In recent years, as the importance of floating offshore structures has grown, several independent studies have identified deficiencies in current code of practice procedures for the calculation of wind-induced overturning moment. These studies have mostly concentrated on specific platforms which are in operation and the authors consider that it is now time to take a step back and consider the behaviour of structures in this class and highlight the inadequacy of calculation procedures. Based on the evidence of published studies, the importance of accurate wind load calculation is discussed from the operator's viewpoint Detailed measurements are presented from a parametric wind tunnel study which examined the influence of freeboard, deck layout and heel angle for a range of azimuth, or wind, angles The results show that lift forces and the platform's freeboard have a major influence on the overturning moment It was found, for example, that the wind can provide a destabilising moment on the platform as it tilts into wind - a situation where consideration of drag force alone would always suggest a righting moment The test results from the study identify a preferred calculation method and show the trends of behaviour for this type of platform. Following the preliminary study in a ‘dry’ wind tunnel, a further investigation was carried out to determine the influence of the sea surface. These additional tests were undertaken m the new combined wind-wave facility at the University of Western Ontario They provide a valuable insight into the suitability of performing wind tunnel tests on offshore platforms in the absence of a realistic sea surface model.
Floating structures are now being more commonly used for both the exploration and production of offshore oil and gas reserves These structures have many forms but all must address the accurate assessment of the environmental loading For floating structures the loading due to the action of the wind can be a major component of the overall load on the structure.
Traditionally, the calculation of the wind loads for offshore structures has been performed using a ‘building block’ approach in which the load on each element of the structure IS calculated independently This approach, borrowed from onshore codes, considers only the along-wind or drag, component of loading.
For a typical floating structure the ratio of above-water to below-water surface area is greater than for a fixed platform and it is not constrained to a fixed position. The traditional methods of calculation, therefore, need to be critically examined in three areas -
Assessment of the degree of conservatism inherent in the simple discrete ‘building block’ approach This is important due to the increased ratio of wind to wave loading
Consideration of the contribution of forces other than drag forces to the overall wind loading
Dynamic wind effects associated with the low frequency content of the wind spectrum coupled with the influence of the sea surface and global body motions