Study on Deepwater Drilling Riser Wear

Abstract

Several riser wear accidents have occurred in offshore oil industry, which threaten offshore drilling safety and cause huge human, economy and environment losses. In 2012 and 2013, several deepwater exploration wells were accomplished in the South China Sea by CNOOC as the operator with the deepwater drilling unit HYSY981. And the assessment and management of deepwater drilling riser wear are problems to be solved urgently. Analysis method of multi-tube coupled system was studied and mechanical analysis models of riser and drillstring were established in this paper. Riser wear of an exploration well in the South China Sea was calculated according to proposed methods. The influence of drilling platform displacement, riser top tension and rotational stiffness of lower flex joint on riser wear was also studied. The results show that the contact between riser and drillstring is interval distribution. The maximum contact force occurs at lower flex joint. The contact force near mudline between casing and drillstring is also large while contact forces of other positions are little. Analysis result of riser-drillstring contact is good agreement with actual phenomena. Drilling platform displacement, riser top tension and rotational stiffness of lower flex joint are important factors influencing riser wear. Riser wear is closely related to flex joint angle, and proper drilling platform displacement leads to small flex joint angle and little riser wear. Riser wear can also be reduced by increasing riser top tension and rotational stiffness of lower flex joint. The related results and conclusions in this paper provide reference for offshore drilling and guide inspection, monitoring and maintenance of riser wear. Then riser integrity and offshore drilling safety can be guaranteed.