Structural Design of the Substructure of a 10 MW Floating Offshore Wind Turbine System Using Dominant Load Parameters

Abstract

Fully coupled integrated load analyses (ILAs) to evaluate not only the load response but also the structural integrity are required to design a floating offshore wind turbine, since there has been no firmly established approach for obtaining the structural responses of a FOWT substructure in the time domain. This study aimed to explore if a direct strength analysis (DSA) technique that has been widely used for ships and offshore structures can adequately evaluate the FOWT substructure. In this study, acceleration and nacelle thrust were used for the dominant load parameters for DSA. The turbine thrust corresponding to the 50-year return period was taken from the literature. The acceleration response amplitude operator (RAO) was obtained through frequency response hydrodynamic analysis. The short-term sea states defined by the wave scatter diagram (WSD) of the expected installation area was represented by the JONSWAP wave spectrum. To account for the multi-directionality of the short-crested waves, the 0th order moments of the wave spectrum were corrected. The probabilities of each short-term sea state and each wave incidence angle were applied to derive the long-term acceleration for each return period. DSA cases were generated by combining the long-term acceleration and nacelle thrust to maximize the forces in the surge, sway, and heave directions. Linear spring elements were placed under the three outer columns of the substructure to provide soft constraints for hive, roll, and pitch motions. Nonlinear spring elements with initial tension were placed on the three fairlead chain stoppers (FCSs) to simulate the station-keeping ability of the mooring lines; they provided initial tension in the slacked position and an increased tension in the taut position. The structural strength evaluation of the coarse mesh finite element model with an element size same as the stiffener spacing showed that high stresses exceeding the permissible stresses occurred in the unstable members of the substructure. The high stress areas were re-evaluated using a fine mesh finite element model with an element size of 50 mm × 50 mm. The scope of structural reinforcement was identified from the fine mesh analyses. It was found that the DSA can be properly utilized for the substructure strength assessment of a FOWT.

Simplified Strength Assessment for Preliminary Structural Design of Floating Offshore Wind Turbine Semi-Submersible Platform

“…Another limitation of the design process specified by the standard is the large number of simulations. It has been shown by Park and Choung [12] that (only for DLC 6.2 specified in IEC 61400-3 [1]), the number of sub DLCs can be up to 5832, not to mention that at least 6 h of simulation is needed for each case. Covering all the DLCs in the preliminary design stage is complex, and only some representative load cases have been assessed in many previous studies.…”

“…Covering all the DLCs in the preliminary design stage is complex, and only some representative load cases have been assessed in many previous studies. Park and Choung [12] generated several direct strength analysis cases combining the long-term acceleration and nacelle thrust to maximise the forces in the surge, sway, and heave directions. Zavvar et al [13] conducted a preliminary structural design considering the still water and above-rated conditions.…”

Design Considerations on Semi-Submersible Columns, Bracings and Pontoons for Floating Wind

“…However, not all cases are equally important; here, we use engineering judgement and compare 12 shapes' performances at critical DLPs and the roughest wave load DLC. This method was employed with great success to reduce the number of analyses when investigating a single platform under different loads [16]. This study is similar in that it picks out separate DLPs to look at structural responses, but the cases are simplified as many different platforms are analyzed and compared with each other.…”

“…with great success to reduce the number of analyses when investigating a single platfo under different loads [16]. This study is similar in that it picks out separate DLPs to lo at structural responses, but the cases are simplified as many different platforms are a lyzed and compared with each other.…”