A Class Perspective for Wind Load Assessment

This study describes the purpose, scope, and acceptance criteria for wind load assessments that are used in the Classification of offshore units and installations. Wind loadings are applied as external environmental forces when evaluating afloat stability, structural strength, and mooring system integrity for mobile offshore units and site-specific installations. Existing criteria in the Rules and industry standards are identified to show the usage of the wind loads and their varying characterization depending on purpose, such as time averaging period and vertical speed distribution. The current requirements are placed in historical context based on the development of the offshore Classification Rules over the last 50 years, and the evolution of tools and technology to carry out the computations over that period are discussed. Practical aspects of calculation and independent analysis are illustrated by applying the standard methodologies in example calculations, using a geometric model that is of similar layout and complexity compared to existing offshore units. Sample calculations are presented as typically performed by Class in an independently developed model, and sensitivity analysis is used to point out areas where results may diverge depending on the approach by the assessor. The effect of these modeling decisions to evaluate wind moment is put in perspective by showing the change in the allowable vertical center of gravity, which relates directly to the deck load capacity of the platform. The result is a consolidated illustration of the Classification methodology for wind load analysis, which has been uniformly applied to a large number of offshore units and installations over a significant period of time. The importance of common assumptions and approach is highlighted, based on the effect that varying certain parameters has on overall results of the wind load calculation. The resulting impact of wind load changes on the stability, mooring, or structural analysis will be further shown. This will assist in providing visibility to industry on what constitutes a complete report of the wind loads for Classification purposes, how detailed a model should be for basic and detail design purposes, and how to improve confidence in the submission and approval process during design.