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For deepwater drilling riser system facing grounding risk in the South China Sea, a mechanical analysis model of deepwater drilling riser grounding accident was established. Focusing on drilling riser grounding accidents of a deepwater floating drilling platform in the South China Sea, mechanical characteristics and grounding accidents of riser were analyzed. And the riser retrieving window was determined. The results showed that the shape of the riser system after riser grounding is catenary. The maximums of bending moment and von Mises stress are located at lower flex joint. Platform moon pool, tensioner and lower flex joint are damaged due to severe bending deformation in the upper and lower drilling riser system after riser grounding accidents. The riser may fracture if drilling platform continues to move toward shallow water. Drilling platform displacement and riser lift height must be within the riser operation window during riser retrieving, or else the riser may fracture and the LMRP may suffer further damage. Residual riser and LMRP can be retrieved in a fixed position when the drilling riser system is separated from seabed. Key words: typhoon, deepwater, drilling, riser, grounding accident, accident analysis

The common critical criterion and nonlinear search method were adopted for the study of connected operation envelopes of deepwater drilling riser and a riser-wellhead-conductor integral finite element model was established. The combination parameters of drilling platform offset, current speed and slip joint stroke were used to determine the riser operability envelopes. The results show that the drilling envelope has an upconing shape and is limited by lower flex joint angle when the surface current speed is low (less than 1.0 m/s). In downstream direction, when the current speed increases, the rotation angle of the lower flex joint increases and the allowable maximum offset of the platform reduces, but in upstream direction, the conditions will be opposite. When surface current speed exceeds 1.0 m/s, the drilling envelope is limited by upper flex joint angle. When it is in upstream direction, the increase of current flow will increase the rotor angle of the upper flexible joint and reduce the drilling envelope rapidly. The non-drilling envelope and emergency disconnect sequence (EDS) actuation envelope are mainly subject to the maximal equivalent stress of the conductor and they will drift towards the upstream direction with the increase of current speed. In addition, through the analysis on influence factors of connection window operation of riser, the tensile force at the top could be increased and drilling fluid density could be reduced properly, so that the drilling envelope could be expanded. Key words: deepwater drilling, riser, drilling envelope, non-drilling envelope, EDS actuation envelope