Fatigue Analysis of Cladded HPHT Pressure Containing Equipment

Abstract

Large, pressure-containing equipment used in subsea production, including Christmas trees (XTs), gate valves, flowloops, jumpers, and connectors, are constructed of low-carbon steel and cladded with corrosion resistant alloys such as Alloy 625. Cladding is a welding process that generates high tensile residual stresses on the cladded layer and on the heat affected zone (HAZ). High pressure high temperature (HPHT) equipment for subsea applications, designed to operate above 15 ksi internal pressure and 350°F, is also cladded. This equipment experiences severe working conditions in the field, plus cyclic loading during operations, such as riser loads, installation, intervention and, most importantly, startup and shutdown sequences. Per API 17TR8 guidelines, all HPHT equipment be hydrostatically tested to 1.5 times the equipment rated working pressure (RWP). For 20 ksi equipment, the hydrostatic test pressure is 30 ksi, which can significantly deform any highly localized stressed regions. These regions deform plastically when test pressure is applied. When the pressure is bled, these regions experience high compressive stresses due to surrounding materials that are still elastic. This paper analyzes a simplified HPHT cladded gate valve (GV) body for fatigue loading (pressure cycles only) using the ASME Sec VIII, Div-3 method. The fatigue stress amplitude is calculated using an elastic-plastic material (E-P method) finite element analysis. In this method, first, the residual stress from the cladding process is simulated, and then the hydrotest is simulated on the component. During the hydrostatic test, fatigue sensitive regions (FSRs) or highly localized stressed regions such as the valve cavity and seat pockets, deform plastically, and the initial weld tensile residual stress turns to compressive (similar to autofrettage). Later, when these components are subjected to working pressure cycles (startup and shutdown), the shear stress range remains the same but the mean stress on the FSR reduces significantly. By considering all types of residual stresses, the high cycle fatigue life can be predicted with a high degree of accuracy.