API 4F pdf download Specifcation for Drilling and Well Servicing Structures 3.34 structural safety levelssL The classification of a drilling structure application by a purchaser to reflect various degrees of consequence offailure, considering life safety and other issues such as pollution, economic loss, and public concern. 3.35 substructure Structure(s) through which the mast load, hook load, rotary load, andor setback...