| Summary: | The rig derrick of a ZJ50/3150DB drilling rig in an offshore jacket platform located in the Bohai bay vibrates obviously when drilling. In order to assess the safety of the derrick when drilling, vibration test for the derrick was carried out and a finite element model of the derrick was built to analyze the impact of vibration on the derrick's structure safety, in combination of the field test data with the calculation results got using the model. Natural frequency, amplitude-frequency response when drilling, vibration amplitude and vibration acceleration of the derrick were tested. Based on the derrick drawing and field situation, the finite element model of the derrick structure was built using ANSYS software. The first-order natural frequency of the derrick calculated using the model was 0.555 8 Hz, which coincided with the filed test data and proved the validity of the model. The analysis of the field test data and the calculation results, show, the derrick is resonance-prone when the rotation speed of the top drive is 36 r/min, the derrick C-style opening has low torsional strength, the two front brace rods of the derrick substructure are crucial components that concern the derrick safety, and derrick vibrations under different working conditions and shock vibration happening when the top drive passing by the monkey board meet safety requirements. Key words: ocean drilling rig, derrick, vibration test, finite element model, vibration assessment
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