Function
Maintenance and service activities ensure the ongoing operational integrity of the wind turbines and associated balance of plant, including planned maintenance and unplanned service in response to faults, either proactively or reactively.
Who is involved
Maintenance and service activities are provided by a combination of the owner’s in-house resources, wind turbine suppliers and third-party service providers.
Key facts
Turbine maintenance
The initial service agreement typically covers the period of the turbine defect warranty, which is usually five years. During this period, turbine technicians are typically employed by the wind turbine supplier. The service agreement may specify that on expiry technicians’ contracts are transferred to the wind farm owner. This ensures continuity of staffing and removes technicians’ disincentive to relocate to the wind farm site.
Activity is divided into preventive maintenance (scheduled) and corrective service (unscheduled) works. The bulk of preventive works will typically be carried out during periods of low wind speeds to minimise the impact on production, however, in practice, this is not always achievable. Corrective service is performed in response to unscheduled outages and is often viewed as more critical, due to accruement of downtime until the fault is resolved. The primary skills required are mechanical or electrical engineering, with further turbine-maintenance training often provided by the relevant turbine provider.
Typical maintenance includes inspection, checking of bolted joints, and replacement of worn parts (with design life less than the design life of the project). Unscheduled interventions are in response to events or failures. These may be proactive, before failure occurs, for example responding to inspections of from condition monitoring or reactive (after failure that affects generation has occurred).
Blade inspections are performed by drones equipped with high-resolution cameras, by rope-access technicians or by high-resolution camera equipment located on the transition piece or vessel.
Notable differences for floating
Conducting maintenance in a moving floating offshore wind turbine presents additional challenges for health and safety when moving items and motion-induced sickness. These issues are more significant at height where any low frequency movement of the floating substructure is amplified.
Substructure maintenance
Maintenance consists of visual inspections, non-destructive testing (NDT) and sea bed survey work with remedial service work completed when required.
Meanwhile inspections focus on structural integrity, lifting, safety equipment, corrosion protection and scour protection.
Routine surveys are likely to be undertaken in the first two years but thereafter on a five- or ten-year cycle. Surveying the status of the protection installed to prevent sediment erosion, where the turbine foundation meets the sea bed (scour), can be carried out by side-scan sonar from a survey vessel or by using a ROV.
Regular inspections are required on secondary steelwork such as ladders, gates, grills and platforms. On some sites, cleaning is needed to remove sea bird guano, which can be a serious health and safety hazard.
Surface inspections and surveys include monopile internal inspections of the grouted or bolted connections and splash zone inspections. Activity needing subsea operations may include infrequent structural and J-tube cathodic protection inspections and weld inspections and can generally be carried out using ROVs.
Diving is required only in exceptional circumstances and efforts are being made to maximise the use of safer, remote techniques.
Notable differences for floating
Monitoring, inspection, and minor repair activities focus on the structural integrity of the floating substructure, the secondary steelwork, its corrosion protection and the various subsystems used on the floating substructure.
For floating wind farm, the mooring system also requires maintenance. Mooring chains are either inspected using general visual inspections or close visual inspections, usually using ROVs. Chain links are cleaned if this is necessary for inspection.
During inspection, ROVs visually inspect the whole chain lengths, especially at critical points such as touch down points which receive the most wear. They use callipers to take measurements of link diameters and link touch points. The data is then compared to measurements taken during manufacturing.
If an anchor or section of mooring line fails, then it is replaced as rapidly as possible.