Fracture Mechanics-Based Fatigue Predictions for HPHT Equipment

ABSTRACT

A methodology covering fatigue life predictions for high pressure high temperature (HPHT) equipment is presented. HPHT equipment is generally thick-walled and by a common definition involves pressure ratings of 15,000 psi and above and temperature ratings of 350ºF and above. A fracture mechanics-based approach is described including the crack growth methodology and the determination of the design life. The approach is similar to that used in the ASME Boiler and Pressure Vessel Code and discussion is presented including a comparison to engineering criticality assessments (ECAs) covered in API recommended practices and in British Standards. The application of safety factors through input variables and through the analysis methodology is discussed. The limitations of S-N fatigue analysis for the evaluation of thick-walled high pressure equipment are also discussed.

Example problems are presented to display the application of the method to subsea equipment. The stresses are examined on a series of cylinders with several wall thickness to diameter ratios. This includes a cylindrical vessel with dimensions similar to a wellhead housing. The finite element method is used to determine the appropriate stress profiles through the thickness for the extreme loading and the cyclic loading.

Several sensitivity studies are performed to display the influence of the inputs. The relative importance of material toughness (i.e. CTOD or KIC) and residual stress are discussed. While their values are generally important, they can vary widely without influencing the fatigue life prediction. The influence of other material properties, such as the crack growth constants, and residual stresses are also discussed.

Cordes, R., San Pedro, R.I., “Fracture Mechanics-Based Fatigue Predictions for HP/HT Equipment,” OTC 19502, Presented at the 2008 Offshore Technology Conference, May 5, 2008, Houston, Texas.