Watch circles are established by operators to ensure a dynamically positioned vessel does not induce loads that exceed wellhead system equipment capacities. In the event the vessel moves off station, beyond the boundary of the watch circle, loads could be transmitted that may approach or exceed the equipment ratings and possibly cause failure. A whole system weak point analysis approach is required to determine and identify the likely failure modes and locations throughout the system.
This paper presents a system weak point analysis using a 3D finite element analysis (FEA) of an API Studded BOP Flange, Hydraulic Wellhead Connector, and Subsea Wellhead System. The FEA models include the first 200 ft below the mudline and are used to evaluate the capabilities of the entire system to failure. To ensure accurate load transfer between the components the actual field installation sequence was modeled, which included contact between components, all preloads, soil resistance, casing string weights, and cements. Subsequently, the global external loads from the rig were incrementally increased beyond the rated load capacities of the entire system to determine failure points. Loads, stresses, and strains in each component were determined as a function of the global external loads. Each component of the system was modeled using elastic-plastic material properties per ASME BPVC, Section VIII, Division 3, Paragraph KD-231.4.
Critical points throughout the wellhead system were monitored for failure which was defined as either through wall plastic strain or through wall stress above yield across the entire wall of a component. A sequential order of limiting locations, or weak points, was determined (first limiting location, second limiting location, etc.). The weak point analysis performed provided valuable information to assess and help mitigate risk. The analysis results also provided valuable insight and context to the equipment manufacturer to develop more robust and reliable system designs.
The insights gathered by doing a weak point analysis using large 3D FEA models of the entire system, inclusive of the wellhead system, wellhead connector, and flanged connection, are presented and provide a better understanding of the system behavior and failure mechanisms.