A Fracture Mechanics-Based Inspection Criterion For Internal Walls Of Offshore Wellhead Equipment

ABSTRACT The purpose of this work is to present an inspection criterion for the internal walls of pressure containing wellhead equipment which is less restrictive than existing limits. The discussion concerns only non-metallic indications oriented parallel to the component bore axis. The proposed criterion is based on a combination of fracture mechanics theory, experimental verification of the presence of non-metal 1ic stringers, estimates of stringer aspect ratios and conservative engineering assumptions. The development starts with the selection of a 1inear elastic fracture mechanics model for a longitudinal crack on the internal wall of a tube. A practical situation with dimensions and conservative assumptions is then used to illustrate the proposed procedure. Interaction of adjacent indications is discussed and a criteria for acceptance in cases of possible interaction is presented. Finally, a simple flow chart showing the various decisions to be made in dispositioning a component with stringer indications is presented. INTRODUCTION Present acceptance criteria for magnetic particle inspection of oil tool pressure containing equipment is based on API specification 14D. This specification states that magnetic particle "linear indications" on forged or wrought products should not exceed 3/16 inch maximum and there should be no more than 10 indications in any 6 square inch area [1]*. The term "linear indications" does not distinguish between actual cracks and non-metallic stringers. Since this 14D criterion is extremely restrictive, it would be useful to develop a more realistic one. The purpose of this paper is to present an approach for setting criteria for stringer type indications on the pressure side walls of tubular products in oilfield service using fracture mechanics technology. The information presented here is for longitudinal indications only. Linear indications transverse to the axis of the tube are rare in practice and if they exist, they indicate severe qua1ity problems. Therefore, transverse 1inear indications should be restricted to 3/16 inch maximum and no more than 10 transverse indications in any 6 square inch area per API 14D. FRACTURE MECHANICS FLAW MODEL Although this work is not intended as a primer on fracture mechanics, several fundamental points must be understood. First, fracture mechanics involves the determination of the intensity of stress, K, at the tip of a sharp crack. (Note: This is not stress intensity as referred to in the ASME Pressure Vessel Code.) The stress intensity for loads (stress) normal to the crack faces, K1 is equal to a flaw geometry factor, ?, times a gross stress, ?g, times a term involving the square root of a crack depth, a. In equation form this relation reads as follows:(Mathematical equation available in full paper) The ? factor accounts for flaw shape (i.e., buried elliptical, buried circular, surface semi-elliptical flaws, etc.), and crack depth to total part thickness ratio. The stress ? g, is the stress at the flaw 1ocation ignoring the fact that the crack is there! "a" is the crack depth.