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Future Trends for Offshore Wind

GENERAL

The offshore wind sector remains relatively immature, and despite the first demonstration project being built in 1991, the total installed capacity only breached the 1,000 MW barrier in 2007/08.  Added to this, experience has shown that the sector presents unique technical challenges that must be addressed through research and development efforts:

• Any project involves multiple distributed installations, spread over much larger areas and in much larger numbers than other offshore industries;
• Nearshore shallow water (for most projects) siting - unlike oil and gas, sea defence works, ports and harbours; and
  
• More stringent economics than oil and gas.

These factors combine so that that there is limited borrowing available from other sectors, and technology has had to evolve within a short timescale on a small number of projects, leaving significant scope for further maturing.  This issue covers all parts of the industry including:

• Wind turbines;
• Wind turbine support structures;
• Modelling tools;
• Electrical infrastructure;
• Assembly and installation; and
  
• Operations and maintenance.

Two drivers cut across all these areas: safety of personnel and the public and environmental protection.

EWEA has led the EU Wind Energy Technology Platform (see Chapter 7) and has convened a working group to identify necessary future technical initiatives for offshore wind.  These issues are discussed here.

WIND TURBINES

Wind turbine technology in general is discussed in Chapter 3, with some future innovative wind energy conversion systems that may be exploited on land or offshore are reviewed under Future Innovations.

It has long been acknowledged that some of the design drivers for a wind turbine installed offshore are fundamentally different from those installed onshore, specifically:

• The non-wind turbine elements of an offshore project represent a much higher proportion of the capital cost, with that cost element only partially scaling with turbine size;
• Acceptable noise levels are much higher offshore; and
  
• Better reliability is required offshore.

These drivers have already influenced the design of wind turbines used offshore and this is leading to the development of wind turbines specifically designed for offshore use with features such as:

• Larger rotors and rated power;
• Higher rotor tip speeds;
• Sophisticated control strategies; and
  
• Electrical equipment designed to improve grid connection capability.
  

Wind resource assessment offshore provides more background, but other technological innovations that may be deployed in future offshore turbines include:

 

• Two-bladed rotors;
• Downwind rotors;
• More closely integrated drive trains;
• Multi-pole permanent magnet generators;
• High temperature superconductors (in generators);
• High voltage output converters (eliminating the need for turbine transformers).

WIND TURBINE SUPPORT STRUCTURES

As shown in Figure 5.2, support structures form a significant proportion of offshore wind development costs. It is expected that there will be both innovation and value engineering of structure designs and improved manufacturing processes to improve the economics and meet the demands for more challenging future sites and wind turbines.

 

Figure 5.13 Typical capital cost breakdown - large offshore wind farm

 

Source: Garrad Hassan and Partners Ltd

Such developments are likely to include modifications to conventional designs, scale-up of manufacturing capacity and processes, and more novel design concepts. Such innovative designs may include:

 

• Suction caisson monotowers;
• Use of suction caissons as the foundation of jacket or tripod structures;
• Application of screw piles;
• Floating structures; and
• Braced supports to monopiles.

There are two key aspects to the maturing of offshore support structures: 

• Acquisition of data on the behaviour of the existing structures in order to support research into the development of improved design tools and techniques and better design standards; this will be used to extend the life of structures, to reduce costs and to develop risk-based life cycle approaches for future designs; and  
• The build-up of scale and speed in production in order to achieve cost reduction and the capacity necessary to supply a growing market 

The development of floating structures, while long-term, will be a major advance if successful.  This is discussed further in Offshore Support Structures.

MODELLING TOOLS

The offshore wind sector will deploy turbines of greater size and in greater numbers than has been done previously.  Understanding of the engineering impacts of this is achieved through modelling, and this increase in scale requires development and validation of the industry’s modelling tools.  Associated with this is the refinement of design standards.  The priority areas that must be addressed for large offshore wind farms are:

• Development of wind turbine wakes within the wind farm;
• Mesoscale modifications to the ambient flow in the immediate environs of the wind farm;
• Downstream persistence of the modification to ambient flow, and therefore the impact of neighbouring wind farms upon each other; and
• Dynamic loads on wind turbines deep within wind farms.

 

ELECTRICAL INFRASTRUCTURE

Incremental development in electrical equipment (switchgear, transformers and reactive power compensation equipment) is to be expected, driven by the wider electricity supply industry.  The offshore wind business will soon be the largest market for subsea cables and so some innovations there may be driven by the specific requirements of the sector, although cables are a relatively mature technology.  Voltage-source high voltage DC transmission is a relatively new commercial technology, and one that will find extensive application in offshore wind.

The major electrical impact of the offshore sector will be the re-shaping of the transmission network of the countries involved in order to serve these major new generating plants.  Also to be expected is an increase in the interconnection of countries to improve the firmness of national power systems, and which may also involve providing an international offshore transmission network dedicated to serving offshore wind projects.

ASSEMBLY AND INSTALLATION

 

Future technical developments in the construction process are likely to be:

• Improvements in harbour facilities that are strategically located for the main development regions;
• Construction of further purpose-built installation equipment for the installation of wind turbines, support structures and subsea cables: vessels and also piling hammers, drilling spreads and cable ploughs; and
• Development of safe, efficient, reliable and repeatable processes to reduce costs, minimise risks, guarantee standards and deliver investor confidence.

 

OPERATIONS AND MAINTENANCE

Successful performance of O&M is most critically dependent on service teams being able to access the wind farm as and when needed.  Good progress has been made on this in recent years and accessibility has improved significantly.  This has been achieved by incremental improvements in:

• Vessels used;
• Landing stages on the wind turbine structures; and
• Procedures.

 
Future offshore wind farms offer new access challenges, being larger and much further offshore.  This will result in increased use of helicopters for transferring service crews, larger vessels to give fast comfortable transit from port to site, and the use of offshore accommodation platforms, combined with evolution of strategies to perform O&M.

FLOATING SYSTEMS

The US DoE has hosted conferences on ‘deepwater’ solutions in recent years. In both the EU and the USfor over 10 years, there has been exploratory research of floating offshore systems and preliminary development of design tools for modelling a wind turbine system on a dynamically active support that is affected by wave climate.  Until recently, such technology, even at the level of a first demonstration, was considered rather far in the future.  However interest has accelerated and demonstration projects have been announced – indeed the first was installed in 2009 (see Statoilhydro-Siemens).

The main drivers for floating technology are:

• Access to useful resource areas that are in deep water yet often near shore;
• Potential for standard equipment that is relatively independent of water depth and sea bed conditions;
• Easier installation and decommissioning; and
• The possibility of system retrieval as a maintenance option.

The main obstacle to the realisation of such technology is:

• Development of effective design concepts and demonstration of cost effective technology, especially in respect of the floater and its mooring system 

STATOILHYDRO-SIEMENS

 

StatoilHydro and Siemens Power Generation entered into an agreement to cooperate on technology to develop floating wind turbines, based on StatoilHydro's Hywind concept.  StatoilHydro has built the world’s first full scale floating wind turbine and test it over a two-year period offshore near Karmøy, an island to the southwest of Norway.  The company announced the investment of approximately NOK 400 million and deployed the field in 2009.

A Siemens 2.3 MW wind turbine (80m rotor diameter) is set on a floating column of the spar-buoy type, a solution long established in oil and gas production platforms and other offshore floating systems.

The flotation element has a draft of 100m below the sea surface, and is moored to the seabed using three anchor points.  The system can be employed in waters depths ranging from 120 to 700m.

Figure 5.14: The SWAY® concept

Source: Sway

The combination of two established technology solutions in the wind turbine and spar buoy may be considered a prudent approach to the development of offshore floating systems.

StatoilHydro has also acquired a substantial share in the technology company Sway, which is developing a highly innovative solution for system support.  The SWAY® system (Figure 5.14) is a floating foundation capable of supporting a 5 MW wind turbine in water depths ranging from 80 m to more than 300m in challenging offshore locations.

In the SWAY® system, the tower is stabilised by elongation of the floating tower to approximately 100m under the water surface and by around 2,000 tons of ballast in the bottom.  A wire bar gives sufficient strength to avoid tower fatigue.  Anchoring is secured with a single tension leg between the tower and the anchor.

The tower takes up an equilibrium tilt angle (typically around 5 to 10°) due to the wind thrust on the rotor.  During power production in storm conditions, there is expected to be a further variation of only +/-0.5 to 1.0° from the equilibrium tilt angle due to wave action.

The concept exploits active control of rotor thrust, and claims to achieve substantial cost savings over competing technology for deep water applications.  The first full-scale wind turbine is expected to be built and installed by 2012.

BLUE H

 

A prototype installation using a concept similar to the tension-leg platform developed in the oil and gas industry was launched in late 2007 by Blue H Technologies of the Netherlands.  The concept was demonstrated through full installation and testing in 2008 in a water depth of over 100 m, approximately 17 km offshore from Puglia, Italy.