Wind load derricks

The derrick or mast must also be designed to withstand wind loads. Wind loads are imposed by the wind acting on the outer and inner surfaces of the open structure. When designing for wind loads, the designer must consider that the drill pipe or other tubulars may be out of the hole and stacked in the structure. This means that there will be loads imposed on the structure by the pipe weight (i.e., setback load) in addition to the additional loads imposed by the wind. The horizontal forces due to wind are counteracted by the lattice structure that is firmly secured to the structure s foundation. Additional support to the structure can be accomplished by the guy lines attached to the structure and to a dead man anchor some distance away from it. The dead man anchor is buried in the ground to firmly support the tension loads in the guy line. The guy lines are pretensioned when attached to the dead man anchor. 

API Standard 4A (superseded by Standard 4F) provides rating of derrick capacities in terms of maximum safe load. This is simply the load capacity of a single leg multiplied by four. It does not account for pipe setback, wind loads, the number of lines between the crown block and the traveling block, the location of the dead line, or vibratory and impact loads. Thus, it is recommended that the maximum safe static load of derricks designed under Standard 4A exceed the derrick load as follows ... 

The value of 1.5 is a safety factor to accommodate impact and vibration loads. Equation 4-1 does not account for wind and setback loads, thus, it may provide too low an estimate of the derrick load in extreme cases. 

All conditions listed in the section titled Load Capacities, subsection titled Mast and Derricks under Dynamic Conditions," are to be specified by the user. Forces resulting from wind and vessel motion are to be calculated in accordance with the formulas presented in the section titled Design Specifications, paragraphs titled Wind, Dynamic Loading (Induced by Floating Hull Motion). ... 

In addition to withstanding some minimum load or loads (sections titled Load Capacities and Design Specifications ), derricks and masts that satisfy API standards must also satisfy certain requirements regarding materials, allowable stresses, wind, dynamic loading, earthquakes and extremes of temperature. 

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