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Support Structure

2-B Energy is developing a dedicated offshore wind turbine with a lower Cost of Energy (CoE) based on: a full jacket support structure concept with integrated design of support structure and foundation; suitable for large distance from the coast with helicopter access; sophisticated control system to reduce dynamic and extreme loads; and optimized for high availability by efficient service & maintenance & repair. The turbine is categorized as a 6MW, 2 blade horizontal axis wind turbine. The turbine is referred to as 2B6.

• This project was initiated after “Upgrade to 140 meter rotor diameter on a two-bladed downwind turbine”, FLOW Project P201204-001-2BE. This project will be denoted as Project ‘130m to 140m’.

Main objective of the project and results
The main objective of this project is to reduce the Cost of Energy by:

• Objective: Have the documented design ready to be submitted to the certification body for A-Design Assessment.
  Results: The documented design is ready. The turbine is currently under assessment for A-Design Assessment. Some parts are submitted to the certification body and already approved. The turbine is demonstrated at the onshore demonstrator project in Eemshaven, Netherland.
• Objective: Have enhanced the support structure readiness by having a documented design method for full jacket support structures ranging between 25m and 60m water depths which can be submitted and assessed by the certification body.
  Result: The design method of the 2-B Energy jacket support structure has been developed. The design method for onshore and offshore is similar. The usability is demonstrated at the onshore demonstrator project Eemshaven, Netherland at similar support structure dimensions (Jacket height of approximately 102m). The design method is certified by the certification body.
• Objective: Preliminary analyses of the impact of the 2B6 turbine on a floating support structure.
  Result: The main conclusion is that mounting the 2-B Energy turbine on a floating support structure is feasible. 2-B Energy still investigates the future possibilities regarding research related to floating support structures. In November 2015 2-B Energy started discussion with TU DELFT regarding the H2020 Marie Curie ITN proposal on floating wind turbines. This resulted in the formalisation of the ITN proposal that was submitted in January 2016.

The integrated design approach of wind turbine, support structure and control system enabled 2-B Energy to increase the rotor diameter of the turbine and achieve an increase of the annual energy production without a large increase of the CAPEX of the turbine. An integrated design approach is necessary as during the design phase the interaction between the support structure and the Rotor-Nacelle Assembly (RNA) has been of major concern. It was observed that the support structure has a significant influence on the load level of the RNA. For offshore conditions, the water depth and soil conditions influence the design of the support structure significantly. Also the performance of the turbine depends upon the understanding of the system as a whole. This interdependency necessitates an iterative design approach, where in each step the system as a whole is assessed, and subsequently the influence, limiting factors and potential contribution to improvement of each component is identified. This integrated design approach leads to improvements resulting in a reduction of Cost of Energy.

The upgraded wind turbine includes optimizations regarding structural components, including the mechanical drive train, and innovative bearing concepts for pitch and yaw while taken into account the certification guideline GL2012 and influenced control and safety strategies. Also the impact of different support structure water depths upon the design is taken into account. The 2B6-Gamma design has been finalised (inside the scope of this project) and is installed (not part of the scope) in Eemshaven, Netherlands as part of 2-B Energy’s onshore demonstrator project. The onshore demonstrator project reveals the feasibility of the offshore wind turbine concept of 2-B Energy. On 18th of December 2015 a major milestone was achieved by the first production of electricity during the commissioning phase of the wind turbine. 

Contribution of the project results to cost and risk reduction for Far-offshore wind energy 
Following the findings of the 130m-140m project, and incorporating the results, it was found that various other structural components were impacted and could be further optimized to facilitate the loads of the lengthened blade. This project incorporates these improvements. It shall be noted that not each individual work package within this Support Structures project leads to a reduction of CoE by itself: these steps are however necessary to allow for the implementation of the rotor increase, which leads to reduction of CoE. In conjunction with FLOW Project P201204-001-2BE: “Upgrade to 140 meter rotor diameter on a two-bladed downwind turbine”, the estimated increase in CAPEX and the increase in AEP are quantified as:

CAPEX                 an increase in blade mass of 4% due to extended blade length

AEP                      yield increase of 6.1% due to larger rotor diameter.

The AEP increase outweighs the CAPEX increase which is reflected in the cost of energy calculation. The Cost of Energy reduction was quantified by using the FLOW Cost Model which results in a reduction of CoE of 5.44%. Although the comparison made here is between two 2-B Energy concepts/designs, the results achieved could also be applicable in a broader sense since the increase in rotor diameter can be applied to other turbine designs to improve the annual energy production.

The fixed support structure project will enhance the installation cycle time. It results in an expected reduction of installation time due to the different strategy of lifting. Only three lifts are required: 2 lifts for the jacket and 1 lift for the RNA. The weather window is increased due less sensitive lifts (no single blade lifting, nor separate rotor lift) and to the installation strategy of using tag lines at the two blades. This improvement contributes to a decrease in cost of energy. Installation time is a key driver for the cost of energy of offshore wind, and weather influence a major part of project risk.

The floating support structure project focused on a small project with 3-5 wind turbines at water depths of 40m-60m. The semi-submersible has a weight range of 1200-1500 ton compared to a fixed foundation of 800-1000 ton. A floating support structure project at water depths up to 60m is comparable to the fixed support structure at 35m water depth. The choice for floating platforms versus fixed foundations is therefore dependent on water depth: at a given point a fixed foundation is no longer economically or technically feasible. For the reference cases within Flow, a floating foundation is for the moment not considered a competitive solution. 2-B Energy is seeking partners to continue investigation of the 2B6 wind turbine design on a floating support structure.

Please visit the website of 2-B Energy for more information: www.2-benergy.com