Operation and maintenance of offshore wind farms is more difficult and expensive than equivalent onshore wind farms. Offshore conditions cause more onerous erection and commissioning operations and accessibility for routine servicing and maintenance is a major concern. During harsh winter conditions, a complete wind farm may be inaccessible for a number of days due to sea, wind and visibility conditions.
Even given favourable weather conditions, operation and maintenance tasks are more expensive than onshore, being influenced by the distance of the OWECS from shore, the exposure of the site, the size of the OWECS, the reliability of the turbines, and the maintenance strategy under which they are operated.
Offshore installations require specialist lifting equipment to install and change out major components. Such lifting equipment can usually be sourced locally and at short notice for onshore wind farms.
The severe weather conditions experienced by an OWECS dictate the requirement for high reliability components coupled with adequate environmental protection for virtually all components exposed to sea conditions.
Consequently, the requirement for remote monitoring and visual inspection becomes more important to maintain appropriate turbine availability levels.
Operational information for onshore wind turbines has been compiled for a number of years which is directly relevant for operation and maintenance issues.
“WindStats” newsletter is a quarterly international wind energy publication with news, reviews, wind turbine production and operating data from over 12,000 wind turbines in Denmark, Germany, Belgium, USA, Sweden, Spain and The Netherlands.
However, WindStats provides very limited information for 1 MW plus turbines. A more relevant source of operating information is provided by turbine manufacturers who either have data in their publicity material or will usually provide data on request.
The overall picture of turbine availability is very good for all major manufacturers who have turbines in full production. For instance, Vestas V66, Enercon E66, Bonus 1.3 MW, Nordex 1.3 MW, Enron/Tacke 1.5 MW all have fleet-average availability of at least 97%. Information on maintenance effort to achieve this is practically unavailable, except through fault reports published in Germany and Denmark (summarised in WindStats).
Monthly wind turbine statistics for Sweden are published by SwedPower AB, and are available on the internet at www.elforsk.se/varme/varm-vind.html .
Published statistical information on the availability, accessibility and reliability of offshore wind turbines is presently limited to site specific information released at the discretion of wind farm operators. Therefore we are dependent on published data from the few existing truly offshore wind farms constructed since 1991. Current offshore wind farms are mostly small in comparison to onshore wind farms, although large scale wind farms, typically around 100 machines, are anticipated.
Operation and maintenance data for onshore wind turbines are readily available as detailed above. However, the environmental conditions associated with offshore installations renders this current machine data inadequate.
Maintenance strategies have been developed in the Opti-OWECS project using Monte Carlo simulations. A simple expert system has subsequently been developed based upon analytical trend curves determined from a large number of Monte Carlo simulations [1].
In the Monte Carlo model, the site accessibility as well as the failures of the wind turbines in the OWECS are simulated stochastically on an hour to hour basis. The response in terms of deployment of maintenance and repair crew, and equipment, is simulated simultaneously in the model. This results in the determination of the instantaneous and overall availability of the OWECS and of the instantaneous and overall costs associated with the adopted maintenance strategy under the assumed site conditions
As mentioned above, ‘expert systems’ [2] have been developed which represent the trend lines found from the far more comprehensive Monte Carlo simulation model. This simple approach enables the assessment of availability and O&M costs for a given OWECS with its O&M strategy as a function of distance to shore and site (wind) conditions. The analytical functions used in this expert system have also been used for the concept evaluation. With them, the OWECS availability and O&M costs could then be determined and optimised for a range of scenarios. [3].
The availability of a wind turbine largely depends on the O&M strategy adopted by the operators of a wind farm. Given the limited amount of offshore O&M data, strategic planning is in its infancy, however a number of options were developed in the Opti-OWECS study:
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1. |
No maintenance: |
Neither preventative nor corrective maintenance are executed, and major overhauls are performed every five years or so. One of the few alternatives is exchanging a whole turbine if availability drops below a predefined minimum or after a certain amount of operational hours. Given the current level of turbine failure rates, this option is not presently viable.
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2. |
Corrective maintenance only: |
Repair carried out soon after a turbine is down, or, alternatively, wait until a certain number of turbines are down. No permanent maintenance crew is needed
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3. |
Opportunity maintenance: |
Executing corrective maintenance on demand and taking the opportunity to perform preventive maintenance at the same time. No permanent maintenance crew is needed
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4. |
Periodic maintenance: |
Scheduled visits performing preventative maintenance, and corrective actions performed as necessary by a permanent dedicated maintenance crew. |
The Opti-OWECS study 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 OWECS 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.
In conclusion, given current reliability and failure modes of commercial offshore wind turbines, which have been adapted from onshore models, a reduced level of preventative and corrective maintenance is not a viable option at this stage in the development of the offshore wind energy industry.
Onshore wind turbines are now enjoying availability levels in excess of 97% with appropriate routine servicing and responsive maintenance actions. However, in practice, this typically equates to visiting a wind turbine four times a year, either for regular service or for repair tasks. [1].
Vestas cite a comparison between availability rates for the Fjaldene onshore wind farm and Tuno Knob offshore wind farm [4]. Average availability for Fjaldene is quoted as 99.3% mainly due to the proximity of this windfarm to Vestas’ Central Service Department.
Tuno Knob average availability is quoted as; 97.9%, 98.1%, and 95.2% for the years 1996 to 1998 respectively. [5].
As stated above, operating expenditure for offshore wind farms is considerably higher than the equivalent onshore facility. Offshore operations are in the region of five and ten times more expensive than work on land, and these costs are exacerbated by inflated prices prevalent within the offshore oil and gas industry. For example, the day rate for an offshore lifting vessel, which will be well over capacity for the wind industry, will typically cost at least ten times that of an appropriate land based crane.
Also, onshore equipment can be sourced and mobilised within a short period of time, usually within hours, and available on site within a day. Offshore lifting cranes are uncommon, and will generally have to travel a considerable distance to an offshore wind farm site, hence the requirement for careful scheduling of such vessels movements. The economics of a large wind farm (e.g. 100 machines) may justify the purchase of a dedicated purpose built lifting vessel which would be available during installation and for maintenance throughout the wind farms lifetime. However, it is commercially expedient to dispense with the need for expensive lifting vessels after installation and hire lifting equipment during scheduled major overhaul. Given relatively calm sea conditions, it is possible to use a floating barge to transport and operate a land based crane offshore. The floating barge need only be a crude construction incurring minimal expenditure, hence be procured and stored for and at a dedicated wind farm.
General maintenance tasks are carried out using less specialised equipment which is generally purchased for the design life of the wind farm.
Operation and maintenance costs mainly related to the wind turbine can account up to 30% and more of the energy costs. [6]. Recent discussions with leading wind turbine manufacturers have indicated that O&M costs, given 95% availability warranties (excluding weather constraints, and dependent on the scale of the project), is approximately £30,000 per turbine per annum for the UK market. The cost of operation and maintenance for the first year of operation may be higher.
The service demand of the present generation of offshore wind turbines in terms of man-hours is in the order of 40 to 80 hours [7]. Service visits are paid regularly, (except in the more demanding first year) about every six months. A more major overhaul will be undertaken every five years, and will take around 100 man hours to complete. [1].
Experience from Tuno Knob show that the total number of service visits have been about 35 to 70 visits per year, an average of approximately 5 visits per turbine per annum. The number of cancelled visits (last moment cancellations due to weather) makes up about 15% relative to the number of service visits realised. [8].
Gaining access to an OWECS for routine servicing and emergency maintenance is difficult or impossible in harsh weather conditions due to wave heights, wind speeds and poor visibility. The traditional and obvious method for transporting personnel and equipment is by boat, which is limited to relatively benign sea states. Wave heights above one metre present serious concerns for health and safety issues and damage to equipment.
Since the beginning of offshore wind farm development, suggested methods for gaining safe access have included:
· Helicopter
· Underwater tunnels
· Wheeled platforms for turbines in close proximity to the shoreline
· Amphibious vehicles where caterpillar tracks transport a platform over a firm and stable seabed
· Small hovercraft or ice roads for frozen seas.
For the present discussion, only the principle advantages and disadvantages of boat (plus jack-up) or helicopter access will be considered:
Boat Access
Advantages:
· well proven method of inshore transportation
· relatively cheap equipment expenditure
Disadvantages:
· impractical for wave heights greater than 1m (dependent on vessel)
· transfer of personnel and equipment difficult in rough conditions
Jack-up
Advantages:
· vessel can be raised above waves to provide a stable access platform
· heavy equipment can be transferred
Disadvantages:
· requires firm seabed conditions
· existing jack-up vessel designs are too large, hence purpose built designs are necessary
· high capital cost of vessel
· installation sequence must be previously defined (cable installation later on)
· sensitive to wave conditions during deployment and retraction of legs
Helicopter Access
Advantages:
· sea state is not a major issue
· quick transfer of personnel and equipment from land to turbines
Disadvantages:
· cost of equipment and qualified operating staff
· turbine must be shut down and locked prior to boarding, and flying is restricted to good visibility and wind conditions
· not possible to use for certain wind turbine fault conditions (for instance yaw bearing failure)
· expensive and cumbersome (landing platforms needed on each turbine)
Helicopter access is routinely used for oil and gas installations and offshore lighthouses, however it is unlikely that this mode of transportation can be reasonably considered for OWECS.
From recent reported experience, it has not been possible to access Vindeby turbines in heights of more than 1 metre using an 8 metre launch, but nevertheless turbines reportedly had an accessibility of 83% for the time during the first 12 months of operation in 1992. However, during the worst month accessibility fell to 45%. It was found that the conical foundation amplified the waves, making boat landing more difficult especially in winds from the north or north-west. Access was limited to wind speeds of less than 7-8 m/s from the north or north-west and 12 m/s from other directions. Solid ice around the foundations and blocking the boat’s nearby home harbour also prevented access for several weeks, although this amount of ice was unusual. The travelling time of approximately 30 minutes in each direction also affected availability and maintenance. [9].
At Tuno Knob a 32 foot fibreglass boat (forward control fishing boat with flat stern) .is used for the service rounds The boat weighs about 11 tonnes and is equipped with a 185 hp diesel engine. [8].
In conclusion, there are a number of current projects addressing the issue of improved access to offshore wind turbine installations. Most focus on maintaining existing boat access methods with emphasis on addressing the issue of motion compensation or complete removal of the vessel from the water at the turbine location. The potential for using small purpose built jack-up vessels with integral craneage is also a possibility assuming a sufficiently large wind farm is to be serviced. However, access using small purpose-built landing craft continues to present the most pragmatic and economic solution.
Improvements made to the base of OWECS to facilitate safe personnel access include:
· Fixed platforms fixed to tower above splash zone with fender posts to absorb vessel impact
· Flexible gangways extended from the vessel and held in the lee of the OWECS base.
· Installation of friction posts against which the vessel maintains a forward thrust during transfer
· Facility for winching the vessel out of the water during harsh sea conditions
· Winch / netting for personnel and equipment
As mentioned above, there are significant advantages in eliminating the need for specialist lifting vessels currently necessary during overhaul or major component replacement. For a number of current offshore wind turbines, craneage facilities (either permanent or temporary) within the nacelle are capable of lifting some of the heaviest components. At Tuno Knob, special electrical cranes were installed in each Vestas V39 turbine to allow replacement of major components, such as rotor blades or generators, without using a large and expensive floating crane. However, all other currently available turbine models require external cranes for the more demanding lifts, although Vestas claim to be able to change rotor blades with on-board cranes on their V80 2 MW machine.
The issue of accessibility can also be addressed by improvements in offshore wind turbine reliability. Both planned and, more importantly, unplanned maintenance levels can be reduced by increasing the reliability and hence availability of the turbine. Particular emphasis is being placed on reliability issues from component level through to overall design improvements such as corrosion protection and component siting.
NEG Micon’s new 2 MW turbine has a fibreglass cabin within the nacelle which encloses the transformer, power and control cabinets within a controlled nacelle environment.
Rotor blades
Current OWECS utilise a three bladed configuration, and it appears that this will continue to be the popular choice of turbine manufacturers. However, two bladed configurations incorporating alternative hub structures may see a rise in popularity given the opportunity to operate turbines at higher rotor speed and without visual constraints. The main advantages from a reliability perspective are the reduction in the number of components, reduced complexity of the hub and easier rotor lifting. The track record of teetering mechanisms is not favourable, and for this reason these may be avoided for offshore use.
Gearboxes
Onshore turbine manufacturers, notably Enercon and Lagerwey, specialise in direct drive generators therefore eliminating the need for a gearbox. Current offshore turbines manufactured by leading manufacturers favour geared drive transmissions. Being the widely recognised as the number one item for mechanical failure and servicing supervision, it would appear a progressive step to move to direct drive systems.
Aerodyn who are currently designing the 5MW Multibrid Technology favour a drive-train consisting of single stage planetary gears, combined with a slow rotating generator, therefore eliminating fast-running components which are prone to wear. [10]
Generators
In general, induction generators require less maintenance than synchronous generators. They do not require a DC source and being inherently more simple and robust are the most common generators in onshore wind turbines.
To protect standard induction generators from marine environments, the generators is totally enclosed with integral insulation to protect the internals from salt and high levels of moisture.
Onshore generators rely on air cooling, which is not recommended for offshore applications. Closed system water cooling or air-to-air heat exchange prevent the risk of corrosion from maritime cooling air.
Direct Drive Systems
Ring type direct drive systems have been developed for onshore wind turbines, primarily by Enercon and Lagerwey. Direct drive systems dispense with the historically problematic gearbox, where the drive train, generator and rotor rotate at the same speed of around 20 rpm for a 2 MW OWECS.
The advantages of direct drive generators are obvious; no gearbox with associated high speed rotating parts, no gearbox oil contamination and leakage, and less routine servicing, to name a few. However, the direct drive generator for megawatt turbines is extremely heavy, bulky and the large diameter required changes the visual appearance of the nacelle. The added tower top mass coupled with increased wind loading increases tower stresses and hence tower dimensions.
The ring generators developed by Enercon are multipole synchronous machines with the copper windings impregnated with resin for environmental protection. Heat is dissipated by conduction via the high surface area steel structure.
ABB’s Windformer is a large diameter gearless generator using permanent magnets rather than coils or electromagnets. No transformer is required as the power is produced at 25 kV DC, compared with AC at less than 1 kV for most turbines. Halved lifetime maintenance costs as well as arguable benefits of up to 20% higher power conversion efficiencies have been claimed [11].
Electrical & Electronic Components
Electrical and control system failures account for the highest percentage of failures. For the year 2000, failures of electrical and controls systems accounted for exactly 50% of the need for wind turbine repairs [12]. Typically, failures of this nature occur due to the number of components, poor electrical connections, corrosion, lightning strikes, etc.
Potting of electronic printed circuit boards and reduction in the number of components are necessary for offshore conditions.
Hydraulic Systems
Elimination of problematic hydraulic systems employed in yaw damping, blade pitching and breaking systems should be realised wherever possible. Electrical actuation is preferable and eliminates the possibility of oil leakage leading to secondary component failure and potential fire risks.
The main methods of marine corrosion protection for offshore installations, recently developed within the offshore oil and gas industry, are selection of corrosion resistant materials, two-pack epoxy coatings, cathodic protection, and creation of controlled environments for sensitive equipment.
The potential wind farm sites being considered in the North and Baltic Seas present harsher maritime conditions in terms of severe sea conditions and higher salinity levels.
More work is needed in developing support structures which can withstand stresses caused by wind and wave loading, together with reductions in material fatigue strength caused by corrosion. Cathodic protection technology of subsea structures is integral in the front end engineering design, with due consideration of state-of-the-art paint systems and metal spray coatings particularly for application within the splash zone.
Surveys of machine outages reveal that around half the unplanned shutdowns on onshore turbines are caused by faults and trips in the electrical and electronic control systems. To reduce the number of unplanned visits to an OWECS, automatic re-set and remote re-set facilities are now becoming common in all new turbines. Increasing numbers of sensors and monitoring equipment are being used, and the signals categorised to register; data, minor faults requiring notification only, or major faults which shut the turbine down automatically.
Using SCADA (System Control And Data Acquisition) systems, monitored signals and alarms are transmitted between the turbine and the onshore control station. Control personnel can interact with the monitoring system to over-ride the turbine controller if necessary.
Internet connections, webcams and sophisticated vibration monitoring for example can now be utilised to detect a limited number of pending failures prior to their occurrence.
Power for the turbine controller, electrical actuators, monitoring and communications systems are drawn from the turbines gross output, or imported from the grid system.
In the event of loss of turbine power generation or lost electrical grid connection, there is no power at the isolated turbine for maintenance work or to keep turbine systems running. At Horns Rev, it is intended to have a back-up diesel generator sited on the substation platform to provide power should the electrical connection to shore be broken.
An important aspect of future wind turbine development is the requirement to adapt existing onshore designs to cope with harsh maritime environments
As indicated in the previous sections, reductions in the lifetime O&M costs of OWECS will require the following to be addressed:
· Development of appropriate maintenance strategies for scheduled and unscheduled maintenance, reflecting the constraints on OWECS in terms of access.
· Improvement of access methods for unscheduled and scheduled maintenance.
· Development of access methods which are less sensitive to wind/wave conditions.
· Reduce time required for offshore working
· Designs for reduced maintenance by:
· Reduction in overall number of components and simplicity of design
· Modular design approach which facilitates the interchange of faulty modules
· Use of high reliability integrated components
· Re-siting of electrical units into an environmentally controlled section of the turbine
· Implementation of offshore corrosion protection technology
· Development of effective conditioning monitoring and remote control systems
1. G W van Bussel – “Reliability, availability and maintenance aspects of large-scale offshore wind farms, a concepts study”, Delft University of Technology, The Netherlands, MAREC 2001 Conference Proceedings, pages 119 – 126.
2. Van Bussel, G.J.W. “The development of an expert system for the determination of availability and O&M costs for offshore wind farms”. Proceedings from the European Wind Energy Conference, Nice, March 1999, pages 402 – 405.
3. Hendriks HB (et. al.) “DOWEC concepts study. Evaluation of wind turbine concepts for large scale offshore application. ”OWEMES 2000 Proceedings, Sicily, April 2000, pages 211 – 219.
4. TK Petersen – “Offshore wind power – the operational aspects”, Vestas Danish Wind Technology A/S, Lem, Denmark.
5. CADDET report “5 MW Offshore Wind Farm”, September 1999, http://194.178.172.86/register/datare/ccr01855.htm
6. Opti-OWECS Final Report, Volume 0, para 5 (v) main conclusions.
7. Chr. Schöntag, “Optimisation of Operation and Maintenance of Offshore Wind Farms”, Report IW-96-108R, Institute for Wind Energy, TU Delft, The Netherlands, November 1996.
8. Tuno Knob - Garrad Hassan questionnaire response, April 2001.
9. Smith, G.S. – “Design for improving the reliability and accessibility of offshore wind plant”, MSc Degree report, Loughborough University, September 2000.
10. Aerodyn Multibrid 5MW machine, www.multibrid.com
11. “Competitive wind farms, does ABB have the answer?” SED Aug/Sept 2000, p27
12. WindStats Newsletter – Autumn 2000, Vol. 13 No.4, page 10.
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Updated September 2008
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