
Reduced wind shear and the costs involved in the support structure, installation and maintenance, will lead to a lower optimum hub height at sea in comparison to land-based turbine towers.

In offshore deployment, the relatively high cost of initial foundations makes it imperative to maximize each foundation by installing the largest possible turbines. Offshore wind farms will use the largest turbines developed and proven for onshore use.
Up till now, operation in the offshore environment is limited to the possible adaptation of land-based wind turbines. This means that extra efforts are made with respect to sealing of bearings, avoiding salt water or salt spray access to the cooling system or generator and gearbox, and corrosion protection. Installation and maintenance procedures are also modifications of the standard onshore practice.
A non-constant speed operation in an offshore environment is favourable for the following reasons:
  • Added compliance
  • Lower structure loading
A non-constant speed operation can only be operated with an indirect grid connection. An indirect connection will also enable the control of the quality of power output to improve the power quality in the electrical grid. This may be useful, particularly if a turbine is running on a weak offshore electrical grid.

Furthermore, on European land based sites the maximum tip speed of wind turbine blades is limited primarily by acoustic noise. Most machines of the leading manufacturers have tip speed lower than 70 m/s although a few machines, not generally market leaders, adopt high tip speeds above 100 m/s. Apart from acoustic considerations, a higher tip speed is advantageous, implying lower torque for a given power rating and lighter and cheaper tower top systems.


Support structure

Although offshore (sub-sea) structures are plentiful, the development of support structures for offshore wind turbines is not considered a common practice. The technology and design of foundations for the offshore exploration of resources is both well developed and well established and therefore there is a huge scope to draw on the accumulated knowledge within the existing practice of offshore technology.
The support structure will be typically designed on a case specific basis. Local conditions will determine loading and demands for fatigue resistance. Therefore support structures will differ for similar size turbines at different locations. It is unlikely that one foundation will be suitable for all situations.
Recent studies determined the piled tripod as being the lightest structure, due to the light foundation piles. However, fabrication of the tripod is likely to be more costly than the monopile and requires more space during transport and handling.
The gravity base structure is more difficult to compare with the other structures, due to the very different type of material and manufacturing process. However, it must be noted that gravity base designs are very case and site specific and this conclusion is not generic. Furthermore, installation, seabed preparation, scour-protection and non-technical issues also differ significantly from those of the steel structures.

It was concluded that the support structure of the monopile type combined with wind turbines up to about 3 MW offers superior performance at demanding Southern North Sea environments with water depths up to 20 m (LAT) and firm soil conditions. Beyond this turbine size, for greater water depth, weaker soils or even more exposed sites other support structure concepts are probably required.


Offshore wind farming

Recent studies concluded: Using a larger scale wind farm does not influence the absolute minimum cost level for which electricity can be produced at prime sites in a significant way. Instead, the benefit offered by large-scale offshore wind energy converters (OWECS) is the greater proportion of sites available for lower cost levels. Therefore exploitation of the sites with superior performance can start on a near to medium time scale based on mature 'megawatt technology'. For an utilisation in the range of (tens of) gigawatt on a longer time scale, development of multi-megawatt technology may offer advantages. The optimum size for an offshore farm today appears to be 100 MW or above. There is no limit to farm size other then the available sites on a given location with respect to water depth and soil conditions, and the individual turbine distance or spacing.

There are several vessel types available for the installation and transport of offshore turbines and components. These are almost all designed for conventional offshore work. A predominant issue with respect to cost seems to be lifting height. The cheaper equipment of the floating sheer legs and self-elevating platform types seems to be no longer available above approximately 85 m hoist height and this can have its impact on the economic viability of certain design concepts.
For the existing wind farms two methods of transport have been used to transport the components to the location:
  • Installation vessel is used for transport and installation. This will have the advantage of 'one-stop-shopping' and single vessel installation.
  • A separate vessel is used for transport and will 'shuttle' between the supply port and the installation site. Separating the transport and the installation activities will enable the use of low-cost transport vessels.
The following assumptions can be made with respect to transport and offshore installations of wind turbines:
  • Where possible, transport of components to a supply port should make use of water-based transport to avoid being limited in size by onshore handling and transport equipment.
  • Pre-assembly will facilitate installation. Pre-assembled turbines and components allow testing and commissioning before installation. A longer-term objective should aim for an integrated design, where the foundation and the turbine are transported and installed as one piece. The installation procedure should at least be simplified and include a minimum of operations offshore. For the near future, this will focus on optimizing separate installation of support and superstructure.
  • An offshore wind farm requires much closer integration of the design and construction activities than an onshore wind farm because of the additional challenges of operating at sea. Using a dedicated installation vessel can make significant time and cost savings.
  • The total build duration for a multi-unit wind farm is likely to take several months. All installation operations will be subject to weather constraints and there will inevitably be periods of non-operation/weather down-time.
  • The highest uncertainty in offshore installations relates to time delays and costs in use of rented equipment. Also, it is important to minimise the time needed for offshore operations as any unscheduled downtime. There is a need for installation vessels that can withstand more severe weather conditions and operate for longer periods of the year.
A comparison showed the one-stop-shopping concept to be the most effective method of installing offshore wind turbines. The possibility to transport and install a completed structure is the main focus for development of a PUFO (Pick-up and Float-over) installation concept.


Availability

Onshore wind turbines are now enjoying availability levels in excess of 97% with appropriate routine servicing and responsive maintenance actions. Recent studies concluded that O&M strategy should be optimised with respect to localised energy production costs rather than pure capital or O&M costs. Further, the availability of offshore wind turbines with commercial offshore wind turbines without significantly improved reliability and without optimised operation and maintenance solution may be unacceptably low, e.g. 70% or less.

Turbine availability is one of the most important parameters to be considered in the design of an offshore turbine. O&M demands will impact considerably on costs of offshore wind turbine systems and affect optimum scale for minimum cost of energy Furthermore, maintenance and repair for offshore turbines will be far more expensive in comparison to land-based installations because of reduced serviceability.


Developments in offshore wind energy

One of the main issues to be addressed in future offshore wind exploration is the location of wind farms further offshore. Public demand in coastal communities and better wind conditions are forcing wind farm developers to move beyond the horizon.
Although a genuine integrated approach will most likely produce an innovative design solution, the evolutionary nature of technical progress and the commercial risks inherent to innovative solutions must be kept in mind.

The main issues can be summarised as follows:
  • Structural dynamics and combined wind and wave loading. Integration of wind and wave loading into the structural dynamics codes is necessary to avoid overdesign.
  • Translating the absence of noise emission constraints (tip speeds of rotors can be higher, leading to smaller transmission ratios and lighter drive trains) and lower turbulence intensities into relative cheap constructions.
  • Development of dedicated offshore concepts.
Wind turbines manufacturers - who have been accumulating considerable experience with the manufacturing and operation of land-based MW+ sized wind turbines, are now developing offshore turbines. Two routes are to be distinguished in their work:
  • Using land turbine concepts and modifying them, so they are suitable to operate offshore (until now in shallow waters);
  • The development of purpose-built offshore designs for installation at greater water depths (15 - 20 m).

Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2003.10.20; logistics@3mE.tudelft.nl , TU Delft / 3mE / TT / LT.