Design Margins for Normal, Extreme, and Survival HPHT Applications
Authors: Grohmann Andrew J. , Selvey Jordan , Ellisor Scott 
Published: April 2017

Typical subsea equipment design uses the widely industry-adopted stress design allowables of 2/3 × Yield, 80% × Yield, and 100% × Yield for normal, extreme, and survival load conditions, respectively. These factors are associated with current API product specifications based upon the ASME linear-elastic (LE) analysis approach and establish the customary relative design margins between the three load event categories. For high-pressure high-temperature (HPHT) and often thick-walled equipment, API17TR8 recommends the use of more advanced elastic-plastic (EP) stress analysis over LE techniques. This is because EP analysis utilizes the actual material constitutive response that more closely approximates the actual structural behavior, better represents the non-linear stress distribution across the component's wall, and models the redistribution of stress that occurs due to inelastic deformation. EP analysis relies on the concept of Load and Resistance Factor Design (LRFD) where the allowable service load(s) are determined by dividing the calculated collapse load by a factor to account for uncertainty and the resistance of the components to the load is thereby assessed. The ASME Section VIII Division 2 and Division 3 codes (hereinafter referred to as Div. 2 and Div. 3) currently only assign LRFD factors for normal load conditions and the applicability of these factors for the other types and magnitudes of loads in a subsea environment need further evaluation. There exists a need for the oil and gas industry to adopt verified and validated normal, extreme, and survival load factors for EP analysis that provide the same relative margins as the LE design allowables.

The ASME EP verification analysis method assesses the acceptability of a component by evaluating several failure modes. The focus of this study is the global collapse failure mode that results from ductile rupture or gross deformation as a result of the applied loads. One of the benefits of the EP method is when a system is analyzed as a whole, consistent design margins are de facto applied to all components. Sound engineering practice dictates that whatever load factors used must be valid for simple geometries if they are expected to be applicable for complex assemblies. Analysis models and test specimens of simplified geometries were analyzed to collapse and tested to failure under combined loads so trends could be identified and established. The LRFD factors are applied to the calculated collapse loads from analysis and correlations are made to the validation tests to failure. The uncertainty of survivability when using the factors is also discussed realizing these design factors have been established for load events whose frequency of occurrence is based on risk assessment.

Physical testing of 43 thick-walled test specimens to failure has been performed. Normal, extreme, and survival load capacities were calculated based on collapse from EP analysis using multiple material models and were compared to the loads at failure in the tested specimens. Probabilistic methods have been applied to assess the likelihood of failure using the various verification methods. This work would be beneficial for use by the industry standards' committees when considering the adoption of normal, extreme, and survival design factors.