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Not all wind turbines are the same, so, not all operations and maintenance (O&M) strategies are the same. While individual models can vary considerably, the range of turbines used in the current global wind industry can be broken down into five basic design strategies.

For decades, the dominant wind turbine design has been the conventional fast-speed geared wind turbine, of which there is a relatively small number of design variations, represented by three examples below. In addition, we have included a typical direct-drive turbine with no gearbox, and a hybrid model known as a fully integrated medium-speed intermediate turbine.

Within these categories individual manufacturers and designers continuously develop new design concepts, which can have a profound effect upon turbine dimensions and weight. While these can lead to specific needs for maintaining the installation, the key features and characteristics of the five groups below highlight the major requirements to keep them operating efficiently.

NON-INTEGRATED FAST-SPEED GEARED
DRIVE SYSTEM
Example shown: Nordex N80 (2000)

The conventional wind turbine design, and partial representation of the "Danish concept" turbine.

The main drive system components are set in a line arrangement with the main shaft and main bearing. This makes the primary components easily accessible and component exchange relatively straightforward.
On a less positive note, fast-speed in-line models have the greatest risk of misalignment, both static and dynamic

SEMI-INTEGRATED FAST-SPEED GEARED
DRIVE SYSTEM
Example shown: 5MW Bard (2007)

This is a different design approach from the Nordex, this has a shorter, more compact drive system. The design is straightforward, but its manufacture is costly.

The gearbox is attached directly to the main machine housing using a flange. There is a single rotor bearing, and either a very short or no main shaft.

Any replacement of main components here is relatively easy (some other concepts are known for complex and time-consuming gearbox replacements).

There is limited risk of generator misalignment, either static or dynamic, but there are some concerns about the large oil seal often applied inside the main bearing. 

SEMI-INTEGRATED DISTRIBUTED FAST-SPEED GEARED DRIVE SYSTEM
Example shown: 2.5MW Clipper Liberty (2005)

This is a Clipper-patented load-splitting design using Clipper's D-Gen gearbox. It was rated highly in a US drive study. It is not yet found in Europe, although it is the basis for the Britannia 10MW offshore turbine being developed by Clipper for UK waters.

It includes four generators connected to the gearbox, which is connected to the main shaft using a flange.
  
Misalignment risks are virtually eliminated and the exchange of generators is relatively straightforward, using the onboard jib crane. The downside is that there is potential for deviations in
the load of individual generators, which can affect operation and lifetime. 

DIRECT-DRIVE SYSTEM
Example shown: 2-2.3MW Enercon E-70 (2003)

The pioneer of direct drive, this design is not the cheapest, but is considered one of the best.
  
The ring-shaped generator and rotor hub turn as a single assembly. The main pin is stationary, with two bearings. An alternative has a rotating main shaft, with the rotor hub located in front and the generator behind the tower.
  
A key advantage of the direct-drive system is that the risk of component misalignment is virtually eliminated. Few and slow-rotating components enhance reliability.
  
Generator exchange, however, is a major operation because of its weight and size.  In the past, the size of the nacelle for a direct-drive turbine has been a disadvantage compared to the equivalent fast-speed geared models. Technology advancements has made this less of an issue.

FULLY INTEGRATED MEDIUM-SPEED GEARED
DRIVE SYSTEM
Example shown: 5MW Areva Multibrid M5000 (initial nacelle design 1996/7)

This model is a hybrid of the fast-speed geared turbine and the direct-drive turbine. Its generator size is somewhere between the two. It has a single-stage gearbox, medium-speed generator and one main rotor bearing integrated in a compact main integrated machine housing.

This is a highly compact drive system which has virtually eliminated the risk of component misalignment.

Reliability is potentially enhanced because of the low speed of the system.
However, if there is a serious main component failure, a full nacelle exchange is required. This is a major operation but does replaces all the main components with new parts, giving it a 100% service life again.