By Doug Lucas; Wind Energy Engineer
The Timken Company
Onshore wind developers and operators deserve enormous credit for their hard work and dedication. If the U.S. is to fully optimize wind capacity in the country, however, it is time to follow the UK and European markets. And this means pushing the offshore wind sector forward in America.
“There’s no end in sight for wind-turbine growth,” reads the lead of a recent investigation into the growing prevalence of wind power and larger turbines, globally. The report anticipates that the average rating of wind turbines worldwide will reach 2.8 MW by 2022, with significantly higher growth in the offshore market. In Europe, it is projected that the average offshore turbine could be 12 MW by 2024.
Typically, initial investments for offshore wind are bigger than onshore but marine projects offer greater generating capacity in the long run. Also, nearly half of the U.S. population lives in coastal areas, which means these regions have extremely high power needs. Offshore projects can leverage strong marine winds to cost-effectively supply electricity to these customers.
To maximize offshore capacity, larger, more powerful turbines are used with components that can withstand the harsh sea-salt conditions while providing reliable operation. However, such high-powered turbines are a challenge to conventional bearing designs. For this reason, offshore turbines rely more on direct-drive technology rather than gearboxes.
Main-shaft bearings are sill used routinely, however — and in just under half of offshore turbines, according to findings from MAKE Consulting. These bearings must endure higher loads, increased deflections, and slower rotational speeds than the ones found in smaller offshore turbines. The lubricants are exposed to saltwater, a corrosive element that may cause unwanted tribology conditions and premature bearing surface damage.
Ultimately, wind operators and turbine OEMs must understand which bearings perform reliably if they are to fully capitalize on offshore opportunities.
Maximizing offshore performance
The sheer size of offshore turbines — which are typically double the capacity of onshore machines — and extreme wind loads mean great demands are placed on a turbine’s gearbox and main-shaft bearings. The potential for costly failures in offshore wind turbines runs high. This means it is vital to choose high-quality components with reliable system designs for efficient operation and minimal downtime.
Ideally, a bearing’s geometry, clearances, and load capacity are custom-engineered for the application’s operational conditions and this is particularly important in offshore turbines.
An improperly engineered bearing may cause too much pre-load, which may result in high stresses, high bearing temperatures, and a shorter life. Additionally, a bearing with too much clearance may experience excessive deflections, improper roller load sharing, higher stresses, misalignment, and edge loading, premature cage, or sliding damage. There’s a lot that can go wrong. (“Clearance” is the total distance that one bearing ring moves relative to another.)
Regardless of the form of damage, however, the result is the same: compromised performance of the turbine. Eventually, the bearing requires repair or replacement, leading to turbine downtime and lost operation. Technical expertise and product quality are necessary to successfully capitalize on the asset’s longevity and potential.
Offshore component repairs and replacements are also typically more costly than onshore ones. Replacing a damaged bearing in an offshore turbine requires a specialized O&M team with a transport vessel and the necessary equipment to correctly diagnose the problem. Additionally, it’s necessary to disassemble the turbine to repair the damage, which requires a costly offshore crane rental.
Weighing the options
Recently, tapered roller bearings have demonstrated desirable performance in a number of new wind-turbine applications when compared to spherical roller bearings, which are the conventional choice in most existing onshore wind applications. In fact, there are several 5-MW+ offshore turbine designs that now employ tapered roller main-shaft bearings.
Spherical bearings are composed of barrel-shaped rollers within a rounded (spherical) race, which behaves like a ball and socket joint (much like a hip or shoulder joint). The rollers in tapered roller bearings are shaped like a truncated cone and are fit within races that are angled (or tapered) to simultaneously support axial and radial loads.
Tapered bearings can be sized smaller and offer an increased power density compared to spherical ones, reducing the overall cost of energy. An ability to properly carry thrust and radial loads typically means high performance in harsh conditions and unpredictable changes in wind speed and direction.
However, it is imperative that the specific demands of the application are considered first. This is because there is no one corrch for offshore applications, offshore wind is relatively uncharted territory in the United States.
For those wanting to take advantage of such project developments, it requires working with the right suppliers who can offer proven expertise on problem-solving and total system design to tackle the challenges of offshore wind.
Making the right choice
Dependable engineering and expertise are critical when selecting bearings capable of the performance required in offshore wind turbines. Here are a few additional tips.
- Reliability of the main shaft requires a bearing that can adequately withstand various loads from ever-changing winds. This feature is particularly critical as wind loads increase, such as in offshore applications.
- Bearings that offer the highest possible performance potential in a compact design are ideal for reducing the overall component size, weight, and manufacturing costs in wind turbines.
- Additional options are available to increase reliability and performance. For example, for onshore turbines, advanced diamond-like carbon (DLC) coatings are available that protect against micropitting and other surface damage.