Behavior of 15-ksi Subsea Wellhead Gaskets as Determined by Analysis and Testing

ABSTRACT Deflection and pressure integrity behaviors are described for the Type VX 15,000 psi service subsea wellhead gasket; loading renditions considered include preload, cyclic pressure, and combined pressure and bending moment. This information has been generated by a combination of direct measurement techniques, finite element analysis, and full scale strain gage testing. Minimum gasket yield strength requirements and pressure integrity with connector separation are discussed. INTRODUCTION During the time period of December 1987 through December 1988, recurring damage to the VX wellhead gasket and mating connector sealing surface was experienced while drilling several 15,000 psi subsea wells in the North Sea. This damage consisted of the development of worm-like grooves in the wellhead connector side of the gasket seating surface, in combination with embedded metal particles in the gasket. While no loss of pressure integrity was experienced during subsea operations, the condition of the sealing surfaces provided a potential leakage problem. Investigation into these field problems resulted in a detailed study of the behavior of the 15,000 psi VX gaskets; the significant results of this study are described in this paper. Initially the focus of the investigation was to determine if dimensional discrepancies, gasket material properties, mechanical malfunction, or operational procedures were the cause of the problems; as the investigation proceeded, low yield strength gasket material was identified as the cause. At this point the focus of the investigation turned to the evaluation of the effects of low yield strength material on gasket behavior through analysis and testing. DESIGN OF VX SUBSEA WELLHEAD GASKET A subsea wellhead is designed to mate with a remotely operated hydraulic connector. A metal gasket, carried by the connector, is used at the wellhead to connector interface to provide pressure integrity; this gasket is routinely replaced each time the connection is remade. The gasket and interface between the subsea wellhead and the mating connector represents a critical pressure barrier between the well bore and the surrounding subsea environment; it is a highly loaded interface subjected to both maximum working pressure and environmentally induced bending moments. The subject VX gasket design and other similar gasket designs have demonstrated reliable service under these conditions for many years. The general design of the subject VX gasket and mating seal surfaces is shown in Figure 1. The design is seen to consist of:Stainless steel inlay at ID of wellhead; surface is at an angle of 23 degrees with the vertical and has a surface finish of 32 microinches.Stainless steel inlay at ID of hydraulic connector; surface is at an angle of 23 degrees with the vertical and has a surface finish of 32 microinches.Gasket main body with ID that provides full bore clearance, along with upper and lower seal surfaces to mate with the wellhead and connector seal surfaces; gasket seal surfaces are at a nominal angle of 22 degrees 45 minutes with the vertical and have a surface finish of 32 microinches.