Offshore electrical infrastructure and networksOffshore electrical infrastructure (including offshore substations, transmission cables to shore and inter-array cabling) presently makes up approximately 14% of offshore wind's levelised cost of energy, although this is expected to increase with wind farm locations further from shore and the expected use of HVDC connections. According to the Offshore Renewable Energy Catapult, between 8,000 km and 12,500 km of inter array cabling is associated with Round 3 offshore wind developments, highlighting the need for cost reduction. Reliability is also a key issue for the cables and associated equipment (transformers and switchgear) given that there is often limited redundancy, offshore repairs/replacements are more costly and the duration of downtime is often much longer. Topics for which the industry has not yet agreed standards include finding the optimal voltage levels, e.g. 66kV or 33kV for array cables, the raising of operating voltages for lower cost wetted cables, long distance transmission (potentially using HVDC or low frequency AC) and developing contingency plans for major repairs using specialist vessels.
Transmission to shoreThe choice between Ac and DC transmission technologies is heavily dependent on the distance from the off-shore collection point to the on-shore grid connection point. At present this is around 100km for the transition from AC to DC technology, but what are the key factors in this choice?
To minimise the number of radial transmission links to shore, multiple off-shore systems can be linked together to form a grid. Thus fewer but larger transmission links to shore would be required. What are the issues in connecting off-shore systems to form an AC grid, e.g. power flow, frequency control, voltage control, etc? An off-shore DC grid could be considered. What are the issues in connecting off-shore systems to form a DC grid, e.g. power flow, fault detection, circuit isolation, etc? A DC grid could be connected to an interconnector, where the off-shore system is a significant distance towards an adjacent AC network, e.g. in Ireland, Scandinavia, or continental Europe.
Electrical system integration
Renewable energy (and nuclear) electricity generation poses increasing system integration challenges. An adequate amount of flexible backup plant is required, DECC scenarios make increasing use of demand side management, and some argue for an increased role for energy storage. Additional system pressure is expected to come from the displacement of gas heating by heat pumps and an increase in the use of electrical vehicles with increased demands on generation transmission and distribution. Development of our modest hydro generation could enlarge the pumped storage capacity to provide more dynamic support of developing renewable energy. The move to renewable power generation will also result in reduced power system inertia and related challenges to power system stability due to an increasing amount of asynchronous generation. Wind energy may have to play a larger part in helping system security through balancing services such as frequency response, reactive compensation and even black start. Increased interconnections with the continent through HVDC will allow greater power system operation flexibility and ease wind energy integration but the extent of this will depend on the generation mix in the other countries.
Offshore wind turbines
The wind turbine drive train presently represents about 16% of offshore wind's levelised cost of energy. Design and reliability issues have been identified and these are expected to become more significant as turbine capacities steadily grow driven by economies of scale offshore. Reliability is also increasingly important as repairs will become more difficult and costly with larger turbines located further offshore. Adopting different technology, for example direct drive turbines as some manufacturers have done, may remove some failure mechanisms but introduce others and consideration of more innovative drive trains would be beneficial. At this stage there is no consensus on the drive train design for large offshore wind turbines and longer term reliability challenges exist for all designs as these are scaled up. Innovations in electrical generator design and its power electronic interface have the potential to reduce costs and increase reliability.
Scaling up also has implications for blade design. Blade technologies must evolve in order to meet the rigorous demands and harsh operating conditions of the offshore sector whilst remaining cost-effective. Future wind turbines will use longer and more flexible blades operating at higher tip speeds and under greater mechanical loadings. Offshore, blades must cope with adverse metocean conditions, including lightning strikes, hail and salt spray. Blade erosion is of increasing concern in this context. Improved blade condition monitoring is a research area with potential to improve blade maintenance but remains technologically challenging.
Foundations and substructures
The expansion of offshore wind involves a move to deeper waters and more cost effective support structures required (covering monopoles, gravity bases, tripods and jackets). Even floating foundations are now being considered. This is an active area of research and innovation and moves towards standardisation are also seen as important.
There is some debate about the correct way to design monopoles and transition pieces to ensure the long term survival of the grouted interface. This needs to be resolved through design & standards to prolong the life of offshore assets to 25 to 30 years if that is practicable.
Scour and seabed movements
Scour is a feature that effects any structure on a movable seabed. Greater than expected local scour can change the eigen-frequency of the turbine leading to the possibility of dangerous resonance with the blade rotation. The combination of wave, tide and wind moments on the structures is not well understood. Global scour and seabed movements are not uncommon occurrences around the UK continental shelf and could have longer term implications for offshore wind farms. Cables can become exposed leading to risks of anchor drags and being caught by trawlers. Accurate and timely DTS (distributed temperature sensing) measurements to find cold spots and/or strain plus cost effective reburial techniques could be required. Improved methods of scour control and exposure protection are to be encouraged for those sites where scour has proved or will prove to be a problem.
The offshore renewable energy industry faces challenges around the need to install ever-larger equipment in deeper waters, further from shore. There are both technical and environmental challenges associated with installation. There are limited vessel designs suitable to manage the growing height and weight of offshore turbines in deeper locations, although $3Bn has been invested in specialist vessels in the last 5 years. Port facilities that can handle offshore equipment for installation purposes are limited in the UK but can be developed within the timescale of projects. Larger wind farms with more turbines, cables, etc. require more sophisticated installation co-ordination and optimisation. Limiting noise when installing foundation is required to reduce harm to marine mammals.
Operations and maintenance
Operations and maintenance (O&M) costs account for up to 25% of the levelised cost of energy from an offshore wind farm. As turbine size increases and water depths and distance from shore also increase, the challenge of operating and maintaining offshore wind farms becomes greater. New maintenance philosophies are required as it becomes uneconomic to send maintenance teams from port on daily basis. The use of helicopters, mother/daughter ships and offshore accommodation bases may be needed. Summer maintenance campaigns, making use of calmer days with less wind, may be required. Technology needs to develop at a fast rate to meet these growing technical demands. Proactive maintenance, making use of state of the art condition monitoring and sophisticated failure mode, effects and criticality analysis is necessary to reduce downtime.
Asset life management
Since offshore wind is relatively new and most operational sites are less than 10 years old a lot of potential aging issues may not be obvious yet. For example, if the main transformers used offshore are oil filled and no special treatment is given in the design, construction and operation, the insulation of the transformers may breakdown well before the end of the expected life due to breathing of salty air and continuous vibration. Also if there is no special consideration in anti-vibration isolators between the transformer and the platform, the rubber isolators may fail much earlier than expected.
As wind farms become operational it is necessary to ensure they generate the energy expected. Issues such as dirty/damaged blades, anemometry errors and yaw misalignments can all reduce generation to below what is expected.
Innovation, such as blade modifications and control system changes, can be used to increase yield above what initial design expectations by improving the power curve at low, medium and high wind speeds and improved forms of wind prediction for the large offshore wind farm sites would be beneficial both for yield prediction and O&M planning as would be improved data management structures.
Wind resource and offshore meteorology
Increasing wind power generation requires improved understanding of the resource and more accurate forecasting tools. Moreover, offshore meteorological and metocean conditions have a significant impact on offshore renewable energy exploitation and improved forms of wind prediction for the large offshore wind farm sites would be beneficial both for yield prediction and O&M planning.
Communication networks & data management infrastructure
Large offshore wind farms are enormous installed assets with a large number of identical capital items in operation. The integration of their communication networks and the management system is proving to be important for achieving the design yield and expected life. The communication networks themselves require consideration and the best ways to integrate data to provide manageable information for asset optimisation is an important topic for discussion and development.