There is the variety of pre-installed equipment, whether service is being conducted under warranty or out of warranty, the budget available to the owner or operator for service, and even what theories about O&M and appetite for risk to which an owner prescribes.

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Some wind plant owners spend every available penny upgrading to sophisticated condition monitoring systems hoping to predict and plan for component repairs. Others minimise service costs, reacting to repairs as they happen.

A wind plant's location can also have a big impact on service. Turbines located in dusty, sandy environments may need different attention than ones in more hospitable grassland. Wind plants operating in the windiest sites will take more strain than those at calmer sites, or sites with less air turbulence or wake effects from other turbines.

Marco Zvanik, vice president of sales for Energy Maintenance Services (EMS), the second-largest wind plant O&M service company in the US, picks out a range of standard services, right, that most O&M companies offer. And, below, he indicates a few of the additional services that are often available.

For example, when blade damages exceed up-tower repair capabilities or a serial design defect is found in a number of blades, many of today's O&M providers can offer down-tower service and repair. The rotor or blades are dropped on site, repaired and re-installed.

Some companies offer gearbox remanufacturing and reconditioning to improve housing and gear ratio. Experienced firms with specialised tooling can perform a range of up-tower changes without using cranes.

To automate some of the day-to-day servicing, gearbox oil auto-lube installation systems, offline oil-filtration systems and upgraded pumps can be installed.

Mechanical gearing in pitch control and yaw control motors are lubricated with grease. Some O&M companies install after-market automated grease application devices.

At some point in a wind turbine's life, it is expected to require some sort of major corrective. This could be the replacement or repair of one of the rotor blades, the gearbox, the main shaft, generator or bearings. Any of these critical components will likely render the turbine inoperable for some considerable time. Some O&M companies can offer comprehensive solutions that integrate the removal, transportation, repair and re-installation of most of the major components, to get the turbine back up and running as soon as possible.

For more see Windpower Monthly's Operations & Maintenance Special Report