Squall Wind Elevation/Gust Factors and Squall Coherence

Abstract A squall is a strong transient wind event generated by a convective storm. Though squalls are an operational concern over a wide range of latitudes, they are typically only design events in tropical areas. At this time, there are no widely accepted elevation and gust factors for squalls that are appropriate for engineering purposes. No specific guidance is given on squall gusts in the present API or ISO standards. The nature of wind squall events is fundamentally different than that of high latitude non-cyclonic storms and tropical cyclones which are presently the only types of wind events specifically addressed by API and ISO standards. Such factors are needed for squall events so that measurements made in different ways may be converted into homogeneous databases at specified elevations and gust intervals such that they can be analyzed in an unbiased way. Such factors are also needed to convert squall criteria values at a specified elevation or gust interval to a relevant elevation and interval required for the engineering of a specific structure or appurtenance. The West Africa Gust (WAG) joint industry project JIP was formed to better understand the nature of wind squall extremes offshore West Africa. The WAG JIP has concluded its third and final planned phase. The first phase of WAG focused on developing regional return period extreme value estimates of squall winds. The second and third phases of WAG focused on understanding the temporal and spatial nature of squall gusts in the context of the requirements needed for offshore engineering. Results were based on an offshore multi-anemometer array set up by the third phase of the JIP (WAG3) offshore West Africa. In this paper, expressions for wind gust elevation profile and gust factors are presented for squall winds over water. Coherence of longitudinal variations in squall winds is compared to the expressions for non-squall winds presently in API and ISO standards. The results presented herein will lead to an improved treatment of squall events for offshore engineering applications.