OFFSHORE WIND TURBINES: 1991 – 2015

Giant technology and scaling steps

The world's first close to shore offshore wind farm, Vindeby, built in Denmark in 1991, features 11 semi-standard 450kW Bonus B35/450kW turbines (now Siemens) and is still in operation today. From today's perspective the look of these tiny three-bladed turbines in shallow water is largely similar with today's 6 to 8MW super class, but physical difference and environmental challenges are both huge.

This article evaluates 24 years of sustained offshore wind progress, and key enabling success factors that made things happen.

Marine modifications

The B35/450kW features a 35m rotor diameter, a high-speed drivetrain, three blades mounted at a fixed angle (passive stall) with tip brakes, and the aerodynamic stall principle applied for limiting maximum power output. The only marine modifications were moving the transformers inside the towers and raising the towers' access door.

The 8MW V164-8.0MW, the world's most powerful turbine in the market, is an offshore-dedicated product development for demanding high-wind, deep-water, far from shore locations. The giant itself operates with variable pitch blades and variable-speed, and features a tube-shape medium-speed drivetrain with a permanent magnet generator (PMG) and a 164m rotor diameter. Between these two pioneering models lies 24 years, a wealth of turbine technology development experience, and factor increments in power rating and rotor swept area of respectively 17.8 and 22m.

Several offshore wind farms built with Siemens turbines at the start of the century apply 2MW and 2.3MW fixed-speed hardware, technically characterised by variable pitch blades capable of rotating around their longitude axis. With a power output control strategy called active-stall, each blade can pitch about 4° in 'positive' direction (with the leading edge towards the wind) and to 90° in the 'negative' direction (i.e. with the trailing edge towards the wind). A 'deep-stall' effect is introduced by actively adjusting the blade angle in the 'negative' direction in the nominal power output range.

A major operating benefit for offshore usage compared to passive stall output limitation is that active-stall control pro-actively retains a near constant output level in the rated power range independent of seasonal air density variations.

The first offshore project with Vestas turbines, the Danish Tun0 Knob project (1995), featured ten 500kW pitch-controlled fixed speed turbines. In 2000 Vestas switched to pitch-controlled variable speed operation that was first applied in the 160MW Horns Rev 1 project (2002). With pitch-control turbines the blade angle is actively adjusted in a 'positive' direction, for limiting output in the nominal power range.

Pitch-controlled variable speed

Siemens during 2003 switched to pitch-controlled variable speed in an upgraded 2.3MW turbine, and reapplied this technology in the 3.6MW model (2004). However, suppliers like Lagerwey and Enercon already introduced pitch-controlled variable speed from the early 1980's, and it has become semi-standard wind technology since about 2003.

Paul Bollwerk, Winergy Vice-President of Sales, recalled a major gearbox innovation first introduced in the B35/450kW: "This new-generation higher rated turbine was characterised by substantially higher gearbox input torque compared to smaller predecessors. Our new planetary -helical gearbox design was at that time a wind industry novelty combining one low-speed planetary gear stage with medium, - and high-speed helical gear stages. A main benefit of planetary -helical gear drives is that they combine compactness with high-torque density, and allow the passage of a hydraulic oil tube for operating the tip brakes and cables for sensors."

One observation in the Vindeby project was that these offshore installations despite operating close to shore generated about up to 30, 40% more energy compared to coastal onshore equivalents. Equipped with only some temperature sensors those first offshore gearboxes are already operating successfully for about 24 years.

Today all offshore turbines are fitted with advanced remote control conditioning monitoring systems and multiple sensors in key components for enhanced overall control capability.

Three-stage high-speed gearboxes are currently applied in a clear majority of all operating offshore turbines including the most powerful 5MW/6.2MW Senvion series. For turbine power ratings above 2MW high-speed gearboxes incorporate two planetary stages, due to the increased input torque levels typically.

Compact and lightweight

When introduced in 2003, the lightweight Vestas V90-3.0MW with a 90m rotor instantly caused wind industry sensation. Especially striking was that it, compared to the smaller V80-2.0MW, offered a 50% increase in power rating and 27% more rotor swept area with almost unchanged head mass and nacelle dimensions. Mass-reducing innovations included a compact high-speed drivetrain concept with a flanged gearbox and single rotor bearing semi-integrated assembly. Also new was a slender lightweight rotor blade incorporating carbon fibres, and a novel mass-optimised tower.

The combination of mass and load-reducing benefits allowed Vestas clients, under certain circumstances, to install V90-3.0MW turbines atop offshore foundation structures originally designed for the V80-2.0MW, together with utilising the same installation methods and vessels. The V90-3.0MW quickly turned into a popular developer choice, but unfortunately serious gearbox issues emerged soon afterwards. The earlier praised compactness of the drive system in practice complicated gearbox exchange, increasing downtime and driving up remedying costs.

The MHI Vestas joint venture in the 3MW class now offers a V112- 3.3MW/3.45MW model with high-speed non-integrated drivetrain solution also common in many other onshore and offshore turbines. The initial 3MW V112-3.0MW version introduced in 2010 offers 55% more rotor swept area compared to the V90- 3.0MW. Expressed in specific power rating the difference is 305 W/m2 versus 472 W/m2, whereby the optimal ratio leading to the lowest possible LCOE is especially dependent upon site-specific mean wind speed. Specific power ratings with the latest offshore turbines range typically between about 300 and 380 W/m2.

The 3.6MW and 4.0MW Siemens, and Senvion 5MW & 6.2MW successor series all feature a main shaft supported by two bearings. This proven solution enables easier gearbox exchange without removing the rotor, or requiring a main shaft-clamping device.

Modular

For the 3.6MW Siemens 'workhorse' turbine introduced in 2004, Winergy developed a modular three-stage gearbox with a physical split between individual stages. Mr Bollwerk: "This design aimed at cost-effective in-board exchange without expensive crane vessel deployment and anticipated at regular gear stage exchange. However, the feature in practice remained unused and was skipped in the gearboxwe developed for the 4MW SWT4.0-130. Despite substantial higher input torque due to the increased rating and larger rotor, this gearbox is shorter and weighs less."

Siemens applies 4-pole induction generators in all its geared turbines models, both fixed speed and variable speed. Senvion instead uses 6-pole doubly fed induction generators (DFIGs) with partial converter (20% of rated capacity) in its 5MW and 6.2MW models. The main benefits of smaller size partial converters are fewer components and reduced power loss.

Vestas uses DFIG's in the V80-2.0MW and V90-3.0MW, switched to PMG's with the V112-3.0MW, and again to induction generators in the V112-3.3MW/4.5MW. PMG's and induction generators require a full converter, whereby 100% of the power generated passes through. Some doubt whether DFIG's can fully meet future grid integration requirements, and switch to especially indication type.

Others consider future availability and price variations of the rare earth metals used for PMG magnets major risk factors, and switch to electrically excited alternative generator solutions.

In 1998 German engineering consultancy aerodyn Energiesysteme presented a unique patented 5MW low-speed turbine overall concept, offering a clever mix between direct drive (no gearbox) and high-speed geared turbines. The trend-setting Multibrid M5000-116 prototype of 2004 comprises a single compact stiff cast structure with single main bearing, a 1.5-stage planetary type gearbox, and a low-speed PMG (148 RPM rated).

Journal bearings

AREVA Wind, now the owner of the Multibrid technology, introduced a new spacious nacelle design incorporating helicopter-hoisting area, while the latest M5000-135 model version features an enlarged 135m rotor diameter with unchanged power rating. Large spacious nacelles are now widely accepted as essential for offshore turbines.

A genuine wind industry novelty in this early 'super class' was the incorporation of gearbox journal bearings, which combine compactness with long service life and high resistance to impact loads. Winergy now incorporates journal bearings in a new generation high speed and medium-speed gearboxes, and the application is expected to develop into a wind industry trend.

The AREVA-Gamesa offshore joint venture Adwen for a new 8MW turbine development chose a mediumspeed drivetrain concept, partially based upon the flanged 'tube-shape' solution of Gamesa's 4.5MW/5MW G128/G132 onshore/offshore series. Medium-speed drive systems are known as compact and typically comprise a two-stage planetary gearbox flanged to a medium speed generator, either PMG, a classic synchronous or induction generator. Flanged connection between main components nearly eliminates misalignment risk, and many consider skipping the high-speed parallel stage a reliability-enhancing benefit. The 8MW Adwen turbine comes with a record 180m rotor diameter.

Siemens and Alstom each developed 6MW direct drive turbines, each with a different drivetrain design and both choosing PMG. Siemens earlier this year installed an upgraded 7MW turbine version, but with unchanged rotor diameter. Direct drive is considered a mature wind technology for onshore with over 30-year track record, but for offshore the application is still new. The market as a sign of confidence has already placed large orders for these turbines, and some argue that direct drive inherently offers superior reliability performance compared to especially high-speed geared turbines, due to gearbox elimination. However, a much-quoted Reliawind study points at the pitch system, frequency converter, and yaw system as the three most trouble-prone areas, and each is integral part of all horizontal-axis wind turbine configurations.

New concepts

Aerodyn pioneered a mediumspeed turbine concept called Super Compact Drive (SCD) back in 2007. The overall design compactness is striking, and includes the elimination of a separate nacelle housing. SCD comes in two-bladed 6MW and 8MW downwind models for offshore. SCD licensee Ming Yang installed last year a 6MW SCD 6.0 prototype in China, a dedicated design adapted for hurricane-prone offshore geographical regions. An enlarged and highly innovative 8MW 'nezzy' sister concept with integrated floating support structure was introduced last year.

Dutch company 2-B Energy has developed a 6MW two-bladed downwind turbine, but incorporating a high-speed drivetrain with DFIG. Two bladed turbines in general enable more efficient sea transport logistics and single-hoist installation of complete heads, and another option is to integrate a helicopter-landing platform. However, a two-bladed rotor is dynamically unbalanced and requires advanced control strategies to overcome this inherent disadvantage.

Hitachi of Japan meanwhile already operates 15 2MW offshore downwind turbines with a three-blade rotor on fixed foundations. It develops a 5MW HWT5.0-126 model version with medium speed drive system, aimed at application in hurricane prone regions and combining with floating substructures.

Again others see the future of offshore wind in large vertical-axis Darrieus type turbines, especially in a combination with floating structures. However, despite several radical designs presented in the past years and with some like the 10MW Aerogenerator raising much publicity there is no offshore experience yet and product developments appear slow.

While experts continue to differ on what could be the most optimal solution(s) for the expanding future offshore market, The fact remains that three bladed high-speed geared turbines still dominate. The next years will no doubt see a fast growing role for direct drive and medium-speed technology, together with the rapid spread of 6 to 8MW class turbines, and perhaps the introduction of even bigger installations. It must be awaited which turbine and interlinked drivetrain concepts will finally prove the absolute winners in the decades to come, but during this growth and maturing process chances of ultimate solutions to emerge seems unlikely.

Eize de Vries

 

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