Simcenter Samcef Wind Turbines prove feasibility of new method for installing offshore wind turbines

Delivering potentially huge savings

The traditional method of installing offshore wind turbines is to assemble the entire three-blade rotor assembly onshore and then use a large ship to install it offshore. AREVA, a leader in the energy industry, had the idea of assembling the rotor blades of their offshore wind turbines during installation, which could potentially save about €500,000 per turbine in logistics and assembly costs by making it possible to use a smaller ship and crane. The critical question in proving this new assembly method was determining the loads that would be applied to the blade, tower and locking system while the turbine was in a partially assembled state.

AREVA Wind’s Yvan Radovcic, head of the loads department, and Edgar Werthen, mechanical engineer, explained how the company used Simcenter Samcef™ Wind Turbines software from Siemens Digital Industries Software to evaluate many thousands of different load cases to prove the viability of the new method that will potentially save several hundred thousand euros in future wind turbine installations.

The European Union is on track to have renewable energy account for 20 percent of total energy consumption by 2020. AREVA Wind has been a player in this segment since 2004, and is one of the top three manufacturers in the offshore wind industry in Europe. The company had an installed base of more than 120 wind turbines in 2013, and has already won firm orders for its M5000 5 MWe (megawatt electrical) wind turbine. The M5000 technology has been further optimized with the development of a new 135-meter rotor that sweeps an area 35 percent larger than the M5000. The first M5000-135 prototype was installed and commissioned in Germany from August to September, 2013.

Wind turbine design challenges

“One of the challenges in designing offshore wind turbines is the high cost and logistical difficulties involved in physical testing,” says Radovcic. “Fully testing a single blade assembly costs several hundred thousand euros. Physical testing is also limited by the wind and wave conditions that happen to be experienced during the testing period.”

AREVA Wind has long been working on improving its ability to simulate the performance of wind turbines prior to the prototyping phase in order to evaluate more design alternatives to improve performance and reduce manufacturing and installation costs.

A major difficulty in simulating wind turbines is the range of physics that must be considered. Such simulations typically require multiple analysis tools. The first main application is assessing the loads on the turbine, which is typically performed with coupled aero-mechanical software. The second is the design of the mechanical components, such as the yaw and gearbox, which are usually analyzed with multi-body dynamics simulation tools. The third is the structural components, such as the bedplate, which typically are analyzed using finite element analysis (FEA) methods.

It usually takes days or weeks to produce results with each of these simulation tools. This is required in order to fully evaluate the proposed design. However, these results are often difficult to integrate with the other tools. Another problem with the use of multiple applications is the need to license, learn and administer each of them.

Capturing the complete system in a single model

On the other hand, Simcenter Samcef Wind Turbines is used to capture the dynamics of a complete wind turbine in a single model that includes hydrodynamics for wave loading, aerodynamic models for wind loading and multi-body models for evaluating the performance of mechanisms, such as the drive train, and finite element (FE) models for simulating the performance of structural elements. Simcenter Samcef™ Solver Suite software is used to compute the solution by direct integration, including multi-body dynamics, control systems, aerodynamic and hydrodynamic forces. The simulation tool includes parametric models of wind turbine components, such as blades, towers and gearboxes that can be quickly adapted to match a specific design. Users also have the option of modeling components and assemblies from scratch.

Predicting dynamic loads on drivetrains “The first challenge that we decided to address with Simcenter Samcef Wind Turbines was accurately predicting the dynamic loads on drive trains,” says Werthen.

Traditionally, AREVA Wind has used analytical methods to perform static analysis to estimate loads on components, such as bearings and gears and the resonant frequencies of the drivetrain. One of the most important cases occurs during a short circuit in the generator. In normal operation, the wind imposes torque on the drivetrain and the generator imposes counter-torque, but if there is a short circuit, the counter-torque is eliminated, generating large loads on the drivetrain. These loads cannot be calculated using traditional analytical methods, and short circuits are difficult and expensive to evaluate physically.

AREVA Wind engineers used the multibody simulation features of Simcenter Samcef Wind Turbines to model the complete drive system.

“This approach made it possible for the first time to accurately estimate the oscillations that occur when we lose counter-torque in the generator,” Radovcic says.

Validating the new assembly method

“Our next major project involved taking advantage of the global simulation capabilities of the software to explore an improvement to our installation process,” says Radovcic. “The traditional installation method involves assembling all three blades onshore. With this approach, a very large ship with a large crane is required to carry and lift the 115-ton blade assembly. Installation makes up 40 percent of the cost of an offshore turbine, so AREVA Wind was interested in reducing the cost of the installation process by installing one blade at a time with a smaller ship and a smaller crane. This approach will potentially save several hundred thousand euros for each wind turbine installed.”

“The challenge in moving to the one-bladeat-a-time assembly method is that we have to ensure that the turbine won’t be damaged by high winds and waves with only one or two blades installed,” says Werthen. “The installation process is often delayed due to weather or other factors, and in this case the turbine remains in a partially assembled state for up to a month. So before using this installation method, it’s essential that we test to ensure that the turbine can withstand a wide range of wind and wave conditions.

“It would cost well over €1 million to address this question with physical testing because we would have to partially assemble the blade, leave it in place for some period of time in order to determine how it would be loaded in various wind conditions. We would also run the risk of damage to the turbine, which would raise the costs even higher.”

Running 1,000 load cases in 16 hours

To evaluate this new installation method virtually, AREVA Wind used Simcenter Samcef Wind Turbines to model the one-blade-at-a-time installation process with a single blade and with two blades installed. The hydrodynamic model was coupled to a detailed FE model of the blade and a coarse multi-body dynamics model of balance of the wind turbine. The simulation computes the loads on the blades, tower and locking system.

Wind and wave loading was modeled with aerodynamic and hydrodynamic elements that can be used to compute one second of simulation time in about one second of clock time compared to the traditional computational fluid dynamics (CFD) method, which could easily take a week of clock time to compute a second of simulation time. This remarkably higher speed made it possible to simulate thousands of load cases for each phase of the assembly process. AREVA Wind uses Simcenter Samcef Wind Turbines on 10 standard personal computers. The simulation software automatically partitions each load case onto a separate processor. The company can run 1,000 load cases in about 16 hours or overnight.

Providing full control over wind turbine geometry

“Most wind turbine simulation solutions are parametric models that let us control the dimensions, but not the geometry of the wind turbine, so they cannot be used for modeling the assembly process,” says Werthen. “On the other hand, Simcenter Samcef Wind Turbines gives us full control over the geometry. This feature is essential for modeling wind turbines during various stages of the assembly process. We are also planning to use it to check loading on components during transportation.”

Radovcic notes, “The simulation has proven that the blade, tower and locking system can withstand a wide range of wind and wave loading during the various stages of the one-blade-at-a-time installation process. We will soon be performing real-world assembly testing to validate the best approach from a logistical standpoint.

“Overall, our two years of working with Simcenter software has been a very positive experience. We got up and running quickly and we have had a very good relationship with the development team. Whenever we’ve run into a problem, they have solved it for us in a reasonable time frame. The software has already paid back far more than the total cost of ownership, and we are looking forward to much greater savings in the future.”