Gas-Tight Rotary Shoulder Connection for Critical High-Pressure Completion, Intervention Riser, and Drilling Applications
Authors: Jellison Michael J. , Muradov Andrei , Brock Jim , Vinson Barry J. 
Published: March 2008

New developments in drilling and completion tubulars are necessary to drill and complete deeper, longer-reach and higher pressure wells in a cost-effective manner. Rotary shoulder style connections traditionally employed on drillpipe can offer significant advantages when coupled with high-pressure, gas tight metal-to-metal seal technology for completion and workover pipe, intervention risers and drillpipe for UBD applications. The paper provides details on how these critical applications can benefit from a new connection technology.

This paper presents the results of a two year comprehensive effort to design, test, and qualify third-generation rotary-shouldered connections for 20,000 psi internal and 10,000 psi external pressure service. ISO13679 testing methodologies for casing and tubing connections were modified to evaluate rotary shoulder connections pressure capabilities. Results from comprehensive finite element modeling and extensive laboratory testing designed to reproduce the harsh, aggressive loading modes and high pressures encountered in field use are presented.

Although drillpipe, drillpipe connections and drill stem materials represent mature technologies, innovations are being developed in these areas. The third generation gas-tight, double-shoulder connection presented here represents several advancements that address some of the challenges ahead.

Introduction

Much focus has been made toward the advancement of connection technology to permit high torque drilling of extended reach, directional and horizontal wells. In response to this need, the development of third generation, ultra-high torque connections was recently announced providing reduced tripping times and the mechanical and hydraulic load requirements for drilling current and future deepwater, extended reach and ultra-deep wells.

This third generation connection incorporates a double-start thread form that reduces the number of revolutions to assemble the connection by 50%, reducing trip time. The thread form also provides a unique dual-radius thread root that offers a step change improvement in fatigue resistance.

In addition to providing a 20,000 psi internal and 10,000 psi external pressure rating, the new connections provide increased mechanical and hydraulic performance compared to second generation high torque connections while also providing fatigue performance greater than standard API connections.

Double-Shoulder Connection Design Development

First generation double-shoulder connections (1st Gen. DSC), see Figure 1, were introduced in the early 1980's.1 1st Gen. DSC's were basically API rotary-shoulder connections (primarily NC or FH) with a second shoulder added inside the box member at the pin nose interface.2,3,4 These 1st Gen. DSC's incorporated the same basic design features in terms of thread form, taper, lead, pitch diameters, etc. as the API connection they were based on. These connections yielded a simple, straight forward solution that increased the connection torsional strength by approximately 40 percent over the corresponding API connection.

The increased torsional strength provided by the secondary shoulder allowed designers to streamline the connection, increasing the internal diameter (ID) and/or decreasing the outside diameter (OD) to improve hydraulic performance of the drill string. As more severe drilling applications were encountered, it became clear that 2nd Gen. DSC's with higher torsional capabilities and more streamlined configurations were required.