Beyond the Watch Circle Wellhead System Global Weak Point Analysis
Authors: Andrew J. Grohmann, Engineering Manager - Analysis and Reliability at Dril-Quip; Jim Kaculi, Vice President of Engineering at Dril-Quip
Presented by: Andrew Grohmann
Watch circles are defined by the offshore drilling contractor as an acceptable zone of operation for a dynamically positioning floating drilling vessel during the drilling process. If the vessel moves outside of this zone, loads can be transmitted into the drilling equipment that may exceed equipment ratings and cause equipment failure. This paper presents a system weak point analysis of this scenario using 3D finite element analysis (FEA) of an API studded BOP flange, hydraulic wellhead connector and typical 15ksi subsea wellhead system with a 27” OD mandrel. The FEA model included all critical well components down to the first 200 ft below mudline to evaluate the equipment as a system. It is important to include the entire system to accurately capture the interactions and correct loads transferred and shared among the various components. The actual field installation sequence was modeled, and included component contact, all preload conditions, soil resistance, casing string weight and cement. Each component of the system was modeled using elastic-plastic material properties as per ASME BPVC Section VIII Div. 3. The paper presents the following conclusions and recommendations. A comprehensive subsea wellhead system weak point 3D analysis was successfully completed. Using a recommended set of example parameters, weak points were identified in the wellhead system. Finally, a failure criteria and method was proposed to help the industry define what constitutes failure and assist in the adoption of common failure criteria.
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