Germanischer Lloyd WindEnergie GmbH (GL-Wind), a subsidiary of the international classification and industrial certification society Germanischer Lloyd, is investigating the use of LMS FALANCS to improve and accelerate their procedures for certifying wind-turbine designs. The certification qualifies turbines on safety, performance and service lifetime. Manufacturers must have the certification to participate in a fast-growing energy production market.
Certification for a growing industry
Certifying turbine designs is an activity that Germanischer Lloyd has been practicing since 1872, beginning with wind-driven water pumps used onboard ships undergoing classification. Today their services are more in demand than ever in a wind-energy market enabled by steady improvements in wind-turbine engineering. It is the engineering-based economy of scale that now makes the industry viable. At the same time that turbine design finally reached the point of cost-effectiveness, governmental directives and compensation plans began guaranteeing return on investments in wind energy. Meanwhile, renewable energy strategies have reached the summits of political and economic agendas worldwide.
The suppliers of components, whole turbines and ‘farms’ to the wind-power industry are gearing up for production levels over the next 5 years that will be double or even triple current levels. In February 2002, the world’s installed wind power capacity totaled 24,000 MW. The European Union Renewables Directive passed in September of 2001 aims at connecting another 40,000 MW of installed wind power to the grid by 2010. In addition, individual countries have their own goals, generating even more demand for new turbines. In Germany, for instance, the federal strategy for wind energy is aiming at 25GW wind energy production by 2025, equivalent to the electric power consumption for 29 million households. This strategy translates into designing, building and installing 5,000 5MW offshore turbines in Germany. This represents the total for only one of 15 member countries committed to meeting the directives of the European Union. The wind-energy market is experiencing a ten to thirty percent growth rate, and GL-Wind is evolving the certification basis with it. A safe and reliable wind-turbine design is of vital importance to give this growing market a stable foundation.
The challenge for GL-Wind
To be certified, manufacturers must design a wind turbine that will reliably produce low cost energy for 20 years while withstanding extreme conditions. Although the average load to which a wind turbine is subjected is minimal, random forces of extraordinary strength hit the structures at unpredictable intervals. The challenge to GL-Wind was finding an improved method of accurately evaluating designs for fatigue-life under these conditions. The method used prior to LMS FALANCS was based on calculations that did not fully take into account the unpredictable random forces, or the interrelationships of multi-axial forces. This oversimplification led to inaccurate fatigue-life predictions, and the only way to secure the operational safety of the turbine was to over-design it. GL-Wind Research and Development has selected LMS FALANCS to be used in the durability simulations to evaluate onshore turbine designs. Ultimately, they plan to use the enhanced methods integrated in LMS FALANCS to standardize the assessment procedure for certification of both onshore and offshore turbine designs.
The importance of obtaining precise internal forces
The critical loads acting on a wind turbine are mainly due to fluctuations in windspeed and direction, but also by starting and stopping. For these events the external loads are known – they are measurable with test equipment. Starting from the external loads, the engineers calculate the internal loads that act on the components such as the rotor hub. A global load spectrum formulated from individual events is used to simulate 20 years of use. The process used prior to the introduction of LMS technology superposes individual internal loads to derive a load histogram (load spectra-range pair) equivalent. This process is unreliable in regard to real mean-stresses because none of the information about phase relationships between the individual loads is included and the inaccurate mean-stresses may lead to non-conservative estimations of fatigue life. By ignoring the phasing relations, the conclusions may even identify the wrong hotspots. This uncertainty generates results that could lead to an overly conservative design, in a business where accurate design is essential to making wind energy a feasible option.
With LMS FALANCS GL-Wind does not have to make assumptions that simplify the data. This leads to more reliable certification analysis, therefore to safer turbines and more economical designs. At GL-Wind, certification analyses more accurately reproduce potential durability scenarios according to specific turbine design and site information.
GL-Wind is adopting the LMS real time domain approach for durability analysis. The internal loads for individual events are applied with LMS FALANCS to the ANSYS model of a structure, then LMS FALANCS test schedules are used for the superposition of events to simulate the 20-year history, ensuring that phasing and mean stresses are calculated into the simulation. In view of the importance of using new, light and strong materials for the components, design evaluation with LMS FALANCS can also include material-based strain-life curves. LMS technology incorporates the full set of load-time series, the wind-speed parameters, and the material-life curves.
Simulations done with LMS FALANCS are more accurate than those based on earlier methods because the efficiency of the calculations makes the time-history approach practicible.
The time-history approach uses all significant damaging events for a 20-year lifetime and accounts automatically for correct phase relations of load components and the correct mean stresses.
Because it can assess the entire model of the structure, hotspots are identified accurately and automatically (no pre-selection necessary as with former methods), and the damage distribution is easily visualized in the displays.
About GL-Wind
Germanischer-Lloyd Windenergie is a subsidiary of Germanischer Lloyd AG, and is the world market leader in wind turbine certification. They are accredited to certify turbines according to international and national regulations and standards including those for Germany, Denmark and the Netherlands. The procedures for certification are under continuous development, and have made gains in accuracy and efficiency.(end)
