Stress wave interactions with holes on a cylindrical structure

Abstract

Cylindrical structures are often used in the offshore industry for their hydrodynamic efficiency, which minimizes drag from waves and currents. Their cylindrical shape provides structural integrity and stability, evenly distributing stress to withstand marine environments. These structures are commonly used in oil rigs, wind turbines, and underwater pipelines.

This thesis investigates the different structural effects of cut-outs on thin-walled cylindrical structures, particularly in the context of pile driving. During the pile driving process, stress waves are created, which create stress concentrations around the cut-outs. A critical review of the literature has identified that the impact of these stress waves on various hole shapes within cylindrical structures is a relevant topic, particularly concerning fatigue.

To find out the impact of different hole shapes on the other limit states of a cylindrical foundation with an emphasis on the fatigue limit state multiple finite element, multiple finite element analysis (FEA) models are made, and the impact of the hole shape is measured.

First, the feasibility of the hole design is tested for the ultimate limit state (ULS) and the static fatigue limit state (FLS) using calculations performed in Ansys. Multiple models are also created in Ansys to calculate the eigenfrequencies of the cylindrical structure with different hole shapes. LS-Dyna is used for stress wave analysis. Initially, the cylindrical structure is given a large length to ensure the stress wave doesn’t reflect during the simulation, allowing the response of the hole to be measured. Finally, the cylindrical structure is placed on a realistic soil model to account for the reflecting stress wave.

The findings of this research have significant practical implications for the design and analysis of cylindrical structures in the offshore industry. They demonstrate that different hole geometries can be successfully implemented in cylindrical structures while satisfying ULS and FLS design requirements. The eigenfrequencies of the holes are excited by the stress wave that occurs during pile driving. When a soil-like structure is added, the reflecting stress wave diminishes this effect. The results suggest that elongated holes may offer superior performance compared to round holes in terms of fatigue damage resistance during pile driving.