Cold-forged connector ties back well risers in Congo field

William M. Taylor
Cameron
Marc Tison, Andre Bahoumina
Elf Congo

The cutaway section of a cold-forged connection, made during lab tests, shows that the tapered actuating pistons have forced dogs outward to press inner sleeve into grooves in outer sleeve (Fig. 2).

A forging tool is being placed into position for lowering into a well riser (Fig. 3).

The tieback of eleven 103/4-in., high-pressure risers in Elf Congo's offshore N'Kossa field used a cold-forge tieback system to create a metal seal.

The time-saving tieback method allows for placing the risers in residual tension.

The installation work was completed in the fall of 1995.

Average time to complete the 103/4-in. casing tiebacks, including test and nipple-up and nipple-down times, averaged 52 hr/well. Tiebacks for all three casing strings averaged 90 hr for all surface and subsea operations including BOP test and nipple-up/nipple-down time.

Slip-type casing hangers have historically been used for such riser tiebacks from mudline wells to platforms. However, this requires considerable time to nipple-up/nipple-down and cut casing. Also, slip and seal hanger systems usually have elastomer seals on rough casing as the primary annulus pressure barrier.

Metal sealing of the primary casing annulus has been made practical because the offshore industry has gone toward compact-wellheads and hanging of the completion on a mandrel.

Hanging the completion on a mandrel, however, has its own set of considerations. Exact riser length may be difficult to predict before running because the riser must first be locked into the mudline casing hanger and then landed out on the support shoulder in the surface head.

Also, a general desire is that riser tieback strings should be in tension after installation. This is not always easy with a passive or "dumb" hanger and fixed shoulder configuration.

Threaded, adjustable mandrel hanger systems exist but can require very close casing string space-out to achieve the desired residual riser tension.

Objectives

Elf's two N'Kossa platforms are about 70 km offshore of Pointe Noire, Congo in 170 m of water. The eleven 41/2-in. and 5-in. subsea wells are rated at 10,000 psi working pressure and have casing programs of 26 in., 133/8 in., and 103/4 in. The 103/4-in. casing is tied back.

In selecting the cold-forged tie-back system, Elf wanted to cut rig time and maximize blow-out preventer (BOP) protection.

A through-the-stack, adjustable tensioning system with metal sealing with a cold forged connection proved to best answer Elf's objectives. Specifically, a cold-forged, tensioned tieback sub was installed in the riser just below the surface casing hanger (Fig. 1).

This system added no additional height to the platform wellheads. The same wellhead can, therefore, be used for both tied back and conventionally drilled wells. The forging tool creates a sound, tensioned joint with all-metal sealing, all at the same time. The 103/4-in. casing string was tied back with 500,000-lb tension.

Forged sleeves

The tieback system includes a forging sleeve that functions much like a slip-joint sub.

The CamForge telescoping sleeve permits the mandrel surface hanger to land out in the platform head. The sub is then stroked out so that mudline tieback operations can be completed and tested conventionally (Fig. 2).

The tensioning/forging tool (Fig. 3) is run into the hole on drill pipe and landed below the hanger in the sub itself. Exact tension, plus a small overpull, is applied to the entire riser against the subsea tieback. While maintaining riser tension, forging pressure is transmitted through the drill pipe to the forging tool to achieve a robust, tensioned, all-metal connection.

The forging tool is stroke limited rather than pressure limited to prevent any possibility of "over forging" the connection.

Running sequence

A typical running sequence includes:

Assemble tieback sub in shop. Make-up just below surface casing hanger. Lock assembly into running tool, and run.

Land surface casing hanger in casing head. Sub will stroke out to allow tieback tool to land in subsea casing hanger. Mudline tieback tool can be torqued in place with running tool, or a dedicated subsea tieback torque tool can be used. Tool is then retrieved.
Run seal assembly and lock in place in surface housing. With hanger locked in place by seal assembly and tieback tool made up in mudline hanger, fix casing riser top and bottom. Run forging tool into hole and lock. Extend hydraulic locking dogs into mating slots in sub to lock in tie-back sub.
Apply upward load to drill pipe and forging tool to create tension in casing riser. At N'Kossa, the 230 metric ton tension resulted in about a 10-cm stretch of the 103/4-in. casing riser.
Apply 5,000 psi pressure down drill pipe to force forging dogs out to contact inner sleeve which, in turn, is forged outward into profiles in outer housing.
Release drill pipe tension and let pipe travel down about 5 in. Rotate drill pipe to left to J-mandrel back into running position. Apply pressure down drill pipe to reset tool by stroking upper body up. This allows forging dogs to retract. Vertical pull then retrieves forging tool.
Test tensioned tieback sub by either pressuring entire casing riser, or by using an isolation test tool, a double-cup tester, to test only tieback sub. N'Kossa risers were tested to 4,800 psi by pressuring the entire string including tieback subs.

Cold forging

The CamForge cold forging process (Fig. 4 [34891 bytes]) has been developed over the previous 10 years. In fact, attachment of N'Kossa's 133/8-in. casing strings to surface wellheads used the CamForge process.

In the tensioned tieback application, forging force is developed by the forging dogs. Close examination of the process reveals that as the forging dogs are forced outward, the inner forge sub reaches its elastic limit as it expands plastically into the inner hardened contours of the outer forge subhousing.

This expansion continues until the outer housing is also stressed well into its elastic range. When forging pressure is removed from the tool, outer housing contraction creates large residual compressive stress between the two sleeves.

It is this residual force that makes permanent and reliable the metal connection to the hardened profiles in the outer housing.

Extensive finite element analysis determined how stresses were distributed in the inner and outer tubular members during forging. Stress distribution also allowed precise prediction of the various forging pressures-vs.-material strengths, wall thickness, and residual stress in the forged sections.

Prototype test

Lab tests on a full-scale prototype tieback connection based on N'Kossa parameters included full function, pressure, and maximum load certification with:

10,000-psi internal hydrostatic pressure
5,000-psi external hydrostatic pressure
1 million-lb tension test
10,000-psi internal hydrostatic pressure with 500,000-lb tension
40-in. adjustment length.

With Elf participation, separate factory acceptance/certification tests were also completed at lower loads of 880,000-lb tension plus 4,800-psi internal hydrostatic pressure, in combination.

The new tieback system is available rated in both NACE or non-NACE. A full 30 day H2S test was successfully performed on the NACE-rated system with 8,000-psi internal pressure and 400,000-lb tension simultaneously maintained through a 1-month evaluation using an aggressive NACE solution for the test medium.

This test and following metallurgical examination were performed by an independent test facility.

The Authors

William M. Taylor is manager of Cameron's subsea project development team in Houston. He has worked with Cameron since 1966. Taylor has a BS in mechanical engineering. He attended both Texas A&M University, and the University of Houston.

Marc Tison is a drilling and completions engineer in Elf Pau's technical department, Pau, France. He has worked in Pau for 17 years.

Andre Bahoumina is a drilling department manager for Elf Congo. He has been with Elf for 23 years, working worldwide as a drilling and completions engineer.