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Summary A subsea blowout preventer (BOP) system is critical in providing safe working conditions for drilling activities in deepwater oil exploration. However, performance evaluation of the shear ram BOP is still a challenge for the industry. In this paper, two aspects of BOP shearing capability are studied: The required shear force to cut the pipe inside BOP successfully and the time it takes to shear the pipe and close the well. A finite element model is used to calculate the maximum shear force of a typical drillpipe, while an analytical hydraulic model is proposed to calculate the BOP closing time. A case study is presented for estimating the emergency disconnect sequence time in a deepwater offshore scenario. It is assumed that the BOP control system is activated from the surface, employing an electrohydraulic multiplexed control device for closing the BOP and rapidly disconnecting the lower marine riser package (LMRP), thus preventing damage to both riser and wellhead.

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During subsea offshore drilling operations, the floating vessel is connected to the wellhead through a series of equipment assemblies. The riser is exposed to dynamic loads from currents, waves, and drilling platform motions. Therefore, a dynamic analysis of the riser system is required to obtain forces and moments in the wellhead. It is even more important in dynamic positioning (DP) operations, where the knowledge of boundary conditions for a safe emergency disconnection is highly relevant. The objective of this paper is to calculate the effects of changing the light and older BOP (typically with 4 rams) for newer and heavier BOPs (after Macondo accident, with 6 rams) on the Emergency Disconnect Sequence (EDS) time and on wellhead equipment strength. A typical scenario of drilling is proposed, in which the marine riser system considers the drilling string, the lower flex-joint, the marine riser, the kill and choke lines and the tensioners. Tensioners are used to top tension the marine riser and to compensate relative heave motion between the riser and the floating vessel. Riser systems were designed using the API RP 16Q and simulated using Orcaflex software. Vessel dynamic motions were calculated according to the response amplitude operator (RAO) data from a typical offshore drilling semi-submersible. The vessel motion is superimposed by a drift-off motion, which consists of a horizontal displacement along time in the direction of the propagating waves. It is employed to simulate the condition of vessel position loss due to thruster and or control system failure. Results indicate that the use of heavier BOPs reduces significantly the available time for initiating the EDS. Results are worse if older generation of wellhead equipment is used, where a smaller drift-off is necessary to safely disconnect the riser before potential equipment and environmental damage occurs.