Comparison of 3-legged and 4-legged jacket for large wind turbines

Abstract

This study entails a detailed comparison of three legged (3LJ) and four legged jacket (4LJ) substructures for large wind turbines. The focus of this study is to determine, structural performance in terms of ultimate strength, fatigue life and natural frequencies. The differentiation is made on the basis of structural performance, material usage and economic feasibility. Using parametric modeling in MATLAB, the research applies to the specific conditions of the Southern North Sea (SN II) site. Key analyses include Ultimate Limit State (ULS), Fatigue Limit State (FLS), and eigenmode evaluations. ULS assessment of both 3LJ and 4LJ is done under various loading scenarios and multiple directions to ensure structural integrity and durability according to NORSOK N-004 guidelines. The FLS analysis indicated that 4LJ structures, with higher redundancy, have longer fatigue lives, making them suitable for harsher environments despite higher initial costs. In contrast, 3LJ structures, with fewer members and welds, offer lower material and construction costs, making them ideal for less demanding conditions. The analysis of the eigen modes for the 3LJ and 4LJ shows that both designs avoid resonance with wind turbine operational frequencies. The 4LJ due to its superior stiffness has higher 1st mode compared to 3LJ. While 3LJ structures achieve significant reductions in steel mass with acceptable compromise on the fatigue life overall leading to economic advantages. The study concludes that the choice between 3LJ and 4LJ jacket substructures should be based on site-specific conditions, cost considerations, and long-term performance requirements to optimize the balance between structural efficiency and economic viability.