About: part 1 of a multi-part article looking at the development of offshore wind turbines as companies look to different approaches to improve reliability.
Outages offshore are more critical than those onshore due to problems of access at sea, limited weather windows during which maintenance work can be carried out, and the sheer size of offshore machines.
Adding to the direct expenses of access and repairs are those due to downtime with its consequent loss of revenue. Historically, the most frequent and expensive result of technical failure in wind turbines has been the need for gearbox repair or replacement.
Debate continues about the extent to which breakdowns originate elsewhere in the drivetrain – in bearings for example – rather than in the gearbox itself, but the result is the same; gearboxes fail. So prevalent has this become that some insurers virtually regard the gearbox as a consumable item and require clients to make provision for periodic replacement. This can happen two or three times during the typical 20-year design lifetime of a wind turbine.
Given that a large gearbox can cost upwards of £200,000, perhaps double this allowing for transport and installation, it is no surprise that the consequent expense can threaten economic viability. The quest for answers is intensifying.
Mitigation
Answers range from proactive operations & maintenance (O&M) measures to designs that dispense with the gearbox altogether. O&M mitigations include fine filtration to keep lubricating oil clean, and use of remote condition monitoring to detect incipient faults so that remedial action can be taken before an actual failure can occur.
Meanwhile, designers have sought to proof their products against the arduous marine environment with corrosion-resistant materials, hermetic sealing and climatic control, while boosting mechanical tolerance through improved component design.
French manufacturer Alstom for example believes that service life can be prolonged by reducing stresses on the gearbox, a result it achieves with its Pure Torque concept. In conventional turbine drivetrains, gearboxes are subjected not only to torque (turning) forces that it is their role to transmit, but also to considerable side forces caused by strong winds and transmitted by the rotor to the main shaft and gearbox.
In the Pure Torque design, the rotor shaft is supported on bearings in a cast iron frame that is an extension of the tower structure. This ensures that deflective stresses are transmitted directly into the tower instead of via the gearbox as happens in most standard designs. Because the gearbox sees only pure torque it should, in common with other downstream drive components, last longer. Alstom is incorporating Pure Torque in the 6 MW offshore turbine that it intends to start producing in 2014. A prototype of its Haliade 150 machine is currently being trialled.
Gamesa and certain other manufacturers similarly transmit side loads to the frame.
This technique still leaves the possibility of misalignments caused by relative movements between the gearbox mounting bed and other structural elements. These are hard to minimise in large machines, but are at least more tolerable if the bending stresses on the gearbox are nullified.
Another type of mechanical stress is that caused when transient high forces due to wind gusts meet the mechanical inertia of the drivetrain, creating oscillatory stresses in shafts and associated components. This can be countered with active damping. A further source of trouble is resonances that can arise within the drivetrain under certain operating conditions and, if prolonged, may cause damage to components. Care at the design stage over structural stiffnesses is needed to avoid harmful resonances.
In part 2 out soon: the drive for fewer components...
About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.