Abstract
As the oil and gas industry moves into drilling and production operations in deeper waters and harsher environments, there is a great demand to develop the next generation of tools and technology to meet these challenges. Subsea wellhead systems form a critical part of the drilling and production equipment, and a proper understanding of their behavior under complex loading conditions is crucial as they provide a critical barrier between the wellbore fluid and the environment.
High-pressure, high-temperature (HPHT) applications can require the use of heavy wall components with significant structural discontinuities and complex geometric features. These conditions result in nonlinear stress gradients across the wall sections and degraded material properties caused by high temperatures. For these applications, API TR 1PER15K-1 [1] suggests the use of elastic-plastic finite element analysis (FEA) in place of the standard linear elastic methodology of stress categorization. An elastic-plastic analysis of a subsea wellhead system with a standard 36 in. ×22 in. ×10¾ in. casing program in accordance with ASME BPVC Section VIII Div. 3 has been performed [2].
Following the recommendation of API TR 1PER15K-1 with regards to system approach evaluation, the full wellhead system was included in the model. To accurately account for the complex interactions and load transfer between system components contributing to the overall structural response, the model used in this analysis included all major components of the system and was sequenced and preloaded in a method analogous to real-world field installation. System preload, global external loads, pressures, and casing loads were applied to the model. The model extended 200 ft below the mudline, a depth considered sufficient for the wellhead system to be free of any boundary effects. The 3D model included non-linear springs to simulate soil resistance. Cements were also included where appropriate.
The wellhead system was required to demonstrate sufficient resistance to failure due to global collapse, local plastic damage, and buckling. Analysis results for each failure mode and a comparison of rated loads using the standard linear elastic and elastic-plastic methodologies is presented. Using more advanced analysis techniques based on industry best available and safest technology, FEA results have allowed for more insightful validation testing, better-informed design decisions, and an improved understanding of system performance in extreme field environments.
