A Damage Mechanics Approach to Fatigue Assessment in Offshore Structures

This article is intended to describe the development of a fatigue damage model capable of assessing fatigue damage in offshore structures. This is achieved by for mulating a set of damage coupled constitutive and evolution equations which make the for mulation of a unified approach possible under both low and high cycle fatigue damage and consistent with the structural dynamic response of the changing/deteriorating material be haviors. The structural analysis for the whole designed period, say about 30 years, can be carried out with the aid of the proposed analytical procedure, in which the fundamental characteristics of sea wave statistics responsible for the structural dynamic response can be sufficiently considered. An offshore structure subject to complex ocean environment is described by a general stochastic system which embeds a group of stochastic subsystems, each characterizing a duty cycle. An effective analytical method is established by introduc ing the concept of duty strain range with a clear mathematical definition and its analytical solution which covers all possible spectral parameters. The history-dependent damage is also included in the damage model so that the overload effects can be analyzed. It should be pointed out that the whole procedure can be fully computerized such that the practical or engineering significance of varying design variables can be readily highlighted.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67255/2/10.1177_105678959300200405.pd

The spectral density for ocean wave forces

A computational formula is developed for determining, from the sea surface spectral density, the spectral density function of the force per unit length at a point on a vertical pile. A surprisingly accurate and simple approximation for the formula is presented and used to explain the near proportionality between the spectral densities of force and sea surface measured in the ocean near Davenport, California. The computational formula and its approximation are extended to provide procedures for the determination of the spectral density of either total force or total overturning moment on a structure consisting of an array of vertical cylinders. The approximation leads to a particularly simple relation in which the spectral density for the total structure is the spectral density for a single pile times a function which characterizes the geometry of the array. As an illustration of the procedure, the total force spectral density is computed for a four-pile instrument platform in 49 feet of water