The natural frequency of oil derrick changes with the changing of the load it bears on, this character of oil derrick is the theoretical basis for establishing the dynamic evaluating method.
石油井架的固有频率随所受荷载的变化而变化这一特性，是建立石油井架承载能力动态测评方法的理论依据。
3
Derrick is the main supporting parts of drilling rig. It bears much static and dynamic load under different conditions. So there are high requirements for design and performance of derrick.