Abstract
This paper presents theoretical aspects and an extensive numerical study of the coupled analysis of tripod support structures for offshore wind turbines (OWTs) by using X-SEA and FAST v8 programs. In a number of site conditions such as extreme and longer period waves, fast installation, and lighter foundations, tripod structures are more advantageous than monopile and jacket structures. In the implemented dynamic coupled analysis, the sub-structural module in FAST was replaced by the X-SEA offshore substructure analysis component. The time-histories of the reaction forces and the turbine loads were then calculated. The results obtained from X-SEA and from FAST were in good agreement. The pile-soil-structure interaction (PSSI) was included for reliable evaluation of OWT structural systems. The superelement concept was introduced to reduce the computational time. Modal, coupled and uncoupled analyses of the NREL 5MW OWT-tripod support structure including PSSI were carried out and the discussions on the natural frequencies, mode shapes and resulted displacements are presented. Compared to the uncoupled models, the physical interaction between the tower and the support structure in the coupled models resulted in smaller responses. Compared to the fixed support structures, i.e., when PSSI is not included, the piled-support structure has lower natural frequencies and larger responses attributed to its actual flexibility. The models using pile superelements are computationally efficient and give results that are identical to the common finite element models.
Highlights
Among the common types of fixed-bottom substructures, shown in Figure 1, monopile structures are most suitable for regions with shallow water depths [1]
Tripod and jacket structures can be constructed in transitional water depths
An alternative design that makes use of the various advantages of both monopile and jacket structures is known as the tripod support structure
Summary
Among the common types of fixed-bottom substructures, shown in Figure 1, monopile structures are most suitable for regions with shallow water depths (less than 30 m) [1]. In a recent design of tripod foundation, the superstructure of the turbine was first simulated under wind and wave dynamic loading using FAST (Fatigue, Aerodynamic, Structures and Turbulence)—a CAE tool developed by the National Renewable Energy Laboratory (NREL), USA, to obtain time histories of internal actions for the pylons [12] using 3D finite element analysis software. These iterative analyses were utilized to obtain the required pile lengths and cross sections for tripod options [12]. A parametric study of an NREL 5MW OWT supported by a tripod structure using coupled analysis with soil-pile interactions is carried out
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