Lateral response enhancement of a conventional offshore monopile using skirts of helical piles and pipe piles with stiffeners

Many vital infrastructures are supported by large-diameter rigid monopiles, such as bridges, wind turbine systems, dolphins, and several other offshore structures. These monopiles, in addition to sustaining large axial loads, are subjected to large lateral loads and moments. However, their lateral load-carrying resistance may not be sufficient to withstand prolonged impact under large wind and wave loading. This capacity depends on the monopile's geometry, the soil properties, and the type of loading. Thus, the lateral monopile capacity can be increased by improving the surrounding soil or the monopile lateral stiffness behavior. The monopile stiffness can be increased by increasing its physical dimensions, but this solution may not be economically or practically feasible. Thus, a range of different foundation solutions in place is being investigated. A 3D finite element analysis FEA is presented in this paper to focus on enhancing the lateral stability of the conventional monopile by attaching a skirt of supporting driven pipe piles, which are distributed around the monopile and fixed with its surface above the mudline. Three different types of supporting piles have been used, conventional small driven pipe piles, helical piles, and pipe piles with stiffeners, which resulted in an increase of the ultimate lateral load capacity by 25.0%, 29.5%, and 41.4%, respectively. The lateral stiffness is accompanied by saving the cross-sectional area of the steel section in case of using unconventional supporting pipe piles such as helical piles or piles with stiffeners. All the output data resulted from a 0.5 m or 0.1D displacement for the monopile at the mudline.