# Parallel Analysis of Offshore Wind Turbine Structures under Ultimate Loads
## 2 Efficient Schemes for the Design of Offshore Wind Turbine Structures  
In the structural design of bridges or buildings, the number of loading combinations is not more than 300 for most codes, such as AASHTO [31] and ASCE 7-98 [32], since the wind and seismic loads are arranged using their ultimate conditions. However, in the design codes of OWT structures, such as IEC61400-3 [1] and DNVR-PC205 [2], the loading combinations of wave and wind loads require multiple directions, and must be able to act in the same or different directions, for both ultimate and service conditions. Thus, we proposed three schemes to increase the efficiency of the numerical computation, as follows:  
(1) The assumption of blade wind forces independent of the support structure We assume that the time-dependent wind forces acting on the blades of a wind turbine are independent of the support structure, so these forces can be calculated at the hub center as three forces and three moments, considering only the blades, without the support structure. This scheme may be useful for real-world design procedures, since the blade information is often confidential in turbine companies, and the calculated hub forces can be used in two-stage analysis. This assumption may cause some deviation in analysis, but can significantly reduce computer simulation time. Since the six force components are the same when changing only the wind direction or wave force data (magnitude and frequency), this calculation can be omitted for many loading cases. For example, we generated a wind turbine structure analysis with 5044 loading cases, including seismic loads according to IEC 61400-3 when there are only 242 different time-history blade wind loads. It was found that this assumption can reduce computation time for blade wind loads, but the accuracy of this assumption is still in question. We used the NREL 5 MW wind turbine [33], with the jacket type foundation, to compare the difference between the two assumptions using Turbsim [34] and FAST [24], where the input data was obtained directly from reference [33] for the flexible support structure analysis, and the degrees of freedom of the tower and support structure were eliminated for the rigid support structure analysis. We analyzed the two conditions under two loading cases: a Design Load Case (DLC) 1.1 normal turbulence model with a \(V_{hub}\) of 25 m/s, and a DLC 6.1 extreme wind speed turbulence model with a \(V_{hub}\) of 47.5 m/s. It should be noted that DLC 1.1 is under the power production condition and DLC 6.1 is under the parked condition. Figure 1 shows the comparisons, which indicate that the results of the two assumptions were not very different. In reality, it is not correct to use the blade forces calculated from a flexible model to analyze the support structure, since the blade forces near the natural frequencies of the support structure are magnified, which causes resonance of the support structure and blade forces. One can omit this scheme to perform the full model analysis, but a considerably increased computer time will be required. Figure 1: Comparisons of the total blade center forces between the rigid and flexible assumptions of the support structure under DLCs 1.1 and 6.1: (**a**) X-direction force of DLC 1.1 (**b**) Y-direction force of DLC 1.2 (**c**) X-direction force of DLC 6.1 (**d**) Y-direction force of DLC 6.1. Using control loads to perform the design procedure between the second and second last design cycles  
In the design procedure, one should first estimate the member sections and perform a structural analysis and steel design to find the appropriate section of each member. Then, the above procedure is performed continuously until all the selected sections are suitable and qualified under all the loads. This means that the structural analyses under all the loads should be analyzed several times.