# Parallel Analysis of Offshore Wind Turbine Structures under Ultimate Loads
## 4 Study of 10 MW OWT under IEC 61400-3 Loads with Earthquake and Typhoon  
The DTU 10-MW OWT [39], with the jacked type support structure, is analyzed as an example in this section, where the average depth of the sea is 50 m, and the soil is moderate-hard sandy soil with a submerge internal frictional angle of 30\({}^{\circ}\). IEC 61400-3 (2019) loads, with the exception of the fatigue and transport loads (DLC 8), are analyzed, where earthquake and typhoon effects are included. Three earthquakes, with a peak ground acceleration (PGA) of 0.32 g and \(T_{s}=1\) s, were also generated, and eight earthquake directions were set at 0\({}^{\circ}\) to 315\({}^{\circ}\), with an increment of 45\({}^{\circ}\), from the X axis, where \(T_{s}\) is the period at the end of constant acceleration in the response spectrum [40]. We assumed that the soil liquefaction occurs 20 m from the sea-bed during the earthquake, and the analysis followed the reference [35]. Moreover, the IEC 61400-3 DLC I.1 and DLC I.2 are used to simulate a class-three tropical cyclone, where the average wind speed at the hub height is 72 m/s, and the significant wave height of a return period of 500 years is 14 m. The finite element mesh and the dimensions of the final design result are shown in Figure 5, where the mesh contains 176 beam elements for columns, 256 beam elements for bracings, 28 beam elements for the tower, 280 beam elements for piles, 36 plate elements for the platform, and 280 p-y curve elements, 280 t-z curve elements, and four Q-z curve elements for soil simulation. In the analysis and design, there are 5044 applied loads listed in Table 2 and the total degree of freedom of the OWT support structure is 4710. For the cases without seismic and tropical cyclone loads, the time step length was set at 0.05 s, and 14,000 time steps (total analysis time = 700 s) were simulated using the direct integration [PERSON]'s method. For the cases with seismic loads, the time step length of 0.01 s, and 18,000 time steps, were used (total analysis time = 180 s), where the seismic load began at 100 s and lasted for 40 s. Rayleigh damping, with a mass damping of 0.04/s and stiffness damping of 0.01 s was used, which produces a damping ratio near 4%at frequencies of 0.3 and 4 Hz. Table 3 shows the computer time, executed in finite element design programs, using one to eight computers. Tables 4-7 show the required steel weight of the design and the control loads of the tower, columns, braces, and piles for various conditions, including IEC 61400-3, class-3 typhoon, or 0.32-g earthquake loads. These results indicate the following features:  
1. The assumption of rigid support structures, to obtain the blade forces and moments to perform the structure analysis, can reduce computer time by nearly three times the amount of the full analysis model, which considers the interaction of blades, support structures, and wind. Moreover, the design of OWT support structures can be fully parallel using independent computers, and the computer time required for the communication between computers is negligible, and can be ignored. 2. As shown in Tables 4-7, the number of necessary loading cases can be over one thousand, but most of these are not dominant in the design of member sizes. Thus, the design cycles between the second and the second-last cases can use a number of control loads, and we suggest finding control loading cases ranging from ten to twenty critical loads for each member (\(10\leq N_{load}\leq 20\)) is appropriate in the design of OWT support structures. 3. Without seismic and typhoon loads, DLC 2.2 may dominate the steel design for the tower and columns. The reason for this is that the blades are out of control at a small instantaneous wind speed, at which the pitch angle is small to obtain wind energy, and the larger turbulent wind later exerts a large force on the blades. Except for this condition, the parked condition, with a large wind speed and wave (DLCs 6.1 and 7.1) will dominate the OWT steel design. 4.