«SYNOPSIS The DOWEC projects aims at implementation of large wind turbines in large scale wind farms. part of the DOWEC project a concepts study was ...»
Reliability, Availability and Maintenance
aspects of large-scale offshore wind farms,
a concepts study.
G.J.W. van Bussel*, PhD; M.B. Zaaijer, M Sc.
Section Wind Energy, Faculty Civil Engineering and Geosciences,
Delft University of Technology, The Netherlands
The DOWEC projects aims at implementation of large wind turbines in large scale wind farms. part of the DOWEC
project a concepts study was performed regarding the achievable reliability and availability levels. A reduction with a factor of 2 with regard to the present state of the art seems fairly easy achievable. This is however not sufficient for application at more exposed sites. Availability levels are lower than targeted, but moreover the O&M cost turn out to be substantially higher than initially anticipated. The main cause for the high O&M costs is the rather frequent need for an expensive external crane vessel. A second design round is necessary to reconsider the reliability levels adopted for almost all concepts. Furthermore a more "farm like design approach" is needed to reduce major maintenance cost and increase availability.
ANALYSIS OF THE O&M PROBLEM
Apart from the size of future offshore wind farms there is another evident and important difference with on shore wind farms. Not only the installation is more difficult and more expensive but also building wind turbines offshore has a major impact on the accessibility for maintenance purposes. It may well be that the complete wind farm is inaccessible by boat or helicopter for a period of one or two months because of harsh weather conditions (wind and waves). And even when weather permits access to the turbines, the cost of offshore maintenance is far higher than the equivalent job on shore.
* Authors Biography Dr. Gerard van Bussel is presently Associate Professor in Wind Energy at the Delft University of Technology. He has been working for more then 20 years in wind energy research. His present research interests cover rotor aerodynamics, offshore wind farm integrated design methods, and wind energy in the built environment. He is a member of the board of NEWIN and EWEA (the Dutch and the European Wind Energy Association respectively).
Michiel Zaaijer is presently research scientist in offshore wind energy at Delft University of Technology. His research focuses on dynamic behaviour of offshore turbines and integrated wind farm design methods. After his M.Sc. in physics in 1993 he has worked for several years as a researcher in the field of aircraft navigation Lifting actions are performed relatively easy on land, but in an offshore environment require special, and therefore expensive and sometimes scarce equipment.
RAMS Terms and definitions
For a proper understanding of this paper it is useful to first clarify a number of terms that will be used frequently:
Reliability of a system is the probability that the system will perform its tasks. This probability is usually determined as a percentage of time. For a wind turbine this indicates percentage of time it is producing the power that corresponds to the acting wind according to its nominal power curve.
Availability is the probability that the system is operating satisfactorily. The major difference between reliability and availability is the O&M strategy of the system. A system can be very reliable: i.e. its failure frequency is extremely low, but when no maintenance or repair action is taken after a failure its availability becomes very poor.
Maintainability is a more qualitative issue that addresses the ease of repair issue. It can though be expressed in terms of hours needed to complete a repair action.
Serviceability regards in a similar way the ease of regular (scheduled) service.
Failure is the termination of the ability to perform a required function of a system.
Accessibility is the percentage of time that a (offshore) construction can be approached. Evidently the accessibility depends upon the equipment used.
Availability of wind turbines
A large number of aspects play a role in the resulting availability of technical systems. Apart from properties of the machine regarding failure frequencies and service demand, also other "external aspects" place a role in the availability level that can be achieved. In the figure below this is shown in a schematic way.
Nowadays commercial onshore wind turbines show very high availability levels. With a proper service organisation and by ensuring that regular maintenance actions are quick and can be performed in time the operators of modern wind turbines show actual availability levels of 98% or sometimes even beyond . It must be stressed however that this is achieved through visiting a wind turbine about four times a year, either for regular service (usually twice a year) or for curing (repair) actions. In situations where both limited access and limited availability of maintenance equipment are at stake, such as for the offshore environment, this may easily lead to an unacceptable down time level. This makes it inevitable to assess the O&M demand of an offshore wind farm in conjunction with the other design parameters in order to achieve the required availability level against optimal cost expenditure. The latter being a trade of between investment costs in order to increase the reliability and the cost of maintenance actions to boost the availability to a high level. Since site accessibility always has a level below 100% for offshore conditions it is paramount to focus first on the decrease of the failure frequency of a an offshore wind energy system.
Reliability of components
Currently, there are a number of resources available that address failure rates of operational commercial (onshore) wind turbines [2,3]. The most useful source to obtain statistical information concerning failure classes of operational wind turbines turned out to . These wind turbines operate on land but to a large extend in the coastal wind conditions of Northern Germany. Analysis of these data over the different type of turbines leads to an estimated total average of 2.20 failures per wind turbine per year. The DOWEC partners considered it reasonable to assume that such level can also be achieved for multi-megawatt wind turbines, at least when developed according to similar design lines as presently used on the current one megawatt scale. For the assessment of the different concepts to be defined later it was however necessary to elaborate the total failure frequency further, in order to come up with average failure frequencies for subsystems. In a workshop with experts from all the partners of the DOWEC team the following table was made up showing a distribution of the total failure frequency over the different components.
Note that in this table the hazardous external conditions that might lead to malfunctioning of the wind turbine, such as a lightning stroke, are internalised. This means that they are taken into account through an assessment of its impact on blades, blade-tips, pitch mechanism, bearings and electronics (the Inverter component and the Control component).
It was realised from the beginning that the on shore level of failure rates (2.20 failures yearly) was not adequate for offshore application, so an effort was made as well to assess improved components. Per component category the
following types of improvements (with respect to ‘average’) are considered:
selection of the most reliable implementation (e.g. an electric yaw/pitch system in stead of an hydraulic system) adding redundancy to (sub-)components using MIL-specs components This resulted in an estimate of reduced average failure frequencies that can be achieved when rather simple measures are taken to modify the current onshore designs, see column 3 of table 1. Evidently there is a cost penalty related to such increase in reliability. This will vary from component to component but, according to the DOWEC experts, will be in the order of 25 (blades) to 100% (blade tips, pitch mechanism, control).
Maintenance (repair) actions have to be taken about two times a year, as could be deduced from the state-of-the-art failure frequency. Usually a repair action is taken by a crew of two persons that drive to the failed wind turbine with a service van. At the spot they enter the wind turbine and try to determine the cause of the failure and either start their repair action or come to the conclusion that extra equipment and/or spare parts are needed for the repair action. The extra equipment can either be "sky-work" utensils or a crane for heavier lifting operations. The repair time can be anything between an hour (a simple inspect and reset action) to some days, when an exchange of a major component turns out to be necessary.
Reduction of maintenance effort is essential when locating wind turbines offshore. At first there is the cost issue.
Offshore work is between 5 to 10 times more expensive than work on land. A second example stressing the need reduction of maintenance demand is the so-called cranage problem. A standard onshore wind turbine requires a lifting operation every 3 to 5 years. The day rate for general purpose lifting equipment from the offshore oil and gas industry is at least a factor 10 higher than a land based crane for similar lifting heights. One of the reasons is that such an offshore crane will be substantially over-dimensioned in terms of lifting weight. Take with that the fact that also mobilising and demobilising such crane, which may well cost several days, must also be paid for, makes is inevitable to reconsider the design of the wind turbine with respect to its maintenance demand, when located offshore.
An alternative for large-scale offshore wind farms (typically of the order of some hundreds of megawatts) may well be to purchase a special purpose crane vessel as maintenance hardware equipment. Such equipment might be used during installation as well. Furthermore it can serve as a permanent basis for the maintenance crew. When the design of the wind turbine is sufficiently adapted it might be even used as a maintenance base where complete nacelles including blades can be overhauled and prepared for replacement of another failed turbine. This maintenance solution was proposed in the OptiOWECS study, for a 100-unit wind farm using 3 MW turbines, ref .
The service demand of the presently manufactured wind turbines in terms of man-hours is in the order of 40 to 80 hours . Service visits are paid regularly and usually (except in the more demanding first year) about every 6 months. Often a more intensive service action has to be taken every five years. At that service shut down, which will take around 100 man-hours, some major components are overhauled and worn out parts are replaced. It will be clear that reduction of the number of service visits to wind turbines in an offshore wind farm must be aimed at. It is virtually impossible to get any significant reduction for the present onshore wind turbines, but in the design process that the DOWEC teams tries to implement a significantly more challenging target has been set.
Promising offshore wind turbine design lines
Taking the current wind turbine designs and their maintenance and service demand into consideration it is clear that future offshore wind turbine designs need to be seriously reconsidered in terms of their adaptation for the marine
environment. Without going into too much details a number of promising design lines can be described:
One design philosophy may be to reduce the number of components to a minimum. This will reduce the number of possible failure modes and therefore lead to a more reliable turbine, which is also easier to maintain and to service.
A two bladed fixed pitch direct drive wind turbine would be a good choice within such a philosophy.
A second approach may be to adopt a modular design line. In such case the modules should be designed and located such that easy maintenance and service is assured. Furthermore the exchange of faulted modules must be possible in a quick and easy way, in order to keep downtime low, and thus keep the availability on the required level.
A third possibility is a design line using integrated components provided these integrated components are easy to service. However their maintenance will probably be difficult and long lasting. Therefore such integrated component approach should make use of highly reliable components resulting in low failure rates.