Structural Requirements for the Effective Transfer of Environmental Loadings in a Subsea Wellhead System

Abstract

ABSTRACT This paper reviews the evolution of design requirements for subsea wellhead housings used in offshore drilling. Structural integrity is paramount in order to resist environmental loadings. A new wellhead design incorporating a pair of axially spaced tapered sockets is presented. Results of detailed computerized analysis along with test data on a full-scale test article are presented. A review of field operations for the new design is presented along with the design of a tool which can be used to seat the sockets, thus making the system useful for extreme fatigue-rated drilling. INTRODUCTION Early subsea wellhead system designs used a single landing or support shoulder between the 18-3/4" subsea wellhead (HP housing) and the 30" conductor housing (LP housing). This type of configuration was found to operate well and provided years of successful operation until two field failures occurred[8][1]. Both involved drilling in high current conditions. Hopper[1] reports that an exploration well being drilled in the North Sea west of Shetland had to be abandoned 29 days after running the HP housing due to the fatigue failure of the weld between the casing extension and the wellhead body. These unexpected failures of subsea wellhead housings during drilling operations prompted an in-depth study into the behavior of a subsea wellhead system when it is subjected to external bending loads. Most of these studies were completed using simple two-dimensional finite element analysis and a maximum bending moment of about 1 million foot pounds, as discussed in references [3], [4], [5], [6], and [7], although reference [2] clearly addresses bending loads above 2 million ft.-lb. The results of all these studies showed that the magnitude of the bending stresses developed in the casing extension below the high pressure wellhead housing is greatly affected by several factors:Type of interface between high and low pressure housings, i.e., single or double socket type.The radial clearances which exist at the socket.The effective cement level in the annulus between the 30" conductor and the 20" casing extension.The size of the wellhead extension.The amount of applied load. Further, these studies showed that to ensure a subsea wellhead system has sufficient structural integrity to endure the static and fatigue bending loads which may be encountered during drilling and production operations, a wellhead system should preferably employ a dual socket interface with the radial clearance at these sockets suitably adjusted to the vertical distance between the sockets. The magnitude of environmental loadings continued to increase with the introduction of Tension Leg Platforms (TLP), because drilling or production risers on these vessels remain connected to the subsea wellhead system in all conditions. This factor automatically results in a considerable increase in the static and fatigue loading which the wellhead system will experience.