A New Approach to the Design of Threaded Connections
Authors: Reynolds G.F. , Summerfield P.D. 
Published: December 2007

A new joint has been designed to overcome the problems associated with running large diameter API buttress thread surface casing. The paper covers the design concept; assessment of design options; initial paper covers the design concept; assessment of design options; initial Finite Element Analysis of design options; detail design and fine mesh elastic/plastic F.E.A. of the final design; product testing under internal pressure, bending, and tension; correlation of strain gauge data with computer stress analysis; The joint is of the threaded and coupled type and the development of machining techniques and novel gauging techniques for producing precision threaded connections on large diameter casing is discused. Experience in initial field trials of the joint is covered.

1. BACKGROUND

Large oil well casing in the diameter range 16 inches to 20 inches is connected generally either by A.P.I. Buttress Threads or by some form of welded on connector which may be threaded or have some other form of latching system. The A.P.I. Buttress threaded and coupled connection is widely used in land wells and from fixed platforms and jack-up rigs offshore but is less suitable for from floating or semi-submersible rigs because of the difficulty of stabbing and running when there is movement between the casing attached to the well and the new casing length which is following the motion of the rig.

These stabbing and running problems arise from the design of the A.P.I. Buttress connection (Fig. 1), which at these casing diameters allows little to tolerance in stabbing accuracy and high risk of cross threading if there is movement of the stabbed casing length during rotation as the joint is made up.

In the event of cross threading of an A.P.I. Buttrees connection the angular misalignment of the stabbed length is so small, approximately .5 degrees, that it is not readily visible under rig conditions. Furthermore, the extra forces generated during the make up of a crossed thread are not always evident because the large diameter casing can readily distort from the circular shape to alleviate any rise in torque generated by the interference in the threads in the early part of make-up. The end result can be a joint made-up to apparent full torque but having partial crest to crest contact and consequent lack of penetration and incomplete load frank contact at make-up. This can cause premature tensile thread pull out failure below the elastic load limit of the casing pipe body. pipe body.

2. THE DESIGN PROBLEM

The primary objective in designing an alternative joint to A.P.I. Buttress was to improve the stabbing and running characteristics of the large diameter casting sizes. The surface casing joint has the following design roles:-To facilitate the assembly of discrete lengths of casing intoa single string.To carry the tensile load of the string up to pipe body yield strength.To carry the compressive load of the string up to the pipe body yield strength.To contain internal pressure in the casing up to pipe body internal yield pressure.To resist external pressure up to pipe body collapse pressure.To resist bending stress equivalent to maximum deviations achievable by the pipe body.

Under these headings the A.F.I. Buttress joint, in surface casing strings, has the following performance:-Buttress has severe limitations on ease and correctness of assembly, particularly under conditions of rig or pipe movement in adverse weather conditions. The joint is slow to make up, requiring some ten turns from stabbing to final make up.But generally has adequate tensile strength although falling marginally short of pipe body yield strength in some size/weight combinations.

P. 959