6.2MW SEAWIND 6

Dutch offshore wind solutions provider Seawind has developed a novel 6.2MW two-bladed upwind turbine with a 126 metre rotor diameter, plus additional technology for bottom-fixed and floating installations together aimed at driving down the cost of energy decisively.

Offshore WIND spoke with chief technical officer Silvestro Caruso about product design philosophy, dedicated technology features and his preference for a two-bladed upwind turbine with active yaw control.

Acceptance

For a given power output, two-bladed wind turbines must rotate substantially faster compared to a three-bladed equivalent, or alternatively switch to blades with wider aerofoils making them inevitably heavy and expensive. However, higher rotor blade tip speeds inherently raises aerodynamic sound level, which together with aesthetic acceptance issues explains why large two-bladed turbines have always remained a minority wind technology.

For offshore these factors are less an issue, and two-bladed turbines could offer at least two main benefits. Complete assembled and pre-commissioned turbine heads (nacelle and rotor) could be stowed on an installation vessel’s deck and installed by a single hoisting operation reducing offshore working time, risk and costs. Alternatively, they could be put more easily atop a support structure at a low-cost assembly location like a pier, towed to a construction site and finally installed.

The Seawind 6 is an offshore-dedicated upwind mediumspeed geared wind turbine, developed for a design life in excess of 30 years, and characterised by a radical concept approach with multiple unusual design choices. The cone-shaped nacelle features distinct left and right side cooling radiators and a central circular-shaped landing platform capable of accommodating large twin-engine helicopters. The nacelle consists of welded semi-circular shaped steel sheets up to 20mm thickness, internally reinforced by rafters to a self-supporting structure like a ship hull, and again structurally bolted to a helicopter landing-deck. The latter enables uncomplicated helicopter-based service access with the turbine in stationary mode and the rotor locked in horizontal position.

Design simplicity

The non-integrated drivetrain comprises a main shaft with two bearings and a large fail-safe double disk brake in between, offering sufficient capacity for bringing the turbine to a full emergency stop. The bearing housings are directly mounted on the rafters. Mr Caruso: “The planetary gearbox has two stages and each housing can be split horizontally and vertically for facilitating easy component exchange and in-board repairs. A gear coupling between main shaft and  gearbox prevents rotor-induced bending moments entering the gearbox. Our overall strategy is for a simple design and overall robustness for minimizing turbine downtime risk offshore.”

Choosing a brushless induction generator with full converter, instead of a permanent magnet generator solution fits into that philosophy as the latter electric machines are in his view more sensitive to magnet temperature management and maintenance issues. For similar reasons the power converter, MV-transformer and switchgear were moved inside the integrated support structure base below water level. This positioning according to Mr Caruso eases service access, reduces vibrations related power electronics failure risk, and enables passive seawater cooling, while also positively impacting turbine head mass (Textbox 1).

Teeter hub

Two-bladed turbines are dynamically unbalanced, which provides major design challenges. One measure to eliminate/minimise high structural (bending) loads during operation, and in particular yawing, is attaching the rotor blades to a flexible structure with limited pivoting capability, a solution called teeter hub. Fast wear of the pivoting mechanism is known as an infamous premature failure cause, putting a brake at its wider turbine application. It could explain why two new 6MW pitch-controlled downwind offshore turbine models, Aerodyn’s SCD 6.0MW and the 2-B Energy 2B6, feature rigid rotor mounting.

The Seawind 6 in contrast incorporates a patent-pending elastic teetering hinge solution, with the blades at a fixed angle rigidly connected to a central square-shape rotor hub. The hub is flanged to the main shaft by a T-shaped intermediate shaft containing two elastic devices. The system allows the movement of the rotor disk around the hinge axis limited to a maximum of around 3 degrees during normal operation, without hitting the bumpers in any normal and extreme situation. The latest teetering hinge design contains multiple individually adjustable and exchangeable elastomer elements and a self-lubricating journal bearing that provides effectively absorption of radial loads. Mr Caruso: “Key benefits are a combination of optimal loads distribution within the elements of the teetering hinge for a long operating life, together with low-level hinge torsional stiffness.”

Active yaw-control

Passive yaw power output limitation by means of a hinged tail system is well known from small wind turbines up to about 10kW. Seawind 6 features active yaw-control with two separate distinct control functions:

1. The full rotor area faces the prevailing wind direction below rated wind speed;

2. Power output control above rated wind speed is achieved by adjusting the rotor angle relative to the wind direction as a function of wind speed.

A hydraulic active yaw system gradually turns the rotor ‘out of the wind’ when wind speed increases, and the full rotor circle that initially faces the prevailing wind gradually changes into an ellipse shape with smaller decreasing projected area. This directly reduces power output, reinforced by a simultaneous drop in aerodynamic efficiency due to inclined less effective airflow at the blades.

Once wind speed calms down the inclined rotor plane turns in opposite direction. The flapping rotor moments, important in rigid rotors, are here low due to the degree of freedom introduced by the teetering hinge. At wind speeds above Cut-out the rotor plane is automatically turned 90 degrees out of the prevailing wind direction into a stationary safe-mode whereby the wind blows against the (smallest) projected rotor plane. In this parked condition the IEC class S Seawind 6 can survive up to 70m/s wind speeds, with the mechanical brake fully locking the rotor. The hydraulic yaw system does not need external electrical energy.

Regarding his choice between an upwind and downwind rotor concept Mr Caruso explained: “Active-yaw control as a principle will work for both upwind and downwind, but during Gamma 60 testing we found that an upwind rotor with active yawing is aerodynamically more stable. An upwind rotor is also not impacted by tower shadowing impact as with downwind turbines each time one of these blades passes the tower, which greatly reduces fatigue loading.”

Yaw moments

Elaborating further on active yaw-control technology features and benefits Caruso said: “Analysis showed our wind induced nacelle yaw moments are only about 15% compared to an equivalent three-blade rigid rotor. We therefore require only three small yaw motors during normal operation and for shut-down, with the fourth motor providing system redundancy.”

The Seawind 6 rated rotational speed is 20rpm, compared to 12.1rpm for a 6.2MW Senvion 6.2M126 with similar rotor diameter, which translates into a high rated blade tip speed of 131.9m/s. State-of-the-art offshore turbine tip speed values are by comparison typically in the 80-95m/s range, with 108m/s for a 5MW XEMC Darwind turbine a current exception. Power is a function of rotational speed and torque (P = f (n * T), and a high rotational speed therefore means lower torque and a smaller-size less expensive gearbox and overall drivetrain. Negative consequence is increased risk of accelerated rotor blade leading to blade edge erosion damage. Mr Caruso: “We cooperate with UK specialist Blade Dynamics in developing a special anti- abrasive coating, with research on going. Higher centrifugal forces are compensated by greater blade robustness. Most critical for the blade design are fatigue-related loads, but these are lower than in three-bladed-turbines.”

Cumulative design choices have resulted in around 295 tonnes head mass (nacelle + rotor + upper tower section + helideck), set against 460T for a conventional three-blade 6.2MW Senvion 6.2M126 without helideck.

Standardised

Finally, the first company priority is to raise sufficient funds for building a prototype envisaged for 2017. “A parallel goal is in commercialising the Seawind 6 with a 126 metre rotor diameter as a standardised product in the world’s main offshore markets. An overall goal is a dramatic CoE reduction to around 80€/MWh by combining advances in turbine design and operation, with integrated support structure added benefits and installation methods without using heavy crane vessels”, Mr Caruso concludes.

Eize de Vries