Assessment of compliant and non-compliant behavior of piles using morison equation

Abstract

The work presented in this thesis aims to investigate the impact of using the non-compliant form of Morison Equation instead of the compliant form, and whether following one of those two methods can have any connection to the fatigue-related failures seen in single vertical free-standing piles. This investigation is performed through a parametric analysis of piles of different sizes in waves of different properties for both forms of the Morison Equation. Each pile is analyzed twice, one analysis to calculate the wave forces using the non-compliant form and the other using the compliant form. Then, the two methods are compared in terms of calculated forces, stresses, accelerations, velocities, and deflections. Wave loads on coastal structures are a crucial design factor and the behavior of a structural element under wave loads is a very complex problem. Due to the dynamic and cyclic nature of wave loads, fatigue-related failure is a common design problem. Especially, in vertical slender structural elements such as piles used in oil and gas terminals. In such structures, fatigue-related issues such as through-the-thickness cracks and excessive deflections can become more prominent, particularly in single free-standing vertical piles. Such a scenario may occur during the construction phase where piles are usually driven and left free-standing without lateral support which is to be provided once the superstructure is installed. Although some lateral support can be secured by connecting piles which are members of a pile group using tie-backs at the top, the piles can still be at a high risk of failure in certain conditions such as storms and rough seas. Therefore, it is necessary to understand the behavior of single free-standing vertical piles under wave loads, particularly; slender or very slender piles which can experience, due to their ability to move more flexibly with waves, a different set of forces compared to less slender piles which exhibit a more rigid-like motion under waves. Wave forces on structures can be estimated using the Morison Equation which is a very simple and very widely used method to calculate in-line forces on a member in the direction of wave motion. Morison Equation can be extended to calculate lift forces (cross-flow forces) which are the forces in the direction perpendicular to the flow direction of the waves. Lift forces appear due to turbulence and vortex-shedding behind the member, and although those forces can sometimes be higher than the in-line component, it is essential to establish a clear understating of in-line forces contribution first and then proceed to study the combined effect of both the in-line and lift forces when they act simultaneously. Therefore, this study only considers the in-line component of wave forces whereas the lift forces and their coupled action with the in-line forces are the focus of future studies