Downhole CT tool connections

There are several connections used in coiled tubing (CT) services for the purpose of isolating pressure and transferring tension, compression, and torsional loads from tools and bottomhole assemblies onto the tube. These connections are typically designed to be field installed and reusable. The most common CT connections are discussed in this article.

Type of CT connections

Nonyielding connections

The following connections have the capability of securing loads and pressure to the end of the CT in a manner that, during makeup, does not result in yielding of the tube body.

External slip type

The external slip-type connection requires the use of a slip or grapple-type load ferrule placed on the outside diameter (OD) of the tube body. The load ferrule is typically constructed with sharp “spiraled” teeth that secure the ferrule onto the CT. The tool connection mechanically wedges the load ferrule onto the CT OD during connection makeup.

Pressure integrity of this connection is typically maintained with the use of O-rings or other types of elastomeric seals on the OD of the CT body. The external CT surface must be prepared to allow for an effective seal.

Internal slip type

The internal slip-type connection requires the use of a slip or grapple-type load ferrule placed within the inside diameter (ID) of the tube body. The load ferrule is typically constructed with sharp “spiraled” teeth that secure the ferrule onto the ID wall of the CT. The tool connection mechanically wedges the load ferrule into the CT ID during connection makeup. Pressure integrity of this connection is typically maintained with the use of O-rings or other types of elastomeric seals applied against the ID of the CT body. The internal CT seam must be removed, and the ID prepared for an effective seal by smoothing and buffing the ID surface scars and imperfections.

It should be noted that these non-yielding connections require that the terminated end of the CT be reasonably round, with OD/ID dimensions within the connector size tolerance. Problems often arise when using these connections on older, used CT that have become oval (distortion of tube roundness) or have experienced diametral growth.

The changes in CT geometry caused by ovality and diametral growth make sealing difficult when using common O-ring technology. Other elastomeric seals are often employed in larger OD CT when the diametric clearance may exceed O-ring sealing parameters.

Changes in CT OD geometry also present problems when trying to install the connectors onto the CT. Where CT geometry changes resulting from ovality are present(typical in larger OD CT sizes), a swaging tool may be used to return the tube body to a “near round” condition. The swaging tool is constructed similar to a muffler-pipe expander, with a hollow-core hydraulic jack pulling a short swaging cone up through a longer split skirt expander. The diameters of the swaging cone and skirt expander are chosen for a given CT size to yield the tube body OD a few thousandths larger than desired. After swaging, the CT body springs back, providing a CT OD with minimal ovality.

Yielding connections.

The following connections have the capability of securing loads and pressure at the end of the CT in a manner that, during makeup, results in yielding of the tube body.

External dimple type

The external-dimple-type connection is secured onto the tube body through the use of numerous mechanical screws. Forces exceeding the CT material yield strength create “dimples” in the tubing. These “dimples” serve as recessed receptacles for the mechanical screws that secure the connection to the tube body OD. Other variations of this basic connection method include pressure actuated dimpling tools used in conjunction with a template. Pressure integrity of this connection is typically maintained with the use of O-rings or other types of elastomeric seals on the OD of the CT body.

Internal dimple type

The internal-dimple-type connection is secured onto the CT body through the use of numerous mechanical indentations into recesses on an internal mandrel insert. Forces exceeding the CT material yield strength create “dimples” in the tubing, serving as the load transfer mechanism that secures the connection to the CT body ID. Pressure integrity of this connection is typically maintained with the use of O-rings or other types of elastomeric seals on the ID of the coiled tube body.

Roll-on type

The roll-on type connection incorporates a machined insert mandrel designed to fit inside the CT. The mandrel is machined with circular recesses or “furrows.” The connection is secured to the tube by means of mechanically yielding the tube body into the machined recesses on the mandrel. Pressure integrity of this connection is typically maintained with the use of O-rings or other types of elastomeric seals on the ID of the tube body.

Weld-on connectors

Weld-on connectors are used in special applications such as coiled tubing drilling (CTD) in which larger CT is used, and the aforementioned connectors impose operating limitations. These limitations include an excessively large OD, reduced torque or other load ratings, vibratory and oscillating load suitability, or a restricted ID that reduces the size of pump-down darts or ball.

Properly designed weld-on connectors will exhibit 100% of the torque and yield ratings of the CT material.

Important parameters to proper weld-on connector design and application include:

• A gradually tapered insertion neck several inches up from a straight-wall section that continues to the weld-bead location (Fig. 1). This taper provides a bend support to prevent concentrating bending loads at a single point where the CT meets the weld-on connector. The straight-wall section of the connector is used to eliminate all bending to the CT heat-affected zone.
• “Chill blocks” aid in limiting the heat-affected zone from welding operations. The connector’s straight-wall OD exceeds the ID of the CT by several thousands of an inch. The weld-on connection is therefore inserted after the CT is preheated in preparation for welding operations.

• 

  Fig. 1—Weld-on connector.

See also

PEH:Coiled-Tubing_Well_Intervention_and_Drilling_Operations

Noteworthy papers in OnePetro

J. J. Kolle, K. Theimer et al. 2007. Development of a Downhole Separator and Intensifier for Coiled Tubing Jetting, Canadian International Petroleum Conference, Jun 12 - 14, 2007. http://dx.doi.org/10.2118/2007-169.

Hubertus V. Thomeer, Rex Burgos et al. 2009. Improving Performance During Coiled Tubing Logging Operations With a Rugged, Modular, Stab-Through Tool System, SPE/ICoTA Coiled Tubing & Well Intervention Conference and Exhibition, 31 March-1 April 2009. 121746-MS. http://dx.doi.org/10.2118/121746-MS.