High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e. S-N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (for fracture mechanics approach) as well as fracture toughness performance.
For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis.
This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection is presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK) and maximum stress intensity factor (Kmax).