INTEGRAL JOINT 16-IN. CASING SETS LENGTH RECORD

A record length and weight of 16-in. casing was set by Louisiana Land & Exploration Co. (LL&E) in its Big Horn 3-36 well in the Madden field in the Wind River basin in Fremont County, Wyo., according to the company.

Ron Crow, Denver operations manager for LL&E, reported that the 14,495 ft of casing had an air weight of 2,149,462 lb. The maximum hook load was 1,900,000 lb because of the buoyancy effect of the drilling fluid. The 146.05 lb/ft P-110 seamless casing string included an integral joint connector design tested to loads and pressures exceeding the safety factors required by LL&E.

The casing string consisted of 344 joints with the new integral connector design on top of 31 joints threaded with an older design from LL&E's inventory (Fig. 1).

Drilling in this part of the Big Horn area targets the Madison formation below 24,500 ft. Several casing and liner strings are required to isolate some troublesome zones, including both lost circulation and high pressure problems. The heavyweight 16-in. string set at a depth around 14,000-15,000 ft provides isolation for several thousand feet of drilling before a 9/8-in. casing string is set around 19,800 ft.

To cut drilling costs by eliminating the need for coupling stock, LL&E approached Baker Premium Threading for a 16-in. version of its NJO integral joint design.

The NJO had only been available previously on 131/8 in. and smaller casing strings.

According to Richard Delange, engineering service manager in Houston for Baker Premium Threading, the " Big NJO" connector had to be fabricated on 16-in. OD, 0.906-in. wall seamless casing and offer at least 70% tension efficiency, along with full internal and external pressure sealing capability.

JOINT DESIGN

The Big NJO integral joint connector resulted from two other designs, the NJO and the HD-L.

The HD-L uses a thread form that provides equal elastic strength in both tension and compression, which is important for bending loads encountered with large-diameter pipe. The NJO has a two-step, swaged box design and has been in use for more than 8 years.

The new connector combines the two-step tapered design and a hooked four-pitch, twin-lead, dual-thread form that makes up with a maximum of three turns after stab and virtually eliminates the possibility of thread jump out (Fig. 2). The connector was customized to larger diameter pipe by using a thread depth of 0.063 in. and a center-step torque shoulder with a negative 15 reverse trapping angle. This type of shoulder resists overtorquing and helps position the pin properly in the box.

The connection is rated equal to the American Petroleum Institute (API) minimum internal yield pressure rating of the pipe; the metal seal is generated by radial interference as the end of the pin contacts a 15 seal surface machined into the base of the box, assisted by applied internal pressure.

Cold forming is used to expand the box end by approximately 5% and slightly contract the pin end for integral BIG NJO threading. The stress is relieved by induction heating.

This method of end finishing provides a joint that meets large-diameter casing requirements at approximately 50% of the cost of weld-on connectors.

TESTING

Fritz Engineering Laboratory at Lehigh University, which has load testing equipment capable of exerting a 5 million-lb static load, performed the verification testing on the connector design.

The tension test reached frame capacity without failure, exceeding the previously calculated minimum parting load of 4,850,000 lb.

Three specimen joints were tested to failure in compression. The lowest failure load was 4,520,000 lb, which exceeded the calculated failure load of 4,395,000 lb.

During internal pressure tests as specified in API RP 5C5, three 16-in. specimen joints were subjected to pressure cycling up to 13,100 psi, and none leaked. A fourth sample was pressure tested to above 16,000 psi before seal leaking occurred. There was no indication of mechanical failure at these pressures.

This proved the casing suitable for the Big Horn well:

The casing had a pressure rating of 10,700 psi, which is equal to the API casing pressure rating.
The finished OD of the integral joint provided the required clearance ratio.
The single connection significantly reduced the potential for leaks, compared to two threaded connections on coupled joints.