Analysis of a Monopile Under Two-Way Cyclic Loading

Introduction

A monopile is a promising foundation for an offshore wind turbine (OWT), which is a simple tubular structure. It provides a low fabrication cost and easy installation. It consists of a large diameter hollow open-ended steel pipe shaft embedded into the ground. Figure 1 shows the schematic diagram of an Offshore Wind Turbine (OWT). Most of the existing OWT lies on a monopile foundation for water depth up to 35m. A monopile is a single-driven pile of 3-8m diameter with a slenderness ratio (L/D) less than 10. These ranges depend on the type of load and the ground condition to support offshore wind turbines (Murphy et al., 2018). The monopile foundation should transfer both vertical loads induced by the self-weight of OWT and the horizontal load from the wind and wave action. The horizontal loads from winds and waves action are cyclic in nature and low in load amplitude (Arshad & O’Kelly, 2014). The foundations of OWT are exposed to a different type of cyclic lateral loading. It needs to be withstood several million cycles in its lifetime. The recommended code (API, 2003; DNV, 2014) describes the four categories of design loads for monopile foundation, which experiences lateral loads. Ultimate Limit State (ULS) relates to an extreme load case that happens once during the OWT lifetime. This analysis verifies the strength and stability of the structure within the acceptable value. Accidental Limit state (ALS) [ULS/1.35] encounter once during the lifetime of the OWT when it undergoes accidental load, such as the impact of a ship on the support structure of the wind turbine. The serviceability limit state (SLS) of the offshore structure is approximately 47% of ULS, which experiences the repeated loading (100 times) over the lifetime of the OWT. The SLS design criteria verify the tolerable deformation of the structure within the acceptable limit. Finally, the Fatigue Limit state (FLS) is approximately 25-30% of ULS, experiences very frequently (10⁷ times) of low amplitude load over the lifespan of OWT that verifies the withstand capacity to failure. These loads on monopile foundation transmit into the soil by mobilizing the lateral resistance of soil around the pile. The accumulated displacement and the changes in stiffness are two important design parameters in addition to the ultimate capacity of the monopile. The accumulated displacement must be limited to prevent the exceeding serviceability limit (Achmus et al., 2009). The cyclic fatigue loading affects the properties of the surrounding soil, which may result in intervention between natural frequency and excitation frequency (Leblanc et al., 2010). The SLS and FLS are the two crucial loading conditions in offshore wind turbine design (Barari et al., 2017). Due to continuous loading, the accumulated displacement and stiffness of the foundation would change and affects the soil-pile system response.

Figure 1.

Schematic diagram of an Offshore Wind Turbine (OWT)