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Task 30

Offshore Code Comparison
Collaboration Continuation
(OC4)

A project to compare dynamic computer codes and models used to design offshore wind turbines and support structures

Placing wind turbines offshore has many advantages. The vast offshore wind resource could power many of the world’s population centers along the coasts. The wind blows harder over the water, and larger turbines can be installed using ships to move the huge components. However, designing turbines and their support structures for offshore operation is challenging, because sites have different water depths, soil types, and severities of wind and waves. To minimize the cost and maximize the durability of offshore wind turbine installations, several new types of support structures are being explored.

Wind turbines are designed and analyzed using simulation tools (i.e., design codes) capable of predicting the coupled dynamic response and the extreme and fatigue loads of the system. Analyses for land-based wind turbines use aero-servo-elastic codes. Some of these codes have been expanded to include the additional dynamics pertinent to offshore installations including the incident waves, sea current, hydrodynamics, and foundation dynamics of the support structure (Figure 1). Aero-hydro-servo-elastic codes are very sophisticated, but limited data are available to validate them in the offshore environment. Ensuring the codes’ accuracy and correctness for designing offshore turbines is a high priority for the wind turbine industry.

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Figure 1 Inputs for code comparison analyses

One way to identify inaccuracies and errors in new computer models and simulations is to compare their outputs under a wide range of conditions. To compare the codes used to design support structures for offshore wind turbines, the Offshore Code Comparison Collaboration (OC3) began in 2004, within Subtask 2 of the IEA Wind Task 23 Offshore Wind Technology and Deployment (see reports). Experts from eighteen organizations in ten countries made code-to-code comparisons to improve the codes’ predictive capability for offshore wind energy structural loads. By 2009, the collaboration completed four work packages or phases, all built on a common 5-MW reference wind turbine model developed in previous work by the National Renewable Energy Laboratory in the United States.

• In Phase I, a monopile with a rigid foundation in 20 m of   water was analyzed.
• In Phase II, the foundation of the monopile from Phase I was made flexible by applying different models to represent the soil-pile interactions.
• In Phase III, the water depth was changed to 45 m and the monopile was swapped with a tripod substructure.
• In Phase IV, the wind turbine was installed on a floating spar buoy in deep water (320 m).

Task 30 OC4 continues the work begun in Task 23. Over an additional four-year period (2010 through 2013). The operating agent organizations (managers) of the work are the National Renewable Energy Laboratory in the United States and the Fraunhofer Institute for Wind Energy and Energy System Technology IWES in Germany. There are two work packages (1.5 years each): focused on jacket support structure and floating semisubmersible, respectively. An experts meeting was held in 2012. Each work package and the experts meeting of verification data sets result in final reports.

The aim of OC4 is not only to benchmark and validate simulation
codes on a structure, but also to encourage model and tool development and the training of new analysts.

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