This paper deals with the dynamic effects of wind on offshore structures. The spatio-temporal characteristics of wind gusts are discussed for the definition of wind loading on offshore structures. The importance of dynamic analysis of structures is emphasized as a rational means of assessing the response of wind-sensitive structures. Procedures for the dynamic response analysis of structures and their components are described in time and frequency domains. Contribution of second order terms at the low frequency range of wind spectrum to wind loading are formulated using properties of Gaussian random process. Extreme wind conditions experienced during a hurricane threaten the servive ability of a structure. Hurricane wind characteristics and their effects on structures are briefly described. Experimental procedures for the determination of wind loads and associated dynamic behavior in a wind tunnel laboratory are presented. Similitude requirements obtained from dimensional arguments derived from the governing equations, of the fluid-structure system are discussed. The potential of wind tunnel testing is fully realized by synthesizing test results with meteorological statistics of local climate to provide predictions of the behavior of wind-sensitive structures expected for certain levels of probability.
Environmental loading has a predominant role in the design of offshore structures for serviceability and servive ability during normal and extreme storm conditions respectively. The predominant loading for structural design of fixed gravity type structures arise through wave and current action, and effects of wind fluctuations are less important. However, wind has significant effects on the floating and compliant platforms with natural periods of 1-100 seconds. Furthermore, some of the auxilliary structures; e.g., communication towers and platform cranes are sensitive to the effects of wind. Therefore, it is important to emphasize the dynamic effects of wind on a class of wind-sensitive structures and to establish rational means of assessing their behavior. Spatial and spectral characteristics of wind fluctuations are needed for the definition of wind loading. Based on the statistical characteristics of wind loading, the dynamic behavior, of structures can be determined using methods of random vibration theory. In the following sections, description of wind characteristics, development of wind loading terms and procedures for evaluating the associated dynamic response are presented.
Atmospheric motion is derived from pressure gradients caused by unequal heating of the Earth's surface by the sun. The resulting atmospheric flow system is influenced by rotation of Earth, topography and surface roughness, reflective and thermal properties of the Earth's surface, cloud cover, and precipitation. Nonlinear interactions between the fluid motions of different scales varying from global dimensions to micrometeorological lengths of less than a millimeter with forces arising from the sources indicated, produce winds of random characteristics. Accordingly the random characteristics of atmospheric wind is best described by statistical methods (1). Therefore, the speed at a given point at a given instant is taken to be the sum of mean speed over a suitable period of time and the fluctuating speed described by standard deviation.