Study on Spudcan Soil-Structure Interaction of Wind Turbine Installation Vessel

Abstract

A jack-up type WTIV (Wind Turbine Installation Vessel) is necessary to install offshore wind turbines. To avoid the effect of waves when installing wind turbines, WTIV should be lifted clear of the water. It is usually supported by several independent truss legs. The spudcans are the inverted cones mounted under each leg of the WTIV which provides stability to both vertical and lateral forces. Before WTIV can operate at a given location, a site-specific assessment should be performed. A risk exists when lifting the WTIV at the site with critical soil conditions especially the strata with strong soil overlying soft soil. Instability may occur when the leg is penetrating a layer of strong soil into underlying soft soil by a rapid leg penetration which is known as punch-through. Thus, it is significant to accurately predict the penetration depth and ensure the stability during spudcan penetration. To avoid punch-through type of failure, prediction of ultimate bearing capacity and corresponding penetration depth is required. For this purpose, the conventional analysis recommended in the SNAME guideline is introduced in this paper. Soil conditions of OW-3 and OW-4 in the wind farm in the Southwest Sea of South Korea are used. WTIV structural assessment of its ability to withstand a storm condition should also be performed. The complex stress and strain state of soil under the spudcan is commonly simplified to a value of soil stiffness that is input as a boundary condition into the structural analysis. These boundary conditions include pinned footings, fixed footings, spring footings based on both SNAME and Model B. Soil-structure interaction effects can be considered when the boundary conditions are set as springs with a corresponding yield surface. A jack-up structural analysis is performed with three kinds of boundary conditions. Structural analysis results with SNAME springs are compared to those with Model B in previous studies. Structural analysis results with SNAME and Model B are almost the same. Structural analysis of WTIV with four legs is also performed with pinned footings, fixed footings and spring footings. From the results, a reduction in stresses of the members at leg-hull connection can be found. Yield first occurs in the leeward spudcan and once yield occurs moment in the spudcan decreases. Besides, soil strength and preload have significant impacts on structural analysis results. Spudcan loads will be redistributed if soil-structure interaction is taken into accounted in a structural analysis.