Reliability of Circular Piles Subjected to Ocean Waves

Abstract

1. Introduction Wave forces exerted on structures are of vital interest to offshore drilling stations and various other marine structures. Because the piles constitute the major or entire support for the whole structure the engineer must ensure that those piles successfully withstand the forces to which they are subjected. In this regard, the most important piece of information is the occurrence of wave forces at the site of the proposed structures. The design problem is complicated by a number of factors. First, wave characteristics change as a wave approaches the shore. Secondly, local wind-generated waves become superimposed on large ocean waves, creating a complicated surface. Thirdly, the forces exerted by waves of similar properties vary greatly. This variation is caused by fluctuations in fluid particle velocities as well as eddies and turbulance around piles caused by the rapidly reversing flow. Consequently, a deterministic approach to wave force prediction appears to be impossible. As an alternative method in design, an engineer might determine his design by using a cumulative distribution function (CDF) of wave forces created from field data. If the distribution of wave forces is known, the engineer can then make reliability statements about his design. The method used here utilizes a multiple linear regression analysis to develop a relationship between wave properties and the parameters 1 used in the Morison force equation. These parameters include velocity and acceleration of water particles and the drag and mass coefficients. Using Monte Carlo simulation these regression relationships are then utilized to develop the probability density formation of wave forces. 2. Description of Force Equation The forces exerted by ocean waves on a pile is one of the most complicated fluid flow problems because of the presence of both unsteady and oscillary motion. The generally accepted formula for a wave force on a pile is the so-called Morison formula,(Mathematical equation available in fullpaper) The first part of this equation represents the for drag caused by surface shear. The second part is the acceleration force on the displaced volume of fluid including the virtual mass effect. The relative importance of inertia force increases with the ratio of the pile diameter to wave height. Although the particle motion in waves is orbital, the method is based on flow. The fluid density and the pile diameter D are assumed to be constant. It is also assumed that the drag force and the inertia force can be treated separately, and the total force can be obtained by adding the solutions linearly. In previous investigations 1,2,3,4,5,6 water particle velocity and acceleration were calculated by stokes7 irrotational theory. The theory considers only waves of small but finite wave steepness.