Development of External Pressure Resistant Tool Joint for Under-balanced Drilling
Authors: Muradov Andrei , Smith Jackie E. 
Published: December 2007

Abstract
This paper presents a new tool joint for 3–1/2 in. drill pipe developed for under-balanced drilling. In addition to the shoulder seal, the tool joint features a radial metal-to-metal seal that prevents leakage from the annulus to the inside of the drill string. It is a 5 in.×2–7/16 in. double shoulder connection with a tensile strength equal to that of the same size NC38 and a torsional strength 85% greater than an NC38. There is no sacrifice in hydraulic efficiency with this connection.

Introduction
Under-balanced drilling has become more widely used in recent years because it results in less formation damage in the production zone and can yield increased penetration rates. Safety issues have been the primary deterrent in under-balanced drilling; however, techniques and equipment have become available to address these problems.
The safety aspects of under-balanced drilling require that the tool joints seal external pressure as well as internal pressure.One major operator specified a sealing capability of 5,000 psi external gas pressure in a tool joint for 3–1/2 in. pipe.
This paper discusses the seal design, the performance characteristics of the tool joint, the results of the finite element analysis and the laboratory testing used to establish the critical performance behavior of the tool joint. The radial metal-to-metal seal configuration was selected over other seal concepts because of its simplicity and reliability and could be integrated into an existing double shoulder tool joint design. Figure 1 is a line drawing of the connection. The finite element analysis studies the stress distribution in the tool joint and the seal integrity with simulated make-up, external pressure, and tension. Laboratory testing verified the effectiveness and durability of the seal.
Seal Design
The design objective was defined to develop a tool joint that would seal 5,000 psi external gas pressure to satisfy the operator's requirements, and could be used in future applications of this nature when a much stronger sealing capabilities (up to 12,500 psi) might be demanded from a tool joint.
Conventional tool joints are designed to be made-up and broken-out quickly to insure fast trip times. They are easy to stab, to spin-up and to make-up power tight. Similarly, they are easy to un-buck and easy to separate. The threads and sealing surfaces are durable and robust enough to withstand the handling damage that comes with tripping the pipe in and out of the hole and racking it back.
The make-up shoulders and thread compound combine to seal drilling fluid in conventional tool joints. These shoulders are flat surfaces perpendicular to the axis of the joint. The widths range from about 1/8 in. on small connections to over ½ in. on larger connections. When the tool joint is in good condition and properly made-up, the contact stress on the shoulder surfaces is near 100,000 psi. The metallic or other solid particles in the thread compound form a gasket between the contacting surfaces. For sealing drilling fluid, a degree of surface damage on the shoulders can be tolerated provided the damage does not go all the way across the face. Tensile loads and bending loads felt by the tool joint during the drilling operation can reduce the contact stress between the shoulders and decrease their ability to seal.