Oscillations Of Vertical Wind Profiles

Abstract

ABSTRACT Most of the time, oceanographic input criteria for design considerations consist of conditions of extreme high winds in storms that have been used to generate the high waves (the so-called "design wave") and currents with a conversion factor to ap proximate a short period gust. Because of this application of high wind conditions, the wind has been considered only at a single level, usually the air-sea interface or at the "gradient level, " with some fixed profile to approximate the air sea interface conditions and the conditions aloft. However, it is the dynamic loading of the entire rig that should be considered in the structural and operational design criteria, and to compute the dynamic load requires knowledge of both the vertical ind profile and its time-dependence. Unfortunately, data does not exist to the extent necessary to specify either the time-dependency of the vertical wind profile itself over a particular locale in the shallow water environment of most offshore exploration and production areas. We, therefore, must rely on the sparse measurements of wind profiles made over relatively homogeneous terrain that can be used to approximate a comparably smooth, pliable ocean surface, o compare with the theory that generates various vertical wind profiles as a function of bottom roughness and thermodynamic considerations. Recent data obtained from meteorologically instrumented towers, measuring winds over marshy terrain during stem conditions, can be used to approximate storm winds over the ocean surface. These data indicate a preferred vertical shear in the storm wind that oscillates on the order of minutes, with additional oscillations on the order of 5 seconds. The length scale of the minute period oscillations is equivalent to the length of semi-submersibles. These oscillations shown in the analyzed data, are related to the trajectory of the wind over the terrain, which is likened to the fetch over rough or smooth, warm or cold water. With the dynamic considerations of oscillating vertical shear taken into account, more efficient design of both mobile and stationary structures is possible, with the added feature of certain safer and more efficient operational procedures during storms. The implications from these data indicate developments that would allow real-time anticipation of impending wind-waves in hostile offshore environments. This short term prediction is possible because waves over which the wind system has passed affect the wind profile. 1. Introduction Recent crisis brought about by shortages of resources have increased the importance of efficiency in construction on one hand, while increasing the need for completed projects on the other hand. These opposing incentives have caused an important design consideration to emerge from its previous state of a "latent headache to offshore designers. " The dynamic effects 6f time changing wind profiles have been accounted for by using standard design factors based on wind measurements over land. For high wind considerations, very little wind-profile data are available over land and in general, are not available over the ocean. However I, since the concern of both designers and operations personnel is with the effects of high winds in a vertical profile that vary with time, then the subject of data presentation and analysis will be restricted to high winds in profile that change with time. There is not sufficient empirical data to specify the time dependent vertical wind profile in high wind conditions at the shallow water sites of offshore exploration and development, however 1 there have been careful measurements made over marshy terrain that can be extrapolat