Experimental and Finite Element-Based Investigation on Lateral Behaviors of a Novel Hybrid Monopile

Abstract

A monopile is the most conventional structure foundation for offshore wind turbines (OWTs) in the world. However, the Korean offshore wind industry has mostly been using the jacket type of foundation. The main reason for the current situation in Korea is that most of the marine soil consists of weak layers of sand and clay. Thus, the monopile foundation depth has to be deep enough to satisfy the intended serviceability design requirement of the monopile and the rotation limit at the seabed; a conventional monopile design concept alone might be insufficient in Korean offshore conditions, or otherwise could be very expensive, e.g., resulting in a rock socket installation at the tip of the monopile. The main objective of this paper is to introduce a novel hybrid monopile that is composed of a monopile and a supplemental support with three buckets, followed by assessing the lateral resistance of the hybrid system through physical experiments and finite element (FE) simulations. Namely, 1/64.5 small-scaled monopile and hybrid physical models with a monopile diameter of 7 m for a 5.5 MW OWT were loaded monotonically. The results show that the hybrid monopile improves the lateral bearing capacity regarding the initial lateral stiffness and ultimate load. The FE analyses of the corresponding physical models were also implemented to support the results from the physical model test. The numerical results, such as the structural member forces and soil deformation, were analyzed in detail. Additionally, a case study using FE analysis was conducted for the 5.5 MW OWT hybrid monopile support installed in a representative Korean weak soil area. The results show that the hybrid monopile foundation has a larger lateral resistance and stiffness than the monopile.

A Study on the Lateral Load Capacity of a Novel Hybrid Monopile via a Centrifuge Model Test

“…Because the failure areas were similar despite the difference in the ground failure geometries depending on the loading direction, the bearing capacity improvement effects compared to the existing monopile were similar. A similar bearing mechanism of the hybrid monopile was also revealed in the results of Kim et al [22].…”

“…The additional structure increases the bearing capacity of the entire system by increasing the resistance area of the ground against the lateral load. This finding was also supported by Kim et al [22]. Kim et al [22] performed a numerical simulation to evaluate the lateral behavior of a hybrid monopile.…”

“…This finding was also supported by Kim et al [22]. Kim et al [22] performed a numerical simulation to evaluate the lateral behavior of a hybrid monopile. They found that the lateral movements of a single monopile cannot be constrained after the soil reaches the ultimate state, while the wider soil around the hybrid monopile is constrained by the additional structure, resulting in increased load capacity.…”

“…This additional support structure has suction buckets arranged at 120 • intervals around the central part. Each bucket is connected to the monopile using a truss member or plate, and shares the load transmitted to the monopile [22]. The central part has a cylindrical sleeve for combination with the monopile, and the monopile and additional support structure can be strongly connected using a high-strength grouting material.…”