Several different types of vessels and platforms are used by the offshore exploration and production industry. Without exception wind loading is a major design consideration during at least one phase of their design lives. In general, wind loadings are most important when the platforms are afloat. Wind overturning moments govern hydrostatic stability requirements and wind forces constitute a major component of the total forces, which may need to be resisted in the mooring, positioning and towing phases.

From an aerodynamic viewpoint these vessels and platforms are complex bluff bodies. The wind loads on them are not amenable to theoretical analysis and empirical calculation procedures or wind tunnel tests on scale models are used to provide estimates of the forces and moments to be expected on the full-scale structures. From a designer’s viewpoint the accuracy of these estimates is important since it influences safety margins, economy of design and operational restrictions.

It is widely accepted that the accuracy of empirical calculation procedures is uncertain. It should be also recognised that these procedures is uncertain. It should also be recognised that these procedures are seldom sufficiently explicit to prevent different individual interpretations leading to appreciable differences in design loading estimates. Largely for these reasons wind tunnel tests are assuming increasing importance and, for example, have been stated as mandatory in the new Norwegian Maritime Directorate rules for floating offshore platforms. For many years Classification Societies have, in presenting their empirical calculation procedures, stated that authoritative wind tunnel tests were acceptable alternatives to calculation. Such tests are themselves subject to uncertainty due to problems of flow simulation on small scale models and cannot be conducted simply by placing an instrumented scale model in the steady airstream of a conventional wind tunnel. Over the past few years a number of techniques have been developed to improve the simulation of full-scale flows. These techniques had not, however, previously been validated for some of the body geometries associated with offshore structures.

The Department of Energy commissioned a series of wind tunnel studies to be undertaken by NMI which had the following main objectives:

i) The measurement of wind forces and moments on models of some typical North Sea platforms.

ii).The application and validation of ‘Reynolds number enhancement’ techniques for improved flow simulation.

ii) Assessment of the calculability of wind forces for a typical semi-submersible.

This report summarises some of the results of these studies as presented in References (1-9) and briefly discusses their application. For detailed results and descriptions, especially of experimental techniques, the original reports should be consulted.

In reviewing the results we have taken the opportunity in some cases to present the results in several ways. The purpose of this approach is to illustrate a point made in Reference (9). The designer often requests forces and moments to be defined with respect to the body axes so that they may immediately be used for hydrostatic calculations. However results in this form rarely provide insight into why the forces and moments vary with wind direction. When results are expressed with reference to wind axes they can be directly compared with calculations and are more readily appreciated.