The 2B6 – A new 2-bladed design
tailored for the new market

Offshore requirements

As we now explore the new market of large scale offshore wind turbine plants, the cornerstones of design and drivers has changed compared to those driving the early design choices. Therefore the foundation for design has shifted where new drivers for offshore technology are challenging the existing design path, possibly being close to its limit for reducing costs further. Offshore specific design drivers needs tailored applications like:

  • transportation and installation processes with high cost
  • electrical grid system are traditional ad-hoc systems and provides low to no redundancy
  • simplicity, service ability and component access – cost of every service hour offshore of much higher impact between labor, equipment and infrastructure
  • turbine access becomes a design driver – up to 50% weather delay in North Sea environment by ship
  • Industry challenged by reactive operational strategies, resulting in costly equipment and repair

With this new environment, there is a renew foundation for the 2-bladed rotor design as many of the early conditions against the 2-bladed concept now are much more aligned and can instead mitigate risk as well as reduce cost.

The new market in combination with new turbine size class opens a new chapter in wind technology and path for the 2-bladed rotor design where risk can be better mitigated and cost savings can reach further. The various new design choices in the 2B6 truly bring opportunity and greater optimization potential, compared to the well-optimized 3-bladed wind turbine used in the market place today. The introduction of the 2B6 will be a design to re-introduce some of these technologies and opportunities. With an overall project perspective and Power Plant view, the 2B6 offers a true potential yet to be discovered for offshore wind where cost of energy is likely to reach well beyond normal cost reduction boundaries by conventional technology.

The 2-bladed rotor

The 2-bladed rotor

The 2-B Energy team out of Hengelo in the Netherlands has been driven by these values and pursued and explored in great detail these new set of conditions. The effort has taken place over a 5 year period and has evolved from conceptual analysis to a complete plant, using latest state of the art controls and design approach. The result, a new turbine platform where the integrated design approach around the rotor and its ripple effects in multiple directions showing evidence of potentially game changing benefits, now ready to be deployed. Below are listed some selected areas to illustrate some specific benefits. Some of these areas are further explored under each segment.

  • The rotor design – control and behavior
  • Transportation and installation
  • Helicopter landing access
  • Service-ability and simplicity
  • Elimination of components

Background of the 2 bladed design

Since the late 70´s we have seen rotor technology evolve in several directions from number of blades to control and aerodynamic designs. Early visions of principles and logic have been taken over by design paths and experiences, driven by market-ability and performance. Since the early days of wind turbine designs the very foundation of market requirements has been one of the leading factors driving technology, especially in rotor designs.

The 2-bladed rotor has always been the ideal choice by the science community. In the early days where the smartest people around the world proposed wind turbine designs to drive cost and rotor engineering forward, the 2-bladed rotor came to be the main path where the vision of turbine size were expected in multi-MW class, very often in downwind designs with multi beam structures. Independent design groupings in USA, Denmark, Sweden, Netherlands, Italy and Germany all brought forward 2 bladed designs and several early prototypes were built to demo these new technologies, some successful, some not.

The technology drive of the 2 bladed rotor were also a split between “stiff – heavier” designs and “soft – lighter” designs. In hindsight the extremer lighter designs as well as the stiffer designs struggled more to find grounds. However, in spite of non-existing design tools and limited experience of larger wind turbines, several successful designs were in operation for many years proving the concept of the 2-bladed rotor, both upwind and downwind.

Downwind

Downwind and high level benefits

There are several factors that drive choice of the rotor orientation. In the 2 bladed case the orientation of the rotor can contribute substantially to the stability or general behavior of the nacelle. Furthermore, orientation can impact structure choices and blade design, where upwind wind turbines are driven by tower clearance requirements for certification and design, driving both structure design choices, blade pre-bending and  stiffness as well as control strategies to avoid extreme blade bending. Given the traditional tubular towers, downwind rotor have experienced more severe wakes due to tubular towers.

For the 2B6, the rotor configuration and choice of orientation was fundamental in defining an optimum between a variety of parameters achieving:

  • Controlled behavior and yaw stability
  • Lower loads generated by the nacelle – lower loads on structure
  • Less blade clearance issues
  • Simpler blade, no pre-bending
  • Allowance of a full integrated jacket design, also providing low wake downwind.

Downwind vs Upwind

Basic principles and facts:

Consequences:

Blade/tower clearance is a main driver for design Full jacket very challenging – tubular tower best upwind option
3 bladed airfoils are inherently softer – 2 blades stiffer Less measures needed (carbon, pre-bending, cone/tilt)
Wind load impacts a blade with downwind thrust loading Upwind rotor – negative, downwind rotor – positive

The conceptual difference to upwind

Upwind rotor with upwind cone angle in operation forces the blades in down wind direction towards the tower due to centrifugal force Thrust load and centrifugal out of plane loading work in the same direction and creates need for more compensation in blade or rotor design
A downwind rotor with downwind cone angle during operation is forcing the blade in upwind direction The loads compensate each other and reduces the total blade bending

Conclusion and 2-B approach:

  • The use of a 2 bladed rotor design with higher stiffness in blades in downwind orientation is favorable where loads counteracts each other, leading to less need for tower clearance measures
  • This has been used to our advantage and enabled us to use a full jacket with wider footprint, still in combination with a simple straight blade with minimal use of stiffer more expensive materials

The full jacket (truss) structure

The full jacket (truss) structure

The jacket design by 2-B Energy is a result of full modeling of wind turbine behavior resulting in both extreme and fatigue loads. As a part of the design integrations, the relationship between nacelle and structure loading has been carefully considered to reach a harmony between the components. The jacket design also integrates the components of traditional tower, transition piece and foundation in to one design. Being a fully integrated process and design from nacelle to seabed, hereby an optimized structure can be provided also eliminating the traditional interface risk.

The 2B6 jacket results in a simpler transition piece (from jacket legs to circular design) with lower weight, also host of elevator and other equipment.
The full jacket design provides for several benefits, of which few can be mentioned as:

  • 3 legs – 15% weight saving to 4 legs and hereby lower weight
  • 3-leg geometry has smaller footprint than 4 legs, allowing for more units / barge
  • Internal nacelle crane reaching deck of ship due to smaller footprint
  • Simpler installation with flanges and both foundation and tower in same sequence
  • Deeper water design with competitive weights
  • HV transformer section can be sandwiched-in with the standard design (1 turbine per 9). Please see illustration on the right and explore sections below.

Safer and better access system

Safer and better access system

One the most promising components of the 2B6 is the helicopter landing access. Today, using larger and larger ships and seeking more and more advanced methods to access wind turbines in higher sea results in a natural limits of only three crews per vessel to avoid increasing transit and waiting time across a workday.  Even at three crews per ship, the efficient time left over to perform service work becomes less of half a twelve hour workday. The helicopter landing is fundamentally different than the current hoisting process and can fundamentally change the way we operate offshore wind turbines in the future. By landing the helicopter, the passive position of the craft makes it possible to safely exit and enter the helicopter without stress and in wind speeds up to 50 knots. A turbine can be reached and accesses within 10 minutes at up to 95% of the time, possibly also 24 hours, 7 days per week. This way we no longer need to consider the driving factors created by ship access restriction such as complex redundancy systems and strategies to avid longer downtime. In principle a fault that may not be defined within a week of high seas in a conventional project with ship access and possibly not be repaired for yet another week can be defined and repaired within the very same day. Hereby availability has the potential to be substantially enhanced and safety for access vastly enhanced.

Another major difference by landing helicopter instead of hoisting is that the helicopter capacity and efficiency is virtually doubled by landing. This is driven by the fuel requirement by hovering in active mode. Assuming site conditions in a North Sea location as Alpha Ventus and a stationed helicopter of same kind offshore, the comparison can be as follows:

As a hoisting helicopter working within a wind farm is required to hold enough fuel to reach a “secondary” landing deck (often onshore), it must carry so much fuel that only two passengers and only 50 kg of tools can be transported (assuming the use of an EC 135 t2) before it needs to refuel and pick up another team of two technicians. For three full service teams of twelve technicians this means six trips and six re-fuelling’s and a total of 300kg of tools.

For the landing craft the fuel restriction goes away due to the multiple “secondary” landing pads on each wind turbine. This means that a helicopter can use its full capacity and transfer four technicians and 275kg of tools each transfer. This would mean three trips of four technicians and 275kg of tools each, resulting in twelve technicians and 825kg of tool over three trips and thereafter refueling.

Furthermore, several systems and processes can be tailored for the use of helicopter making full integration possible and resulting in both higher efficiency and lower cost. For a 300 MW wind farm, a craft may use in rough assumption 500 -700 hours per year. Should problems arise requiring more technicians in the field, the helicopter can easily double its operating hours per year, where the alternative with ship use would require an additional ship and related infrastructure.

From 10 kV to HV in one transformation

From 10 kV to HV in one transformation

The 2-B Energy strategy is to take integrated system views, which is also the case for the electrical generation system. The 2B6 offshore wind turbine is based on a doubly fed induction generator with small converter (DFIG-light) concept. A significant aspect where the electrical configuration of the 2B6 WTG distinguishes itself in comparison to turbines of other manufacturers, is  with respect to the Stator Voltage.

It’s common that the generation voltage is set to 400 V or 690 V and is thereafter transformed by means of a step-up Transformer to a MV grid voltage, most often 33 kV. The 2B6 has a direct rotor output Voltage of 10 kV and is therefore able to directly connect into a MV grid, without the use of a step-up Transformer.

This aspect has far going consequences within the MV grid of the Wind park and gives rise to an unique park design, known as the “PowerTown grid”. Instead of a String topology, the PowerTown grid makes use of a Star topology, where 8 wind turbines connects in a star to the center wind turbine and a 3 x 3 pattern (individually called a “PowerBlock”). The multiple of PowerBlocks grids then constitutes the PowerTown grid.  The PowerBlocks can be connected on HV level through string or loop. With loop there is a very high degree of redundancy in the system.

A benefit of this topology is that the cable lengths within the PowerBlock (10 kV) are kept small and identical with very simple equipment. But the real benefit lays in the fact that this design can step the grid voltage up to HV (110 – 220 kV) with an group transformer integrated in to the center turbine in a “sandwich” design, built in between the two jacket sections. Hereby there is no need or of a separate offshore transformation platform. See Illustration…

Hereby the power generated from the wind turbine benefit from less losses (no full converter, no full size transformer) and less equipment as a full transformation step is eliminated. Furthermore spare transformer on 60 MVA level is a fairly cost efficient and risk mitigating measure.

Installation benefits

Installation benefits

One of the greater costs and project risk areas beside normal wind turbine components is the transportation and installation process. The high cost of the new generation of larger vessels in combination with multi-discipline processes requiring narrow weather conditions (wave height and wind speeds), results in more time on the sea with high risk for delays. Ripple effects through a full project can have substantial economical outcome. The benefit of the 2-bladed rotor benefits this process in multiple ways. The principle of having full rotors mounted on nacelle during shipping and installation increases efficiency and reduces installation time. The stacking and transportation of nacelle and rotors also maintains a narrow width of vessel that otherwise can be a hindrance on rivers or through narrow channels.

The installation process of a 2 bladed rotor brings the most benefits where full nacelle and rotor can be lifted and installed in only one lift with higher degree of control by help of taglines. Hereby installation can fully eliminate blade handling, rotor turning and tool changes as a result where no vertical flanges needs precision and alignment but only mating of horizontal flange where bolt alignments are easily made by tag line control. This enables installation in higher wind conditions in much quicker and less complex process.

By tailored installation process also including full jacket structure, the installation capacity is expected to reach more than double of that of a standard 3-bladed wind turbine of today.

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